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

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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] fabri- cated 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 ofD 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. DPc ≥ DPv + DPl + DPg (1) where DPv is the working fluid’s pressure loss during the vapor state’s motion. DPl is the working fluid’s pressure loss during the liquid state’s motion.
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