Experimental Study on Thermal Performance of a Loop Heat Pipe with Different Working Wick Materials

energies

Article Experimental Study on Thermal Performance of a Loop Heat Pipe with Different Working Wick Materials

Kyaw Zin Htoo 1 , Phuoc Hien Huynh 2, Keishi Kariya 3 and Akio Miyara 3,4,*

1 Graduate School of Science and Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan; [email protected] 2 Department of Heat and Refrigeration Engineering, Ho Chi Minh City University of Technology—VNU—HCM (HCMUT), 268 Ly Thuong Kiet, Ho Chi Minh City 72409, Vietnam; [email protected] 3 Department of Mechanical Engineering, Saga University, 1 Honjo-machi, Saga 840-8502, Japan; [email protected] 4 International Institute for Carbon-Neutral Energy Research, Kyushu University, Nishi-ku, Motooka, Fukuoka 819-0395, Japan * Correspondence: [email protected]

Abstract: In loop heat pipes (LHPs), wick materials and their structures are important in achieving continuous heat transfer with a favorable distribution of the working fluid. This article introduces the characteristics of loop heat pipes with different wicks: (i) sintered stainless steel and (ii) ceramic. The evaporator has a flat-rectangular assembly under gravity-assisted conditions. Water was used as a working fluid, and the performance of the LHP was analyzed in terms of temperatures at different

locations of the LHP and thermal resistance. As to the results, a stable operation can be maintained in the range of 50 to 520 W for the LHP with the stainless-steel wick, matching the desired limited ◦ −2 Citation: Htoo, K.Z.; Huynh, P.H.; temperature for electronics of 85 C at the heater surface at 350 W (129.6 kW·m ). Results using ◦ Kariya, K.; Miyara, A. Experimental the ceramic wick showed that a heater surface temperature of below 85 C could be obtained when −2 Study on Thermal Performance of a operating at 54 W (20 kW·m ). Loop Heat Pipe with Different Working Wick Materials. Energies Keywords: electronics cooling; loop heat pipe; wick materials; thermal conductivity 2021, 14, 2453. https://doi.org/ 10.3390/en14092453

Academic Editors: Alessandro Mauro 1. Introduction and Gabriela Huminic Loop heat pipes use a passive two-phase heat transport device in which advantageous characteristics are offered, such as operation against the gravity through the capillary effect Received: 1 March 2021 and ﬂexible characteristics. They have been widely used in energy applications, spacecraft Accepted: 20 April 2021 Published: 25 April 2021 thermal control, electronic device cooling, and commercial radiators [1]. Zhou [2] fabricated a plate-type loop heat pipe (LHP) and investigated the effect on the heat transfer

Publisher’s Note: MDPI stays neutral performance of the LHP with multilayer metal foams as a wick structure. Their flat evapo- with regard to jurisdictional claims in rator’s experimental results show that multilayer copper foams have better performance published maps and institutional affil- than nickel foam because of higher thermal conductivity and smaller pore size. Siedel [3] iations. presented a numerical simulation, comparing it with the experimental data of a flat disk- shaped evaporator. The results showed that the larger effective thermal conductivity of wick has an influence on the overall thermal performance of the LHP. However, other researchers, Maydanik [4] and Hoang [5], suggested the wick material, with its lower effective thermal conductivity, to handle the problem of heat leakage. In addition, the Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. hydrophilic effect on the ceramic wick structure is higher than that of stainless steel [6]. This article is an open access article Therefore, the performances of stainless-steel wicks and ceramic wicks in LHPs are com- distributed under the terms and pared in this study, of which the ceramic wick has much lower thermal conductivity and a conditions of the Creative Commons higher hydrophilic effect [7,8]. In an LHP system, the porous wicks’ structure is essential Attribution (CC BY) license (https:// because it provides the capillary force for working fluid circulations, a liquid flow path, and creativecommons.org/licenses/by/ a place for phase change heat transfer. Moreover, the porosity effect of the wick materials 4.0/). inside the evaporator is necessarily considered for improving the overall performance of

Energies 2021, 14, 2453. https://doi.org/10.3390/en14092453 https://www.mdpi.com/journal/energies Energies 2021, 14, × FOR PEER REVIEW 2 of 20

Energies 2021, 14, 2453 2 of 23 wick materials inside the evaporator is necessarily considered for improving the overall performance of LHPs [9]. Therefore, the porosity, pore diameter, and thermal conductiv- LHPsity of the [9]. wicks Therefore, were themeasured. porosity, The pore influence diameter, of the and different thermal wicks conductivity on the performance of the wicks wereof LHPs measured. is tested The with inﬂuence the designed of the evaporator different wicks. Although on the this performance is a simple of case LHPs study is tested of an witharrangement the designed with different evaporator. wicks, Although the approp this isriate a simple engineering case study approach of an of arrangement the calcula- withtion procedure different wicks, related the to appropriateapplying the engineering designed evaporator approach ofis thepresented calculation in this procedure paper. It relatedmay improve to applying the understanding the designed evaporator of the problems is presented that exist in this in paper. the LHP It may evaporator improve and the understandingfuture LHP systems. of the problems that exist in the LHP evaporator and future LHP systems. The LHP hashas somesome similarsimilar operationoperation principlesprinciples withwith conventionalconventional HPs.HPs. TheThe phasephase changing process is happened in the LHP systemsystem and the capillarycapillary force isis usedused asas thethe motivation for the operation. Figure1 1 demonstrates demonstrates the the scheme scheme of of the the analytical analytical LHP LHP and and an operation cycle diagram.

(a) (b)

FigureFigure 1.1. Principles of LHP: (a) Scheme ofof analyticalanalytical LHP.LHP. ((bb)) LHPLHP workingworking cyclecycle diagram.diagram. Reproduced from [[1],1], Yu.F.Maydanik:Yu.F.Maydanik: 2005.2005. At first, the system is not supplied with heat load. In this case, the liquid stays at LevelAt A–A first, as the an assumption.system is not The supplied liquid with found he atat the load. evaporator In this case, zone the (Point liquid 1) andstays the at compensationLevel A–A as an chamber assumption. evaporates The fromliquid the found wick at when the evaporator heat is supplied zone to(Point the evaporator. 1) and the Thecompensation vapor from chamber the evaporator evaporates flows. from Then, the wick it approches when heat to is the supplied heating to wall. the evaporator. As a result, theThe vaporvapor pressurefrom the reduces.evaporator Simultaneously, flows. Then, itthe approches temperature to the rises heating a little wall. (Point As a 2)result, and isthe higher vapor than pressure the vapor reduces. located Simultaneously, at the compensation the temperature chamber. rises In this a little situation, (Point the2) and wick is behaveshigher than as a the thermal vapor barrier. located The at vaporthe compensation which is superheated chamber. condition In this situation, in the evaporator the wick zonebehaves cannot as a passthermal the barrier. compensation The vapor chamber which through is superheated the wick condition which is in saturated the evaporator due to thezone force cannot of capillarypass the thatcompensation keeps the chamber liquid inside. through Then, the the wick hydraulic which is lock saturated is happened due to inthe the force wick. of capillary Afterwards, that keeps the vapor the liquid continuously inside. Then, flow tothe the hydraulic condenser’s lock is inlet happened (Point 3).in Whenthe wick. vapor Afterwards, flows from the Point vapor 2 continuously to Point 3, both flow temperature to the condenser’s and pressure inlet (Point decrease. 3). When The de-superheat,vapor flows from condensation, Point 2 to andPoint subcooled 3, both temperature processes happen and pressure from Point decrease. 3 to Point The 5. de- In thissuperheat, case, there condensation, is no pressure and loss subcooled from Point processes 3 to Point happen 5 as anfrom assumption. Point 3 to Point Because 5. In of this the pressurecase, there loss is andno pressure the hydrostatic loss from resistance Point caused3 to Point by friction,5 as an theassumption. pressure differenceBecause of∆ Pthe56 includespressure theloss pressure and the hydrostatic loss. Afterward, resistance the liquid caused located by friction, at Stage the 6 pressure flows into difference the chamber ∆P56 forincludes compensation. the pressure Moreover, loss. Afterward, some parts the of liqu theid heat located load areat Stage provided 6 flows to theinto evaporator the chamber at thefor expensecompensation. of the workingMoreover, fluid. some This parts condition of the isheat heated load toare temperature provided toT 7the. The evaporator progress fromat the Point expense 7 and of the Point working 8 are relative fluid. This to the condition filtration is ofheated the liquid to temperature through the T7. wick The intopro- thegress evaporation from Point place.7 and Point In this 8 are way, relative the liquid to the may filtration prove of to the be superheated.liquid through However, the wick its boiling-up does not happen because of its short duration in such a state. The state of the working fluid in the vicinity of the evaporating menisci is represented by Point 8.

Energies 2021, 14, 2453 3 of 23

Pressure loss (dP1–8) corresponds to the total value of pressure losses in all the working- fluid circulation sections. From the above analysis, the function of the LHP is considered in three conditions. At first, the capillary condition is the condition for conventional HPs for an operation. ∆Pc ≥ ∆Pv + ∆Pl + ∆Pg (1) where ∆Pv is the working fluid’s pressure loss during the vapor state’s motion. ∆Pl is the working fluid’s pressure loss during the liquid state’s motion. ∆Pg is the pressure loss because of the hydrostatic of the liquid column. ∆Pc is the wick capillary pressure. The second condition is just for the LHP. At the LHP’s startup, it ensures that the liquid is exhibited from the evaporating zone to the compensation chamber.

∂P ∆T − = ∆P (2) T 1 7 EX ∂ Tv

where ∂P/∂T is the derivative of the saturated line’s slope. Tv is the mean temperature between T1 and T7. ∆PEX is the total pressure loss in all the sections of the working ﬂuid’s circulation except the wick. At the third condition, the liquid is prevented from steaming in the liquid line because of the ambient pressure loss and heating.

∂P ∆T − = ∆P − (3) T 4 5 5 6 ∂ Tv

where ∂P/∂T is the derivative of the saturation line’s slope. Tv is the mean temperature between T4 and T5. ∆P5–6 is the pressure loss in total from State 5 to State 6. The working fluid should own a high value of dP/dT to minimize the temperature difference ∆P1–7 and ∆P4–6 from the second condition. The purpose of the tested LHP in this study is the cooling of electronics such as processors located at Data Centers (DCs). Therefore, the evaporator surface which is heated needs to be flat to improve the evaporator and electronics’ contact quality. Moreover, it will make the heat flux q and distribution of temperature on the active surfaces uniform and eliminate the occurrence of mounting blocks at the evaporator. The investigated evaporator design belongs to the evaporator group with opposite replenishment (EOA). In this design, liquid flows from the top to the bottom surface of the wick structure, as shown in Figure2a. Figure2b shows the evaporator with longitudinal replenishment (ELR). The liquid from ELR is provided from the compensation chamber which is located behind the wick. Otherwise, the liquid flows perpendicular to the heat flow rate. The evaporator with opposite replenishment own a simple structure and the liquid absorption surface is large. However, the evaporator can become thicker because the compensation chamber is located above the capillary system. Moreover, the heat leak from the evaporator to the compensation chamber through the wick will be more severe because of the large cross-section of the wick. Energies 2021, 14, 2453 4 of 23 Energies 2021, 14, × FOR PEER REVIEW 4 of 20

(a) (b)

FigureFigure 2. 2.LHP LHP evaporators evaporators (a )(a Opposite) Opposite replenishment, replenishment, (b) Longitudinal(b) Longitudinal replenishment. replenishment. Reproduced Reproduced from from [10], Y.F.[10], Maydanik:Y.F.Maydanik: 2014. 2014.

TheThe investigated investigated evaporator evaporator has has two two parts: parts: the the vapor vapor collector collector and and the the wick’s wick’s space space and compensation chamber. The copper plate is used for the separation between two and compensation chamber. The copper plate is used for the separation between two ele- elements. The brazing method is used to fix them. Additionally, to install the thermo- ments. The brazing method is used to fix them. Additionally, to install the thermocouples, couples, a hole with a 1-mm diameter and 22.5 mm for the length was fabricated at the a hole with a 1-mm diameter and 22.5 mm for the length was fabricated at the evaporator’s evaporator’s base. The heating block’s top surface and the evaporator’s bottom surface base. The heating block’s top surface and the evaporator’s bottom surface have the same have the same area: 27 cm2 (45 × 60 mm). This dimension was considered based upon area: 27 cm2 (45 × 60 mm). This dimension was considered based upon the specifications the specifications of some processors, as presented in Table1. Different LHP technologies of some processors, as presented in Table 1. Different LHP technologies focussing on var- focussing on various designs of the LHP’s evaporator and other types of LHPs such as ious designs of the LHP’s evaporator and other types of LHPs such as miniature LHPs, miniature LHPs, micro LHPs, evaporator with longitudinal replenishment (ELR) LHPs, micro LHPs, evaporator with longitudinal replenishment (ELR) LHPs, and LHPs with and LHPs with parallel condensers are compared and summarized in Table2. parallel condensers are compared and summarized in Table 2. Table 1. Processor specifications. Table 1. Processor specifications. Thermal Power Case Dimensions Heat Flux No. Modern 2 2 No. Modern Thermal Power Design (W) Design Case (W) Dimensions(mm (mm× mm) × mm) Heat(W/cm Flux (W/cm) ) 1 Core i7 5960X 140 52.5 × 45 5.9 1 Core i7 5960X 140 52.5 × 45 5.9 2 Core i7 5930K 140 52.5 × 45 5.9 2 Core i7 5930K 140 52.5 × 45 5.9 3 Core i7 4960X 130 52.5 × 45 5.48 3 Core i7 4960X 4 Core 130 i7 4930X 130 52.5 × 52.545 × 45 5.48 5.48

4 Core i7 4930X 6 Core 130 i7 3790X 150 52.5 × 52.545 × 45 6.3 5.48 7 Xeon E7 8891 v3 165 52 × 45 7.05 6 Core i7 3790X 150 52.5 × 45 6.3 8 Xeon E7 8880 v3 150 52 × 45 6.41 7 Xeon E7 8891 v3 165 52 × 45 7.05 9 Xeon E7 8890 v2 155 52 × 45 6.62 8 Xeon E7 8880 v3 10 Itanium 150 9300 185/155/130 52 × 48.545 × 40.25 9.47/7.94/6.66 6.41

9 Xeon E7 8890 v2 11 Itanium 155 9500 170/130 52 × 48.545 × 40.25 8.71/6.66 6.62

10 Itanium 9300 185/155/130 48.5 × 40.25 9.47/7.94/6.66

11 Itanium 9500 170/130 48.5 × 40.25 8.71/6.66

Table 2. Different LHP technologies from the literature.

Specifications of LHPs from the No. References Objects and Results Literature K. Fukushima— Objects: A wick with a liquid core is proposed. 2017 [11] Flat—rectangular evaporator An experimental and computational investigation was conducted. Keywords: New-fashioned flat evaporator The temperature distribution inside the evaporator and the heat 1. Capillary force, structure, micro-LHP (Evaporator 20 × load’s break down from the mathematical model are obtained. heat transport, 10 × 3 mm, 200 mm in length transport Results: Minimum RLHP is 1.2 K/W and maximum Q = 11 W, from LHP, porous line) the experiment. PTFE

Energies 2021, 14, 2453 5 of 23

Table 2. Different LHP technologies from the literature.

Speciﬁcations of LHPs from No. References Objects and Results the Literature Objects: A wick with a liquid core is proposed. An experimental and computational investigation was conducted. Energies 2021, 14, × FOR PEER REVIEW The temperature distribution inside the evaporator and 5 of 20 Energies 2021, 14, × FOR PEER REVIEW the heat load’s break down from the mathematical 5 of 20

EnergiesEnergies Energies 20212021,, 1414 ,2021, ×× FORFOR, 14 PEER,PEER × FOR REVIEWREVIEW PEERFlat—rectangular REVIEW evaporator model are obtained. 55 ofof 2020 5 of 20

New-fashioned flat Results: Minimum RLHP is 1.2 K/W and maximum Porous evaporatorpolytetrafluorethylene structure, wick 17 Q = 11 W, from the experiment. K. Fukushima—2017 [11Porous] polytetrafluorethylenemicro-LHP (Evaporator wick 17 × 9 × 2 mm (PTFE) (50%, 2.2 μm, 6.48 × Keywords: × Porous9Porous × 2 mm20 Porous polytetrafluorethylenepolytetrafluorethylene (PTFE)× 10 polytetrafluorethylene× (50%,3 mm, 2.2 200 μm, mm wickwick6.48 in × 1717 wick 17 10−14 m2, 0.25 W/m·K) 1. Capillary force, 10××− 14 99 m ×× 22, mmlength0.25×mm 9 × (PTFE)W/m·K) (PTFE)2 transportmm (PTFE) (50%,(50%, line) 2.22.2 (50%, μμm,m, 2.2 6.486.48 μ m, ×× 6.48 × Working fluid: ethanol. heat transport, Working1010−−1414 mm2Porous10 2,fluid:, 0.250.25−14 m W/m·K)W/m·K)ethanol.2 polytetrafluorethylene, 0.25 W/m·K) LHP, porous PTFE WorkingWorkingwickWorking fluid:fluid: 17 × ethanol.ethanol. fluid:9 × 2 ethanol. mm (PTFE) (50%, 2.2 µm, 6.48 × 10−14 m2, 0.25 W/m·K) Working fluid: ethanol.

Objects: It presents a method for the wick shape optimization via Objects:Objects: It Itpresents presents a amethod method fo forr the the wick wick shape shape optimization via calculation and experiment. Using only the three-phase contact calculationoptimizationObjects:Objects:Objects: ItIt and presentspresentsvia experiment. It calculationpresents aa methodmethod aUsing method and foforr experiment.theonlythe wickfowick ther the three-phaseshapeshape wick Using optimization optimizationshape contact optimization viavia via line’s length, the hevap is maximized (q = 2 W/cm2). line’sonlycalculationcalculation length, thecalculation three-phase andtheand h experiment.experiment.evap and is contact maximized experiment. Using line’sUsing (q length, only only Using= 2 W/cm thethe theonly three-phasethree-phase2h). evapthe isthree-phase contactcontact contact Effects of the case, wick material,2 and the working2 fluid,2 are Effectsmaximizedline’sline’s of length,length,line’s the (case,q length, thethe= 2 hwickh W/cmevapevap the isis material, maximized maximizedh).evap is maximized and (( qtheq == 2 2working W/cmW/cm (q = 22 ).).W/cm fluid, ).are PTFE wick (bulk thermal conductivity = discussed. PTFE wick (bulk thermal conductivity = discussed.EffectsEffects ofEffectsof thethe case, case,of the wickwick case, material,material, wick material, andand thethe and workingworking the working fluid,fluid, areare fluid, are 0.25 W/m·K).PTFE wick (bulk thermal PTFEPTFE wickwickPTFE (bulk(bulk wick thermalthermal (bulk thermal conductivityconductivity conductivity ==Results:fluid, are= discussed. 0.25 W/m·K).conductivity = 0.25 W/m·K). Results:discussed.discussed. discussed. Stainless steel case (k = 16 W/m·K) TheResults: effect of the three-phase contact line (TPCL) on the groove Stainless0.250.25 W/m·K).W/m·K).Stainless 0.25steel W/m·K). case steel (k = case16 W/m·K) TheResults:Results: effectResults: of the three-phase contact line (TPCL) on the groove Machined widths (minimum): 0.3 pressureThe effect loss of the (by three-phase calculation) contact is shown. line (TPCL) on the MachinedStainlessStainless(Stainlessk widths =steelsteel 16 W/m casecase steel(minimum): ((·kkK) =case= 1616 W/m·K)(W/m·K)k =0.3 16 W/m·K) pressureTheThe effecteffect Theloss ofof effect(by thethe calculation) three-phase three-phaseof the three-phase is contcont shown.actact cont lineline act (TPCL)(TPCL) line (TPCL) onon thethe grooveongroove the groove (circumferential groove) and 0.4 mm Comparisongroove pressure between loss (by evaporator calculation) heat-transfer is shown. coefficient (HTC) M. (circumferentialMachinedMachinedMachined widthswidths groove) widths (minimum):(minimum): and (minimum): 0.4 0.3mm0.3 0.3Comparisonpressurepressure pressure lossloss between (by(by loss calculation)calculation) evaporator(by calculation) isis shown.heat-transfershown. is shown. coefficient (HTC) M. (axial groove) Comparison between evaporator heat-transfer Nishikawara—(axial(circumferential(circumferential groove)0.3(circumferential (circumferential groove)groove) groove) andand groove) 0.40.4 and mmmm 0.4 obtainedmm using Equation (9) and that obtained through the Nishikawara—M.M. M. obtainedcoefficientComparisonComparison Comparisonusing (HTC) between betweenEquation obtained between evaporatorevaporator (9) an using devaporator that Equation heat-transferheat-transfer obtained heat-transfer (9) through and coefficientcoefficient that thecoefficient (HTC)(HTC) (HTC) 2017 [12] (axial(axial groove)groove)and(axial 0.4 groove) mm (axial groove) experiments (ethanol) is presented. 2017Nishikawara—Nishikawara— [12] Nishikawara— experimentsobtainedobtainedobtained throughusingusing (ethanol) EquationEquation using the is experiments Equationpresented. (9)(9) anandd (9) thatthat (ethanol) an obtainedobtainedd that obtained throughthrough through thethe the Keywords: Keywords:20172017 [12][12]2017 [12] isexperimentsexperiments presented.experiments (ethanol)(ethanol) (ethanol) isis presented.presented. is presented. Capillary CapillaryKeywords:Keywords:Keywords: evaporator, M. Nishikawara—2017evaporator,CapillaryCapillaryCapillary [12 ] Keywords:2. capillary 2. capillaryevaporator,evaporator, evaporator, Capillarypumped evaporator, loop, capillary 2. 2.2. pumped2.capillarycapillary capillaryloop, Increasing axial grooves will reduce the pumped loop,evaporator evaporator Increasing axial grooves will reduce the evaporatorpumpedpumpedpumped loop, loop, loop,number of circumferential grooves. HTC, LHP,HTC, optimized LHP, wicknumberIncreasingIncreasing ofIncreasing circumferential axialaxial groovesgrooves axialaxial groovesgrooves willgrooves.will reducereduce willwill reduce thethe the HTC,evaporatorevaporator LHP,evaporator Three wicks were fabricated for shape, 3-phaseoptimized contact wick lineThree numbernumber wicksreducenumber ofof werecircumferentialcircumferential the of fabricated numbercircumferentialof grooves.forgrooves. grooves. optimizedHTC,HTC, LHP,LHP,HTC, wick LHP, experimental examination: shape, 3-phase experimentalThreeThree wickswickscircumferentialThree examination: were werewicks fabricatedfabricated were grooves. fabricated forfor for shape,optimizedoptimized 3-phaseoptimized wickwick L triwick = 3150/m (4 axial × 71 contact line Ltriexperimentalexperimental = 3150/mThreeexperimental (4 wicksaxial examination:examination: × were 71examination: fabricated contactshape,shape, line 3-phaseshape,3-phase 3-phase circumferential) for experimental examination: circumferential)LLtritri == 3150/m3150/mLtri = 3150/m (4(4 axialaxial (4 ×× 71axial71 × 71 contactcontactcontact lineline lineLtri = 2630/mLtri = 3150/m(16 axial × 56 Ltricircumferential)circumferential) = 2630/mcircumferential) (16 axial × 56 circumferential)(4 axial × 71 circumferential) circumferential)LLtritri == 2630/m2630/mLtri = 2630/m (16(16 axialaxial (16 ×× 56axial56 × 56 ClassicalLtri wick= 2630/m with only 16 axial Classicalcircumferential)circumferential) circumferential)wick with only 16 axial grooves(16 and axial 1 mm× 56 width circumferential) of the groove The heat transport’s contribution at TPCL was estimated at 0.87 groovesClassicalClassical andClassical wickwick 1 mm withwith wick width onlyonly with of 1616 theonly axialaxial groove 16 axial The heat transport’s contribution at TPCL was estimated at 0.87 Classical wick with only 16 when fitting to experiment results and 0.63 in simulation. groovesgroovesaxialgrooves andand grooves 11 mmmm and width width1 and mm 1 ofofwidth mm thethe width groove grooveof thewhen groove TheThe heatfittingheat The transport’stransport’s toheat experiment transport’s contributioncontribution re sultscontribution and atatat TPCLTPCLTPCL 0.63 at in waswas wasTPCL simulation. estimatedestimated was estimated atat 0.870.87 at 0.87 Comparison of various working fluids and wick materials, htri of the groove Comparisonestimatedwhenwhen fittingfittingwhen at of 0.87toto fittingvarious experimentexperiment when to working experiment fitting reresultssults to fluids experiment reandandsults and 0.630.63 wickand inin results simulation.0.63simulation. materials, in simulation. h tri increased with wick’s thermal conductivity. The value of htri was increasedandComparisonComparison 0.63Comparison with in simulation. of ofwick’s variousvarious ofthermal various workingworking conductivity. working fluidsfluids and andfluids The wickwick and value materials,materials, wick of hmaterials,tri was hhtritri htri higher for ammonia due to the changes in interfacial HTC. higherComparisonincreasedincreased forincreased ammonia withwith of wick’s variouswick’s with due thermalwick’sthermal to working the thermalchanges conductivity.conductivity. fluids conductivity.in and interfacial wick TheThe valuevalue HTC.The ofof value hhtritri was was of h tri was Flat-disk shape evaporator. Objects:materials, It histri toincreased enhance withheat transfer wick’s thermal of pool boiling using the Flat-disk shape evaporator. Objects:higherhigher It forhigherfor is ammoniaammoniato enhance for ammonia duedue heat toto transfer theduethe changes changesto the of poolchanges inin interfacial interfacialboiling in interfacial using HTC.HTC. the HTC. Tilt angles: −90°, −60°, −30°, 0°, 90° (“−”: conductivity. The value of h was higher for ammonia TiltFlat-diskFlat-disk angles:Flat-disk shapeshape−90°, − evaporator. evaporator.60°,shape −30°, evaporator. 0°, 90° (“ −”: modulated porous wick sinteredtri on the heater wall. Three types of modulateddueObjects:Objects: to theObjects: ItIt porous ischangesis toto enhanceItenhance wickis into interfacial enhancesintered heatheat transfertransfer heaton HTC. the transfer ofof heater poolpool of boilingwall.boiling pool Three boiling usingusing types thethe using of the antigravity) evaporators: MWE (microchannel/wick evaporator), MME antigravity)TiltTilt angles:angles:Tilt angles: −− 90°,90°, −−60°, 60°,−90°, −−30°, 30°,−60°, 0°,0°, − 30°, 90°90° (“(“0°,−−”: ”:90°evaporators: modulatedmodulated(“−”: modulated MWEporousporous (microcha porous wickwick sinteredsintered wicknnel/wick sintered onon thethe evaporator), heateronheater the wall.heaterwall. MME ThreeThree wall. types typesThree ofof types of Forced convective air cooling (Ta = 22 to (modulated monoporous wick evaporator), and MBE (modulated Jinliang Xu— Forcedantigravity)antigravity) convectiveantigravity) air cooling (Ta = 22 to (modulatedevaporators:evaporators:evaporators: monoporous MWEMWE (microcha (microchaMWE wick (microcha evnnel/wicknnel/wickaporator),nnel/wick evaporator),evaporator), and MBEevaporator), (modulated MMEMME MME Jinliang Xu— 24 °C) biporous wick evaporator) were fabricated. 2014 [13] 24ForcedForced °C) Forced convectiveconvective convective airair coolingcooling air cooling ((TTaa == 2222 ( toTtobaiporous (modulated (modulated= 22 to (modulated wick monoporous monoporousevaporator) monoporous werewickwick fabricated.evev wickaporator),aporator), evaporator), andand MBEMBE and (modulated(modulated MBE (modulated 2014JJinlianginliang [13] J inliang Xu—Xu— Xu—Water as working fluid Keywords: Water2424 °C)°C) as 24working °C) fluid bbiporousiporousbiporous wickwick evaporator)evaporator) wick evaporator) werewere fabricated.fabricated. were fabricated. Keywords:20142014 [13][13]2014 [13] There are three layers of porous LHP, ThereWaterWater are asasWater three workingworking aslayers working fluidfluid of porous fluid 3. LHP,Keywords:Keywords: Keywords: materials, primary layer (as shown in 3. evaporator, heatmaterials, ThereThere areareThere primary threethree are layerslayers threelayer of oflayers(as porousporous shown of porous in evaporator,LHP,LHP, LHP, heat table), secondary copper table (2 mm– 3.3. 3.transfer, table),materials,materials, secondarymaterials, primaryprimary copper primary layerlayer table (as(as layer shown shown(2 mm– (as showninin in transfer,evaporator,evaporator, evaporator, heatheat 149 heatμm), and third absorbent wool layer modulated 149table),table),μm), secondaryandsecondarytable), third secondary absorbent coppercopper copper tabletable wool (2(2 table mm–layermm– (2 mm– modulatedtransfer,transfer,transfer, (2 mm–rpore = 20 μm) porous wick (2149 149mm–μμm),m),rpore149 andand =μ 20m), thirdthird μ m)and absorbentabsorbent third absorbent woolwool layerlayer wool layer porousmodulatedmodulated wickmodulated Evaporator: ϕ80 × 10 (without CC Evaporator:(2(2 mm–mm–(2rrporepore mm– ϕ ==80 2020 r× pore μ μ10m)m) = (without 20 μm) CC porousporous porous wickwick wickthickness); Aheating = 5 cm2 thickness);Evaporator:Evaporator:Evaporator: Aheating ϕϕ8080 = × ×5 10 10 cmϕ 80 (without(without2 × 10 (without CCCC CC Results: MBE LHP reduces the startup time and achieves more Vapor line: ID6/OD8 × 5502 mm 2 Vaporthickness);thickness); line:thickness); ID AA6/heatingheatingOD = 8=A 5×5heating cm550cm 2 = mm 5 cm Results: MBE LHP reduces the startup time and achieves more Liquid line: ID6/OD8 × 300 mm stable operation than MWE. LiquidVaporVapor line: line:Vaporline: ID IDID6/ line:6/6/ODODOD 8ID 88× ×6/×300 550OD550 mm 8 mmmm × 550 mm stableResults:Results: operationResults: MBEMBE LHPLHP than MBE reduces reducesMWE. LHP reduces thethe startupstartup the startup timetime andand time achievesachieves and achieves moremore more LiquidLiquid Liquidline:line: IDID line:6/6/ODOD ID88 ×6/× 300OD300 8 mmmm × 300 mm stablestable operationoperationstable operation thanthan MWE.MWE. than MWE.

Energies 2021, 14, × FOR PEER REVIEW 5 of 20

Porous polytetrafluorethylene wick 17 × 9 × 2 mm (PTFE) (50%, 2.2 μm, 6.48 × 10−14 m2, 0.25 W/m·K) Working fluid: ethanol.

Objects: It presents a method for the wick shape optimization via calculation and experiment. Using only the three-phase contact line’s length, the hevap is maximized (q = 2 W/cm2). Effects of the case, wick material, and the working fluid, are PTFE wick (bulk thermal conductivity = discussed. 0.25 W/m·K). Results: Stainless steel case (k = 16 W/m·K) The effect of the three-phase contact line (TPCL) on the groove Machined widths (minimum): 0.3 pressure loss (by calculation) is shown. (circumferential groove) and 0.4 mm M. Comparison between evaporator heat-transfer coefficient (HTC) (axial groove) Nishikawara— obtained using Equation (9) and that obtained through the 2017 [12] experiments (ethanol) is presented. Keywords: Capillary evaporator, 2. capillary pumped loop, Increasing axial grooves will reduce the evaporator number of circumferential grooves. HTC, LHP, Three wicks were fabricated for optimized wick experimental examination: shape, 3-phase Ltri = 3150/m (4 axial × 71 contact line Energies 2021, 14, 2453 circumferential) 6 of 23 Ltri = 2630/m (16 axial × 56 circumferential) Classical wick with only 16 axial grooves and 1 mm widthTable of the 2. Cont.groove The heat transport’s contribution at TPCL was estimated at 0.87 when fitting to experiment results and 0.63 in simulation. Specifications of LHPs from No. References ComparisonObjects and of Results various working fluids and wick materials, htri the Literature increased with wick’s thermal conductivity. The value of htri was Flat-disk shape evaporator. higher for ammonia due to the changes in interfacial HTC. ◦ ◦ ◦ Objects: It is to enhance heat transfer of pool boiling Flat-diskTilt shape angles: evaporator.−90 , −60 , −30 , ◦ ◦ Objects:using the It modulatedis to enhance porous heat transfer wick sintered of pool on boiling the using the 0 , 90 (“−”: antigravity) Tilt angles: −90°, −60°, −30°, 0°, 90° (“−”: modulated porous wick sintered on the heater wall. Three types of Forced convective air cooling heater wall. Three types of evaporators: MWE antigravity) ◦ evaporators:(microchannel/wick MWE (microcha evaporator),nnel/wick MME evaporator), (modulated MME (Ta = 22 to 24 C) Forced convective air cooling (Ta = 22 to Jinliang Xu— Water as working fluid (modulatedmonoporous monoporous wick evaporator), wick ev andaporator), MBE (modulated and MBE (modulated 24 °C) biporous wick evaporator) were fabricated. 2014 [13] There are three layers of Water as working fluid biporous wick evaorator) were Keywords: porous materials, primary There are three layers of porous fabricated. LHP, layer (as shown in table), 3. materials,secondary primary copper layer (as table shown in evaporator, heat Jinliang Xu—2014 [13] table), secondary(2 mm–149 copperµm), and table third (2 mm– transfer, Keywords: 149μm),absorbent and third wool absorbent layer wool layer modulated 3. LHP, (2 mm–(2rpore mm– = 20rporeμm)= 20 µm) porous wick evaporator, heat transfer, Evaporator:Evaporator: ϕ80 × 10φ (without80 × 10 CC modulated porous wick (without CC thickness); thickness); Aheating = 5 cm2 Energies 2021, 14, × FOR PEER REVIEWA = 5 cm2 6 of 20 Vapor line:heating ID6/OD8 × 550 mm Results:Results:MBE MBE LHP LHP reduces reduces the the startup startup time time and and achieves more Vapor line: stableachieves operation more stable than operationMWE. than MWE. Liquid line: ID6/OD× 8 × 300 mm ID6/OD8 550 mm At heat flux of 40 W/cm2 (heater heat flux), the MBE Liquid line: ◦ LHP can operate when T2 c is around 63 C; Condenser:ID6/ OD130 8× ×130300 × 25 mm At heat flux of 40 W/cm (heater heat flux), the MBE LHP can (Rt = 0.12 K/W) Optimum CR = 51.3%. CR = 38.5%, 51.3%; 64.1%,× 64.1%,× 76.9% operate when Tc is around 63 °C; (Rt = 0.12 K/W) Optimum CR = Condenser: 130 130 25 Operation antigravity condition is better than others Sintering process: oven temperature 51.3%. CR = 38.5%, 51.3%; 64.1%, with the MBE LHP’s proper design. 900 °C 64.1%,for 4 h. 76.9% BestOperation fin’s geometric antigravity parameters: condition his =better 1.5 mm; than others with the MBE Sintering process: oven LHP’s proper design. ◦ p = 1.5 mm; w = 3 mm; best particle size: 88 µm temperature 900 C for 4 h. Best fin’s geometric parameters: h = 1.5 mm; p = 1.5 mm; w = 3 mm; Objects:best particle The size: effects 88 ofμm increasing the number of groovesObjects: onThe a effects wick’s of surface increasi onng LHP the performancenumber of grooves on a were investigated. were investigated. LHP with cylindrical wick’s surface on LHP erformance LHP withstainless-steel cylindrical evaporatorstainless-steel (φ16 × 65) evaporator (ϕ16 × 65) S.C. Wu—2014 Water cooling Water cooling [14] Wick material: nickel S.C. Wu—2014 [14] Wick material:(ID/OD =nickel 9/12.5); (ID/OD largest = 9/12.5); Keywords:Keywords: 4. largest rrporepore == 1.9 1.9 −− 2.52.5 μµm;m; K = 1.3 − 3.25 Wick4. structure,Wick structure, LHP, −13 2 × 10 mK =, porosity: 1.3 − 3.25 63%× 10 − −67%.13 m 2, evaporatorLHP, area, grooves Vapor line:porosity: ID5/OD 63–67%.6 × 470 evaporator area, Results: The wick with 16-groove16-groove waswas quicklyquickly damage.damage. The other Liquid Vaporline: ID line:4.5/ODID5/6 ×OD 5856 × 470 grooves Thewicks other have wicks similar have properties, similar properties, such as porosity, such as pore radius, K. Liquid line: ID4.5/OD6 × 585 Condenser: ID5/OD6.4 × 800 porosity,Sintering porecondition: radius, 45K .min at 600 °C. Condenser: ID5/OD6.4 × 800 ◦ Ammonia as working fluid. SinteringIncreasing condition: the groove 45 number min at 600increasesC. the LHP’s performance (Q Ammonia as working fluid. Increasing= 500 W, Rt the = 0.14 groove K/W). number increases the LHP’s performanceAn optimal number (Q = 500 of W, groovesRt = 0.14 fabr K/W).icated on the wick surface is An optimal number of grooves fabricated on the wick presented. surface is presented. Objects: For fabricating the LHP’s evaporator, a low-cost sintering method is explored. The porous material partially fills the vapor collection channel embedded in the evaporator’s base; two evaporators were fabricated.

Flat-disk shape evaporator. Wick material = Nickel (ϕ42 × 3), Jeehoon Choi— particles size: 3 μm, Pcapillary= 401. kPa, K 2013 [15] = 0.99·10−11 m2, porosity = 64%, keff = 9 Keywords: W/K·m Miniature LHP, CC: stainless-steel (ϕ46 × 7) evaporator, Vapor line: ID4.95/OD6.35 × 250 (made A sintering procedure is used to fabricate the evaporator shown in 5. sintering, by copper) Figure (b). It is presented in Section 2.2 of the paper. contact Liquid line: ID4.95/OD6.35 × 300 (made Results: The startup of LHP with the second evaporator was conductance, by stainless-steel) shorter in time and more stable than one with the traditional LHP. thermal Forced convective air cooling By using the evaporator with the interpenetrated wick, the resistance Working fluid: water temperature on the CC reduced significantly which is 34 °C. Horizontal orientation Traditional LHP operated at 30–165 W; 80–141 °C; 1.81–0.71 K/W Heater surface area: 30 × 30 mm2 LHP using interpenetrated wick/base plate: 30–180 W; 47–102 °C; 0.76–0.43 K/W The lower temperature of the CC was due to the design of the second evaporator. It helped the wick and evaporator base contact excellently. It reduced heat loss to the CC through the evaporator’s wall. Randeep Miniature LHP using flat disk-shaped Singh—2008 copper evaporator (ϕ30 × 10 mm) Objects: For the thermal control of small electronic equipment, it [16] Working fluid: water addresses the thermal characteristics of mLHP using a flat disk- shaped evaporator. Keywords: Nickel wick (thickness = 3 mm, rpore = 3– 6. LHP, heat 5 μm; porosity: 75%) transfer, Condenser using Air forced cooling (Ta thermal = 22 ± 2 °C) performance, Copper Vapor line: ϕ2 × 150 mm miniature LHP, Copper Liquid line: ϕ2 × 290 mm Energies 2021, 14, × FOR PEER REVIEW 6 of 20

Energies 2021, 14, × FOR PEER REVIEW 6 of 20

Condenser: 130 × 130 × 25 At heat flux of 40 W/cm2 (heater heat flux), the MBE LHP can CR = 38.5%, 51.3%; 64.1%, 64.1%, 76.9% operate when Tc is around 63 °C; (Rt = 0.12 K/W) Optimum CR = Condenser:Sintering 130 process: × 130 × oven 25 temperatureAt heat51.3%. flux of 40 W/cm2 (heater heat flux), the MBE LHP can CR =900 38.5%, °C for 51.3%; 4 h. 64.1%, 64.1%, 76.9% operateOperation when Tantigravityc is around condition63 °C; (Rt is= 0.12better K/W) than Optimum others with CR the = MBE Sintering process: oven temperature 51.3%.LHP’s proper design. 900 °C for 4 h. OperationBest fin’s antigravity geometric condition parameters: is better h = 1.5 than mm; others p = 1.5 with mm; the w MBE = 3 mm; LHP’sbest proper particle design. size: 88 μm BestObjects: fin’s geometric The effects parameters: of increasi h ng= 1.5 the mm; number p = 1.5 of mm; grooves w = 3on mm; a bestwick’s particle surface size: 88 on μ LHPm performance were investigated. Objects: The effects of increasing the number of grooves on a wick’s surface on LHP performance were investigated. LHP with cylindrical stainless-steel evaporator (ϕ16 × 65) S.C. Wu—2014 LHPWater with cylindricalcooling stainless-steel [14] evaporatorWick material: (ϕ16 × 65) nickel (ID/OD = 9/12.5); S.C. Keywords:Wu—2014 Waterlargest cooling rpore = 1.9 − 2.5 μm; K = 1.3 − 3.25 4. [14] Wick structure, Energies 2021, 14, 2453 Wick× 10material:−13 m2, porosity: nickel (ID/OD 63% − =67%. 9/12.5); 7 of 23 Keywords:LHP, largestVapor rpore line:= 1.9 ID − 2.55/OD μm;6 × K 470 = 1.3 − 3.25 4. Wickevaporator structure, area, Results: The wick with 16-groove was quickly damage. The other × 10Liquid−13 m2, porosity: line: ID4.5/ 63%OD −6 67%. × 585 LHP,grooves wicks have similar properties, such as porosity, pore radius, K. VaporCondenser: line: ID5/ IDOD5/6OD × 4706.4 × 800 evaporator area, Results:Sintering The wick condition: with 16-groove 45 min at was600 °C.quickly damage. The other LiquidAmmonia line: ID as4.5/ workingOD6 × 585 fluid.Table 2. Cont. grooves wicksIncreasing have similar the groove properties, number such in ascreases porosity, the LHP’spore radius, performance K. (Q Condenser: ID5/OD6.4 × 800 Specifications of LHPs from Sintering= 500 W,condition:Rt = 0.14 45 K/W). min at 600 °C. No. References Ammonia as working fluid. Objects and Results the Literature IncreasingAn optimal the groove number number of grooves increases fabricated the LHP’s on the performance wick surface (Q is = 500 W, Rt = 0.14 K/W). Objects:presented. For fabricating the LHP’s evaporator, a An optimallow-costObjects: numberFor sintering fabricating of method grooves the is LHP’sfabr explored.icated evaporator, on The the porous wick a low-cost surface sintering is presented.materialmethod is partially explored. fills The the porous vapor collectionmaterial partially channel fills the vapor Objects:embeddedcollection For fabricating channel in the evaporator’s embedded the LHP’s inevaporator, base; the twoevaporator’s evaporators a low-cost base; sintering two methodwere is fabricated. explored. The porous material partially fills the vapor Flat-disk shape evaporator. collection channel embedded in the evaporator’s base; two Flat-diskWick shape material evaporator. = Nickel evaporators were fabricated. Wick material(φ42 × 3),= Nickel particles (ϕ42 size: × 3), 3 µm, Jeehoon Choi—Flat-diskparticles shapePcapillary size: evaporator. 3=μ 401.m, P kPa,capillary = 401. kPa, K −11 2 2013 [15] Wick= 0.99·10materialK −=11 =0.99m Nickel2, ·porosity10 (ϕm42 = ,× 64%, porosity3), keff = = 9 JeehoonKeywords: Choi— particlesW/K·m size:64%, 3 μkeffm,= P 9capillary W/K= ·401.m kPa, K −11 2 φ × Jeehoon2013 Choi—2013Miniature [15] LHP, [15= ]0.99·10 CC: stainless-steelCC:m , porosity stainless-steel ( ϕ= 4664%, × (7) k46eff = 9 7) Vapor line: Keywords:Keywords:evaporator, W/K·mVapor line: ID4.95/OD6.35 × 250 (made AA sinteringsintering procedureprocedure isis usedused toto fabricatefabricate thethe evaporator shown in 5. ID4.95/OD6.35 × 250 (made 5. MiniatureMiniaturesintering, LHP, evaporator,LHP, CC: bstainless-steely copper) (ϕ46 × 7) evaporatorFigure (b). It shown is presented in Figure in (b).Section It is 2.2 presented of the paper. in by copper) sintering,evaporator,contact contact conductance,VaporLiquid line: line:ID4.95/ ID4.95/OD6.35OD 6.35× 250 × (made300 (made A sintering SectionResults: 2.2procedure The of startup the paper. is ofused LHP to withfabricate the second the evaporator evaporator shown was in 5. Liquid line: thermalsintering, resistance by copper) FigureResults: (b). It The is presented startup of in LHP Section with 2.2 the of second the paper. conductance, by stainless-steel)ID4.95/OD 6.35 × 300 (made shorter in time and more stable than one with the traditional LHP. evaporator was shorter in time and more stable than contactthermal LiquidForced line:by convective ID stainless-steel)4.95/OD air6.35 cooling × 300 (made Results:By using The startupthe evaporator of LHP withwith thethe secondinterpenetrated evaporator wick, was the one with the traditional LHP. By using the evaporator conductance,resistance by stainless-steel)WorkingForced fluid: convective water air coolingshortertemperature in time and on morethe CC stable reduced than significantly one with the which traditional is 34 °C.LHP. with the interpenetrated wick, the temperature on the thermal ForcedHorizontal convectiveWorking orientation air fluid: cooling water By usingTraditional the evaporator LHP operated with the at 30–165 interpenetrated W; 80–141 wick, °C; 1.81–0.71 the K/W CC reduced significantly which is 34 ◦C. resistance WorkingHeater fluid:Horizontal surface water area: orientation 30 × 30 mm2 temperatureLHP using on interpenetrated the CC reduced wick significantly/base plate: which 30–180 is 34 W; °C. 47–102 °C; Traditional LHP operated at 30–165 W; 80–141 ◦C; HorizontalHeater orientation surface area: Traditional0.76–0.43 LHP K/W operated at 30–165 W; 80–141 °C; 1.81–0.71 K/W 2 1.81–0.71 K/W 30 × 30 mm 2 Heater surface area: 30 × 30 mm LHPLHPThe using lower using interpenetrated temperature interpenetrated wickof the wick/base/base CC wasplate: plate:due 30–180 to 30–180 the W; design 47–102 W; of °C;the 0.76–0.4347–102second K/W ◦evaporator.C; 0.76–0.43 It K/W helped the wick and evaporator base contact TheThe excellently.lower lower temperature temperature It reduced of theheat of theCC loss CCwas to was duethe due CCto the through to thedesign the of the seconddesignevaporator’s evaporator. of the secondwall. It helped evaporator. the wick It and helped evaporator the wick base contact Randeep Miniature LHP using flat disk-shapedexcellently. and evaporator It reduced base heat contact loss to excellently. the CC through It reduced the Singh—2008 copper evaporator (ϕ30 × 10 mm) evaporator’sheatObjects: loss For towall. the the CC thermal through control the evaporator’s of small electronic wall. equipment, it Randeep[16] MiniatureWorking LHPMiniature fluid: using water LHPflat disk-shaped using flat addresses the thermal characteristics of mLHP using a flat disk- Objects:shaped For evaporator. the thermal control of small electronic equipment, it Singh—2008Keywords: copperNickel evaporator disk-shapedwick (thickness (ϕ30 copper× 10 = mm)3 mm, rpore = 3–Objects: For the thermal control of small electronic addresses the thermal characteristics of mLHP using a flat disk- 6. [16] LHP, heat Working5 μm; fluid: porosity:evaporator water 75%) (φ30 × 10 mm) equipment, it addresses the thermal characteristics of Working fluid: water shapedmLHP evaporator. using a flat disk-shaped evaporator. Keywords:transfer, NickelCondenser wick (thickness using Air = 3forced mm, r coolingpore = 3– (Ta Nickel wick 6. LHP,thermal heat 5 μm;= 22 porosity: ± 2 °C) 75%) (thickness = 3 mm, Randeeptransfer, Singh—2008performance, Condenser [ 16]Copper using Vapor Air line: forced ϕ2 × cooling150 mm ( Ta rpore = 3–5 µm; porosity: 75%) Keywords:thermal = 22 ± 2 °C) miniature LHP, CopperCondenser Liquid line: using ϕ2 × Air 290 forced mm LHP, heat transfer, thermal 6. performance, Copper Vaporcooling line: ( Tϕ2= × 22150± mm2 ◦C) performance, miniature LHP, a miniature LHP, Copper LiquidCopper line: Vapor ϕ2 × 290 line: mm flat evaporator, Results: Startup condition at different heat loads, φ2 × 150 mm Q R thermal control = 5–70 W, mLHP = 5.66–0.17 K/W. The evaporator Copper Liquid line: wall’s temperature was lower than 100 ◦C. φ2 × 290 mm Oscillating behavior was found when Q was between Heater surface area: 10 and 20 W. This oscillation occured due to the 25 × 25 mm fluctuation of heat loss from the evaporator to the CC Condenser: φ2 × 50 mm and the subcooled liquid temperature. Energies 2021, 14, × FOR PEER REVIEW 7 of 20

Energies 2021, 14, × FOR PEER REVIEW 7 of 20 flat evaporator, Heater surface area: 25 × 25 mm Results: Startup condition at different heat loads, Q = 5–70 W, thermal control Condenser: ϕ2 × 50 mm RmLHP = 5.66–0.17 K/W. The evaporator wall’s temperature was flat evaporator, Heater surface area: 25 × 25 mm Results:lower thanStartup 100 °C.condition at different heat loads, Q = 5–70 W, Oscillating behavior was found when Q was between 10 and 20 thermal control Condenser: ϕ2 × 50 mm RmLHP = 5.66–0.17 K/W. The evaporator wall’s temperature was W. This oscillation occured due to the fluctuation of heat loss from Energies 2021, 14, 2453 lower than 100 °C. 8 of 23 Oscillatingthe evaporator behavior to the was CC foun and dthe when subcooled Q was liquidbetween temperature. 10 and 20 W.Objects: This oscillation A cooling occuredsystem withdue toan the LHP fluctuation for a supercomputer’s of heat loss from Table 2. Cont. thethermal evaporator control to is the presented. CC and the subcooled liquid temperature. Speciﬁcations of LHPs from Copper loop heat pipes with different vapor pipe IDs (4 and 3 mm No. References Objects:Objects and A Results cooling system with an LHP for a supercomputer’s M.A. the Literature Flat-oval evaporator (longitudinal thermaleach) were control fabricated. is presented. Chernysheva— Objects: A cooling system with an LHP for a replenishment evaporator) 80 × 42 × 7 Coppersupercomputer’sThe test loop was thermal heat carried controlpipes out is with presented. with different a heat load vapor from pipe 20 IDto s600 (4 andW during 3 mm 2014 [17] Copperthe cooling loop heat water’s pipes with differenttemperature vapor pipe wasIDs changed from 20 to 80 °C. M.A. mm, Aactive = 32 × 42 mm2 each) were fabricated. Keywords: Flat-oval evaporator (longitudinal (4 and 3 mm each) were fabricated. Chernysheva— 12 Vapor grooves Flat-ovalϕ1.8 × evaporator33 TheThe test test was was carried carried out with aout heat with load from a heat 20 to load from 20 to 600 W during replenishment evaporator) 80 × 42 × 7 600 W during the cooling water’s temperature was LHP, (longitudinal replenishment ◦ 2014 [17] Copper wick (porosity 243%, rpore = 27 changed from 20 to 80 C. was changed from 20 to 80 °C. mm, Aactive = 32 × 42evaporator) mm 80 × 42 × 7 mm, 7. supercomputer, A = 32 × 42 mm2 Keywords: M.A. Chernysheva—2014μm) [17] active the coolin 12 Vapor grooves ϕ121.8 Vapor × 33 grooves φ1.8 × 33 cooling system,Keywords: LHP, Copper wick (porosity 43%, water’s LHP, Vapor line: (1) ID4/OD5 × 305pore mm, (2) operating Copper wick (porosityr = 2743%,µm) r = 27 7. supercomputer,7. supercomputer, cooling pore temperature ID3/OD4 × 305 mmVapor line: system,μm) operating temperature, (1) ID4/OD5 × 305 mm, cooling system,temperature, Liquid thermal line: ID3/OD4 × 810 mm Vapor line: (1) ID4/(2)ODID3/5 OD× 3054 × 305 mm, mm (2) Results: LHP’s operating temperature varied slightly when the operatingthermal resistance IDCondenser:3/OD4 × 305 ID mm4/ODLiquid 5 × line:160 mm condenser cooling temperature changed in the range below 40 °C temperature,resistance ID3/OD4 × 810 mm Liquid line: ID3/ODCondenser:4 × 810 mm Results:(called LHP’s as variable operating temperature conductance varied slightly mode). × Results: LHP’s operating temperature varied slightly when the thermal ID4/OD5 160 mm when the condenser cooling temperature changed in Condenser: ID4/OD5 × 160 mm It is more applicable◦ to use copper-water LHPs when cooling resistance condenserthe range below cooling 40 C (called temperature as variable changed in the range below 40 °C conductancetemperatures mode). of condenser is above 50 °C. (calledIt is more as applicable variable to use conductance copper-water LHPs mode). when Flat evaporator (thickness, δ = 1.2 mm),It cooling is more temperatures applicable of condenser to use is above copper-water 50 ◦C. LHPs when cooling vapor line, liquid Flatline, evaporator and condenser (thickness, δ = 1.2 mm), vapor line, liquid temperatures of condenser is above 50 °C. line (δ = 1 mm) line, and condenser line Flat evaporator (thickness, δ = 1.2 mm), Objects: mLHP for mobile electronics. Evaporator: 60 × 23(δ =× 1 1.2 mm) mm vapor line, liquid line,Evaporator: and 60condenser× 23 × 1.2 mm Obects: mLHP for Aactive: 15 × 9 mm Aactive: 15 × 9 mm Guohui Zhou—line (δ = 1 mm) Primary porous material mobile electronics. Primary porous material (inside Objects: mLHP for mobile electronics. 2016 [18] Evaporator: 60 × 23(inside × 1.2 evaporator): mm sintered evaporator): sinteredfrom 10from layers 10 of 500layers mesh of GuohuiAactive Zhou—2016: 15 × 9 [18 mm] copper wire mesh GuohuiKeywords: Zhou— Keywords:Primary500 mesh porous copper material wire(50 × 21 mesh× (inside0.8 mm) (50 × 21 × 2016Miniature [18]8. LHP,Miniature LHP, ultrathin, (porosity: 65.2%) thermal0.8 resistance, mm) (porosity:Secondary 65.2%) wick (in liquid line) evaporator): sintered from 10 layers of Results: Startup of LHP happened at 2 W with 8. ultrathin, mobile electronics sintered from 4 layers of Results: Startup of LHP happened at 2 W with evaporator Keywords: Secondary wick (in liquid line) sintered evaporator temperature 43.9 ◦C. 150 mesh copper wire mesh thermal 500 mesh copper wire mesh (50 × 21 × WhentemperatureQ is at 11 W, R43.9= °C. 0.11 K/W Miniature LHP, (δ = 0.43 mm) LHP from 4 layers of 150 mesh copper wire There is no noticeably different performance with resistance, 0.8 mm) (porosity: Liquid65.2%) line: 105 mm, vapor LHP 8. ultrathin, Results:differentWhen orientations.QStartup is at 11 of W, LHP R happened = 0.11 K/W at 2 W with evaporator mesh (δ = 0.43 mm)line: 105 mm. Secondary wick (in liquid line) sintered For cooling mobile electronics: a tablet or smartphone, mobile Condenser: 125 mm There is no noticeably different performance with different thermal Liquid line: 105 mm, vapor line: 105 temperaturethis mLHP achieves 43.9 a promising °C. thermal electronics from 4 layers of 150(Natural mesh cooling) copper wire orientations. resistance, Whenmanagement Q is solution. at 11 W, RLHP = 0.11 K/W meshmm. ( δ = 0.43 mm)LHP’s inclination: horizontal, mobile anti-, and assisted gravity. ThereFor cooling is no noticeably mobile electronics: different performancea tablet or smartphone, with different this mLHP LiquidCondenser: line: 105 125 mm, mmWater vapor(Natural as working line: cooling) ﬂuid 105 electronics orientations.achieves a promising thermal management solution. mm.LHP’s inclination: horizontal, anti-, and For cooling mobile electronics: a tablet or smartphone, this mLHP Condenser:assisted gravity. 125 mm (Natural cooling) achieves a promising thermal management solution. LHP’sWater inclination: as working horizontal, fluid anti-, and assisted gravity. Objects: This micro LHP was fabricated for mobile electronic Water as working fluid devices. Objects:The effect This of microvapor LHPand condenserwas fabricated thickness for mobile on μLHP electronic Micro loop heat pipe performance was investigated. Takeshi devices. Chemical-etching and diffusion Shioga—2015 The effect of vapor and condenser thickness on μLHP bonding process for fabrication [19] Micro loop heat pipe performance was investigated. Takeshi Evaporator: 20 × 17 × 0.6 mm Keywords: Chemical-etching and diffusion Shioga—2015 Vapor line: (1) 5.6 × 0.4 and (2) 1 × 75 9. Micro LHP, bonding process for fabrication [19] mm in length thermal Evaporator: 20 × 17 × 0.6 mm Keywords: Liquid line 4 × 0.4 × 120 mm resistance, heat Vapor line: (1) 5.6 × 0.4 and (2) 1 × 75 9. Micro LHP, Condenser (1) 5.6 × 0.4 and (2) 1 × 110 leak, operation mm in length thermal mm in length Results: The μLHP could not operate with vapor line and orientation Liquid line 4 × 0.4 × 120 mm resistance, heat condenser line thickness at 0.4 mm. CondenserWorking fluid: (1) 5.6 water × 0.4 and (2) 1 × 110 leak, operation When Q is at 5 W, RLHP = 0.8 K/W, Tevaporator = 50.5 °C. When Q = 15 mm in length Results: The μLHP could not operate with vapor line and orientation W, RLHP = 0.32 K/W. Working fluid: water condenser line thickness at 0.4 mm. Heat loss was estimated at around 11%. When Q is at 5 W, RLHP = 0.8 K/W, Tevaporator = 50.5 °C. When Q = 15 W, RLHP = 0.32 K/W. Heat loss was estimated at around 11%. Energies 2021, 14, × FOR PEER REVIEW 7 of 20

flat evaporator, Heater surface area: 25 × 25 mm Results: Startup condition at different heat loads, Q = 5–70 W, thermal control Condenser: ϕ2 × 50 mm RmLHP = 5.66–0.17 K/W. The evaporator wall’s temperature was lower than 100 °C. Oscillating behavior was found when Q was between 10 and 20 W. This oscillation occured due to the fluctuation of heat loss from the evaporator to the CC and the subcooled liquid temperature. Objects: A cooling system with an LHP for a supercomputer’s thermal control is presented. Copper loop heat pipes with different vapor pipe IDs (4 and 3 mm M.A. Flat-oval evaporator (longitudinal each) were fabricated. Chernysheva— replenishment evaporator) 80 × 42 × 7 The test was carried out with a heat load from 20 to 600 W during 2014 [17] the cooling water’s temperature was changed from 20 to 80 °C. mm, Aactive = 32 × 42 mm2 Keywords: 12 Vapor grooves ϕ1.8 × 33 LHP, Copper wick (porosity 43%, rpore = 27 7. supercomputer, μm) cooling system, Vapor line: (1) ID4/OD5 × 305 mm, (2) operating ID3/OD4 × 305 mm temperature, Liquid line: ID3/OD4 × 810 mm thermal Results: LHP’s operating temperature varied slightly when the Condenser: ID4/OD5 × 160 mm resistance condenser cooling temperature changed in the range below 40 °C (called as variable conductance mode). It is more applicable to use copper-water LHPs when cooling temperatures of condenser is above 50 °C. Flat evaporator (thickness, δ = 1.2 mm), vapor line, liquid line, and condenser line (δ = 1 mm) Objects: mLHP for mobile electronics. Evaporator: 60 × 23 × 1.2 mm Aactive: 15 × 9 mm Guohui Zhou— Primary porous material (inside 2016 [18] evaporator): sintered from 10 layers of Keywords: 500 mesh copper wire mesh (50 × 21 × Miniature LHP, 0.8 mm) (porosity: 65.2%) 8. ultrathin, Results: Startup of LHP happened at 2 W with evaporator Secondary wick (in liquid line) sintered thermal temperature 43.9 °C. from 4 layers of 150 mesh copper wire resistance, When Q is at 11 W, RLHP = 0.11 K/W Energies 2021, 14, 2453 mesh (δ = 0.43 mm) 9 of 23 mobile There is no noticeably different performance with different Liquid line: 105 mm, vapor line: 105 electronics orientations. mm. For cooling mobile electronics: a tablet or smartphone, this mLHP Condenser: 125 mm (Natural cooling) Table 2. Cont. achieves a promising thermal management solution. LHP’s inclination: horizontal, anti-, and assisted Specificationsgravity. of LHPs from No. References Objects and Results Water asthe working Literature fluid Objects:Objects: This This micro micro LHP LHP was was fabricated fabricated for for mobile mobile electronic devices.electronic devices. TheThe effect effect of of vapor vapor and and condenser condenser thickness thickness on on μµLHPLHP Micro loop heat pipe performance was investigated. Takeshi performance was investigated. Chemical-etchingMicro loop and heat diffusion pipe Shioga—2015 bondingChemical-etching process for fabrication and [19] Evaporator:diffusion 20 × 17 bonding × 0.6 mm process TakeshiKeywords: Shioga—2015 [19] for fabrication Vapor line: (1) 5.6 × 0.4 and (2) 1 × 75 9.Keywords: Micro LHP, Evaporator: 20 × 17 × 0.6 mm mm in length 9. Micro LHP,thermal thermal resistance, Vapor line: (1) 5.6 × 0.4 and Liquid line 4 × 0.4 × 120 mm heat leak,resistance, heat (2) 1 × 75 mm in length Condenser (1) 5.6 × 0.4 and (2) 1 × 110 operationleak, orientation operation Liquid line 4 × 0.4 × 120 mm mm in lengthCondenser (1) 5.6 × 0.4 and Results:Results: The The μµLHPLHP could could not not operate operate with with vapor vapor line line and orientation Energies 2021, 14, × FOR PEERWorking REVIEW(2) fluid: 1 × 110water mm in length condenserand condenser line thickness line thickness at 0.4 atmm. 0.4 mm. 8 of 20 ◦ Working fluid: water WhenWhen QQ isis at at 5 5 W, W, RRLHPLHP = =0.8 0.8 K/W, K/W, TevaporatorTevaporator = 50.5= 50.5°C. WhenC. Q = 15 W,When RLHPQ = =0.32 15 K/W. W, RLHP = 0.32 K/W. HeatHeat loss loss was was estimated estimated at at around around 11%. 11%. Based on the operating orientation,orientation, thethe LHP’sLHP’s performance was performanceslightly changed. was slightly changed. Objects: The investigation of copper-water LHP using dual dualparallel parallel condensers condensers was conducted was conducted primarily primarily for LED for illumination Evaporator: 30 × 30 × 15 LEDapplications illumination with applicationshigh-power. with high-power. Ji Li—2013 [20] Evaporator:(in mm). 30 × 30 × 15 (in mm). Keywords: GravityGravity assisted assisted LHP LHP Ji Li—2013LED [20 ]cooling, ConnectingConnecting line: ID line: 5 mmID 5 mm Keywords:10. LHP, parallel Wick materialWick material is copper is copper (porosity 50%, 10. LED cooling,condensers, LHP, parallel rpore = 65(porosityμm, K = 6 50%, ×10−r11pore m2)= 65 µm, − condensers, thermal resistance × 112 2 thermal A Heater =K 25= × 6 2510 mm m ) Results: At Q = 300 W, the value of R is 0.4◦ °C/W; with Tair = 15 °C, 2 Results: At Q = 300 W, the value of R is 0.4 C/W; with A Heater = 25 × 25 mm ◦ ◦ resistance Condenser size: 120 × 80 × 50 mm TQ = =0–100 15 C, W,Q T=junction 0–100 < 75 W, °C.T < 75 C. Condenser size: air junction At low heatheat loads,loads, thethe condenser’scondenser’s unpredictable nonuniform 120 × 80 × 50 mm nonuniformperformance performance caused the unstable caused the behavior unstable of behaviorthe LHP. of the LHP. 2. Experimental Setups and Data Reduction 2. Experimental2.1. Description Setups and of Test Data LHP Reduction 2.1. DescriptionThe of Test elevation LHP difference between the evaporator and condenser is 350 mm in this The elevationexperiment. difference Cartridge between heaters the were evaporator used and andinserted condenser into the is copper 350 mm heating in this block at the experiment.evaporator Cartridge base. heaters The were heat used is rejected and inserted at the intowater-cooled the copper condenser. heating block The at mass the flow rate evaporatorand base. inlet The temperature heat is rejected of cooling at the water-cooled water were condenser.around 7.5 The× 10 mass−3 kg flows−1 and rate 27.5 and °C, respec- inlet temperaturetively. The of cooling power watersupplied were to around the evaporator 7.5 × 10− wa3 kgs adjusted s−1 and 27.5and ◦measuredC, respectively. by a volt slider The powerand supplied a digital to power the evaporator meter, respectively. was adjusted The and thermal measured contact by aresistance volt slider between and a the heat- digital powering meter,block and respectively. the evaporator The thermal is minimized contact resistanceby thermal between grease, thewhich heating was blockfilled in the in- and the evaporatorterface and is fixed minimized with screws. by thermal Pressure grease, transd whichucers was and filled four in thermocouples the interface andwere inserted fixed withdirectly screws. into Pressure the path transducers of the working and four fluid thermocouples at different positions were inserted of the directly LHP, such as at into the pathevaporator of the working outlet T fluideo, condenser at different inlet positions Tci, condenser of the LHP,outlet such Tco, and as at the evaporator inlet of compensa- outlet Teo,tion condenser chamber inlet TcciT.ci These, condenser temperature outlet T transducersco, and the inlet detect of compensation the temperature chamber distribution in- Tcci. Theseside temperature the LHP. transducersTherefore, the detect characteristics the temperature of circulation, distribution as well inside as phase the LHP. distribution, Therefore,can the be characteristics evaluated. Pressure of circulation, transducer as well Pe aswas phase installed distribution, at the outlet can be of evaluated.the evaporator. Fig- Pressure transducerures 3 andP 4e wasand installedTable 3 show at the the outlet schematic of the evaporator. diagram, photo Figures of3 the and real4 and experimental Table3 showsetup, the and schematic the main diagram, specifications photo of of the the real LHP. experimental setup, and the main specifications of the LHP.

(a) (b)

Energies 2021, 14, × FOR PEER REVIEW 8 of 20

Based on the operating orientation, the LHP’s performance was slightly changed. Objects: The investigation of copper-water LHP using dual parallel condensers was conducted primarily for LED illumination Ji Li—2013 [20] Evaporator: 30 × 30 × 15 (in mm). applications with high-power. Keywords: Gravity assisted LHP LED cooling, Connecting line: ID 5 mm 10. LHP, parallel Wick material is copper (porosity 50%, −11 2 condensers, rpore = 65μm, K = 6 ×10 m ) 2 thermal A Heater = 25 × 25 mm Results: At Q = 300 W, the value of R is 0.4 °C/W; with Tair = 15 °C, resistance Condenser size: 120 × 80 × 50 mm Q = 0–100 W, Tjunction < 75 °C. At low heat loads, the condenser’s unpredictable nonuniform performance caused the unstable behavior of the LHP.

2. Experimental Setups and Data Reduction 2.1. Description of Test LHP The elevation difference between the evaporator and condenser is 350 mm in this experiment. Cartridge heaters were used and inserted into the copper heating block at the evaporator base. The heat is rejected at the water-cooled condenser. The mass flow rate and inlet temperature of cooling water were around 7.5 × 10−3 kg s−1 and 27.5 °C, respectively. The power supplied to the evaporator was adjusted and measured by a volt slider and a digital power meter, respectively. The thermal contact resistance between the heating block and the evaporator is minimized by thermal grease, which was filled in the interface and fixed with screws. Pressure transducers and four thermocouples were inserted directly into the path of the working fluid at different positions of the LHP, such as at evaporator outlet Teo, condenser inlet Tci, condenser outlet Tco, and the inlet of compensation chamber Tcci. These temperature transducers detect the temperature distribution inside the LHP. Therefore, the characteristics of circulation, as well as phase distribution, Energies 2021, 14, 2453 can be evaluated. Pressure transducer Pe was installed at the outlet of the evaporator.10 of Fig- 23 ures 3 and 4 and Table 3 show the schematic diagram, photo of the real experimental setup, and the main specifications of the LHP.

Energies 2021, 14, × FOR PEER REVIEW 9 of 20

(a) (b)

FigureFigure 3.3. TestTest LHP: LHP: (a ()a Schematic) Schematic diagram diagram of the of theLHP LHP and andtemperature temperature measurement measurement points. points. (b) The ( blocations) The locations of thermo- of thermocouplescouples on heating on heating block and block evaporator. and evaporator.

Figure 4. Photograph of the real experimental setup under gravity-assisted conditions. Figure 4. Photograph of the real experimental setup under gravity-assisted conditions.

Table 3. Main parameters of the LHP.

Evaporator Body Values Material Copper Length (mm) 80 Width (mm) 70 Height (mm) 24.5 Active area (mm2) 60 × 45 Fin geometry Cross area (mm2) 2 × 2 Height (mm) 1.5 Fin pitch (mm) 4 Wick structure Material Stainless steel Bulk volume (mm3) 50 × 41 × 5 Material Ceramic Bulk volume (mm3) 50 × 41 × 5 Vapor line OD/ID (mm) 6.35/4.35 Length (mm) 800 Condenser line OD/ID (mm) 6.35/4.35 Length (mm) 600 Liquid line OD/ID (mm) 3.2/1.7 Length (mm) 1300 Working fluid Water

Energies 2021, 14, 2453 11 of 23

Table 3. Main parameters of the LHP.

Thermocouples T1, T2, T3 were installed, as shown in Figure3b, to consider the temperature gradient caused by the heat flux and the temperature on the top surface of heating block Ts1, and the correct values of heating power and heat flux supplied to the evaporator were accessed. In the evaporator base, thermocouple T4 was inserted to estimate the temperature at the evaporator’s bottom surface; Ts2 measures the temperature at the base of fin Tbf. The heat released from the condenser, Qc, is obtained from the temperature difference of cooling water Twa-i, Twa-o, and the mass flow rate of cooling water. All measured data were collected and recorded by the KEITHLEY 2701 data acquisition system. Moreover, the level of liquid inside the compensation chamber can be observed by the polycarbonate evaporator cover. Table4 describes the uncertainties of the mass flow meter and thermocouples (obtained from the calibration process in which a Pt100 thermometer (Chino Co. Model—R900- F25AT) was used as the standard source). Figures5 and6 demonstrate the structure of the evaporator and the geometry of the evaporator’s inner surface. Energies 2021, 14, × FOR PEER REVIEW 10 of 20

Amount (mL) 33

Thermocouples T1, T2, T3 were installed, as shown in Figure 3b, to consider the temperature gradient caused by the heat flux and the temperature on the top surface of heat- Energies 2021, 14, 2453 12 of 23 ing block Ts1, and the correct values of heating power and heat flux supplied to the evaporator were accessed. In the evaporator base, thermocouple T4 was inserted to estimate Energies 2021, 14, × FOR PEERTable REVIEW 4. Uncertainty values. s2 10 of 20 the temperature at the evaporator’s bottom surface; T measures the temperature at the base of fin TParametersbf. The heat released from the Uncertainty condenser, Qc, is obtained from the temperature ◦ T1, T2, T3 wa-i wa-o, 0.06 C difference of coolingAmount water (mL) T , T and the mass 33 flow rate of cooling water. All meas- ◦ ured data wereT4 collected and recorded by0.07 theC KEITHLEY 2701 data acquisition system. ◦ ThermocouplesTeo T1, T2, T3 were installed, as shown in0.06 FigureC 3b, to consider the tem- Moreover, the level of liquid inside the compensation◦ chamber can be observed by the perature gradientTci caused by the heat flux and the temperature0.06 C on the top surface of heat- polycarbonateing block Ts1, and evaporator the correct values cover. of heating power and heat◦ flux supplied to the evap- Tco, Tcci 0.1 C orator were accessed. In the evaporator base, thermocouple◦ T4 was inserted to estimate Table 4 describesTwa-i the uncertainties of0.1 thCe mass flow meter and thermocouples (ob- the temperature at the evaporator’s bottom surface; Ts2 measures the temperature at the T ◦ tainedbase fromof fin T bfthe. Thewa-o heatcalibration released from process the condenser, in whQc, is0.06 ichobtainedC a Pt100 from the thermometer temperature (Chino Co. Model— R900-F25AT)differencePressure of cooling was transducer usedwater T aswa-i, Tthewa-o, and standard the mass flow source). 1.5rate kPa of cooling Figures water. 5 Alland meas- 6 demonstrate the structure ured dataMass were ﬂow collected meter and recorded by the KEITHLEY 0.18% of 2701 reading data acquisition system. of theMoreover, evaporator the level andof liquid the inside geometry the compensation of the chamber evaporator’s can be observed inner by thesurface. polycarbonate evaporator cover. Table 4 describes the uncertainties of the mass flow meter and thermocouples (obtained from the calibration process in which a Pt100 thermometer (Chino Co. Model— R900-F25AT) was used as the standard source). Figures 5 and 6 demonstrate the structure of the evaporator and the geometry of the evaporator’s inner surface.

Figure 5. Schematic design Figureof the 5. evaporator.Schematic design 1: of vapor the evaporator. collector; 1: vapor 2: collector;compensation 2: compensation chamber; chamber; 3: 3: polycarbonate lid; 4: O-ring; polycarbonate lid; 4: O-ring; 5: charging pipe; 6: copper evaporator body; 7: wick; 8: vapor grooves; Figure 5. Schematic design of the evaporator. 1: vapor collector; 2: compensation chamber; 3: polycarbonate lid; 4: O-ring; 5: charging pipe; 6: copper 9:evapor copperplate;ator 10: body; vapor pipe. 7: wick; 8: vapor grooves; 9: copperplate; 10: vapor pipe. 5: charging pipe; 6: copper evaporator body; 7: wick; 8: vapor grooves; 9: copperplate; 10: vapor pipe.

(a) (b)

Figure 6.Figure The inner 6. The surface inner geometry surface geometry of the evaporator of the evaporator (a) before (a )and before (b) after and (installingb) after installing the wick. the wick.

Furthermore,Table 4. Uncertainty the experimental values. uncertainty analysis of the computed heat transfer coefﬁcients is given by the following equation (Equations (4)–(13)). Parameters Uncertainty The(a experimental) result R is assumed to be calculated from a set of measurements(b) using a data interpretationT1,program T2, T3 presented by [21]. 0.06 °C Figure 6. The inner surface geometryT4 of the evaporator (a) before and0.07 (°Cb) after installing the wick. Teo R = R(X1, X2, X3,..., XN)0.06 °C (4) Tci 0.06 °C Each measurement uncertainty’s effect on the calculated result if only that one mea- Table 4. UncertaintyTco, Tvalues.cci 0.1 °C surement were in error would be Twa-i 0.1 °C ParametersTwa-o ∂R 0.06 °C Uncertainty δRXi = δXi (5) Pressure transducer ∂Xi 1.5 kPa Mass flowT1, meter T2, T 3 0.18% of reading 0.06 °C T4 0.07 °C Furthermore, the experimental uncertainty analysis of the computed heat transfer coefficients is given by theT followingeo equation (Equations (4)–(13)). 0.06 °C Tci 0.06 °C

co cci T , T 0.1 °C Twa-i 0.1 °C Twa-o 0.06 °C Pressure transducer 1.5 kPa Mass flow meter 0.18% of reading

Furthermore, the experimental uncertainty analysis of the computed heat transfer coefficients is given by the following equation (Equations (4)–(13)).

Energies 2021, 14, 2453 13 of 23

The partial derivative of R with respect to Xi is the sensitivity coefﬁcient for result R with respect to measurement Xi. The single terms are combined by a root-sum-square method when respective independent variables are used in function R.

( ) 1 N ∂R 2 2 δRXi = ∑ δXi (6) i=1 ∂Xi

Then, the parameter ∆T12 can be deﬁned as,

∆T12 = T1 − T2 (7)

The uncertainty for ∆T12 is shown in Equation (8).

1 2 2 2 δ(∆T12) = δT1 + δT2 (8)

where ∆T12 is the temperature difference between T1 and T2 at the heating block. The parameter ∆T23 can be deﬁned as,

∆T23 = T2 − T3 (9)

The uncertainty for ∆T23 is shown in Equation (10).

1 2 2 2 δ(∆T23) = δT3 + δT2 (10)

where ∆T23 is the temperature difference between T2 and T3 at the heating block. The parameter ∆T13 can be deﬁned as,

∆T13 = T1 − T3 (11)

The uncertainty for ∆T13 is shown in Equation (12).

1 2 2 2 δ(∆T13) = δT1 + δT3 (12)

where ∆T13 is the temperature difference between T1 and T3 at the heating block. The parameter q can be deﬁned as,

∆T q = k 12 (13) δ1 The uncertainty for q is shown in Equation (14).

k δq = δ(∆T12) (14) δ1 where q is the heat ﬂux; k is the thermal conductivity of copper. δ1 is the distance between the heating block’s thermocouples. The parameter Tbf can be deﬁned as,

qδ T = T − 2 (15) b f 4 k Energies 2021, 14, 2453 14 of 23

The uncertainty for Tbf is shown in Equation (16).

! 1 δ 2 2 δ T = (δT )2 + 2 δq (16) b f 4 k

where Tbf is the evaporator ﬁn’ base temperature. T4 is the evaporator’s base temperature. The parameter he can be deﬁned as, q he = (17) Tb f − Teo

The uncertainty for he is shown in Equation (18).

1   2  2 2 !2 δq qδT qδTb f δ(h ) =  +  eo  +    e   2   2   (18) Tb f − Teo Tb f − Teo Tb f − Teo

where he is the heat transfer coefﬁcient of the evaporator. Teo is the evaporator outlet temperature. The parameter hesat can be deﬁned as, q hesat = (19) Tb f − Tesat

The uncertainty for hesat is shown in Equation (20).

1   2  2 2 !2 δq qδT qδTb f δ(h ) =  +  sat  +    esat   2   2   (20) Tb f − Tesat Tb f − Tesat Tb f − Tesat

where hesat is the heat transfer coefﬁcient of the evaporator, calculated from the saturation temperature. Tesat is the saturation temperature accessed from vapor pressure.

2.2. Description of the Wick’s Thermal Conductivity Measuring System The wick materials’ thermal conductivity was measured by the transient hot-wire method. Figure7 shows a schematic diagram of the thermal conductivity measuring device and sensor unit. It consists of a power supply device, a measuring device, a sensor unit, and a recording device (Personal Computer, PC). At the time of measurement, a constant current is applied from the power supply to the sensor. In addition, the voltage drop of the sensor unit is measured by a measuring device at regular intervals and recorded by a recording device. Energies 2021, 14, 2453 15 of 23 Energies 2021, 14, × FOR PEER REVIEW 13 of 20

Nichrome Wire L=40.14 mm D=0.1 mm R=0.3824 Ω

Kapton Tape

(a) (b)

FigureFigure 7. Thermal 7. Thermal conductivity conductivity measuring measuring device device (a) Schematic(a) Schematic Diagram Diagram (b) Sensor(b) Sensor Unit. Unit.

TheThe basic basic equation equation for for thermal thermal conductivity conductivity measurement measurement is expressed is expressed by theby the follow- follow- inging equations equations (Equations (Equations (21)–(24)). (21)–(24)). TheThe thermal thermal conductivity conductivity of theof the wicks wicks (Equation (Equation (21)) (21)) [22 ],[22], ∗ q∗ d𝑞 ln t𝑑 𝑙𝑛 𝑡 λ =𝜆 = − λ − 𝜆 (21)(21) 2 2π d2𝜋∆T𝑑 Δ𝑇1 where where λ2 is the thermal conductivity of the wick sample. λ2 is the thermal conductivity of the wick sample. λ1 is the thermal conductivity of the insulator. λ1 is the thermal conductivity of the insulator. q* isq* the is the amount amount of heating of heating per per wire wire length. length. t ist the is the time time taken. taken. ∆TΔisT the is the temperature temperature change change from fromt = 0t = s. 0 s. ResistanceResistance of thinof thin wire wire at a at certain a certain temperature, temperature,

𝑅= 𝑅 (1 + 𝛼Δ𝑇) (22) R = Ro(1 + α∆T) (22) where where Ro is the initial electrical resistance. Ro isα is the the initial temperature electrical coefficient resistance. of resistance. α isFrom the temperature Equation (22) coefﬁcient and Ohm’s of resistance. Law, From Equation (22) and Ohm’s Law, 1 𝑉(𝑡) Δ𝑇 = ( − 1) (23) 1 V𝛼(t) 𝐼𝑅 ∆T = − 1 (23) α IR where o where V is the voltage drop on the thin wire. V isI theis the voltage current. drop on the thin wire. I isBy the substituting current. ΔT into Equation (21), By substituting ∆T into Equation (21), 𝑑 𝑉(𝑡) 𝛼𝐼 𝑅 = =𝐶𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (24) d V(t)𝑑 𝑙𝑛 𝑡 αI2𝜋(𝜆3R2 +𝜆) = o = Constant (24) Then, the wick materials’d ln t thermal2π(λ conductivity1 + λ2) can be obtained from the slope of the relationship between the voltage drop of the thin wire and the logarithm of the heating time.Then, The the measured wick materials’ thermal thermal conductivity conductivity of the can tested beobtained wicks and from the the comparison slope of the with relationship between the voltage drop of the thin wire and the logarithm of the heating literature data [7,8] are presented in Table 5. time. The measured thermal conductivity of the tested wicks and the comparison with

literature data [7,8] are presented in Table5.

Energies 2021, 14, 2453 16 of 23 Energies 2021, 14, × FOR PEER REVIEW 14 of 20

Table 5. Thermal conductivity of wicks. Table 5. Thermal conductivity of wicks.

Present Study Present StudyValue from Value References from References −1 −1 Wick λ2 [W Wick(m K)−1] λ2 [W (m K) ]Wick Wick λλ22 [W[W (m(m K) K)−1]] Stainless Steel Stainless 6.4 Steel 6.4 SP-Sintered SP-Sintered [8] [8] 11.87 Ceramic Ceramic3.5 3.5 Ceramic Ceramic [7] [7] 44

2.3. Description of Condenser InIn this study, study, the the condenser condenser is is a adouble-p double-pipeipe heat heat exchanger exchanger with with a counter a counter flow ﬂow ar- arrangement.rangement. Cooling Cooling water water from from the the constant constant temperature temperature pump pump flows ﬂows in the annular areaarea whilewhile thethe vaporvapor condensescondenses inside inside the the copper copper tube. tube. The The condenser condenser structure structure is describedis described in Figurein Figure8 and 8 and Table Table6. 6.

Figure 8.8. Photograph of the condenser.

TableTable 6.6. SpeciﬁcationSpecification ofof thethe condenser.condenser. Parameters Inner Tube Outer Tube Parameters Inner Tube Outer Tube Material Smooth Copper tube Poly-carbonated resin Material Smooth Copper tube Poly-carbonated resin Length,Length, mm mm 600 mm 600 mm 600 600 mm mm OD/IDOD/ID, mm, mm 6.35/4.35 6.35/4.35 13/9 13/9

Furthermore, the same copper smooth tube is used for the vapor pipe and liquid pipe Furthermore, the same copper smooth tube is used for the vapor pipe and liquid pipe of the LHP, whose OD/ID is 6.35/4.35 and 3.2/1.5 mm, respectively. of the LHP, whose OD/ID is 6.35/4.35 and 3.2/1.5 mm, respectively.

2.4. Data Reduction From Figure 3b, the values of heat flux q, heat transfer coefficient he, and thermal From Figure3b, the values of heat flux q, heat transfer coefficient he, and thermal resistances Re, Rc, and Rct can be estimated by the following equations (Equations (25)– resistances Re, Rc, and Rct can be estimated by the following equations (Equations (25)–(36)): (36)): A = 272 cm2, δ1 = 5 mm, δ2 = 2.5 mm. A = 27 cm , δ1 = 5 mm, δ2 = 2.5 mm. Heat fluxflux flowingflowing fromfrom thethe heatingheating blockblock toto thethe evaporator’sevaporator’s activeactive areaarea isis calculatedcalculated by Fourier’s Law of heat conduction as

∙ 𝑇 −𝑇 𝑇 −𝑇 𝑇 −𝑇 · 𝑞 =𝑘T1 − T2 =𝑘T2 − T3 =𝑘T1 − T3 (25) q = k 𝛿 = k 𝛿 = k 2𝛿 (25) δ1 δ1 2δ1 where where𝑞∙ is the heat transfer rate per unit heating block’s surface area. q· is the heat transfer rate per unit heating block’s surface area. k is the thermal conductivity of the copper heating block. k is the thermal conductivity of the copper heating block. T1 to T3 is the heater temperature. T1 to T3 is the heater temperature. δ1 is the distance between the thermocouples inside the heating block. δ is the distance between the thermocouples inside the heating block. Then,1 the mean of 𝑞∙ can be expressed by Equation (26). Then, the mean of q· can be expressed by Equation (26). 1 Δ𝑇 Δ𝑇 Δ𝑇 𝑞=1 ∆(𝑘T +𝑘∆T +𝑘∆T ) (26) q = 3k 12𝛿+ k 23𝛿+ k 2𝛿13 (26) 3 δ1 δ1 2δ1 𝑄=𝑞∗𝐴 (27)

Energies 2021, 14, 2453 17 of 23

Q = q ∗ A (27)

Additionally, the temperature of the heating block’s top surface, Ts1, and the evaporator bottom’s temperature, Ts2, can be determined by Equations (28) and (29).

1 3(qδ ) 2(qδ ) (qδ ) T = T − 1 + T − 1 + T − 1 (28) s1 3 1 k 2 k 3 k

qδ T = T + 2 (29) s2 4 k T − T R = s2 eo (30) e Q

Tci − Twa−i Rc = (31) Qc

Qc = mwaCp(Twa−o − Twa−i) (32) T − T R = s1 s2 (33) ct Q q he = (34) Tb f − Teo q hesat = (35) Tb f − Tesat qδ T = T − 2 (36) b f 4 k

3. Results and Discussion Figure9 displays the values of temperature at various locations in the experiment. The results show that the stainless-steel wick exhibits higher heat flux when compared to the ceramic wick. During the heat load range of 50 to 520 W in Figure9a and 53 to 155 W in Figure9b, the values of Teo and Tci are almost similar, and Tco and Tcci are nearly equal. We affirm that the working fluid circulates stably inside the loop heat pipe. However, in the LHP with a ceramic wick, some fluctuations were found in the range of heat power, from 53 to 80 W, where the values of Tcci were higher than Tco. This phenomenon happened because there was the intermittent appearance of a vapor–liquid interface near the position of thermocouple Tcci in the liquid line. In the experiment of the LHP with a stainless steel wick, the supplying liquid for the compensation chamber was more stable than the LHP with the ceramic wick. Moreover, in this experimental research, temperature Ts1 on the heating block’s top surface can be viewed as the electronics temperature, which is normally recommended to be lower than 85 ◦C for reliable and effective operation [23]. Therefore, this present LHP, with stainless steel and ceramic wicks, can satisfy the recommendation until the heat load reaches 350 and 54 W, respectively. The measured values and frequency of pore distribution by the mercury injection method [24] are given in Table7 and Figure 10. Energies 2021, 14, × FOR PEER REVIEW 15 of 20

Additionally, the temperature of the heating block’s top surface, Ts1, and the evaporator bottom’s temperature, Ts2, can be determined by Equations (28) and (29). 1 3(𝑞𝛿) 2(𝑞𝛿) (𝑞𝛿) 𝑇 = 𝑇 − +𝑇 − +𝑇 − (28) 3 𝑘 𝑘 𝑘 𝑞𝛿 𝑇 =𝑇 + (29) 𝑘 𝑇 −𝑇 𝑅 = (30) 𝑄 𝑇 −𝑇 𝑅 = (31) 𝑄

𝑄 =𝑚𝐶(𝑇 −𝑇) (32)

𝑇 −𝑇 𝑅 = (33) 𝑄 𝑞 ℎ = (34) 𝑇 −𝑇 𝑞 ℎ = (35) 𝑇 −𝑇 𝑞𝛿 𝑇 =𝑇 − (36) 𝑘

3. Results and Discussion Figure 9 displays the values of temperature at various locations in the experiment. The results show that the stainless-steel wick exhibits higher heat flux when compared to the ceramic wick. During the heat load range of 50 to 520 W in Figure 9a and 53 to 155 W in Figure 9b, the values of Teo and Tci are almost similar, and Tco and Tcci are nearly equal. We affirm that the working fluid circulates stably inside the loop heat pipe. However, in the LHP with a ceramic wick, some fluctuations were found in the range of heat power, from 53 to 80 W, where the values of Tcci were higher than Tco. This phenomenon happened because there was the intermittent appearance of a vapor–liquid interface near the position of thermocouple Tcci in the liquid line. In the experiment of the LHP with a stainless steel wick, the supplying liquid for the compensation chamber was more stable than the LHP with the ceramic wick. Moreover, in this experimental research, temperature Ts1 on the heating block’s top surface can be viewed as the electronics temperature, which is normally recommended to be lower than 85 °C for reliable and effective operation [23]. Therefore, this present LHP, with stainless steel and ceramic wicks, can satisfy the recom- Energies 2021, 14, 2453 mendation until the heat load reaches 350 and 54 W, respectively. The measured 18values of 23 and frequency of pore distribution by the mercury injection method [24] are given in Table 7 and Figure 10.

Energies 2021, 14, × FOR PEER REVIEW 16 of 20

(a) (b) Figure 9. LHP’s temperatures distribution at different heat loads when installed with (a) a stainless-steel wick and (b) a Figure 9. LHP’s temperatures distribution at different heat loads when installed with (a) a stainless-steel wick and (b) a ceramic wick. ceramic wick.

TableTable 7. 7.Test Test results results of of pore pore distribution distribution measurement measurement by by the the mercury mercury injection injection method. method.

WickWick Materials Materials Ceramic Ceramic Stainless Stainless Steel Steel Pore volume in total (cm3.g−1) 0.20 0.07 Pore volume in total (cm3·g−1) 0.20 0.07 CentralCentral pore pore diameter diameter (µm) (μm) 1.291.29 16.4016.40 TotalTotal pore pore speciﬁc specific surface surface area (m area2·g −(m1) 2.g−1) 0.660.66 0.02 0.02 AverageAverage pore pore size size (µm) (μm) 1.181.18 11.8511.85 −3 BulkBulk density density (g.cm (g.cm) −3) 2.012.01 4.344.34 Porosity (%) 39.3 31.5 Porosity (%) 39.3 31.5

(a) (b)

FigureFigure 10. 10.Pore Pore frequency frequency distributiondistribution graphsgraphs (aa)) Ceramic wick ( (bb)) Stainless Stainless steel steel wick. wick. Reproduced Reproduced from from [25], [25 Saga], Saga Ce- Ceramicsramics Research Research Laboratory: Laboratory: 2019. 2019.

3.1.3.1. Performance Performance Evaluation Evaluation TheThe thermal thermal performanceperformance can be be evaluated evaluated in in terms terms of ofheat heat flux flux q, heatq, heat transfer transfer coef- coefficientficient he, hande, and thermal thermal resistances, resistances, which which are are evaporator evaporator Re,R condensere, condenser Rc,R andc, and contact contact sur- surfaceface RctR, ctas, asshown shown in Figures in Figures 11 and11 and 12. In12 .this In study, this study, the evaporator the evaporator heat transfer heat transfer coeffi- coefficientcient was wascalculated calculated from from vapor vapor temperature temperature Teo atT eotheat outlet the outlet of the of evaporator the evaporator and andsatu- saturationration temperature temperature TesatT,esat accessed, accessed from from the the vapor vapor pressure pressure measured measured at atthe the outlet outlet of ofthe theevaporator. evaporator.

(a) (b) Figure 11. SEM images (100 μm magnification) of (a) a stainless steel wick and (b) a ceramic wick. Reproduced from [25], Saga Ceramics Research Laboratory: 2019.

Energies 2021, 14, × FOR PEER REVIEW 16 of 20

Figure 9. LHP’s temperatures distribution at different heat loads when installed with (a) a stainless-steel wick and (b) a ceramic wick.

Table 7. Test results of pore distribution measurement by the mercury injection method.

Wick Materials Ceramic Stainless Steel Pore volume in total (cm3.g−1) 0.20 0.07 Central pore diameter (μm) 1.29 16.40 Total pore specific surface area (m2.g−1) 0.66 0.02 Average pore size (μm) 1.18 11.85 Bulk density (g.cm−3) 2.01 4.34 Porosity (%) 39.3 31.5

(a) (b) Figure 10. Pore frequency distribution graphs (a) Ceramic wick (b) Stainless steel wick. Reproduced from [25], Saga Ce- ramics Research Laboratory: 2019.

3.1. Performance Evaluation The thermal performance can be evaluated in terms of heat flux q, heat transfer coefficient he, and thermal resistances, which are evaporator Re, condenser Rc, and contact surface Rct, as shown in Figures 11 and 12. In this study, the evaporator heat transfer coefficient was calculated from vapor temperature Teo at the outlet of the evaporator and saturation temperature Tesat, accessed from the vapor pressure measured at the outlet of the Energies 2021, 14, 2453 19 of 23 evaporator.

(a) (b)

Figure 11. SEM images (100 µm magniﬁcation) of (a) a stainless steel wick and (b) a ceramic wick. Reproduced from [25], Energies 2021, 14, × FOR PEER REVIEW 17 of 20 Figure 11. SEM images (100Saga μm Ceramics magnification) Research Laboratory: 2019.of (a) a stainless steel wick and (b) a ceramic wick. Reproduced from [25], Saga Ceramics Research Laboratory: 2019.

(a) (b)

FigureFigure 12. he 12., R,h evs., R, qvs. ofq (ofa) ( aa) stainless a stainless steel wick wick and and (b) ( ab ceramic) a ceramic wick. wick.

In bothIn both experiments experiments of of stainless stainless steel steel and and ceramic ceramic LHPs, LHPs, the evaporator the evaporator heat transfer heat transfer coefficient obtained from Teo was higher than the values calculated from saturation temper- eo coefficientature T obtainedesat. Observing from the T difference was higher in the results,than the the vaporvalues might calculated be superheated from saturation before tem- peratureleaving Tesat the. Observing evaporator. the This difference superheating in process the result coulds, happen the vapor when might vapor flowsbe superheated in the before leavingcrossing grooves,the evaporator. and the heat This of this superheati process comesng process from the could surrounding happen area when of the finsvapor flows in theand crossing the grooves’ grooves, surface. and The the possible heat operatingof this process heat flux comes for the stainless-steelfrom the surrounding wick was area of higher than the ceramic wick. Comparing the evaporator heat transfer coefficient of the the finsLHPs, and the the evaporator grooves’ operating surface. with The the possible ceramic wickoperating had the heat higher flux evaporator for the stainless-steel heat wick transferwas higher coefficient than when the theceramic heat transfer wick. coefficient,Comparinghe, linearly the evaporator increased with heat the transfer heat coefficient fluxof the in theLHPs, LHP the with evaporator the stainless steeloperating wick. However, with the the ceramic ceramic wick LHP didhad not the operate higher evapo- better than the stainless steel one. The significant mass flow rate of vapor requires large rator heat transfer coefficient when the heat transfer coefficient, he, linearly increased with pore sizes in the ceramic wick to reduce resistance. Otherwise, the performance of the the heat flux in the LHP with the stainless steel wick. However, the ceramic LHP did not operate better than the stainless steel one. The significant mass flow rate of vapor requires large pore sizes in the ceramic wick to reduce resistance. Otherwise, the performance of the condenser decreases. The numerical values obtained from the experimental uncertainty analysis of the computed heat transfer coefficients are presented in Tables 8 and 9.

Table 8. Uncertainty values of the computed heat transfer coefficients for LHPs with stainless steel wicks.

Q q δ(q) he δ(he) hesat δ(hesat) W W/m2 % W/(m2 K) % W/(m2 K) % 50.22 18,598.44 30.42 4592.67 30.46 1345.93 62.64 100.29 37,299.05 15.17 5588.10 15.25 2150.30 37.71 147.91 54,451.96 10.39 8458.43 10.39 3984.10 32.14 188.17 69,832.60 8.10 8323.83 8.12 4804.16 25.30 237.16 87,723.91 6.45 8368.00 6.43 5676.28 17.73 290.03 107,645.92 5.26 8427.34 5.14 6428.54 12.52 350.72 129,896.27 4.35 9388.90 4.31 7633.87 9.80 401.67 148,767.07 3.80 10,023.39 3.77 8228.43 7.74 445.87 165,137.27 3.43 10,651.51 3.46 9177.51 6.74 503.47 186,469.50 3.03 11,785.25 3.08 10,499.12 5.67 520.70 192,826.71 2.93 11,900.59 2.99 10,742.33 5.26

Table 9. Uncertainty values of the computed heat transfer coefficients for LHPs with ceramic wicks.

Q q δ(q) he δ(he) hesat δ(hesat) W W/m2 % W/(m2 K) % W/(m2 K) % 53.41 19,782.71 28.59 13,195.93 26.67 13,932.62 48.68 79.59 29,477.46 19.19 11,352.25 16.61 9709.18 21.12

Energies 2021, 14, 2453 20 of 23

condenser decreases. The numerical values obtained from the experimental uncertainty analysis of the computed heat transfer coefﬁcients are presented in Tables8 and9.

Table 8. Uncertainty values of the computed heat transfer coefﬁcients for LHPs with stainless steel wicks.

Q q δ(q) he δ(he) hesat δ(hesat) W W/m2 % W/(m2·K) % W/(m2·K) % 50.22 18,598.44 30.42 4592.67 30.46 1345.93 62.64 100.29 37,299.05 15.17 5588.10 15.25 2150.30 37.71 147.91 54,451.96 10.39 8458.43 10.39 3984.10 32.14 188.17 69,832.60 8.10 8323.83 8.12 4804.16 25.30 237.16 87,723.91 6.45 8368.00 6.43 5676.28 17.73 290.03 107,645.92 5.26 8427.34 5.14 6428.54 12.52 350.72 129,896.27 4.35 9388.90 4.31 7633.87 9.80 401.67 148,767.07 3.80 10,023.39 3.77 8228.43 7.74 445.87 165,137.27 3.43 10,651.51 3.46 9177.51 6.74 503.47 186,469.50 3.03 11,785.25 3.08 10,499.12 5.67 520.70 192,826.71 2.93 11,900.59 2.99 10,742.33 5.26

Table 9. Uncertainty values of the computed heat transfer coefﬁcients for LHPs with ceramic wicks.

Q q δ(q) he δ(he) hesat δ(hesat) W W/m2 % W/(m2·K) % W/(m2·K) % 53.41 19,782.71 28.59 13,195.93 26.67 13,932.62 48.68 79.59 29,477.46 19.19 11,352.25 16.61 9709.18 21.12 105.72 39,154.18 14.45 10,783.11 14.27 8046.37 16.92 132.38 49,028.99 11.54 11,805.62 11.73 8359.43 14.03 155.45 57,574.38 9.83 10,737.34 9.87 7810.08 11.71

Performance of Evaporator and Condenser

Figure 12a,b displays the evaporator and condenser thermal resistances, Re and Rc, of the stainless steel and ceramics LHPs. In the LHP with the stainless steel wick, the values of Re became smaller with the increase in heating power. However, thermal resistance Re for the ceramic one gradually increased in the same operating conditions, although the performance of the evaporator with the ceramic wick was more effective than the one with the stainless steel wick. For the thermal resistances of the condenser at the stainless steel LHP, it was reduced to the minimum value of 0.09 K.W−1, then raised up slightly, as shown in Figure 12a. The higher the heat power supplied to the evaporator, the less liquid exists inside the compensation chamber. As a result, with more liquid present in the condenser with the increasing heat load, the performance of the condenser was slightly reduced. For the LHP with the ceramic wick, Rc’s values were significantly high, i.e., 1.8 K.W−1 at 53 W, due to the lower performance of the condenser. Additionally, particle differences between the stainless steel wick and ceramic wick are apparent in Figure 11a,b; the inter- faces between the particles in Figure 11b are not as straightforward as those in Figure 11a. In the loop heat pipe with the stainless-steel wick, the working fluid was smoothly cir- culating inside the LHP because the flow resistance through the stainless-steel wick was lower than that of the ceramic wick. Therefore, the heat supplied made the working fluid evaporate, and the heat leak through the stainless-steel wick became small. In the ceramic wick’s case, the working fluid was not able to circulate correctly inside the LHP due to the high hydraulic resistance in the ceramic wick. As a result, heat flow, called a heat leak, became dominant in the evaporator with the ceramic wick. Energies 2021, 14, 2453 21 of 23

4. Conclusions The influence of two different wicks was tested in this study and used to describe the LHPs’ performance with a flat rectangular evaporator. The experimental results demonstrate that the LHP with the stainless-steel wick has better cooling performance than the LHP with the ceramic wick. The heater surface temperature of the LHP with the stainless steel wick increased from 40 to 105 ◦C in the range of a heat load from 50 to 520 W. In the ceramic wick LHP, this surface temperature reached 106 ◦C at a heat load of 155 W. Under gravity-assisted conditions, the LHP with the stainless steel wick could keep the temperature on the heater surface at 85 ◦C for a heat load of 350 W. However, the LHP with the ceramic wick could operate only at lower than 118 W for the same LHP, and the heater’s surface temperature approached 85 ◦C when the LHP was conducted at 54 W (20 kW·m−2). The ceramic wick cannot handle a high heat flux because of the conflict between vapor release and liquid suction. Large pore sizes are required to reduce resistance for the significant mass flow rate of vapor. It has been explained by Meléndez and Reyes [26]. 3 π ρvσ εde mv = (37) 128 µv d where mv is the vapor mass flow rate; ρv, σ, µv, and ε are vapor density, surface tension, viscosity, and porosity, respectively; δ is the wick thickness, and de is the effective pore diameter. On the other hand, large capillary pressure is achieved by tiny pores for liquid suction, according to the Laplace-Young equation.

4σcos ∝ ∆P = , ∝ is the contact angle (38) de As a result, the vapor release and liquid suction of the ceramic wick require different pore sizes for better heat performance in LHPs.

Author Contributions: Conceptualization, A.M. and K.K.; methodology, A.M., K.K. and P.H.H.; software, P.H.H. and K.Z.H.; validation, P.H.H. and K.Z.H.; formal analysis, P.H.H. and K.Z.H.; investigation, P.H.H.and K.Z.H.; resources, A.M. and K.K.; data curation, P.H.H.and K.Z.H.; writing— original draft preparation, K.Z.H.; writing—review and editing, K.Z.H. and A.M.; visualization, P.H.H. and K.Z.H.; supervision, A.M.; project administration, A.M. and K.K.; funding acquisition, A.M. and K.K. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Not applicable. Conﬂicts of Interest: The authors declare no conﬂict of interest. Energies 2021, 14, 2453 22 of 23

Nomenclature

◦ ◦ Ts1 temperature at heater surface ( C) Tbf fin base temperature ( C) ◦ ID.OD pipe inner.outer diameter (mm) Tci condenser inlet temperature ( C) thermal conductivity of copper heating k T condenser outlet temperature (◦C) block [W·(m K)−1] co compensation chamber inlet temperature q heat flux (kW·m−2) T cci (◦C) ◦ Q heat load (W) Teo evaporator outlet temperature ( C) evaporator bottom surface temperature R thermal resistance of evaporator (K.W−1) T e s2 (◦C) cooling water temperature at inlet position R thermal resistance of condenser (K.W−1) T c wa-i (◦C) cooling water temperature at outlet R thermal contact resistance (K.W−1) T ct wa-o position (◦C) distance between the thermocouples inside T to T heater temperature (◦C) δ 1 3 1 heating block (m) distance between the thermocouple T and T evaporator base temperature (◦C) δ 4 4 2 the bottom surface of evaporator (m) −1 Qc heat released from condenser (W) CP specific heat of cooling water [J·(kg K) ] saturation temperature accessed from m mass flow rate of cooling water (kg·s−1) T wa esat vapor pressure evaporator heat transfer coefficient heat transfer coefficient of h h calculated from saturation temperature e evaporator(kW·m−2 K−1) esat (kW·m−2 K−1) ∆T temperature change from t = 0 (◦C) t time (s) amount of heating per wire length q* T initial temperature (◦C) (W·m−1) o ◦ −1 Ro initial electrical resistance (Ω) α temperature coefficient of resistance ( C ) V voltage drop on thin wire (V) L thin wire length (mm) thermal conductivity of wick sample A area of heating block surface (m2) λ 2 [W·(m K)−1] thermal conductivity of insulator I electric current (I) λ 1 [W·(m K)−1] heat transfer rate per unit heating block’s q· surface area (kW·m−2)

References 1. Maydanik, Y.F. Loop heat pipes. Appl. Therm. Eng. 2005, 25, 635–657. [CrossRef] 2. Zhou, W.; Ling, W.; Duan, L.; Hui, K. Development and tests of loop heat pipe with multi-layer metal foams as wick structure. Appl. Therm. Eng. 2016, 94, 324–330. [CrossRef] 3. Siedel, B.; Sartre, V.; Lefèvre, F. Numerical investigation of the thermo hydraulic behaviour of a complete loop heat pipe. Appl. Therm. Eng. 2013, 61, 541–553. [CrossRef] 4. Maydanik, Y.; Fershtater, Y.G.; Pastukhov, V.G. Loop Heat Pipes: Development, Investigation and Elements of Engineering Calculations; Technical Report; Ural Division of the USSR Academy of Sciences: Moscow, Russia, 1989. 5. Hoang, T.T.; O’Connell, T.A.; Ku, J.; Butler, C.D.; Swanson, T.D. Miniature loop heat pipes for electronic cooling. In Pro- ceedings of the ASME 2003 International Electronic Packaging Technical Conference and Exhibition, Maui, HI, USA, 6–11 July 2003; pp. 517–525. 6. Kondou, C.; Umemoto, S.; Koyama, S.; Mitooka, Y. Improving the heat dissipation performance of a looped thermosyphon using low-GWP volatile ﬂuids R1234ze(Z) and R1234ze(E) with a super-hydrophilic boiling surface. Appl. Therm. Eng. 2017, 118, 147–158. [CrossRef] 7. Santos, P.H.; Bazzo, E.; Becker, S.; Kulenovic, R.; Mertz, R. Development of LHPs with ceramic wick. Appl. Therm. Eng. 2010, 30, 1784–1789. [CrossRef] 8. Canti, G.; Celata, G.P.; Cumo, M.; Furrer, M. Thermal hydraulic characterization of stainless steel wicks for heat pipe applications. Rev. Gén. Therm. 1998, 37, 5–16. [CrossRef] 9. Ji, X.; Wang, Y.; Xu, J.; Huang, Y. Experimental study of heat transfer and start-up of loop heat pipe with multiscale porous wicks. Appl. Therm. Eng. 2017, 117, 782–798. [CrossRef] 10. Maydanik, Y.; Chernysheva, M.; Pastukhov, V.G. Review: Loop heat pipes with ﬂat evaporators. Appl. Therm. Eng. 2014, 67, 294–307. [CrossRef] 11. Fukushima, K.; Nagano, H. International Journal of Heat and Mass Transfer New evaporator structure for micro loop heat pipes. Int. J. Heat Mass Transf. 2017, 106, 1327–1334. [CrossRef] 12. Nishikawara, M.; Nagano, H. International Journal of Heat and Mass Transfer Optimization of wick shape in a loop heat pipe for high heat transfer. Int. J. Heat Mass Transf. 2017, 104, 1083–1089. [CrossRef] Energies 2021, 14, 2453 23 of 23

13. Xu, J.; Ji, X.; Yang, W.; Zhao, Z. Modulated porous wick evaporator for loop heat pipes: Experiment. Int. J. Heat Mass Transf. 2014, 72, 163–176. [CrossRef] 14. Wu, S.; Wang, D.; Gao, J.; Huang, Z.; Chen, Y. Effect of the number of grooves on a wick’s surface on the heat transfer performance of loop heat pipe. Appl. Therm. Eng. 2014, 71, 371–377. [CrossRef] 15. Choi, J.; Sung, B.; Kim, C.; Borca-Tasciuc, D.-A. Interface engineering to enhance thermal contact conductance of evaporators in miniature loop heat pipe systems. Appl. Therm. Eng. 2013, 60, 371–378. [CrossRef] 16. Singh, R.; Akbarzadeh, A.; Mochizuki, M. Operational characteristics of a miniature loop heat pipe with flat evaporator. Int. J. Therm. Sci. 2008, 47, 1504–1515. [CrossRef] 17. Chernysheva, M.; Yushakova, S.; Maydanik, Y. Copper–water loop heat pipes for energy-efficient cooling systems of supercom- puters. Energy 2014, 69, 534–542. [CrossRef] 18. Zhou, G.; Li, J.; Lv, L. An ultra-thin miniature loop heat pipe cooler for mobile electronics. Appl. Therm. Eng. 2016, 109, 514–523. [CrossRef] 19. Shioga, T.; Mizuno, Y. Micro loop heat pipe for mobile electronics applications. In Proceedings of the 2015 31st Thermal Measurement, Modeling & Management Symposium (SEMI-THERM), San Jose, CA, USA, 15–19 March 2015; pp. 50–55. 20. Li, J.; Lin, F.; Wang, D.; Tian, W. A loop-heat-pipe heat sink with parallel condensers for high-power integrated LED chips. Appl. Therm. Eng. 2013, 56, 18–26. [CrossRef] 21. Moffat, R.J. Describing the uncertainties in experimental results. Exp. Therm. Fluid Sci. 1988, 1, 3–17. [CrossRef] 22. Takekoshi, E.; Imura, S.; Hirasawa, Y.; Takenaka, T. Method for Measuring Thermal Conductivity of Solids by Transient Thin Wire heating Comparison Method (Edition. B); Japan Society of Mechanical Engineers: Tokyo, Japan, 1981; Volume 47, pp. 1307–1316. [CrossRef] 23. Ebrahimi, K.; Jones, G.F.; Fleischer, A.S. A review of data center cooling technology, operating conditions and the corresponding low-grade waste heat recovery opportunities. Renew. Sustain. Energy Rev. 2014, 31, 622–638. [CrossRef] 24. Nourzadeh, N.; Shadizadeh, S.R.; Manshad, A.K. Determination of pore size distribution profile along wellbore: Using repeat formation tester. J. Pet. Explor. Prod. Technol. 2017, 7, 621–626. [CrossRef] 25. Saga Ceramics Research Laboratory. Available online: https://www.scrl.gr.jp/main/ (accessed on 28 November 2019). 26. Meléndez, E.; Reyes, R. The pool boiling heat transfer enhancement from experiments with binary mixtures and porous heating covers. Exp. Therm. Fluid Sci. 2006, 30, 185–192. [CrossRef]