Study on Heat Transfer Performance of Antifreeze-R134a Heat Exchanger (Arhex)

Article Study on Heat Transfer Performance of Antifreeze-R134a Heat Exchanger (ARHEx)

Liping Pang 1, Kun Luo 1, Shizhao Yu 2, Desheng Ma 1, Miao Zhao 1 and Xiaodong Mao 3,*

1 School of Aviation Science and Engineering, Beihang University, Beijing 100191, China; [email protected] (L.P.); [email protected] (K.L.); [email protected] (D.M.); [email protected] (M.Z.) 2 AVIC Xinxiang Aviation Industry (Group) Co., Ltd., Xinxiang 453049, China; [email protected] 3 School of Aero-engine, Shenyang Aerospace University, Shenyang 110136, China * Correspondence: [email protected]

Received: 5 October 2020; Accepted: 16 November 2020; Published: 23 November 2020

Abstract: In this paper, the liquid cooling and vapor compression refrigeration system based on an Antifreeze-R134a Heat Exchanger (ARHEx) was applied to the thermal management system for high-power avionics in helicopters. The heat transfer performance of the ARHEx was studied. An experimental prototype of ARHEx was designed and established. A series of experiments was carried out with a ground experimental condition. A heat transfer formula for the antifreeze side in the ARHEx was obtained by means of the coeﬃcient of Nusselt number with experimental analysis. The performance of heat transfer and pressure drop for the refrigerant side of the ARHEx was deduced for the given condition.

Keywords: avionics; Antifreeze-R134a heat exchanger; heat transfer performance; thermal management

1. Introduction The demand for avionics cooling is increasing with its onboard integration development. In the traditional air-cooling system, the outside ram air compressed by an aircraft engine flows through a series of components, and finally it transfers the waste heat to the outside environment and becomes bleed air [1]. The avionics are cooled by the bleed air, and the heat is brought out. An environment control system (ECS) with air cycle is often used to cool the avionics, but the air bled from the engine decreases the engine performance, and fuel consumption increases [2,3]. Some military helicopters have a large number of high-power airborne electronic equipment that generates a great amount of heat. Both an air cooling system (ACS) and liquid cooling system (LCS) can be used for the cooling of electronic equipment in helicopters. For avionics with low heat flux, the ACS can be used. However, for avionics with high heat flux, there may be many problems when the ACS is adopted. First, the ACS will inevitably bleed an amount of air from the engine, which in turn will result in excessive fuel penalty. Second, excessive air flow cannot be provided by some engines. In addition, it is difficult to arrange the air-cooling pipeline in the electronic equipment cabin due to limited space. Therefore, a new type of helicopter thermal management system should be used to cool the avionics instead of ECS [4]. Based on the above reasons, the vapor compression refrigeration (VCR) system, the LCS, and effective heat transfer have been researched in past years [5,6]. Out of all these cooling systems, the VCR system has a huge advantage because of its high-performance heat transfer and because it is less affected by the external environment. The thermal management system combining the antifreeze liquid cooling (ALC) loop and the VCR loop has been increasingly favored. The ALC loop is composed of an antifreeze tank, pump, Antifreeze-R134a Heat Exchanger (ARHEx), and a high-power electric

Energies 2020, 13, 6129; doi:10.3390/en13226129 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 14

composed of an antifreeze tank, pump, Antifreeze-R134a Heat Exchanger (ARHEx), and a high- Energiespower 2020electric, 13, 6129 equipment cold plate, and the VCR loop is composed of a compressor, condenser,2 of 14 electronic expansion valve (EEV), and ARHEx. The compressor and pump provide power for the equipmentloop, and the cold heat plate, can and be thetransferred. VCR loop When is composed the high-pressure of a compressor, refrigerant condenser, liquid electronic condensed expansion by the valvecondenser (EEV), passes and ARHEx. through The the compressor EEV, the pressure and pump drops provide because power of forthe theobstruction, loop, and which the heat leads can beto transferred.vaporization When of the the refrigerant high-pressure liquid. refrigerant Then, the liquidlatent heat condensed of vaporization by the condenser is absorbed passes at the through same thetime, EEV, so thethat pressure its temperature drops because is correspondin of the obstruction,gly reduced, which leadsbecoming to vaporization low-temperature of the refrigerant and low- liquid.pressure Then, vapor. the latentThe required heat of vaporizationcooling capacity is absorbed can be at satisfied the same by time, adjusting so that itsthe temperature speed of the is correspondinglycompressor and reduced,the opening becoming of the low-temperature EEV. The antifreeze and low-pressure absorbs a large vapor. amountThe required of wastecooling heat capacitydischarged can from be satisfiedthe electronic by adjusting avionics. the Then, speed its of pump the compressor forces the antifreeze and the opening to pass ofthrough the EEV. its TheARHEx, antifreeze and the absorbs waste aheat large is amounttransferred of waste into th heate VCR discharged loop. The from VCR the loop electronic finally avionics. discharges Then, the itswaste pump heat forces to the the environment. antifreeze to In pass addition, through this its sy ARHEx,stem also and has the high waste cooling heat is efficiency transferred and into stable the VCRworking loop. characteristics The VCR loop [7,8]. finally Therefore, discharges many the waste types heat of toaircraft, the environment. such as military In addition, aircraft this F-22 system and alsomilitary has highhelicopter cooling AH-64, efficiency NH-90, and etc., stable have working adopted characteristics this type of thermal [7,8]. Therefore, management many system types ofto aircraft,remove the such waste as military heat of aircraft electronic F-22 equipment and military [9,10]. helicopter AH-64, NH-90, etc., have adopted this type ofAlthough thermal the management ARHEx has system been toused remove in some the wastetypes heatof helicopter, of electronic its equipmentapplication [is9, 10limited.]. In orderAlthough to improve the its ARHEx performance has been in used aviation in some application, types of helicopter,a study based its application on a certain is helicopter limited. In design order toshould improve be carried its performance out. In this in paper, aviation a series application, of experiments a study based were carried on a certain out to helicopter assess the design heat transfer should beperformances carried out. of Inan this ARHEx. paper, The a series experimental of experiments results wereunder carried certain outexperimental to assess theconditions heat transfer were performancesobtained. Based of on an the ARHEx. experimental The experimental analysis, th resultse heat transfer under certain performances experimental were deduced. conditions were obtained. Based on the experimental analysis, the heat transfer performances were deduced. 2. Helicopter Thermal Management of Avionics Based on ARHEx 2. Helicopter Thermal Management of Avionics Based on ARHEx 2.1. Helicopter Thermal Management System 2.1. Helicopter Thermal Management System With the increase in heating power of electronic avionics, high-efficiency heat transfer technologyWith the has increase received in heating more powerattention of electronicin recent avionics,years. Figure high-e 1ffi ciencyshows heatthe transferhelicopter technology thermal hasmanagement received more system attention (TMS) in for recent avionics years. based Figure on1 showsa VCR the loop helicopter and an thermalALC loop, management equipped system on the (TMS)equipment for avionics cabin in based the helicopter. on a VCR In loop the helicopter and an ALC TMS, loop, its equippedARHEx is ona key the component. equipment cabinIts cold in side the helicopter.fluid is R134a In the refrigerant helicopter in TMS, the itsVCR ARHEx loop, is and a key its component. hot side fluid, Its cold or the side antifreeze, fluid is R134a is 65% refrigerant glycol inaqueous the VCR solution. loop, and The its antifreeze hot side fluid, absorbs or thea large antifreeze, amount is of 65% waste glycol heat aqueous discharged solution. from The the antifreezeelectronic absorbsavionics. a largeThen, amount its pump of wasteforces heat the dischargedantifreeze fromto pass the through electronic its avionics. ARHEx. Then, In the its pumpARHEx, forces the theantifreeze antifreeze exchanges to pass heat through with itsthe ARHEx. low-temperature In the ARHEx, refrigerant the antifreezeand transfers exchanges the waste heat heat with to thethe low-temperatureVCR loop. The heated refrigerant refrigerant and transfersfinally transfers the waste the heatwaste to heat the VCRto the loop. outside The ram heated air. In refrigerant this way, finallythe waste transfers heat in the the waste ALC heat loop to is the transferred, outside ram an air.d the In temperature this way, the of waste the electronic heat in the equipment ALC loop is transferred,maintained andin a good the temperature working range. of the electronic equipment is maintained in a good working range.

Air

Condenser VCR loop EEV Compressor

ARHEx Equipment Cabin Cockpit

Pump High-Power Electric ALC loop Equipment Cold Plate

Antifreeze Tank

Figure 1.1. Helicopter thermal management system (TMS).

The advantages of this helicopter TMS are as follows: (1) it is less aﬀected by the external ﬂight environment; (2) the varied cooling demands will be met by adjusting the speed of the compressor

Energies 2020, 13, x FOR PEER REVIEW 3 of 14 Energies 2020, 13, x FOR PEER REVIEW 3 of 14 Energies 2020, 13, 6129 3 of 14 The advantages of this helicopter TMS are as follows: (1) it is less affected by the external flight The advantages of this helicopter TMS are as follows: (1) it is less affected by the external flight environment; (2) the varied cooling demands will be met by adjusting the speed of the compressor environment; (2) the varied cooling demands will be met by adjusting the speed of the compressor and the opening of the EEV; (3) the cooling capacity per unit area and heat transfer coefficient of the andand thethe openingopening ofof thethe EEV;EEV; (3) (3) the the cooling cooling capacitycapacity perper unitunit areaarea andand heatheat transfertransfer coecoefficientﬃcient of of the the ARHEx are higher than the evaporator cooled by air. ARHExARHEx are are higher higher than than the the evaporator evaporator cooled cooled by by air. air.

2.2.2.2. StructuralStructural ParametersParameters ofof ARHExARHEx 2.2. Structural Parameters of ARHEx TheThe heat heat transfer transfer performance performance of of ARHEx ARHEx is is importantimportant forfor thethe performanceperformance ofof thethe helicopterhelicopter TMS.TMS. The heat transfer performance of ARHEx is important for the performance of the helicopter TMS. TheThe ARHEx ARHEx should should be well-designed,be well-designed, otherwise otherwise the reliability the reliability of electronic of electronic avionics avionics cannot be cannot ensured. be The ARHEx should be well-designed, otherwise the reliability of electronic avionics cannot be Hence,ensured. it isHence, necessary it is necessary to test its heatto test transfer its heat performance. transfer performance. ensured. Hence, it is necessary to test its heat transfer performance. TheThe structure structure of of the the studied studied compact compact plate plate fin fin ARHEx ARHE isx shown is shown in Figure in Figure2. It is2. composedIt is composed of fins of The structure of the studied compact plate fin ARHEx is shown in Figure 2. It is composed of andfins flowand channels.flow channels. The flow The channels flow channels of antifreeze of antifreeze and refrigerant and refrigerant are arranged are arranged alternately. alternately. FinPeva, fins and flow channels. The flow channels of antifreeze and refrigerant are arranged alternately. FinLFinPevaeva,, FinL FinTevaeva, FinT, FinHeva,eva FinH, andeva, FinOand FinOeva representeva represent the the fin fin parameters. parameters. In In the the present present study, study, FinP FinPevaeva= = FinPeva, FinLeva, FinTeva, FinHeva, and FinOeva represent the fin parameters. In the present study, FinPeva = 11 mm, mm, FinL FinLevaeva == 11 mm, mm, FinT FinTevaeva = 0.1= 0.1mm, mm, FinH FinHeva = eva1.5 =mm,1.5 and mm, FinO andeva FinO = 0.5eva mm.= 0.5 The mm. height The of height the flow of 1 mm, FinLeva = 1 mm, FinTeva = 0.1 mm, FinHeva = 1.5 mm, and FinOeva = 0.5 mm. The height of the flow thechannel flow channelis 1.5 mm. is The 1.5 mm.length, The height, length, and height, width and of the width condenser of the condenserare 270 mm, are 160 270 mm, mm, and 160 80 mm,mm, channel is 1.5 mm. The length, height, and width of the condenser are 270 mm, 160 mm, and 80 mm, andrespectively. 80 mm, respectively. The heat transfer The heat coefficient transfer coe offfi ARHEcient ofx ARHExis increased is increased by arranging by arranging the channels the channels to be respectively. The heat transfer coefficient of ARHEx is increased by arranging the channels to be tomulti-channel be multi-channel countercurrent. countercurrent. The number The number of antifreeze of antifreeze and andrefrigerant refrigerant channels channels is 21 is 21and and 20, multi-channel countercurrent. The number of antifreeze and refrigerant channels is 21 and 20, 20,respectively. respectively. respectively.

Flow channel FinDeva FinOeva Flow channel FinDeva FinOeva

Cold side Flow channel FinHeva FinHeva

FinLeva FinLeva Hot side Flow channel FinT Hot side Flow channel FinTeva eva Fin FinPeva Fin FinPeva Antifreeze Refrigerant Antifreeze Refrigerant Figure 2. Structure of Antifreeze-R134a Heat Exchanger (ARHEx). FigureFigure 2.2.Structure Structure ofof Antifreeze-R134aAntifreeze-R134a Heat Heat Exchanger Exchanger (ARHEx). (ARHEx).

Figure 3 shows the principle of ARHEx. The antifreeze flows into the cylindrical cavity, then FigureFigure3 shows3 shows the the principle principle of ARHEx.of ARHEx. The The antifreeze antifreeze flows flows into theinto cylindrical the cylindrical cavity, cavity, then flows then flows into the 21 channels, and finally flows out of the ARHEx, while the refrigerant flows into the intoflows the into 21 channels,the 21 channels, and finally and finally flows outflows of theout ARHEx,of the ARHEx, while thewhile refrigerant the refrigerant flows intoflows the into other the other 20 channels. The antifreeze and refrigerant exchange heat in the form of cross-flow. 20other channels. 20 channels. The antifreeze The antifreeze and refrigerant and refrigerant exchange exchange heat in heat the formin the of form cross-flow. of cross-flow.

Refrigerant channel Antifreeze channelRefrigerant channel Antifreeze channel Refrigerant channel Antifreeze channelRefrigerant channel Antifreeze channel

Inlet of refrigerant Inlet of refrigerant

outlet of refrigerant outlet of refrigerant

Inlet of antifreeze Inlet of antifreeze

outlet of antifreeze outlet of antifreeze

Figure 3. Principle of ARHEx. Figure 3. Principle of ARHEx. Figure 3. Principle of ARHEx.

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3. Experimental3.3. Experimental Prototype Prototype and anda Experimentalnd ExperimentalExperimental Conditions ConditionsConditions

3.1. 3.1.3.1.Experimental Experimental Prototype Prototype In thisInIn thisthisstudy, study,study, a high-efficiency aa high-ehigh-efficiencyﬃciency heat heatheat transfer transfertransfer scheme, scheme,scheme, which whichwhich refers refersrefers to Reference to Reference [8], [was[8],8], was studied. studied. FiguresFiguresFigures 4 and4 4and and5 present5 present5 present the the experimethe experimental experimental ntalprinciple principle principle and and andthe the experimentthe experimental experimental device, device,al device, respectively. respectively. ItIt cancan It becan be seenseenbe seen that that that the the experimentthe experiment experiment was was was composed composed composed of of a aofVC VCR aR VC loop, loop,R loop, an an ALC an ALC ALC loop, loop, loop, a adata dataa data acquisition acquisition acquisition unit, unit, unit, and and and a a controlcontrola control unit. unit. unit.

AntifreezeAntifreeze DataData singal singal RefrigerantRefrigerant ControlControl singal singal

1 1 2 2 P1 P1T1 T1 3 3 4 4 P0 P0 P2 P2T2 T2 5 5

6 6 7 8 8 P3 P3T3 T3 7 P4 P4T4 T4 9 9

10 10 12 12 11 11 13 13

DataData acquisition acquisition unit unit ControlControl unit unit

FigureFigureFigure 4. Schematic 4.4. SchematicSchematic of the ofof theexperimentalthe experimentalexperimental facility facilityfacility [8], [(1[8],8],: condenser (1:(1: condensercondenser dryer; dryer;dryer; 2: water-cooled 2:2: water-cooledwater-cooled plate plateplate fin finfin condenser;condenser;condenser; 3: electronic 3:3: electronicelectronic expansion expansionexpansion valve valveva (EEV);lve (EEV);(EEV); 4: compressor; 4:4: compressor;compressor; 5: mass 5:5: mass flow flowflow meter; meter; 6: non-return 6: non-return valve; valve; 7: ARHEx;7:7: ARHEx; 8: volume 8: volume volume flow flow flowmeter; meter; meter; 9: control 9: 9: control control valve; valve; valve; 10: electronic 10: 10: electronic electronic heating heating heating device; device; device; 11: antifreeze 11: 11: antifreeze antifreeze tank; tank; 12: tank; 12: bypass12:bypass bypassvalve; valve; 13: valve; antifreeze 13: 13:antifreeze antifreeze pump). pump). pump).

ARHExARHEx

Figure 5. Experimental device. FigureFigure 5.5. ExperimentalExperimental device.device.

As shownAsAs shown in Figure in Figure Figure 4, the 44,, fluidthe fluid fluid in the in in VCRthe the VCR VCRloop loop was loop was R134a, was R134a, R134a, while while whilethe fltheuid the fl uidin fluid the in ALCthe in theALC loop ALC loop was loop was 65%was65% glycol 65%glycol aqueous glycol aqueous aqueous solution. solution. solution. In the In VCRthe In VCR theloop, VCRloop, R134a loop,R134a flowed R134a flowed successively flowed successively successively through through the through ARHEx,the ARHEx, the mass ARHEx, mass flowmassflow meter, flowmeter, compressor, meter, compressor, compressor, water-cooled water-cooled water-cooled plate plate fin plate condenser,fin fincondenser, condenser, condenser condenser condenser dryer, dryer, dryer, and andthe and EEV.the the EEV. EEV. When When the thethe vaporvaporvapor compressed compressedcompressed by the byby thecompressorthe compressorcompressor flowed flowedflowed through throughthrough the thecondenser,the condenser,condenser, the the heatthe heatheat was waswas dissipated dissipateddissipated into intointo the thethe ramram air, air,and andthen then the vaporthe vapor flowed flowed through through the EEtheV EE withV with a two-phase a two-phase state. state. Finally, Finally, the two-phasethe two-phase refrigerantrefrigerant absorbed absorbed the heatthe heat in the in ARHEx,the ARHEx, flowed flowed back back to the to compressor,the compressor, and andachieved achieved a cycle. a cycle. In In

Energies 2020, 13, 6129 5 of 14 ram air, and then the vapor flowed through the EEV with a two-phase state. Finally, the two-phase refrigerant absorbed the heat in the ARHEx, flowed back to the compressor, and achieved a cycle. In the ALC loop, antifreeze flowed successively through the antifreeze tank, antifreeze pump, control valve, ARHEx, and non-return valve. An electric heater was used in our experiment to simulate the heating of electronic avionics. The mass flow meter was used to measure the value of the refrigerant mass flow rate, and the temperature sensors T1, T2, T3, T4 and the pressure sensors P0, P1, P2, P3, P4 were installed in the appropriate location, as shown in Figure4; therefore, the temperature or pressure value of the corresponding position was measured. Based on the measured temperature and pressure of the fluid, the enthalpy change value or pressure difference of the corresponding liquid could be calculated. It should be noted that the uncertainty of the volume flow meter was 5%, and there were some ± errors in the use of experimental measurement equipment, so the measurement results needed to be corrected with a data processing method. The designations related to the position of experimental devices in Figure4 are listed as follows: . m, kg/s, marked with 5, mass flow rate of refrigerant; • tin , C, marked with T1, inlet temperature of EEV; • EEV ◦ pin , kPa, marked with P1, inlet pressure of EEV; • EEV tout, C, marked with T2, refrigerant outlet temperature of ARHEx; • c ◦ pout, kPa, marked with P2, refrigerant outlet pressure of ARHEx; • c ∆P, kPa, marked with P0, refrigerant pressure drop of AREHx; • . V ,L/min, marked with 8, volume flow rate of antifreeze; • h tin, C, marked with T4, antifreeze inlet temperature of ARHEx; • h ◦ pin, kPa, marked with P4, antifreeze inlet pressure of ARHEx; • h tout, C, marked with T3, antifreeze outlet temperature of ARHEx; • h ◦ pout, kPa, marked with P3, antifreeze outlet pressure of ARHEx. • h To minimize the leaking heat, the ARHEx and the connected pipes had to be wrapped with polyurethane foam and aluminum foil, as shown in Figure5. The experiment was designed to study the heat transfer performance of the ARHEx. The experimental data were not recorded until the system worked steadily. The flow rates, pressure, and temperature of the fluids were monitored and then used in the subsequent calculation and analysis. It is noted that the measurement data were not recorded until the system stabilized for 25 min.

3.2. Experimental Conditions In the ground experimental tests, the low-temperature refrigerant ﬂowed into the cold side of ARHEx, and the high-temperature antifreeze ﬂowed into the hot side of ARHEx. According to the design requirements of the experiments, the superheat, SH, was required as the calculated parameter. SH of the refrigerant at the ARHEx outlet is expressed as

out SH = t teva (1) c − out where tc is the refrigerant outlet temperature of the ARHEx obtained by resistance thermometers out marked with T2, ◦C; teva is the evaporation temperature, ◦C, and is uniquely determined by pc . The unknown properties were calculated using the REFPROP software in this study. Assuming the ﬂow process in the EEV is an adiabatic expansion process, the refrigerant enthalpy at the ARHEx in inlet, Hc , is deﬁned as Hin Hin (2) c ≈ EEV in in where HEEV indicates the inlet refrigerant enthalpy of EEV, kJ/kg, and can be calculated with tEEV in and pEEV. Energies 2020, 13, 6129 6 of 14

out out out Meanwhile, the refrigerant enthalpy at the ARHEx outlet, Hc , can be calculated by tc and pc . The pressure of the refrigerant at the ARHEx inlet is expressed as

in out pc = ∆P + pc (3)

in where pc is the inlet pressure of the ARHEx, kPa; ∆P is the refrigerant pressure drop of the ARHEx, out kPa; pc is the outlet pressure of the ARHEx, kPa. in in Based on Hc and pc , the thermodynamic state of refrigerant at the ARHEx inlet could be obtained. The heating powers of the electronic equipment were 5, 8, 10, 12, and 15 kW, respectively. In this . out in study, pc , Vh, and th could be regulated. However, they were not adjusted together. The experimental conditions are shown in Table1.

Table 1. Experimental conditions.

No. Symbol Parameter Value Unit in 1 tEEV Inlet temperature of EEV 50 ◦C in 2 pEEV Inlet pressure of EEV 1492 kPa out 3 pc Outlet pressure on refrigerant side 338, 362, 388, 415 kPa 4 SH Outlet superheat on refrigerant side 5 ◦C . 5 Vh Antifreeze volume ﬂow 20, 35, 50, 60 L/min tin 6 h Inlet temperature on antifreeze side 20, 25, 30, 35, 40 ◦C pout 7 h Outlet pressure on antifreeze side 100 kPa

3.3. Experimental Stability Based on the basic heat transfer calculations, for a group of setting experimental data, heat transfer quantity for antifreeze and refrigerant are defined, respectively. The heat transfer quantity for the antifreeze can be calculated by Equation (4) [11]: . Q = Cp V ρ tin tout . (4) A · h · · h − h where QA is the heat transfer quantity for the antifreeze, kW; Cp is the antifreeze specific heat capacity, . 3 in J/(kg ◦C); ρ is the antifreeze density, kg/m ; Vh is the volume flow rate of antifreeze, L/min; th is the · out antifreeze inlet temperature of ARHEx, ◦C; th is the antifreeze outlet temperature of ARHEx, ◦C. The heat transfer quantity for the refrigerant is as follows:

. out in Q0 = m H H (5) R · c − c . where QR0 is the heat transfer quantity for the refrigerant, kW; m is the mass ﬂow rate of refrigerant, in out kg/s; Hc is the inlet refrigerant enthalpy of ARHEx, kJ/kg; Hc is the outlet refrigerant enthalpy of ARHEx, kJ/kg. For each experimental condition, the unsteadiness can be calculated using Equation (6) [12]: q 2 2 (QA Qm) + (QR Qm) ∆Q = − − (6) Qm

and Q + Q Q = A R (7) m 2 where ∆ is the unsteadiness; Qm is mean value of heat transfer quantity, kW. Q Hence, the heat transfer quantity of ARHEx is expressed as Qm 1 ∆ . In engineering practice, · ± Q it is acceptable that the heat balance dispersion or unsteadiness is in the range of 3–7% [11–15]. We have adopted ∆Q < 6% as a criterion in our analysis. Energies 2020, 13, x FOR PEER REVIEW 7 of 14 Energies 2020, 13, 6129 7 of 14 In this study, the measurement data were modified with the least-squares method; the accuracy of measurement sensors was corrected. Suppose that the data measured by the standard sensors is y, and theIn this data study, measured the measurement by the experimental data were sensors modified is withx. Then the least-squaresthe data can method;be corrected the accuracy by the followingof measurement linear fitting sensors formula: was corrected. Suppose that the data measured by the standard sensors is y, and the data measured by the experimental sensors is x. Then the data can be corrected by the ˆ following linear fitting formula: y = aˆx +b (8) y = axˆ + bˆ (8) By measuring a series of different values, 𝑎 and 𝑏 can be calculated by the following formula: ˆ By measuring a series of differentnn values, aˆ and b can be calculated by the following formula:  2 axynxyxnxˆ =−/ 2 −()   n ! n !  P P 2  aˆ =ii==11xy nxy / x2 n(x) (9)  − − (9) ˆ =−i=1 i=1 byaxbˆ = y ˆ aˆx − 4. Experimental Result Analysis 4. Experimental Result Analysis 4.1. Heat Transfer Quantity 4.1. Heat Transfer Quantity . In the heat exchanger experiments, the antifreeze volume flow rate, Vh, could be regulated, and the 𝑉 remainingIn the twoheat variablesexchanger remained experiments, constant, the antifreeze namely, tin volume= 30 C flow and rate,pout = 338, could kPa. be regulated, and h ◦ c the remaining two variables remained constant, namely,. 𝑡 = 30 °C and 𝑝 = 338 kPa. Figure6 shows the relationship of QA and Vh for the given conditions, where QA is calculated Figure 6 shows the relationship of QA and 𝑉 for the given conditions, where QA is calculated with Equation (4). In Figure6, the dotted line is the experimental data, and the solid line is the fitting with Equation (4). In Figure 6, the dotted line is the experimental data, and the solid line is the fitting curve. With the increase in volume flow rate, the heat transfer quantity obviously increases. When the curve. With the increase in volume flow rate, the heat transfer quantity obviously increases. When volume flow reaches 60 L/min, the heat transfer quantity is 15 kW. the volume flow reaches 60 L/min, the heat transfer quantity is 15 kW.

12 (kW)

A 9 Q

15 30• 45 60 V h (L/min) . Figure 6. Relationship of QA and Vh. Figure 6. Relationship of QA and 𝑉 4.2. Efficiency of Heat Transfer 4.2. Efficiency of Heat Transfer The efficiency of heat transfer is calculated by Equation (8) [2,10]: The efficiency of heat transfer is calculated by Equation (8) [2,10]: tin tout hin− h out η0 = tthh− (10) η = tin tout (10) 0 hin− c out tthc− in where η is the efficiency of heat transfer, %; t is the inlet temperature of the hot side antifreeze, C; where 𝜂0 is the efficiency of heat transfer, %;h 𝑡 is the inlet temperature of the hot side antifreeze,◦ out out t is the outlet temperature of the hot side antifreeze, ◦C; tc is the outlet temperature of the cold side °C;h 𝑡 is the outlet temperature of the hot side antifreeze, °C; 𝑡 is the outlet temperature of the coldrefrigerant, side refrigerant,◦C. °C. in In the experiment, a constant inlet enthalpy, Hc, and a constant outlet superheat, SH, were kept In the experiment, a constant inlet enthalpy, 𝐻. , and a constant outlet superheat, SH, were kept for the cold side refrigerant. The inlet volume flow, V , and the temperature of the hot side antifreeze, for the cold side refrigerant. The inlet volume flow, 𝑉 h, and the temperature of the hot side antifreeze, tin, were changed. The calculation relationship of η = f(pout, tin, tin) could be obtained. More than 𝑡h , were changed. The calculation relationship of η0 =0 f (𝑝 c,𝑡 ,𝑡h )c could be obtained. More than 80 80 experiments were carried out to obtain this relationship. In these ground experiments, Hin experiments were carried out .to obtain this relationship. In these ground experiments, 𝐻 ≈ 271.52c ≈ in out 271.52 kJ/kg, SH 5 ◦𝑉C, and Vh was controlled at 20, 35, 50, and 60 L/min, respectively.𝑡 t𝑝 and pc kJ/kg, SH ≈ 5°C, and≈ was controlled at 20, 35, 50, and 60 L/min, respectively. and h were

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out in varied,were varied, respectively. respectively. Figure Figure 7a–d7 a–dshows shows the therelationship relationship of η of0 andη0 and 𝑝 p cforfor the the given given values values of of 𝑡t . h andand 𝑉V h. .

out out p = 338kPa pc = 362kPa (a) c (b) 60 in t = 20℃ tin = 20℃ 50 h h in = 25℃ in ℃ th 50 th = 25 in t = 30℃ tin = 30℃ 40 h h in ℃ in th = 35 40 th = 35℃ in ℃ in th = 40 th = 40℃ (%) (%)

0 30 0 η η 30

20 20

10 10 20 30• 40 50 60 20 30• 40 50 60 V h（L/min） V h（L/min）

pout = 388kPa out c pc = 415kPa (c) (d) 100 75 in = 20℃ in th th = 20℃ in t = 25℃ tin = 25℃ h 80 h in ℃ in 60 th = 30 th = 30℃ in t = 35℃ tin = 35℃ h 60 h in ℃ in 45 th = 40 th = 40℃ (%) (%) 0 0 η η 40 30

20 15

20 30• 40 50 60 20 30• 40 50 60 V h（）L/min V h（L/min）

Figure 7. Relationship of η andpout for given tin and tin. Figure 7. Relationship of η0 and0 𝑝 c for given 𝑡h and c𝑡 .

out From Figure7, some conclusions can be observed when pcvaries from 338 kPa to 415 kPa: From Figure 7, some conclusions can be observed when 𝑝 varies from 338 kPa to 415 kPa: . out out (1)(1) WithWith the the increase increase of of 𝑉V, hη,0η decreases0 decreases gradually. gradually. For For𝑝 p c = 338= 338 kPa kPa and and 𝑝 p c = 362= 362 kPakPa under under the in samethe same volume volume flow ﬂowrate, rate,η0 doesη0 doesnot necessarily not necessarily decrease decrease with withthe increase the increase of 𝑡 of. However,th . However, for out out 𝑝for pc= 388= 388 kPa kPa andand 𝑝 pc = =415415 kPa kPa under under the the same same volume volume flow ﬂow rate, rate, ηη00 decreasesdecreases with with the the in increaseincrease of of 𝑡th .. out out (2)(2) ηη0 0increasesincreases gradually gradually with with the increase ofof p𝑝c . For. For all all experimental experimental results, results, when whenpc 𝑝= 415 = kPa,415 . kPa,in 𝑡 = 20 °C and 𝑉 = 20 L/min, η0 reaches its maximum value and is 86%. Whenout 𝑝 = 338 t = 20 ◦C and Vh = 20 L/min, η0 reaches its maximum value and is 86%. When pc = 338 kPa, h . kPa,in 𝑡 = 40 °C and 𝑉 = 60 L/min, η0 reaches its minimum value and is 16%. th = 40 ◦C and Vh = 60 L/min, η0 reaches its minimum value and is 16%. Figure 8 shows the relationship of η0 = f (𝑝 out,𝑡in,𝑡in). In Figure 8, letters A, B, C, and D indicate Figure 8 shows the relationship of η0 = f (pc , th , tc ). In Figure8, letters A, B, C, and D indicate that 𝑝out is equal to 415, 388, 362, and 338 kPa, respectively. The relationship between η0 and 𝑝out,𝑡in that pc is equal to 415, 388, 362, and 338 kPa, respectively. The relationship between η0 and pc , th and 𝑡in is clear. and tc is clear.

Energies 2020, 13, 6129 9 of 14 Energies 2020, 13, x FOR PEER REVIEW 9 of 14

pout tin tin Figure 8. Relationship of η0 = f ( c , h , c ). Figure 8. Relationship of η0 = f (𝑝 ,𝑡 ,𝑡 ). 4.3. Heat Transfer Formula for Antifreeze 4.3. HeatOn theTransfer basis Formula of the experiments, for Antifreeze the heat transfer formula of the antifreeze in the ARHEx can be deduced,On the and basis then of the the convective experiments, heat the transfer heat transfer coefficients formula can of be the calculated. antifreeze in the ARHEx can be deduced,The heatand transferthen the betweenconvective the heat antifreeze transfer and coefficients the wall of can the be ARHEx calculated. can be calculated as follows (9) [11–13]: The heat transfer between the antifreeze and the wall of the ARHEx can be calculated as follows Q = h A T T (11) (9) [11–13]: A eva,A · eva,A · eva,A − eva,wall 2 2 where heva,A is the heat transfer coeffi=−cient, W⋅⋅/(m ◦C);()Aeva,A is the heat transfer area, m . Teva,A is the QhA eva,,, A Aeva,inA· Tout eva A T eva wall (11) mean value of antifreeze temperature, Teva,A = (th +th )/2, ◦C; Teva,wall is the wall temperature, ◦C. The heat transfer coefficient is as follows: 2 2 where heva,A is the heat transfer coefficient, W/(m ·°C); Aeva,A is the heat transfer area, m . Teva,A is the eva,A 𝑡 𝑡 eva,wall mean value of antifreeze temperature, T = ( Nu+ A )/2,λA °C; T is the wall temperature, °C. heva,A = · (12) The heat transfer coefficient is as follows: DA ⋅ λ The Nusselt number of antifreeze side is calculated= Nu A byA the following formula [14–19]: heva, A (12) DA b c NuA = a Re Pr (13) The Nusselt number of antifreeze side is calculated· · by the following formula [14–19]:

and bc Nuρ v =D Aa ⋅⋅ Re Pr µ Cp (13) Re = · A· ,... Pr = · (14) µ λA and where NuA is the Nusselt number; Re is the Reynolds number; Pr is the Prandtl number; a is correction coefficient considering various factors; bρis⋅⋅ experiencevD index;μc⋅isCp a constant value, 0.4; ρ is antifreeze Re = A ， Pr = density, kg/m3; v is the antifreeze velocity, mμ/s; D is the antifreezeλ hydraulic diameter, m; Cp is(14) the A … A antifreeze specific heat capacity, J/(kg C); µ is the antifreeze dynamic viscosity, Pa s; λ is the thermal ·◦ · A whereconductivity NuA is ofthe antifreeze, Nusselt number; W/(m C).Re is the Reynolds number; Pr is the Prandtl number; a is correction ·◦ coefficientThe calculated considerin valuesg various of factors; NuA are b is plotted experience against index; various c is a constant Re and value, are shown 0.4; ρ is in antifreeze Figure9. density,With 80 experimentalkg/m3; v is the results, antifreeze regression velocity, analysis m/s; DA has is the been antifreeze used tofit, hydraulic and the diameter, regression m; resulted Cp is the in antifreezethe following: specific heat capacity, J/(kg·°C); μ is the antifreeze dynamic viscosity, Pa·s; λA is the thermal conductivity of antifreeze, W/(m·°C). Nu = 0.5 Re1.396 Pr0.4 (15) A · · The calculated values of NuA are plotted against various Re and are shown in Figure 9. With 80 experimental results, regression analysis has been used to fit, and the regression resulted in the following: ⋅⋅1.396 0.4 NuA = 0.5 Re Pr (15)

Energies 2020, 13, 6129 10 of 14 Energies 2020, 13, x FOR PEER REVIEW 10 of 14

0.4 1.396 18 NuA/Pr = 0.5·Re ) 0.4 15 /Pr

ln(Nu 12

9 9.0 9.5 10.0 10.5 11.0 11.5 ln(Re)

0.4 FigureFigure 9.9. Plot of ln(NuAA/Pr/Pr0.4)) as as a a function function of of ln(Re) ln(Re) for for the the experimental experimental data. data. 4.4. Heat Transfer and Pressure Drop Formula for Refrigerant 4.4. Heat Transfer and Pressure Drop Formula for Refrigerant In our 80 experiments, refrigeration was subcooled by a circulation of cold water in the condenser. In our 80 experiments, refrigeration was subcooled by a circulation of cold water in the It became two-phase fluid with 0.3–0.35 dryness by the EEV. Then it became slightly superheated vapor condenser. It became two-phase fluid with 0.3–0.35 dryness by the EEV. Then it became slightly after it flowed through the ARHEx. superheated vapor after it flowed through the ARHEx. In this experiment, the SH was about 5 C. However, there was always the deviation of tout in the In this experiment, the SH was about 5◦ °C. However, there was always the deviation ofc 𝑡 in actual experiment, which made the SH distribute around 5 C. Figure 10a–d shows the SH during the the actual experiment, which made the SH distribute around◦ 5 °C. Figure 10a–d shows the SH during actual experimental process. the actual experimental process. In the refrigerant side, the heat transfer between the refrigerant and the wall of heat exchanger can be calculated by Equation• (14) [20,21]. • V h = 20L/min V h = 35L/min 15 15 (a) = (b) out QoutR heva,R Aeva,R teva Teva,wall out out (16) p c = 338kPa p c = 362kPa · · − p c = 338kPa p c = 362kPa out out out out p = 388kPa p = 415kPa p c = 388kPa p c = 415kPa where QR is the heatc transfer quantity,c W; heva,R is the convective heat transfer coefficient between 10 2 10 2 the refrigerant and the wall, W/(m C); A is the heat transfer area, m ; teva is the refrigerant ·◦ eva,R evaporation temperature, C; T is the wall temperature,) C. C

C) ◦ eva,wall ◦ o o ( ( Gnielinski correlation is used to calculate the convective heat transfer coeﬃcient as follows [22–24]: SH SH 5 5 q = 3 3 + 3 heva,R hcv hNcB, (17)

0 hcv = hLO FTP,0 (18) 20 25 30 35 40 · 20 25 30 35 40 in o in o ( /8) (Re 1000) Pr t ( C) ξ λR th ( C) h hLO = · − · . (19) 0.5 2/3 1.0 ·D • 1 + 12.7 (ξ/8) Pr − R • V h = 50L/min · · V h = 60L/min 15 2 15 where(c) hNcB is the nucleate boiling contribution, W/(m(d)◦C); FTP is the correlation coefficient; ξ is the out out p = 338kPa p = 362kPa · pout = 338kPa pout = 362kPa friction factor coefficient;c DR is the hydraulicc diameter in cold side, m; λc R is the thermalc conductivity pout = 388kPa pout = 415kPa pout = 388kPa pout = 415kPa of refrigerant, W/(m c C); Re is the Reynoldsc number; Pr is the Prandtl number.c c ·◦ The10 pressure drop on the refrigeration side consists10 of three parts [25]: the friction pressure drop, ) the acceleration pressure drop due to a change in the fluid) density along the flow axis, and the pressure C C o o ( drop due to gravity. Compared with the other two pressure( drop components, the friction pressure SH dropSH has5 a large proportion. Hence, the other two pressure5 drops are ignored, and the pressure drop of cold side refrigerant is calculated by the following formulas [26–28]: ! dP ξ G2 v . 0 ∆P = k · · 0, G = A m (20) 20 25 30 35 40 dp 20R 25 30 35 40 ≈ dz F 2DR · in o in (oC) th ( C) th

Figure 10. Relationship of SH for given 𝑡 and 𝑝 .

Energies 2020, 13, x FOR PEER REVIEW 10 of 14

0.4 1.396 18 NuA/Pr = 0.5·Re ) 0.4 15 /Pr

Energies 2020, 13, 6129 A 11 of 14 ln(Nu  12   3/21/12  12 " 0.9 !#16 16 −   8  7 ε 37530   ξ = 8 +  2.457 ln + 0.27 +   (21)  Re  Re D R Re   9 2 . where kdp is the pressure drop9.0 gain coe 9.5fficient; 10.0AR is the 10.5 cross-area 11.0 of refrigerant 11.5 side, m ; m is the ln(Re) refrigerant mass flow rate, kg/s; v is refrigerant flow velocity, m/s; ε is the absolute roughness; DR is the hydraulic diameter of refrigerant side, m. Figure 9. Plot of ln(NuA/Pr0.4) as a function of ln(Re) for the experimental data. In the Equation (16), heva,R is the heat exchange coefficient computed from the heat exchange 4.4.correlations Heat Transfer without and any Pressure modifications, Drop Formula so the for heat Refrigerant transfer quantity QR computed with the experimental temperature data also needed to be modified. The equation can be established as follows: In our 80 experiments, refrigeration was subcooled by a circulation of cold water in the . condenser. It became two-phase fluid= with 0.3–0.35= drynessin byout the EEV. Then it became slightly QA keva,R QR Cp Vh ρ th th (22) superheated vapor after it flowed through the· ARHEx.· · · − whereIn kthiseva,R experiment,is the heat transfer the SH was gain about coeffi cient.5 °C. However,keva,R and therekdp in was the heatalways transfer the deviation and pressure of 𝑡 drop in theformulas actual ofexperiment, the refrigerant which side made can the be SH deduced distribute using around the experimental 5 °C. Figure 10a–d data. shows Figure the 11 SHshows during the thefitting actual result experimental of heat transfer process. gain coefficient, and keva,R = 8. In the same way, kdp = 1.

• • V h = 20L/min V h = 35L/min (a) 15 (b)15 out out out out p c = 338kPa p c = 362kPa p c = 338kPa p c = 362kPa out out out out p c = 388kPa p c = 415kPa p c = 388kPa p c = 415kPa 10 10 ) C C) o o ( ( SH SH 5 5

0 0 20 25 30 35 40 20 25 30 35 40 in o in o t ( C) th ( C) h • • V h = 50L/min V h = 60L/min (c)15 (d)15 pout = 338kPa pout = 362kPa out out c c p c = 338kPa p c = 362kPa pout = 388kPa pout = 415kPa out out c c p c = 388kPa p c = 415kPa 10 10 ) ) C C o o ( ( SH SH 5 5

0 0 20 25 30 35 40 20 25 30 35 40 in o in (oC) th ( C) th

in out Figure 10. Relationship of SH for given t and pc. Figure 10. Relationship of SH for given 𝑡h and 𝑝 .

Energies 2020, 13, 6129 12 of 14 Energies 2020, 13, x FOR PEER REVIEW 12 of 14

15 keva,R = 8

10 (kW) R Q

20 25• 30 35 40 V h (L/min)

Figure 11. HeatHeat transfer transfer gain gain coefficient. coefficient. 5. Conclusions 5. Conclusions The aim of this paper was to study the helicopter electronic equipment cooling technology basedThe on aim a of compact this paper plate was fin to study ARHEx. the helicopte Its experimentalr electronic prototype equipment was cooling designed technology and based made. onMore a compact than 80 experimentsplate fin ARHEx. were conducted Its experimental to assess prototype its heat transfer was designed performance and made. in certain More conditions. than 80 experimentsThe experimental were results conducted show to the as following:sess its heat transfer performance in certain conditions. The experimental results show the following: (1) The heat transfer quantity, QA, in the ground experiment can reach 15 kW; (1) The heat transfer quantity, QA, in the ground experiment can reach 15 kW; (2) The heat transfer efficiency, η0, in the ground experiment can reach 40–80%; (2) The heat transfer efficiency, η0, in the ground experiment can reach 40%–80%; (3) The heat transfer formula of the antifreeze side can be deduced as Nu = 0.5 Re1.396 Pr0.4; (3) The heat transfer formula of the antifreeze side can be deduced as Nu = 0.5·Re· 1.396·Pr· 0.4; (4) The heat transfer gain factor of the refrigerant side, keva,R, is 8, and the pressure drop gain factor (4) The heat transfer gain factor of the refrigerant side, keva,R, is 8, and the pressure drop gain factor of refrigerant side, kdp, is 1. of refrigerant side, kdp, is 1.

Author Contributions: The contributions of the authors are presentedpresented asas follows.follows. Conceptualization, K.L; K.L; Methodology, K.L.; Software, Software, D.M.; D.M.; Validation, Validation, M.Z.; M.Z.; Fo Formalrmal Analysis, Analysis, L.P. L.P. and K. K.L.;L.; Investigation, M.Z.; M.Z.; Resources, X.M.; X.M.; Data Data Curation, Curation, S.Y.; S.Y.; Writing Writing Origin Origin Dr Draftaft Preparation, Preparation, L.P., L.P., K.L. and and M.Z.; M.Z.; Writing Writing Review Review & & Editing, L.P.; L.P.; Visualization, K.L.; Supervision, L.P.; L.P.; Proj Projectect Administration, S.Y. S.Y. and M.Z.; Funding Funding Acquisition, Acquisition, X.M. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by The Liao Ning Revitalization Talents Program, grant number XLYC1802092. Funding: This research was funded by The Liao Ning Revitalization Talents Program, grant number XLYC1802092.Conﬂicts of Interest: The authors declare no conﬂict of interest.

ConflictsNomenclature of Interest: The authors declare no conflict of interest.

NomenclatureFinP spacing of the fin FinL length of the fin FinT length of theFinP fin spacing of the fin FinH height of the fin FinL length of the fin FinO offset of the fin t temperatureFinT (◦C) length of the fin p pressure (kPa)FinH height of the fin ∆P refrigerant pressure drop (kPa) . FinO offset of the fin V volume flow rate (L/min) . m mass flow ratet (kg/s) temperature (°C) SH superheat (◦pC) pressure (kPa) H specific enthalpy (kJ/kg) ΔP refrigerant pressure drop (kPa) Q heat transfer quantity (kW) Cp specific heat𝑉 (J /kg/K) volume flow rate (L/min) v velocity (m/s)𝑚 mass flow rate (kg/s) h heat transfer coefficient (W/m2/K) SH superheat (°C) H specific enthalpy (kJ/kg) Q heat transfer quantity (kW)

Energies 2020, 13, 6129 13 of 14

Greek symbols ρ density (kg/m3) ∆ unsteadiness (%) η heat transfer eﬃciency λ thermal conductivity (W/m/K) µ dynamic viscosity (Pa s) · ξ friction factor coeﬃcient ε absolute roughness (mm) Subscripts eva evaporator c cold side of the ARHEx h hot side of the ARHEx A antifreeze R refrigerant EEV electronic expansion valve eva evaporator

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