Thermal Characteristics of an Oscillating Heat Pipe Cooling ... - MDPI

energies Article

Thermal Characteristics of an Oscillating Heat Pipe Cooling System for Electric Vehicle Li-Ion Batteries Ri-Guang Chi, Won-Sik Chung and Seok-Ho Rhi * Applied Thermal Engineering Lab, School of Mechanical Engineering, Chungbuk National University, 1 ChungDae-ro, SeoWon-gu, Cheongju 28644, Korea; [email protected] (R.-G.C.); [email protected] (W.-S.C.) * Correspondence: [email protected]; Tel.: +82-43-261-2444 Received: 5 March 2018; Accepted: 13 March 2018; Published: 15 March 2018

Abstract: The heat generation of lithium ion batteries in electric vehicles (EVs) leads to a degradation of energy capacity and lifetime. To solve this problem, a new cooling concept using an oscillating heat pipe (OHP) is proposed. In the present study, an OHP has been adopted for Li-ion battery cooling. Due to the limited space in EVs, the cooling channel is installed on the bottom of the battery module. In the bottom cooling method with an OHP, generated heat can be dissipated easily and conveniently. However, most studies on heat pipes have used bottom heating and top or side cooling methods, so we investigate the various effects of parameters with a top heating/bottom cooling mode with the OHP, i.e., the inclination angle of the system, amount of working fluid charged, the heating amount, and the cold plate temperature with ethanol as a working fluid. The experimental results show that the thermal resistance (0.6 ◦ C/W) and uneven pulsating features influence the heat transfer performance. A heater used as a simulated battery was sustained under 60 ◦ C under 10 W and 14 W heating conditions. This indicates that the proposed cooling system with the bottom cooling is feasible for use as an EV’s battery cooling system. Keywords: oscillating heat pipe (OHP); Li-ion battery cooling; electric vehicles (EVs)

1. Introduction Because of the large amount of fossil fuel consumption in recent years, the climate and the ecosystem have become unstable, and other environmental problems have become increasingly serious. According to a study by the International Energy Agency, almost 50% of oil is consumed by transportation systems [1], but the highest energy efficiency of their engines is only 40% and large quantities of pollutants are generated in the conventional combustion process [2]. To address the limited energy resources and environmental problems caused by fossil fuels, considerable research has been carried out by a number of researchers. The ratio of electricity production through fossil fuels, including coal, is gradually decreasing, while the ratio of non-fossil fuels from sources such as nuclear power plants and hydroelectric power plants is gradually increasing. Research investments in energy technologies related to batteries have been increasing recently in the EV industry [3]. Batteries are energy storage and supply devices which are closely related to the performance of electric vehicles (EVs). Because of the high energy density and long life, many lithium batteries are needed in various devices. However, Li-ion batteries have limited heat resistance, due to the heat generated during charging and discharging, battery capacity and lifetime reduction [4,5]. Recently, the battery heat generation of LiFePO4 (20 Ah) batteries was investigated by Panchal et al. [6]. They reported that battery heat generation rate was a function of the discharge capacity. Their results showed that the highest rate of heat generation was found to be 91 W for 4 C discharge rate and 13 W for 1 C discharge rate. Their one pouch cell was cooled by two active water flow cold plates. Energies 2018, 11, 655; doi:10.3390/en11030655

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discharge rate and 13 W for 1 C discharge rate. Their one pouch cell was cooled by two active water 2 of 16 flow cold plates. To solve this thermal problem, conventional air cooling [7–12] and water cooling methods have solve this thermal problem, conventional air tried cooling and water cooling variation methods have been To used [13,14]. In particular, Panchal et al. [15] to [7–12] investigate temperature with been used [13,14]. In particular, Panchal et al. [15] tried to investigate temperature variation with water water cooling as a function of different discharge capacities. However, due to the limited thermal cooling as a function of different capacities. due toof theair limited thermal conductivity conductivity and specific heatdischarge of air, the coolingHowever, performance cooling methods is not and specificfor heat of air, the cooling performance of airiscooling is not for high satisfactory high performance EVs. Water cooling a good methods solution for the satisfactory high heat generated performance EVs. Water coolingofisleakage a good and solution thechannels, high heatfriction generated byetc. batteries but itnew has by batteries but it has problems largefor flow losses, Therefore, problems of leakage and channels, friction losses, etc. Therefore, cooling technologies cooling technologies withlarge heatflow pipes and phase change materials are beingnew developed. Researchers with pipeson and phaseexamples change materials arecooling being developed. reported several have heat reported several of battery ideas usingResearchers heat pipes have [16–20]. Theseon heat pipe examples of battery cooling ideas pipeswith [16–20]. Theseheating heat pipe applications have applications have used many heatusing pipe heat designs a bottom structure and top or used side many heat pipe designs with a bottom heating structure and top or side cooling structure. cooling structure. As shown shown in in Figure Figure 1, 1,an anoscillating oscillatingheat heatpipe pipe(OHP), (OHP),also alsocalled called a pulsating heat pipe (PHP), a pulsating heat pipe (PHP), is is proposed in the present study a promising transfer technology for battery cooling proposed in the present study as aaspromising heatheat transfer technology for battery cooling due todue its to its high transmitting performance simple wickless structure,compact compactsize, size,and and low high heat heat transmitting performance withwith its its simple wickless structure, manufacturing cost [21–24]. An OHP could be an ideal candidate for Li-ion battery cooling for future EVs compared to other conventional cooling technologies technologies [25–27]. [25–27]. Energies 2018, 11, 655

Figure 1. Oscillating heat pipe (OHP).

In In 1990, 1990, Akachi Akachi in in Japan Japan invented invented this this new new heat heat transfer transfer device, device, the the OHP OHP (or (or PHP). PHP). It It consists consists of of aa curved curved channel channeland andsaturated saturatedworking workingfluid, fluid, and can be divided into the evaporator, insulation, and can be divided into the evaporator, insulation, and and condenser sections. Inoperating the operating the working fluid inside the is OHP is separated by a condenser sections. In the state,state, the working fluid inside the OHP separated by a liquid liquid slug and a vapor plug [23], and when the heat is supplied to the heating section, the liquid slug slug and a vapor plug [23], and when the heat is supplied to the heating section, the liquid slug starts starts to oscillate the pressure difference the flow between the evaporator to oscillate drivendriven by thebypressure difference alongalong the flow pathpath between the evaporator andand the the condenser [24]. This self-activated oscillation of the working fluid leads to the transfer of heat condenser [24]. This self-activated oscillation of the working fluid leads to the transfer of heat through through the liquid slug. Subsequently, theslug liquid slug into moves the expansion or compression space the liquid slug. Subsequently, the liquid moves theinto expansion or compression space of the of the vapor plug [25,26]. These oscillations in each channel affect each other and can be linked to the vapor plug [25,26]. These oscillations in each channel affect each other and can be linked to the liquid liquid slug and vapor plug motion. slug and vapor plug motion. In In contrast contrast with with its its simple simple structure, structure, the the operating operating mechanism mechanism of of OHPs OHPs seems seems quite quite complicated complicated and has been studied in theory and experimentally by many researchers. Such studies and has been studied in theory and experimentally by many researchers. Such studies have have mainly mainly focused focused on on the the effects effects of of various various parameters, parameters, such such as as the the tube tube diameter, diameter, filling filling ratio, ratio, inclination inclination angle, angle, number of turns, and working fluid properties; the its flow visualization. number of turns, and working fluid properties; the its flow visualization. Tong et al. al. [28], [28], Qu Qu et et al. al. [29], [29], Katpradit Katpradit et et al. al. [30], [30], Xu Xu et et al. al. [31], [31], Das Das et et al. al. [32], [32], and and Tong et Soponpongpipat et al. [33] among others have investigated OHPs via visualization studies using Soponpongpipat et al. [33] among others have investigated OHPs via visualization studies using glass tubes. They Theycarried carriedout outexperimental experimentaland and analytical research to visualize their structure glass tubes. analytical research to visualize their flowflow structure and and liquid film behavior. liquid film behavior.

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Ma et al. [34,35], Qu et al. [36,37], and Ji et al. [38] achieved higher heat transfer performance with Ma etasal.the [34,35], Qu et al. [36,37], and Jioperation. et al. [38] In achieved higher heat transfer performance a nanofluid working fluid in the OHP addition, many researchers [39–47] have with a nanofluid as the working fluid in the OHP operation. In addition, many researchers [39–47] attempted to investigate the effects of various parameters such as the tube diameter, the length of the have attempted to investigate the effects of various parameters such as the tube diameter, the length evaporation section and the condensation section, the bend diameter, the number of turns, and the of the evaporation andWang the condensation section, the diameter, the number of on turns, inclination angle. In section addition, et al. [48] conducted anbend experimental investigation an and OHP inclination angle. In addition, et al. [48] conducted an experimental investigation anto as the a battery cooling system, but theyWang used the bottom heating/top cooling method. However,on due OHP as a battery cooling system, but they used the bottom heating/top cooling method. However, the limited space available in the structure of EVs, the cooling channel should be installed under the due tothus the leading limited to space available the structure of EVs,and the replacement. cooling channel should be installed battery, more frequentinbattery maintenance under the battery, thus leading to more frequent battery maintenance and replacement. The main objective of this paper is to investigate the heat transfer performance of an OHP The main objective of this paper is to investigate the heat transfer performance of an OHP to cool to cool an EV’s Li-ion batteries, and thus to determine its operating mechanism at different input an EV’s Li-ion batteries, and thus to determine its operating mechanism at different input powers, powers, charging ratios and inclinations. To achieve this objective, it is worthwhile to begin by charging ratios and inclinations. To achieve this objective, it is worthwhile to begin by providing an providing an overall perspective on the principal ideas behind various cooling technologies for EVs. overall perspective on the principal ideas behind various cooling technologies for EVs. This will This will ascertain the relative position of OHPs as a heat transfer solution for EV’s battery cooling. ascertain the relative position of OHPs as a heat transfer solution for EV’s battery cooling. Such OHPs Such OHPs should be designed to be operated with efficient cooling performance at any inclination should be designed to be operated with efficient cooling performance at any inclination angle. The angle. The inspiration for theresearch presentisresearch is to find ways to achieve a high performance passive inspiration for the present to find ways to achieve a high performance passive system as system as well as, to drastically reduce the manufacturing complexity involved in OHPs. The concept well as, to drastically reduce the manufacturing complexity involved in OHPs. The concept of an of OHP an OHP cooling system is expected to addresses these[49,50]. issuesIn [49,50]. In addition, we attempted cooling system is expected to addresses these issues addition, we attempted to design to adesign a novel battery cooling system with top heating/bottom cooling with an OHP with a long novel battery cooling system with top heating/bottom cooling with an OHP with a long evaporator evaporator section and a short condensing section and a short condensing section. section. 2. Experiments 2. Experiments Figure 2a–c shows OHP; itit also alsoshows showsthe thethermocouple thermocouple Figure 2a–c showsthe thepresent presentbattery batterycooling coolingsystem system with with OHP; positions. AsAsshown is an an application applicationfor forEV EVbattery battery positions. shownininFigure Figure2a, 2a,the thepresent present OHP OHP cooling cooling system system is cooling. Therefore, in the experiment, appropriate working conditions were simulated for the OHP cooling. Therefore, in the experiment, appropriate working conditions were simulated for the OHP system, with rolling and and pitching orientations. As shown in Figure 2b, the 2b, present wasOHP fabricated system, with rolling pitching orientations. As shown in Figure the OHP present was byfabricated a copper capillary tube, divided into three parts: evaporator, adiabaticadiabatic and condenser sections, by a copper capillary tube, divided into three parts: evaporator, and condenser sections, with lengths 190total mm.width The total width with of 152turns 2 mm outer diameter copper tube with lengths of 190 mm.ofThe with 15 turns mm of outer diameter copper tube (0.5 mm (0.5wall) mm thick lesslength than the of aThe heater. The experimental setup illustrated in Figure 2c thick is lesswall) thanisthe of length a heater. experimental setup illustrated in Figure 2c mainly mainlyofconsists of an end-opened OHP assembly having wired heating water consists an end-opened OHP assembly having 15 turns,15a turns, wired aheating system,system, a wateraflowing flowing cooling system, and a multichannel data acquisition system. heater (190 × 150 cooling system, and a multichannel data acquisition system. The heaterThe (190 mm × 150mm mm) wasmm) used was used to simulate a Li-ion battery of an EV. It has been manufactured with a special design to to simulate a Li-ion battery of an EV. It has been manufactured with a special design to simulate an EV’s simulate an EV’s rectangular pouch battery. The benchmarked model of the present cell heater was rectangular pouch battery. The benchmarked model of the present cell heater was essentially a cell of essentially a cell car of aproduced Hyundai hybrid produced by LGoutput Chem. power Its nominal output was 19.9 a Hyundai hybrid by LG car Chem. Its nominal was 19.9 W. power Heat generation W. Heat generation rate is depending on discharging C-rate. In the present study, the extra high rateis rate is depending on discharging C-rate. In the present study, the extra high rate heat generation heat generation is not considered. not considered.

(a) Electric car cooling model using OHP

Figure 2. Cont.

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(b) Experimental OHP Module

(c) Experimental Setup Figure 2. Experimental System. Figure 2. Experimental System.

The thermostat providing a cooling water (0.01 kg/s, 23 °C) system was connected to a cooling Theflowing thermostat providing a ×cooling (0.01 kg/s, 23 ◦ C) system was connected to a cooling water channel (190 mm 15 mm)water comprised of aluminum alloy and with a wall thickness of water flowing channel (190 mm × 15 mm) comprised of aluminum alloy and with a wall thickness 2.5 mm. To reduce heat losses, an extruded polystyrene (thickness = 10 mm) insulation plate covered of 2.5both mm.outer To reduce heat losses, an extruded polystyrene (thickness = 10 mm) insulation plate covered cover sides of the OHP and heater, respectively. To sustain the shape ofOHP the liquid columnrespectively. inside the tube, the tube diameter of the OHP should both outer cover sides of the and heater, meet the working criteria of the tube’s inner inside diameter in Equation (1)of[49]. Whenshould the To sustain the shape of the liquid column the described tube, the tube diameter the OHP working fluid is ethanol, this critical inner diameter is about 0.035 m. Therefore, the present 2 mm meet the working criteria of the tube’s inner diameter described in Equation (1) [49]. When the working ODiscopper tube with 0.5 mm thickness is suitable the operation limit the of the OHP: 2 mm OD copper fluid ethanol, this critical inner diameter is aboutfor 0.035 m. Therefore, present tube with 0.5 mm thickness is suitable for the operation limit of the OHP: = 2s (1) − σ Dcr = 2 (1) g i ρ=l − ρ g Do = 2 mm); it has 15 turns, the bend The present OHP is made of a copper tube (D 1 mm, curvature diameter is 5 mm, and the total volume of heat pipe inside is about 4.5 mL. The charged The present OHPasisthe made of a fraction copper tube Do = of 2 mm); it has 15 turns,volume the bend i = 1 mm, amount is described volume (VF =(D ((charged volume working fluid)/(total curvature diameter is 5 mm, and the total volume of heat pipe inside is about 4.5 mL. The charged of the OHP)) × 100) ratio compared to the total volume of the OHP. Its evaporator and condenser amount is described as to thethe volume (VF =channel, ((charged volume of connected working fluid)/(total sections are attached heaterfraction and cooling respectively, by a 25 mm volume long of adiabatic the OHP)) × 100) ratio compared to the total volume of the OHP. Its evaporator and condenser section. sections are attached to the heater and cooling channel, respectively, connected by a 25 mm long adiabatic section.

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As shown in K-type thermocouples (TT-K-36-SLE, Omega, Stamford, CT, USA) were As inFigure Figure3,3,1919 K-type thermocouples (TT-K-36-SLE, Omega, Stamford, CT, USA) installed and and connected to the data acquisition module to were installed connected to the data acquisition module(MX-100, (MX-100,Yokogawa, Yokogawa,Tokyo, Tokyo, Japan) to measure the temperature of the OHP cooling system. The accuracy of temperature measurement was measure the temperature of the OHP cooling system. The accuracy of temperature measurement was ± (rgd (rgd 0.05% 0.05% ++ 0.7). ±

(a) Thermocouple Position in Heater Surface

(b) Thermocouple Position in OHP surface Figure 3. 3. Thermocouple Thermocouple Positions. Positions. Figure

As shown in Figures 3a,b, thermocouples were attached on the surface at 19 positions : Nos. 1– As shown in Figure 3a,b, thermocouples were attached on the surface at 19 positions : Nos. 1–5 5 in five positions on the heater surface, Nos. 6–8 in the upper part on the evaporator surface of the in five positions on the heater surface, Nos. 6–8 in the upper part on the evaporator surface of the OHP 25 mm away from the top end, Nos. 9–11 in the middle section of evaporator section with 50 OHP 25 mm away from the top end, Nos. 9–11 in the middle section of evaporator section with mm pitch, Nos. 12–14 in the lower part of the evaporator section with a 50 mm pitch, Nos. 12–14 close 50 mm pitch, Nos. 12–14 in the lower part of the evaporator section with a 50 mm pitch, Nos. 12–14 to the 25 mm adiabatic section, Nos. 15–17 in the middle of the condensing section, and Nos. 18–19 close to the 25 mm adiabatic section, Nos. 15–17 in the middle of the condensing section, and Nos. in the coolant inlet and outlet. We used the DC power supply to adjust the power of the heater. The 18–19 in the coolant inlet and outlet. We used the DC power supply to adjust the power of the heater. minimum scales of the ammeter and voltmeter are 0.01 A and 0.01 V, respectively. The working fluid The minimum scales of the ammeter and voltmeter are 0.01 A and 0.01 V, respectively. The working of the OHP was anhydrous ethanol, and it was injected by a syringe into the OHP with a perfect fluid of the OHP was anhydrous ethanol, and it was injected by a syringe into the OHP with a perfect vacuum of 10−−55 torr. The minimum scale of the charging syringe was 0.01 mL. vacuum of 10 torr. minimum scale of the syringe 0.01 mL. To estimate theThe thermal performance of charging the present OHPwas system, the average temperature difference of the evaporator and effective thermal resistance was determined from steady-state data. The average temperature difference was computed by obtained experimental data as Equation (2): ∆ =

−

(2)

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To estimate the thermal performance of the present OHP system, the average temperature difference of the evaporator and effective thermal resistance was determined from steady-state data. The average temperature difference was computed by obtained experimental data as Equation (2): ∆T = TEvap − TCond Energies 2018, 11, x

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where TEvap and TCond are the average of all nine evaporator and three condenser temperatures, respectively. The resistance is an important indicating thecondenser thermal performance of where andthermalare the average of all nineparameter evaporator and three temperatures, an OHP; it is defined as shown in Equation (3) [45]: parameter indicating the thermal performance respectively. The thermal resistance is an important of an OHP; it is defined as shown in Equation (3) [45]: TEvap − TCond R= − Q =

(3) (3)

where Q is the heating power input added to the evaporator, which is calculated by: where Q is the heating power input added to the evaporator, which is calculated by: Q= =VI

(4) (4)

where V V and voltage and and electric electric current, current, respectively. respectively. where and II are are the the input input voltage Experimental uncertainties of the direct measurement V, and were Experimental uncertainties of the direct measurement parameters parameters such such as as T, T, V, and II were synthesized by the system uncertainty. According to the methods in [3], the maximum uncertainty synthesized by the system uncertainty. According to the methods in [3], the maximum uncertainty of of thermal resistance in this study was ±3.2%. thermal resistance in this study was ±3.2%. 3. Results and Discussion In the present experimental study, the operation and performance of the OHP was associated supplied heat, amount of coolant charged, number of turns, tube with various various parameters parameterssuch suchasasthe the supplied heat, amount of coolant charged, number of turns, diameter, tube tube length, heating and and cooling length and and inclination angle. The The potential of using the tube diameter, length, heating cooling length inclination angle. potential of using OHP as aas battery cooling module waswas verified. the OHP a battery cooling module verified. Figure 4a,b shows the experimental results with different charged amounts amounts and and supplied supplied heats. heats. heat flux increased, thethe maximum charged amount (charging limit)limit) and As shown shown in inFigure Figure4a, 4a,asasthe the heat flux increased, maximum charged amount (charging optimum charged amount showing the best performance increased. As illustrated in Figure 4b, the and optimum charged amount showing the best performance increased. As illustrated in Figure 4b, thermal resistance waswas decreased byby increasing the heat the thermal resistance decreased increasing the heatflux fluxand andthe thecharging chargingamount; amount; itit decreased decreased by VF == 15.6% 15.6% (Q == 20 20 W) W) and and VF VF ==20% 20%(Q (Q==25 25W), W),and andafter afterthat, that, itit increased increased again. again. This This means that the OHP system requires an optimum charging amount to achieve achieve the the best best heat heattransfer transferperformance. performance. In addition, it can be associated with oscillation behaviors. This This trend trend on on the effect of the charged amount is similar to the results of Lee [39]. [39].

(a) Effect of Q on VF Figure 4. Cont.

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Ethanol, Tc = 23 oC Q=10W Q=14W Q=20W Q=25W

0.9

Q=10W Q=14W Q=20W Q=25W

0.7 0.9 0.6

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Ethanol, Tc = 23 oC

0.8 1

Thermal resistance(oC/W)


Thermal resistance(oC/W)

1

0.8 0.7

0 0.6

5

10

15

20

25

30

35

40

45

VF, %

(b) Effect of VF on Thermal resistance with different Q 0 5 Amount 10 15(VF:20Volume 25 30 35 40 45 Figure 4. Effect of Charged Fraction) on thermal resistance. Figure 4. Effect of Charged Amount (VF:VF, Volume Fraction) on thermal resistance. %

(b) Effect of VF on Thermal resistance with different Q

This is because when the supplied heat is increased, the temperature difference between the This is and because when theofsections supplied increased, the temperature between the Figure 4. Effect Charged Amount Volume Fraction) on thermal resistance. evaporator condensation isheat also is(VF: increased. Thus, this leads todifference an increased pressure evaporator and condensation sections is also increased. Thus, this leads to an increased pressure differenceThis between bubbles. If the charged amount is insufficient, the difference working between fluid cannot be is because when the supplied heatamount is increased, the temperature the be difference between bubbles. If the charged is insufficient, the working fluid cannot sufficiently supplied to the vapor region of the heat pipe, resulting in a dry-out phenomenon. Since evaporator and condensation sections is also increased. Thus, this leads to an increased pressure sufficiently suppliedcoefficient to the vapor regionlarger of the heatvapor, pipe, resulting in of a dry-out Since the liquid viscosity in the case a large phenomenon. amount of working difference between bubbles.isIfmuch the charged than amount is insufficient, the working fluid cannot be the liquid viscositydrop coefficient is much larger than vapor, in pipe the case ofthe a large amount of working fluid, the pressure will between liquid wall, motion of working sufficiently supplied to theincrease vapor region of thethe heat pipe, and resulting in a dry-out phenomenon. Since fluid fluid, the pressure drop will increase between the liquid and pipe wall, the motion of working theresisted, liquid viscosity coefficient is muchthe larger than vapor, in the case of a large amount of working fluid will be and then subsequently thermal resistance will increase. will be resisted, and then subsequently the thermal resistance will increase. fluid, the pressure drop will is increase between the liquid and pipe theFigure motion5ofshows working The present OHP system a battery cooling application forwall, EVs. thefluid thermal The present OHP system is a battery cooling application for EVs. Figure 5 shows the thermal will be resisted, and then subsequently the thermal resistance will increase. resistance of the OHP with the heating and inclination angles with different charging ratio. As shown The present system is a battery cooling application for EVs. Figurecharging 5 shows ratio. the thermal resistance of the OHPOHP with the heating and inclination angles with different As shown in Figure 5a,b, roughly increasing the α-angle, the thermal resistance was increased. Especially in the resistance of roughly the OHP with the heating and inclination angles with different charging ratio. As shown in Figure 5a,b, increasing the α-angle, the thermal resistance was increased. Especially case of a low heat flux (Q = 10~14 W), the thermal resistance was increased sharply with increasing incase Figure 5a,b, roughly increasing the α-angle, thermal resistance was increased. Especially in the with in the of a low heatin flux (Q 10~14 W),the the thermal resistance increased sharply the α-angle. Conversely, the case=of high heat flux as Q = 20~25 W, thewas thermal resistance variation case of a low heat flux (Q = 10~14 W), the thermal resistance was increased sharply with increasing increasing the with α-angle. Conversely, in the case of high heat flux as Q= 20~25 W, the thermal resistance was not large different α-angles. As shown in Figure 5b, the β-angle also influences the OHP the α-angle. Conversely, in the case of high heat flux as Q = 20~25 W, the thermal resistance variation variation was not large with different α-angles. As shown in Figure 5b, the β-angle also influences performance. Similar Figure 5a for the As α-angle, low heat fluxβ-angle (Q = 10also W),influences the thermal was not large withtodifferent α-angles. shown at in aFigure 5b, the the resistance OHP the OHP performance. Similar to Figure 5a for This the α-angle, at a low heatisflux (Q = 10 with W), the thermal increased sharplySimilar with increasing the effect consistent Rittidech’s performance. to Figure 5a forβ-angle. the α-angle, attrend a low on heatthe flux (Q = of 10 W), the thermal resistance resistance increased sharply with increasing the β-angle. This trend on the effect of is consistent increased sharply withheating/bottom increasing the β-angle. This trend on the effect of is consistent with Rittidech’s with [50] results based on top cooling. Rittidech’s [50] based resultsonbased on top heating/bottom cooling. [50] results top heating/bottom cooling.

(a) Effect of α-angle orientation with different Q and VF

(a) Effect of α-angle orientation with different Q and VF Figure 5. Cont.

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(b) Effect of β-angle orientation with different Q and VF Figure 5. Effect of Orientation with different Q and VF.

Figure 5. Effect of Orientation with different Q and VF.

At a low heat flux (Q = 10 W), thermal resistance is strongly influenced by increasing the heat flux angles both α and resistance β. This is due to the minimal pressure difference between At awith low inclination heat flux (Q = 10 of W), thermal is strongly influenced by increasing the heat flux and cold region gravity effect of working fluid with pressure increasingdifference charged amount at hot with hot inclination anglesinduced of bothfrom α and β. This is due to the minimal between low heat flux. In high heat flux it can be disappeared due to highly increased vapor pressure. and cold region induced from gravity effect of working fluid with increasing charged amount at low transient temperature power vapor spectrum analysis. The heat flux.Figures In high6–8 heatshow flux the it can be disappeared duevariations to highlyand increased pressure. temperature behavior of the wall surface close to the inlet (or outlet) of the evaporator section of the Figures 6–8 show the transient temperature variations and power spectrum analysis. The temperature OHP is one of the most important factors to investigate the chaos that can be induced in the system. behavior of the wall surface close to the inlet (or outlet) of the evaporator section of the OHP is one of the Thus, the temperature time series analysis (No. 13 as the surface close to the inlet (or outlet) of the mostevaporator important section) factors to investigatetothe chaosthe that can beoscillation induced in system. Thus, the temperature is significant analyze chaotic of the the OHP. In the present study, time the series analysis 13 as the surface close by to various the inlet (or outlet)such of the evaporator is OHP’s chaotic(No. oscillatory behavior induced parameters, as charging ratio,section) heat significant to inclination analyze the oscillation of thethe OHP. the of present study,fluctuations the OHP’satchaotic flux, and waschaotic investigated. To analyze timeIn series temperature a specificbehavior location on the OHP wall, power spectrum analysis using Fast Fourier oscillatory induced bytube various parameters, such as charging ratio, heatTransform flux, and (FFT) nclination was proposed [21]. In this work, No. 13 installed in the bottom of evaporator is dedicated to analyzing was investigated. To analyze the time series of temperature fluctuations at a specific location on the interfacial work between theanalysis evaporator andFast condenser. power spectrum density (PSD) [21]. OHPthe tube wall, power spectrum using FourierThe Transform (FFT) was proposed approach is useful to investigate the data module of temperature time series. The PSD of a time In this work, No. 13 installed in the bottom of evaporator is dedicated to analyzing the series interfacial can explain the power distribution into frequency components. work between the evaporator and condenser. The power spectrum density (PSD) approach is useful to The statistical average of a certain signal or sort of signal (including noise) as analyzed in terms investigate the data module of temperature time series. The PSD of a time series can explain the power of its frequency content is called its spectrum by signal analysis tool Origin 8.5. The PSD can be distribution into frequency components. defined as Equation (5) [26,51]. By definition, PSD can be computed with the following equation:

The statistical average of a certain signal or sort of signal (including noise) as analyzed in terms of (5) =∑ its frequency content is called its spectrum by signal analysis tool Origin 8.5. The PSD can be defined where (5) [26,51]. is the auto-correlation function of the input signal. as Equation By definition, PSD can be computed with the following equation: In the present FFT, to estimate the power spectrum with a finite number (about 5000–13,000 method ∞ samples for the input signal), the periodogram was used. It estimates the power as the mean jω = ∑ r xx (Fourier m)e− jωm xx eamplitude squared amplitude (MSA) from Pthe of the transformed data. The frequency m=−∞ column is obtained from the sampling interval ∆ and the number of input data points N. The nth frequency datum is given by: where r (m) is the auto-correlation function of the input signal.

(5)

xx

In the present FFT, to estimate the power spectrum with a finite number (about 5000–13,000 = (6) ∆ samples for the input signal), the periodogram method was used. It estimates the power as the mean Figure 6a–c(MSA) shows the transient temperature variation of transformed the OHP system with 14 frequency W (VF = 6.6%) squared amplitude from the amplitude of the Fourier data. The column heat supplied in vertical orientation. As shown in Figure 6a, Nos. 13 and 16 were oscillating in a is obtained from the sampling interval ∆t and the number of input data points N. The nth frequency certain range of amplitude and periodical wave, but Nos. 7 and 10 hardly vibrated. When the charged datum is given by: amount was VF = 11.1%, as shown in Figure 6b, all the n thermocouples installed in the OHP system f n the = temperature of the heater was reduced more than (6) showed oscillatory temperature variations, and N∆t Figure 6a–c shows the transient temperature variation of the OHP system with 14 W (VF = 6.6%) heat supplied in vertical orientation. As shown in Figure 6a, Nos. 13 and 16 were oscillating in a certain range of amplitude and periodical wave, but Nos. 7 and 10 hardly vibrated. When the charged

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amount was Energies 2018, 11,VF x

= 11.1%, as shown in Figure 6b, all the thermocouples installed in the OHP system 9 of 16 showed oscillatory temperature variations, and the temperature of the heater was reduced more than in the the case case of of VF VF==6.7%. 6.7%.When Whenthe thecharged chargedamount amountwas wasVFVF = 15.6%, temperature oscillation in = 15.6%, thethe temperature oscillation of of the OHP intermittently stopped. could be due to the pressure adjusting of the system. the OHP intermittently stopped. ThisThis could be due to the pressure adjusting timetime of the system. In In addition, when heater reached a certain temperature, oscillatory operation restarted, addition, when thethe heater reached a certain temperature, thethe oscillatory operation waswas restarted, as as shown in Figure This shows that operating statusofofthe theOHP OHPisisclosely closelyrelated relatedto tothe thecharged charged shown in Figure 6c.6c. This shows that thethe operating status amount. Figures Figure 6d–f No. 13 13 with withVFs, VFs,and and14 14W, W, amount. 6d–f show show the the time series of temperature fluctuations at No. ◦ condenser temperature temperature of 23 °C, C, the the filling ratio of VF == 6.6~15.6%, 6.6~15.6%,ethanol ethanol as as the the working working fluid fluid and and condenser its PSD PSD diagram. diagram. The The spectral spectral analysis analysis of of the the time time series series in in Figure Figure 66 indicates indicates aadominant dominant peak peak around around its frequencies of 0–0.1 Hz. frequencies

(a) VF = 6.7%

(d) VF = 6.7%

(b) VF = 11.1%

(e) VF = 11.1%

(c) VF = 15.6%

(f) VF = 15.6%

Figure and FFT FFT PSD PSD Distribution Distribution (#No. (#No. 13), Figure6.6.Transient TransientTemperature Temperature Variation Variation and 13), Q Q == 14 14 W. W.

Figure 6d–f indicates an intense periodic or quasi-periodic oscillation of temperature at this dominant frequency with a PSD of 0.05 Hz with VF = 6.7%. The PSDs of oscillations at other frequencies are in an order to be neglected compared with this dominant peak. As shown in Figure 6d–f, when increasing the VF the PSD distribution was strongly reduced. This means that the system


10 of 16

was unstable or had no oscillatory working behavior. Further, in the present OHP system, heat Energies 2018, 11,was 655 induced from repeated pressure fluctuations, and hence the strong cyclic pressure10 of 16 transfer

fluctuation means more heat transfer.

(a) VF = 6.7%

(e) VF = 6.7%

(b) VF = 11.1%

(f) VF = 11.1%

(c) VF =15.6%

(g) VF =15.6%

(d) VF = 24.4%

(h) VF = 24.4%

Figure 7. Transient Temperature Variation and FFT PSD Distribution (#No. 13), Q = 20 W.

Figure 7. Transient Temperature Variation and FFT PSD Distribution (#No. 13), Q = 20 W.

Figure 6d–f indicates an intense periodic or quasi-periodic oscillation of temperature at this dominant frequency with a PSD of 0.05 Hz with VF = 6.7%. The PSDs of oscillations at other frequencies are in an order to be neglected compared with this dominant peak. As shown in Figure 6d–f, when increasing the VF the PSD distribution was strongly reduced. This means that the system was unstable or had no oscillatory working behavior. Further, in the present OHP system, heat transfer was induced from repeated pressure fluctuations, and hence the strong cyclic pressure fluctuation means more heat transfer.

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(a) VF = 6.7%

(f) VF = 6.7%

(b) VF = 13.3%

(g) VF = 13.3%

(c) VF = 20.0%

(h) VF = 20.0%

(d) VF = 26.7%

(i) VF = 26.7%

(e) VF = 40.0%

(j) VF = 40.0%

Figure 8. Transient Temperature Variation and FFT PSD Distribution (#No. 13), Q =25 W.

Figure 8. Transient Temperature Variation and FFT PSD Distribution (#No. 13), Q =25 W.

Like Figure 6, Figure 7 shows the individual frequency distribution for each applied filling ratio Like Figure the individual frequency distribution applied fillingare ratio for 20 Figure W heat6, input. For7ashows VF = 6.7%, the filling ratio in Figure 7a, the PSDfor andeach frequency number not comprehensive, of different ThisPSD means the OHPnumber workedare for 20 Washeat input. For a with VF =many 6.7%,peaks the filling ratio inmagnitudes. Figure 7a, the andthat frequency

not as comprehensive, with many peaks of different magnitudes. This means that the OHP worked

Energies 2018, 11, 655 Energies 2018, 11, x

12 of 16 12 of 16

withpotentially potentiallylarge large chaotic temperature fluctuations. Thedecreased PSD decreased an increasing with chaotic temperature fluctuations. The PSD with an with increasing charging charging and the frequency was nearly zero for large amounts of working fluid. Thisthat implies ratio and ratio the frequency was nearly zero for large amounts of working fluid. This implies the that thewas system was unstable with decreased oscillatory behavior. It is clear the system in system unstable with decreased oscillatory behavior. It is clear that thethat system was in was stable stable oscillatory operation a low charging ratio. Figure 8 shows the effect the charging ratio oscillatory operation with awith low charging ratio. Figure 8 shows the effect of theof charging ratio with with = 25 It isthat clear the needed system aneeded a low charging rate to ensure proper oscillatory Q = 25QW. It isW. clear thethat system low charging rate to ensure proper oscillatory operation. ◦C operation. AsFigures shown6–8, in Figures average surface temperatures for different heat fluxes As shown in average6–8, heater surfaceheater temperatures for different heat fluxes were 46.67 ◦ ◦ ◦ were °C for 10 Q W,=56.3 °Cfor for65.3 Q = 14 W, Q for=65.3 °Cand for Q = 20CW, °C for Q = 25 W. for 10 46.67 W, 56.3 C for 14 W, C for 20 W, 73.6 forand Q =73.6 25 W. Figure 9 shows the PSD series of the wall temperature at No. 13 under a heating power of 25 Figure 9 25 W, W, ◦ VF = 20% and a condenser temperature of 23 °C and its its PSD PSD aa VF C and ethanol as the working fluid and diagram with different α and β diagram β orientations. orientations. The ThePSD PSDdistribution distributionwith withααand andββorientations orientationsshows showsa avery verysimilar similartrend trendasasshown shownininFigure Figure9.9.Increasing Increasingthe theααand andββangles anglesreduced reduced the the PSD PSD distribution. distribution.

Figure 9. 9. FFT FFT PSD PSD Distribution Distribution with with Different Different Orientation Orientation (#No. (#No. 13), 13), Q Q == 25 25 W. W. Figure

As explained above in Figures 6–8, a decreased PSD and frequency distribution led to reduced thermal performance. It can be seen in Figure 9 that there are different PDF distributions and frequency spreading in the PSD diagram in the range of 0–0.1 Hz. Upon increasing orientation angles, it is evident that there is dominant peak in the PSD diagram. Moreover, the PSD decayed by the

Energies 2018, 11, 655

13 of 16

As explained above in Figures 6–8, a decreased PSD and frequency distribution led to reduced thermal performance. It can be seen in Figure 9 that there are different PDF distributions and frequency spreading in the PSD diagram in the range of 0–0.1 Hz. Upon increasing orientation angles, it is evident that there is dominant peak in the PSD diagram. Moreover, the PSD decayed by the frequency increment. This indicates the chaotic state in the present OHP system. Thus, the PSD with different orientation in Figure 9 can be explained as chaos working states under the given operating conditions. 4. Conclusions In the present study, the specially designed OHP has a 150 mm long evaporator section and a 15 mm short condensing section. The conclusions may be summarized as follows: -

-

The thermal performance was mainly dominantly dependent on the amount of working fluid charged. The system worked properly with a low charged VF of around 10% of the total volume. Increasing the heat input, the working range of the charged amount varied in the 10–26% range. The optimum charged amount with supplied heat input should be determined by the thermal resistance behavior. The α and β-angle inclination of the OHP system minimally influenced the thermal resistance with a high heat input, but at a low heat, the thermal resistance sharply increased when the α and β-angles increased. However, the working behavior with FFT analysis was not significantly affected by the orientation. The average heater surface temperature was maintained around 60 ◦ C for heat fluxes under Q = 20 W.

Acknowledgments: This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No. NRF-2016R1D1A1B03930495). Author Contributions: Ri-Guang Chi, worked for whole experiments and analysis. Won-Sik Chung supported the experiments. Seok-Ho Rhi advised as a supervisor for this research. And this research was managed and supported by Seok-Ho RHI. Conflicts of Interest: The authors declare no conflicts of interest.

Nomenclature Dcr Di Do g I P TEvap Tc TCond R Q V VF Greek Letters α β σ ρl ρg ∆T

Critical diameter [m] Inner diameter [m] Outer diameter [m] Gravitational acceleration [m/s2 ] Electric current [A] Power Spectrum Density [Energy/frequency] Evaporator section of average temperature [◦ C] Temperature of cooling water [◦ C] Condition section of average temperature [◦ C] Thermal resistance [W/◦ C] Heating of heater [W] Voltage [W] Volume fraction, ((Volume of charged working fluid)/ (Total volume of OHP))×100 Rolling angle [◦ ] Pitching Angle [◦ ] Turface Tension [N/m] Density of liquid [kg/m3 ] Density of gas [kg/m3 ] Temperature difference [◦ C]

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