Application of bio-wick in compact loop heat pipe

Journal Pre-proofs Application of bio-wick in Compact Loop Heat Pipe A. Brusly Solomon, Akhilesh Kumar Mahto, Catherine Joy, A. Albert Rajan, Dubey Abhishek Jayprakash, Abhinav Dixit, Abhinav Sahay PII: DOI: Reference:

S1359-4311(19)35028-8 https://doi.org/10.1016/j.applthermaleng.2020.114927 ATE 114927

To appear in:

Applied Thermal Engineering

Received Date: Revised Date: Accepted Date:

25 July 2019 8 January 2020 8 January 2020

Please cite this article as: A. Brusly Solomon, A. Kumar Mahto, C. Joy, A. Albert Rajan, D. Abhishek Jayprakash, A. Dixit, A. Sahay, Application of bio-wick in Compact Loop Heat Pipe, Applied Thermal Engineering (2020), doi: https://doi.org/10.1016/j.applthermaleng.2020.114927

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Application of bio-wick in Compact Loop Heat Pipe A. Brusly Solomon*, a, Akhilesh Kumar Mahtoa, Catherine Joyb, A. Albert Rajanc, Dubey Abhishek Jayprakasha, Abhinav Dixita, Abhinav Sahaya

aMicro

and Nano Heat Transfer Laboratory, Centre for Research in Material Science and Thermal Management, Department of Mechanical Engineering, Karunya Institute of Technology and Sciences, Coimbatore, India. bDepartment

of Electronics and Communication, Karunya Institute of Technology and Sciences, Coimbatore, India.

cDepartment

of Electrical and Electronics Engineering, Karunya Institute of Technology and Sciences, Coimbatore, India.

Abstract Compact loop heat pipes are widely used in high-performance compact electronic devices. The wick structure plays a vital role in the design of compact loop heat pipes. The performance of a wick dramatically influences the performance of the heat pipe. There are different kinds of wick structures used to enhance the performance of heat pipes such as screen wick, sintered wick, and grooved wick. In this study, a natural bio-carbon based wick structure is introduced to study the performance of a compact loop heat pipe. The Carbon material for the wick structure is prepared by carbonizing the Karuvelam wood as it possesses high waterabsorbing capacity and ability to withstand high temperatures. The wick parameters such as porosity, capillary rise, pore size, and contact angle are investigated in order to study the effect of heat input, the impact of wick structure on temperature distribution and equivalent thermal resistance of the heat pipe. The results reveal that the temperature difference between the evaporator and condenser decreases effectively so that the thermal resistance decreases. This work reveals that the bio-carbon wick structure shall be a cost-effective alternative solution for a loop heat pipe mechanism. Keywords: bio- wick, loop heat pipe, thermal resistance, heat transfer, porous structure 1. Introduction

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Compact Loop Heat Pipes (CLHPs) are passive heat transfer devices that work based on a phase-change heat transfer mechanism which are widely used in spacecraft for thermoregulation purposes and anti-icing, cooling of electronic components, as well as in computers [1-2]. The heat transport capacity of CLHP is much higher than that of the traditional heat pipes. The high heat transport shall be realized if the liquid and vapor are transported effectively through separate channels. Studies were conducted to explain the performance characteristics of the CLHPs by varying the shape of the evaporator, size of the compensation chamber, number of evaporators, condensers as well as types of wick materials. The wick structure is an essential element in the CLHPs as the capillary pressure developed by the wick is the primary driving mechanism for its operation. Different varieties of wick structures have been examined to enhance the heat transfer process in the heat pipes. Screen mesh is one of the basic wick structure used in heat pipes due to its easy installation procedure, and the same has been applied and tested in many CLHPs[3-6]. Zhou et al. [7] have developed an ultra-thin loop heat pipe with an evaporator thickness of 1.2mm and a condenser thickness of 1mm for cooling mobile phone electronic system. The wick structure was made of a fine copper mesh using the sintering process. The experimented results revealed that the heat pipe exhibited a stable startup, and it could transfer to a maximum of 12W with the minimum thermal resistance of 0.11oC/W. Solomon et al. [8] numerically analysed the effect of thin porous coating on the screen wick of the heat pipes that are charged with Cu nanofluids. The impact of nanoparticle deposition on various parameters such as vapor and liquid velocities, as well as wall temperatures, were discussed. It was observed that the deposition on the wick increases the capillarity and increase the heat transfer performance of the heat pipe by lowering the wall temperature and increasing the liquid as well as vapor velocity.

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Though the screen mesh or wired mesh was employed in many applications, sintered wicks are preferred over screen wicks due to its good permeability, high porosity and small pore size [9]. To meet the demand in high heat flux removal up to 500W/cm2 and to extend the critical heat flux limit, Weibel et al. [10] have developed and tested the capillarity of a thin porous wick having a thickness of 1mm sintered with 100µm size copper powder. Three different types of capillary material having 50% porosity were fabricated through the sintering process and tested. The first one was with uniform homogeneous sintered powder, the second one was with a grid pattern fabricated using copper powder and the third one was with a radial wedge formed by the sintering process. The capillarity results were compared with uncoated homogeneous wick material. In all those structures, in-situ visualization was made to study the vapor formation and found that the patterning of wicks introduces high-permeability liquid paths into the wick structure and thereby reduces the thermal resistance.

Deng et al. [11]

developed loop heat pipes with four different sintered wick structures as capillary material and estimated its capillary performance through the infrared(IR) thermal imaging process. The Inco type 255 with a particle size 2.2-2.8µm, 123 nickel wick with a particle size of 3-7 µm, two spherical and irregular shaped with a particle size of 75-110 µm were fabricated and characterized. It was observed that the copper wick presented a higher permeability and capillary performance than that of the nickel wick. Also, it was noticed that the performance of Inco type 255 nickel wick was better than that of type 123 in terms of permeability and capillary performances. Li et al.[12] conducted a study on the performance of copper –water CLHP with a sintered wick. The heat pipe was tested and found that the dry out did not occur even after 600W of heat input. Li et al. [13] developed a thin 1mm flat heat pipe based heat spreader with a sintered hybrid wick made up of fine copper powder. The improved heat pipe has effectively transferred the heat as much as 120W, and its thermal resistance was found to be lower than that of the 1mm thick copper plate as a heat spreader. In order to improve the

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thermal performance, Li et al. [14] have developed an ultra-thin flattened heat pipe with three different kinds of wick structures namely single arch-shaped sintered–grooved wick (SSGW), bilateral arch-shaped sintered–grooved wick (BSGW), and mesh–grooved wick (MGW). The test results indicated that the evaporation and condensation resistance of an ultra-thin flattened heat pipe was increasing with the increase in fill ratio until it reached a dry out. Also, the heat pipe with SSGW offered the lowest evaporation resistant; at the same time, the heat pipe with MGW showed the most moderate condensation resistance. In the recent past, research on CLHPs have gained its momentum as there were many industries tried to operationalize this concept for electronic cooling applications. To commercialize the loop heat pipes in civil fields, Wang et al.[15] have conducted a comparative study with Nickel and Stainless steel sintered wick using R134 as a working medium. Both LHPs charged with R134 showed excellent startup behavior and transferring more than 100W. Also, the LHPs with nickel wick showed excellent antigravity capability. Wan et al.[16] experimented the thermal performance of a miniature loop heat pipe contains copper –water nanofluid as working fluid with the copper particle size of 50nm. In this study, the wick structure was made as arrays of Sintered red copper sheets from the compensation reservoir to the evaporator. It was observed that the application of nanofluid reduced the thermal resistance by about 22% and enhanced the heat transfer coefficient by about 20% at a volume concentration of 1%. In order to reduce the heat leakage experienced during the operation of the loop heat pipe, Wu et al. [17] have applied a sintered polytetrafluoroethylene (PTFE) material as wick structure and compared its performance with a nickel wick. It was observed that the PTFE material particle size of 300-500µm, an effective pore radius of 1.7 µm, the porosity of 50% and permeability of 6.2 x 10-12 m2 has the best wick property. The application of the above PTFE wick in the loop heat pipe showed a lower evaporator temperature indicating

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low leakage of heat while maintaining the similar performance of heat pipe with nickel wick structure. Apart from the screen wicks mentioned above and sintered wicks, few specially designed wick materials were applied in heat pipes to meet the demand in increasing heat flux density in electronic devices. Liu et al. [18] analysed the operating characteristics of a loop heat pipe with polyacrylonitrile-based carbon fiber wick for higher heat flux electronic cooling devices. The wick surface was coated with copper using a chemical plating method in order to strengthen the wettability of the wick. The modified wick has offered a high wetting surface and resulted in a reduction in thermal resistance, and stability has improved during its operational range of 15-75W. Ji et al. [19] have developed a novel integrated flat heat pipe using a porous network wick prepared with compressed copper foam and compared its performances with the conventional flat heat pipes. The author presented that the new integrated flat heat pipe exhibited a better performance over the traditional flat heat pipe with an improved fin efficiency of 93%. Maydanik et al.[20] experimented with a flat disc-shaped loop heat pipe made up of stainless steel along with bi-porous wick and ammonia as a working fluid. The performances were analyzed at different inclinations ranging from -90o to +90o and with a heat sink temperature range of 0 to 40 oC. The results indicated that the maximum evaporator temperature is less than 80 oC at a maximum heat input of 300W for the horizontal position. Also, the evaporator and total resistance of LHP were 0.067 °C/W and 0.084 °C/W, respectively at a heat input of 220W. Yang et al. [21] introduced a low cost oxidized braided wires in mono and composite wick structure in an ultra-thin flattened heat pipe. The results indicated that the surface oxidization process increased the roughness of the braided wick which in turn enhanced the capillary force and heat pipe performance. Lee et al. [22] developed and tested a submillimeter-thick flexible flat heat pipe with a mesh wick having a nanostructured superhydrophilic surface. It was recorded that the nanostructured 5

superhydrophilic surface brought significant enhancement in the thermal performances compared to the heat pipes with the conventional copper base. Jiang et al. [23] developed a flattened heat pipe for cooling the microchips with a porous crack composite wick structure. The wick structure consists of microcrack channels in which the unbending section was filled with porous sintered powder. It was observed that the heat pipe has reached a steady-state within the 30s and also found that there was a significant reduction in temperature between the evaporation and condenser section as well as an enhancement in heat transfer limit. Abdulshaheed et al. [24] modified the evaporator by integrating hydrophilic CuO nanowires on the inner surface of the heat pipe and observed that there was a considerable improvement in performance compared to that of the same without coating. In order to bring down the manufacturing cost and to avoid complex fabrication processes of the wick structure, He et al. [25] has tested a loop heat pipe with pouring porous wick. The wick material was prepared by molding cement with additives. The transient performance of the loop heat pipe at various heat sink temperatures was studied and found that this heat pipe could start quickly up to 80W with an evaporator temperature of less than 100 oC.

Esarte et al. [26] have prepared a loop heat pipe with a 3D printed wick using a selective

laser melting process and tested in an 80W LED lamp. It was observed that the junction temperature of the LED lamp was maintained below 100oC for a maximum heat input of 80W. Wits et al. [27] has formed an array of 3D cubic fin structures in the evaporator to increase the capillarity and evaporation rates using additive manufacturing technology in the hybrid heat pipe – thermosyphon device. In this hybrid heat pipe device, the heat pipe technology was used in the evaporator and the thermosyphon technology was used in the condenser. This method was found to be operating well with the lowest evaporator resistance of 0.086 oK/W, the condenser resistance of 0.17 oK/W and with the total resistance of 0.26 oK/W at a fill ratio of 30% and heat flux of 35W/cm2. Further, Pastukhov et al. [28] developed a loop heat pipe 6

charged with ammonia as a working fluid to dissipate heat from different sources. To analyze the viability of biomaterial as wick structure in loop heat pipes, Putra et al. [29] have studied and compared the performances of loop heat pipes with sintered wick structure and biomaterial (collar) wick structure with nanofluid as working fluid. It was observed that the pore distribution of collar material was much smaller and homogenous compared to the sintered powder wick, which resulted in lower the thermal resistance of the loop heat pipe. From the above literature, it is clearly understood that different types of wick structures and materials have been applied for heat transfer enhancement in various heat pipes as mentioned in Table 1. Also, studies are going on for improving the performance of heat pipes by modifying the wick and also to identify alternate wick materials. It is noticed that most of the wick structures are metal-based and are coated with metal or metal oxides. Very few research has been done on experimenting the viability of applying biomaterials as a wick structure. The commercial wick structures which are available in the market and the wick structures fabricated with the latest 3D printing technologies are found to be costly. Moreover, the additional surface modifications techniques used in the above-reported studies are found to be a time-consuming process [25]. Moreover, there is a high demand for applying the biomaterials based wick structures in heat pipes due to its environmentally friendly nature, cost-effectiveness, availability, and also exhibiting significant improvement in performance compared to the commercial wick structures [14,25,26]. Though there were studies related to the application of biomaterials in CLHPs, there were no studies reported about the application of charcoal especially carbonized karuvelum wood as wick material. Therefore, the present study is unique in introducing a new kind of bio wick material and analyzing its performance in Compact loop heat pipes. 2. Experimental details 2.1 Preparation and characterization of charcoal wick

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A Carbonization process was performed to prepare a carbon wick from karuvelam wood. Initially, the wood was cut and shaped into a size of 100mm x 50mm x 50mm pieces and heated in a preheating chamber of a furnace with a steady temperature. Carbonization was done in the presence of minimum air supply to avoid the complete burning of wood. The wood pieces were made into adequate charcoal with different carbonizing times to obtain suitable charcoal, and the quality of each charcoal was analyzed. The optimum temperature and time required for carbonization were identified and the samples were analysed. Good quality charcoal was obtained when the wood was heated at a temperature of 300 oC for 15 minutes. The prepared charcoal was found to be stable, insoluble in water and rigid enough to carry out various tests. Then the charcoal was reshaped into a size of 35mm × 25mm × 3mm by polishing and removing the excess carbon using sandpaper to fit sturdily into the evaporator of the compact loop heat pipe. After the preparation of a charcoal wick material, the characterization was done using the SEM images. The pore size and porosity of the same were estimated and analysed for understanding the viability of using it as a wick material. The porosity of the material was also determined experimentally by using the displacement method. In this method, the weights of the carbon wick were measured when it was dry (Wd) and when it was saturated with water (Ws). The difference between the above two was the volume of water absorbed or the void volume available in the charcoal. The porosity of the charcoal was estimated by dividing the void volume by the bulk volume of charcoal. Also, the porosity was estimated by using an image processing technique and cross verified with the experimental method. The SEM image of the bio wick material shown in Fig. 1(d) was processed and converted into a black and white image. The white area and black area were estimated using MatLab image processing technique, and the porosity was calculated by dividing the black area with the total area of the image. The processed image is presented for reference in Fig 2. A simple capillarity test was 8

performed to estimate the capillary strength of charcoal. The contact angle was measured to study the wettability of charcoal when it was dry as well as in wet conditions using a Drop Shape Analyzer –Model DSA25 manufactured by KRUSS. The evaporator of the heat pipe was made by machining a cavity in a solid copper block and then fixing the wick structure and closing the cavity with a top plate, as shown in Fig 3. A copper block of dimensions 80mm × 70mm × 50mm was machined to make an inner cavity of dimensions 36mm × 26mm × 16mm and the outer sizes of 40mm × 30mm × 13 mm. The thickness of the bottom wall and side walls were 1mm and 2mm, respectively. A 4 mm thick plate-like structure was also machined with the same cavity to connect the top plate using bolts and nuts. A groove structure with rubber sealing was provided to avoid any leakage from the evaporator. There were 12 holes drilled with the diameter of 5mm at the top plate and the evaporator plate to connect the both with bolts and nuts. A 6mm diameter copper tube was then combined with the evaporator section to transport the vapor to the condenser section and to return the condensate liquid to the evaporator section. A capillary tube was also attached to the vapor line to charge the heat pipe with working fluid. The working fluid in the present study is DI water. 2.2 Experimental Procedure A well-established experimental set up used for characterizing the traditional heat pipes and thermosyphon [21–25] was used to study the performance of this heat pipe. The experimental set up is shown in Fig.4. This consists of a compact loop heat pipe, digital ammeter, voltmeter, chilling unit, data logging system, flow meter, and a personal computer and the testing procedure was similar to the previous studies[21–25]. A copper block comprising of 2 cartridge heater with a heating capacity of 200W was attached to supply heat input to the evaporator surface of CLHP. The heater block was covered with the hylam block, and the vapor line and liquid line were covered with fiberglass insulation to minimize the heat 9

loss. Thermocouples were mounted over the surface of CLHP at different positions for measuring and recording the temperature variations as marked in Fig 4. There was twelve number of T-type thermocouples with an accuracy of ±0.2oC were used to measure the temperature responses at a different location at various heat loads. Two thermocouples for measuring the evaporator temperature, three thermocouples for measuring the vapor temperature, two thermocouples for measuring the condenser wall temperature and two thermocouples for measuring the liquid line temperature were used in this experimentation. Also, one thermocouple each was fixed at the inlet and exit of the condenser to measure the temperature variation. The condenser section was covered with the cooling jacket made up of transparent epoxy material and cold water at a temperature of 20 oC was supplied from the chiller. The flow rate of the cooling water was monitored using a flow meter, and the flow rate was maintained at 180 ml/min. The uncertainty in the measurement of the cooling water flow rate was ±3%. The CLHP was tested in different heat load ranges from 50-250W, and temperatures at various points were recorded until a steady state occurs. The CLHP operation is considered a steady state, when the wall temperature, as well as the outlet cooling water temperatures, are constant about 15 minutes. In order to verify the performance of the CLHP with charcoal material, a CLHP with a traditional wick with screen mesh was fabricated and tested with similar experimental constraints. Four layers of copper screen mesh with 100 mesh/inch and with the wire diameter of 90µm were used, which have the same thickness of a charcoal wick. The porosity and the permeability of the wick was calculated using equation (1) and (2), 𝜑=1―

1.05𝜋𝑁𝑑𝜔

(1)

4

where N is the mesh number and 𝑑𝜔 is the pore size of the wick. For screen wick, the wire diameter was considered as pore size. 10

𝐾=

𝑑2𝜔𝜑3

(2)

122 (1 ― 𝜑)2

The porosity and permeability of the wick structure were 0.63 and 3.4 x 10-6 m2, respectively. The calculated permeability of the charcoal wick was 2.56 x 10-7m2. The performance of the charcoal wicked CLHP was compared with the screen meshed wick CLHP. The screen meshed CLHP was selected as the porosity can easily be adjusted to the porosity of charcoal wick by varying the number of layers of screen mesh. In order to ensure good contact between the evaporator wall and screen wick, a small spring system has been used. The fill ratio is a crucial parameter for the operation of the CLHP. Literature reveals that the performance of the heat pipe was better when the fill ratio was between 30-70%. Ji et al. [19] and Tarayil et al. [4] used 30% fill ratio, Zhu et al. [37] used 30-70% fill ratio, and Zhou et al. [7] used 37% fill ratio and have observed better performances in their respective studies. From the above literature, it is evident that a minimum fill ratio of 30% is required to maintain better performance in wide ranges of heat pipes. Therefore, in the current study, 30% fill ratio corresponding to the total volume was used. The total volume of CLHP includes the volume of the evaporator, compensation chamber, liquid, and vapor line as well as the condenser volume. 2.3 Data Reduction Heat transferred by the condenser of the loop heat pipe was calculated using the Newton law of cooling as 𝑄𝑜𝑢𝑡 = 𝑚 𝐶𝑝 (𝑇𝑜𝑢𝑡 ― 𝑇𝑖𝑛),

(3)

and the heat supplied to the evaporator was taken as 𝑄𝑖𝑛 = 𝑉 𝑥 𝐼,

(4)

The total resistance of the heat pipe was calculated using equation (5) as 𝛥𝑇

𝑅𝑇 = 𝑄𝑜𝑢𝑡,

(5) 11

where 𝛥𝑇 = 𝑇𝑒 ― 𝑇𝑐, 𝑇𝑒 and 𝑇c is the average evaporator and condenser wall temperatures The evaporator and condenser resistances were calculated using equations (6) and (7) respectively 𝑅𝑒 =

𝛥𝑇𝑒

(6)

𝑄𝑖𝑛

𝛥𝑇𝑐

(7)

𝑅𝑐 = 𝑄𝑜𝑢𝑡 where 𝛥𝑇𝑒 = 𝑇𝑒 –𝑇 𝑣 , 𝛥𝑇𝑐 = 𝑇𝑣 –𝑇 𝑐

The average wall temperature in the vapor line was considered as the saturation or vapor temperature 𝑇𝑣. After the above calculations, the uncertainty present in the heat transfer rate and total resistance calculations were estimated using equations (8-10) and found that the maximum uncertainty in the thermal resistance was 4.5%. ∆𝑄 𝑄

=

∆𝑚 2

∆(∆𝑡) 2

( ) +( ) 𝑚

(8)

∆𝑡

where ∆𝑚 is the error in the mass flow rate measurement, and ∆𝑡 is an error in the temperature difference of cooling fluid. The term

∆𝑄 𝑄

represents the uncertainty present in the estimation of

the heat transfer rate.

∆𝑞 𝑞

∆𝑅 𝑅

∆𝑄 2

(∆𝐴) 2

=

( ) +( )

=

( ) +(

𝑄

∆𝑄 2 𝑄

(9)

𝐴

∆(∆𝑇ℎ𝑝) 2 ∆𝑇

where the ∆𝑇ℎ𝑝 and

)

∆𝑅 𝑅

(10)

are the error in the temperature difference and uncertainty in the

thermal resistance of heat pipe, respectively. 12

3. Results and Discussion The wick material charcoal was precisely prepared with a suitable dimension to apply in CLHPs. Before testing the performance of CLHP, the charcoal material was characterized using SEM measurements. Fig 1 shows a photograph of a prepared wick structure and SEM image of wick material at different magnifications 100X, 1000X, and 3000X. The charcoal wick structure was found with two different pore sizes. The first category of pores was in the average size of 100µm, and the second category of pores was in the average size of 6 µm. The porosity of the wick structure is measured using the displacement method as well as the image processing method, as mentioned in the previous section. The porosity estimated using a displacement method was almost 20%, while the porosity calculated using image processing was nearly 52%. The first category of pores was found to be long pores, passing from one side to the other side of the wick structure like arteries; contrary, the second category of pores were found to be spherical shaped air pockets. Therefore, the first category of pores supports capillary action leading to the transportation of working fluid, which is essential for the CLHP operation. Whereas the later does not transfer but absorbs. Further, the capillary rise was measured vertically and was found to be 7 mm rise. The capillary pressure developed by both screen wick and carbon wick was estimated using the expression ∆𝑝𝑐𝑎𝑝 = 2𝜎 𝑟𝑐, in which 𝜎 is the surface tension of the working fluid and 𝑟𝑐 is the pore size. The pore size of the screen wick was 90 µm, which was equal to the wire diameter in the screen wick. The estimated pore size of the carbon wick was 100 µm and 6 µm respectively for the first and second categories. It was found that the capillary pressure generated by the screen wick was 1600 Pa with the pore size of 90 µm, the capillary pressure generated by the charcoal wick was 1440 Pa for the pore size of 100µm and 20571 Pa for the pore size 6 µm. The capillary pressure of carbon wick with 100 µm pore was found to be lower than that of the screen wick. However, with the 6 µm pore size, the capillary pressure was 20571 Pa, which was much higher than that of the screen wick. 13

Therefore, the combined capillary effect of charcoal wick was much higher than that of the screen wick, which enhances the capillary pumping of working fluid and leading to better heat transfer capacity. Further, to understand the capillary pumping effect and to recognize the heat transfer process of CLHP during its startup period, the contact angle of the wetted and non-wetted charcoal was measured. In the case of wetted charcoal, it was saturated with water before analyzing the contact angle. The contact angle variation and the absorbing characteristics of the working fluid are shown in Figure 5. It was observed that the contact angle of the wetted wick was found to be lower than that of the non-wetted wick. Moreover, the non-wetted charcoal quickly absorbs the working fluid than the wetted one even though the contact angle was less than that of the non-wetted one. As these properties of the charcoal wick material were found to be suitable for heat pipe applications, the charcoal wick structure was applied in CLHP to investigate the performance. During the experiment, the heat was applied to the evaporator wall, and the same was transferred to the fluid through wall and wick. The working fluid present in the wick got heated up and became vapor at the liquid-vapor interface through the evaporation process. This vapor was transferred through the vapor line and condensed in the condenser. Then the condensate returned to the evaporator through a compensation chamber and this cycle continued as long as the heat was supplied. Fig. 6 presents the transient evaporator temperature of CLHP at 40W and 80W for both screen and charcoal wicked CLHP. It was found that the temperature at the evaporator raised to 75 oC within 7 minutes from the startup and then it reached a steady-state condition. A similar trend was found while testing the charcoal wicked CLHP; however, there was a reduction in wall temperature.

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Further, it was noticed that the evaporator wall temperature of CLHP with screen wick was uniform and no fluctuation was observed. However, a temperature fluctuation was observed in the CLHP with charcoal wick, suggesting that there may be a liquid pool exists in the evaporator at low heat inputs. This liquid pool maybe exists due to the low wetting characteristics of carbon wick during start-up as mentioned earlier. During the start-up, the screen wick may be saturated with liquid while the carbon wick may not be saturated with a liquid which in turn results in the existence of a liquid pool. When there is a liquid pool, formation of bubbles and collapse leading to the wall temperature fluctuation during the startup. A similar fluctuation in temperature due to the liquid pool was reported in the literature[25]. However, the fluctuations in the evaporator temperature vanished after 80W, suggesting that the liquid pool disappeared, leading to the uniform wall temperature. The liquid pool may be disappeared due to the adequate circulation of working fluid at higher heat inputs. The above phenomenon can also take place due to the wetting characteristics of the charcoal wick, as mentioned earlier. It is seen in Fig. 5 that the contact angle of the wetted carbon is low compared to non-wetted charcoal. Also, the wetted carbon absorbs the working fluid faster than the non-wetted charcoal. Therefore, during the startup, the fluid may not be absorbed 100% by the wick material resulting in the presence of a liquid pool. As time goes by and the heat input increased, the wetting characteristics of charcoal improved that leading to an increase in waterabsorbing characteristics. Therefore the liquid pool might vanish at higher heat inputs resulting in uniform evaporator temperature. Fig. 7 presents the evaporator wall temperature of both screen and charcoal wicked CLHP at different heat inputs. It is observed that the evaporator temperature increases linearly as the heat input increases. Also, it is noticed that the temperature of carbon wicked CLHP is lower than that of the screen wicked CLHP at all heat inputs except the 50W. This higher temperature at 50W may have resulted due to the existence of a liquid pool in the evaporator 15

during startup. As the liquid pool in the evaporation section vanishes at higher heat inputs, the evaporator temperature also reduces for the charcoal wick. This reduction in wall temperature may also due to the variation in the effective thermal conductivity of the wick material or the change of thermal contact resistance on the wall/wick interface. The variation of condenser wall temperature of both screens wicked and charcoal wicked CLHP at different heat inputs shown in Fig. 8. It is noticed that there is a significant variation in temperature at a lower power level and less significant variation at a higher power level for both cases. However, there is a variation in condenser temperature when comparing both CLHP’s. The charcoal wicked CLHP showed a low condenser temperature than the screen wicked CLHP. Moreover, the condenser temperature is sharply rising up to a heat input of 100W, and then it is almost constant even after increasing the heat input. This may be because of the steady cooling water supply at the condenser section. Also, the temperature deviation between the carbon wicked and screen wicked CLHP is significant up to 100W, and then the variation is less significant at higher heat inputs. The vapor temperature variation with respect to different heat input is presented in Fig. 9, it is observed that that the temperature is increasing for both charcoal wicked and screen wicked CLHP up to 150W and there is no significant variation in vapor temperature after 150W for CLHP having charcoal and there is an increase in temperature for the screen meshed CLHP. Fig. 10 shows the heat transferred by the condenser of CLHP at different heat inputs to the evaporator. It is noticed that the heat transferred by the CLHP is significant at higher heat inputs and less significant at lower heat inputs. Moreover, the difference between the heat supply to the evaporator and the heat transferred by the condenser is ±8 percentage which is within the acceptable heat leakage limit.

16

The evaporator resistance variation of both the charcoal and screen wicked CLHP decreases exponentially and is presented in Fig. 11. There is no significant variation in resistance observed between the charcoal and screen wicked CLHP. The total resistance also followed a similar trend of evaporator resistance for both CLHPs. The total resistance variation of both the CLHPs at various heat input was presented in Fig. 12. The total resistance reduced from 0.75 to 0.17 oC/W when the heat input raised from 50 to 250W. There is a slight difference in resistance at lower heat inputs and no change in resistance at higher heat inputs between both the heat pipes. These results suggest that the screen wick can be replaced with charcoal as a wick structure. It is fascinating to note that the carbon wick with 20% porosity and screen wick with 63% porosity showed similar performance in terms of thermal resistance. It suggests that the charcoal is maintaining a better performance than the screen wick even though the porosity of both materials were different. Though the carbon wick is supposed to perform poorly due to the low porosity, it is performing as similar to the screen wick due to a better capillary effect. The combined capillary force produced by both 90 µm and 6 µm pores, as mentioned earlier, enhance the wetting characteristics as well as the liquid pumping from the compensation chamber to the evaporator surface, leading to lower thermal resistance. Hence, the performance of the charcoal wicked CLHP can be further enhanced via adjusting the porosity of wick by means of adding more liquid paths using mechanical drilling. The charcoal wick is found to perform better than the metallic one, which is non-degradable and not cost-effective. Therefore, the conventional wick materials can be replaced with a low cost, environment-friendly, lightweight Charcoal wick material. Considering the high heat transfer capacity while maintaining low evaporator temperature, notably less than 100oC, the operating conditions of the charcoalbased CLHP matching with the cooling requirements of IGBT module in power electronic converters [37,38]. Therefore, the developed CLHP with Charcoal wick material can effectively be utilized for cooling IGBT power electronic modules and similar applications.

17

Conclusions Heat transfer performance of a Compact loop heat pipe with charcoal as a wick structure was tested, and the results were compared with traditional screen mesh wicked Compact loop heat pipe. It is interesting to note that the charcoal material possesses suitable properties to be used as wick material for heat pipe applications. The application of charcoal in the CLHP reduces the total resistance from 0.75 to 0.17 oC/W, maintains the evaporator temperature between 65 to 95oC for the heat input range of 50 to 250W. This behavior of heat pipe is suitable for cooling IGBT modules and other electronic applications in which the junction temperature to be less than 100oC. As both the charcoal wicked and conventional wicked CLPHs exhibit similar performances and the fact that the charcoal posses the advantages such as lightweight, high capillarity, availability, and environmentally safe nature, it could be a potential alternative for traditional wick materials. However, the long-term performance of this heat pipe has to be investigated. Acknowledgment The first author would like to thank the DST-SERB for financially assisting this research work under the project number YSS/2015/001084. References [1]

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Nomenclatures A C ℎ I m 𝑄 𝑞 r 𝑅 𝑇 V Δt ΔT

: : : : : : : : : : : : :

area (m2) specific heat (J/kg K) heat transfer coefficient (W/m2 K) current (Amps) coolant mass flow rate(kg/s) heat transfer rate (W) heat flux (W/m2) pore size(m) resistance (oC/W) temperature (oC) voltage (volts) temperature difference of cooling water (oC) temperature difference (oC) SYMBOLS

𝑐 𝑒 𝑖𝑛 𝑜𝑢𝑡 𝑇 𝑠𝑎𝑡 𝑣 σ

: condenser : evaporator : input : output : total : saturated : vapor : surface tension

List of figures Figure 1(a) Photographic view of prepared charcoal (b) SEM image of Charcoal at 100X (c) at 1000X and (d) at 3000X Figure 2 Processed image of Fig. 1(d) Figure 3 View of the evaporator (a) Exploded and (b) Sectional view Figure 4 Schematic of experimental set-up with thermocouple positions Figure 5 Contact angle of water on charcoal with non-wetted and wetted case Figure 6 Transient temperature variation in the evaporator of carbon and screen wick heat LHP Figure 7 Evaporator temperature variation of CLHP at various heat loads Figure 8 Condenser Temperature variations of CLHP at different heat loads

21

Figure 9 Vapor Temperature variations of CLHP at various heat loads Figure 10 Heat Transfer capacity of CLHP Figure 11 Resistance at the evaporator of CLHP at various heat loads Figure 12 Total thermal resistance of CLHP at various heat loads

Table 1 Review of wick structures applied in different heat pipes Sl. No

Authors

Wick type

Type of heat pipe

Screen mesh Screen mesh Screen mesh Screen mesh Sintered Screen mesh Nanoparticle coated screen wick

Compact loop heat pipe Compact loop heat pipe Compact loop heat pipe Compact loop heat pipe Ultra-thin loop heat pipe

1 2 3 4 5

Stephen et al.[3] Tharayil et al.[4] Tharayil et al.[5] Tharayil et al.[6] Zhou et al.[7]

6

Solomon et al.[8]

7

Weibel et al.[10]

Thin porous sintered wick

8 9 10

Deng et al.[11] Li et al.[12] Li et al.[13]

Sintered wick Sintered wick Sintered hybrid wick

11

Li et al.[14]

Sintered wick (arch shape)

12

Wan et al.[15]

13

Wu et al.[17]

14

Liu et al.[18]

15

Ji et al.[19] Maydanik et al.[20]

Sintered red copper sheep Sintered polytetrafluoroethylene (PTFE) Polyacrylonitrile-based carbon fiber as wick Porous network wick

16 17

Yang et al.[21]

18

Lee et al.[22]

19

Jiang et al.[23] Abdulshaheed et al.[24]

20 21

He et al.[25]

22 23 24

Esarte et al.[26] Wits et al.[27] Putra et al.[29]

Bi-porous wick Oxidized composite braided wires Nanostructured superhydrophilic surface Porous crack composite wick Hydrophilic CuO nanowires Pouring porous wick (Cement with additives) 3D printed wick 3D cubic fin structures Biomaterial as wick structure (collar)

22

Cylindrical heat pipe In-situ flow visualization study Loop heat pipe Compact loop heat pipe Flat heat pipe heat spreader Ultra-thin flattened heat pipes Mini loop heat pipe Loop heat pipe Loop heat pipe Flat heat pipe Flat disc-shaped loop heat pipe Ultra-thin flattened heat pipe Sub millimeter-thick flexible flat heat pipe Flattened heat pipe Grooved heat pipe Loop heat pipe Loop heat pipe Hybrid heat pipe device Loop heat pipe

(a)

(b)

(c)

(d)

Figure 1(a) Photographic view of prepared charcoal (b) SEM image of Charcoal at 100X (c) at 1000X and (d) at 3000X

23

Figure 2 Processed image of Figure 1(d)

24

(a) Bolts

Top plate

O-ring

Wick

Evaporator cavity Nuts

Bolts & Nut

(b)

Top plate

O-ring

Vapor space

Liquid line

Vapor line Compensation chamber

Carbon wick

Figure 3 View of the evaporator (a) Exploded and (b) Sectional view

25

Figure 4 Schematic of experimental set-up with thermocouple positions

26

Time (S)

Nonwetted wick surface

Wetted wick surface

0

4

10

14

22

Figure 5 Contact angle of water on charcoal with non-wetted and wetted case

27

120

Evaporator Temperature (°C)

100

80

60 carbon wick Screen mesh wick

40

20

0 0

10

20

30 Time (minutes)

40

50

60

Figure 6 Transient temperature variation in the evaporator of carbon and screen wick heat LHP

28

Temperature (oC)

90

80

Screen 70

Carbon

60 0

50

100

150 Heat input (W)

200

250

300

Figure 7 Evaporator temperature variation of CLHP at various heat loads

29

Temperature (oC)

60

50

40 Screen Carbon

30

20 0

50

100

150 Heat input (W)

200

250

300

Figure 8 Condenser Temperature variations of CLHP at different heat loads

30

90

Vapour Temperature (oC)

80 70 60 Screen

50

Carbon 40 30 0

50

100 150 Heat input (W)

200

250

300

Figure 9 Vapor Temperature variations of CLHP at various heat loads

31

-8

%

300

250

%

Heat Transfered (W)

+8 200

150

100 Screen Carbon

50

0 0

50

100

150

200

250

Heat Input (W)

Figure 10 Heat Transfer capacity of CLHP

32

300

0.725

Resistance (oC/W)

0.625 0.525 0.425 0.325

Screen Carbon

0.225 0.125 0.025 0

50

100 150 Heat input (W)

200

250

300

Figure 11 Resistance at the evaporator of CLHP at various heat loads

33

0.9

Total resistance (oC/W)

0.8 0.7 Screen

0.6

Carbon

0.5 0.4 0.3 0.2 0.1 0

50

100

150 Heat input (W)

200

250

300

Figure 12 Total thermal resistance of CLHP at various heat loads

34

Highlights

 Novel bio-wick is prepared and characterized for loop heat pipe  Porosity, capillary rise, pore size, and contact angle are measured  Significant temperature difference between evaporator and condenser is noticed  Maximum capillary pressure generated in the carbon wick is 20571 Pascal  Evaporator temperature decreased in carbon wicked heat pipe about 7oC

35

CRediT Author Statement To The Associate Editor Applied Thermal Engineering

Dear Sir, We wish to confirm that the following are the contribution made by each author of this article titled “Application of bio-wick in Compact Loop Heat Pipe” authored by A. Brusly Solomon, Akhilesh Kumar Mahto, Catherine Joy, A.Albert Rajan, Dubey Abhishek Jayprakash, Abhinav Dixit, Abhinav Sahay. A. Brusly Solomon

:Conceptualization, Methodology, Supervision, Validation, Writing - Original Draft, Funding acquisition, Project administration

Akhilesh Kumar Mahto

: Investigation, Resources

Catherine Joy A. Albert Rajan

: Software : Writing- Reviewing and Editing

Dubey Abhishek Jayprakash: Investigation Abhinav Dixit

: Investigation

Abhinav Sahay

: Investigation

We confirm that this manuscript is not published before nor submitted to any other journal for the consideration of publication. Also, we wish to confirm that there is no conflict of interest among the co-authors.

Yours truly Dr. A. Brusly Solomon, M.E., Ph.D., Associate Professor, Department of Mechanical Engineering, Karunya Institute of Technology and Sciences. Coimbatore – 641 114

36

To The Editor Applied Thermal Engineering Dear Sir,

We wish to confirm that the present manuscript titled “Application of bio-wick in Compact Loop Heat Pipe” authored by A. Brusly Solomon, Akhilesh Kumar Mahto, Catherine Joy, A.Albert Rajan, Dubey Abhishek Jayprakash, Abhinav Dixit, Abhinav Sahay is new research work. We also confirm that this manuscript is not published before nor submitted to any another journal for the consideration of publication. Also, we wish to confirm that there is no conflict of interest among the co-authors. Thank you

Yours truly Dr. A. Brusly Solomon, M.E., Ph.D., Assistant Professor, Department of Mechanical Engineering, Karunya Institute of Technology and Sciences. Coimbatore – 641 114 Tel: 0422-2614059 Mobile: 8220023860 Email: [email protected] [email protected] [email protected]

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