Thermal Performances of Loop Heat Pipe with Hybrid Wick Structures in Evaporator

Thermal Performances of Loop Heat Pipe with Hybrid Wick Structures in Evaporator 10th IHPS, Taipei, Taiwan, Nov. 6-9, 2011 Xiang jun a, Chang, Shyy Woei b, Escada c a. Senior Engineer, Vital Components Co. Ltd., Shenzhen, Guangdong, 518104, China b. Professor, Thermal Fluids Laboratory,National Kaohsiung Marine University, 811,Taiwan Tel:+86-0755-3366-8888-51117,Fax: +86-0755-3366-8887,E-mail: zeus.xiang81@gmail.com. ABSTRACT Loop heat pipe (LHP) is an efficient and effective heat transmitting device with practical cooling applications for electronic chipsets of high power density. This experimental study examines the thermal performances of a copper-water LHP with a specific rectangular evaporator. This evaporator is constructed by a copper base and a stainless steel end cover, within which the primary and secondary wicks are respectively fabricated from the sintered copper powder and the stainless steel meshes. Influences of the fluid inventory, heat load and the constituent wick structures on the thermal performances of this LHP are experimentally examined. With the specific wick structures designed to suppress the heat leakage from the evaporator to its compensation chamber, the overall thermal resistance of this LHP can be reduced to 0.187 /W with satisfactory start-up performances at horizontal orientation. Temperature oscillations over the heating area on the evaporator only arise at low heat loads in the range of 10-50W, while the stable operations are ensured throughout the higher power ratings up to 250W. Keywords: Loop Heat Pipe (LHP), Thermal Performances, Cooling, Heat Transmitting Device, Wick Structures 1. INTRODUCTION Following by the progress in semi-conductor industries, the speed and power of the intensified electronic circuits are accordingly increased. The dissipated heat from these intensified electronic circuits is dramatically increased (Maydanik,2005; McGlen R. J.,2004; Reay D.,2006). In order to keep the operating temperatures for these intensified electronic chipsets in the sustainable range to ensure their life spans, the conventional cooling devices, such as fan-fin and heat pipe, may not be able to cope with the cooling duties required for these intensified electronic chipsets. By way of employing other advanced cooling measures for these intensified electronic chipsets, the loop heat pipe appears as one favorable solution by the researches (Pastukhov V.G.,2009;Maydanik Y.F., 2010). However, the cylindrical shape of the evaporator for a conventional loop heat pipe (Maydanik Y.F., 2005; Ku J.,1999) may be inconvenient for several practical applications in the respect of electronic cooling. With the cylindrical shape of the evaporator, the space requirement is generally increased and the special treatment is often needed to improve the thermal contact between the heat source and the evaporator. As a result, the researchers have developed the plate-type loop heat pipes (Wan zh.x.2007; Dong X.G.2007). For this type of loop heat pipe, the evaporator generally takes two configurations by either allocating the compensation chamber (CC) on the back surface of the evaporator (Becker S.,2010) or installing on top of the surface wick in the evaporator (Wan zh.x.2007; Dong X.G.2007). For the loop heat pipe with the CC on top of the surface wick in the evaporator, the wick structure is generally unified in the evaporator (Wan zh.x.2007; Dong X.G.2007). However, the various requirements for the effective capillary radius and the permeation rate are usually defined by the applications for a LHP. In general, the small effective capillary radius with high permeation rate permeation rate is desirable. If the thermal conductivity and the capillary force are the only parameters considered, the serious heat loss from the LHP may occur, leading to the extremely high saturation temperature and pressure in CC. The high saturation pressure in CC undermines the circulation of return flow from the liquid line into the evaporator, which causes the hard start-up. For such plate-type loop heat pipe, due to the single wick structure in the flat evaporator, either the overall thermal resistance is relatively large (Wan zh.x.2007; Becker S.,2010) or the considerable heat loss could incur the start difficulty or the poor instabilities (Dong X.G.2007). In order to resolve these difficulties caused by the configurations of the loop heat pipe, this research work proposes the hybrid two-layer wick - 97 -

structure in evaporator. The smaller capillary radiuses with high thermal conductivity are used to construct the wick structure nearby the vapor grooves, which is referred to as the primary wick. On the back of this primary wick, the capillary material having the lower thermal conductivity and the higher permeation rate formulates the secondary wick. Through this experimental study to comparatively examine the thermal performances of the loop heat pipes, it confirms that the hybrid two-layer wick structure in the evaporator gives rise the improved starting performance with the reduced overall thermal resistances. 2. EXPERIMENTAL APPARATUS AND PROCEDURES Figure 2 depicts the constructional details of the evaporator for the LHP tested. The hybrid wick is consolidated by the primary and secondary wicks. The primary wick is constructed by a layer of sintered powders of 200-300µm. The secondary wick is constructed by 11 layers of stainless meshes of 50 mesh µm, The end-cover of the evaporator is made of 1mm thick stainless steel walls, while the bottom wall of the evaporator is 1mm thick C1100 copper plate. The vapor line, condenser and liquid line are made of C1100 copper tube. Table 1 summarizes the geometric parameters of the present LHP. Prior to filling the working fluid of de-ionized water, the entire LHP is vacuumed to the pressure level of 4.5 10-4 Torr. Having filled the working fluid into the LHP, the secondary vacuum process reduces the pressure to 2 10-2 Torr, after which the LHP is sealed. All the temperature measurements are detected by 10 type-t thermocouples connecting with the Fluke Hydra 26A data logger. The maximum precision error for the temperature measurement is defined as 0.10C. In addition to the ambient temperature, all the locations where the thermocouples are embedded are indicated in Fig. 1. The water flow rate for condenser cooling is 1.2L/min. With forced convective airflow cooling conditions, the fin area for the condenser is 0.369m 2 at the cooling airflow rate of 32.7CFM. To simulate the uniform heat flux heating condition, two cylindrical heater bars are embedded in a 25mm 25mm mm copper base plate. The heater power for the evaporator of the tested LHP is controlled by adjusting the electrical power fed into these heater bars. The precision of the Wattage gauge is ±0.1W. To minimize the heat loss flux to the surrounding ambience, the 10mm thick Tunnel plate of thermal conductivity 0.23Wm -1 K -1 is used to encapsulate the heater assembly. At the maximum heater power of 250W, the heat loss is 1.85W. The experimental uncertainties for this heating system are less than 0.74%. (a) Fig.1LHP with a flat evaporator. (b) (1).base, (2).vapor groove, (3).vapor collector, (4).vapor line, (5).liquid line, (6).cover, (7).compensation chamber, (8).secondary wick, (9).primary wick, (10).wick fixing column, (11).gas-liquid isolation plate Fig.2 Evaporator of LHP. Table 1. Geometric parameters of present LHP Evaporator Length(mm) Width(mm) Thickness(mm) Body thickness(mm) wall 1 43 43 21.72 Compensation chamber Length(mm) Width(mm) Thickness(mm) Vapor line 36 12 Primary/Secondary wick Length/ID(mm) 65/4.4 Length(mm) 38/38 Condenser Width(mm) / Thickness(mm) 2/2 Length/ID(mm) 373.2/4.4 Porosity (%) 48/63 Liquid line Pore radius(µm) 102/352 Length/ID(mm) 136.8/4.4-98 -

3. RESULTS AND DISCUSSIONS 3.1. Comparative performances of thermal resistance The thermal resistance is one of the most important performance factors for conventional heat pipes. Based on the consideration for the heat dissipation performance for a heat sink with the electronic chipset(s), the total thermal resistance of the present LHP (Rt) is defined as R T T Q h s t = (1) in which T h is the surface temperature of the heat source, Ts is the temperature of heat sink and Q is the heater power. 3.1.1 Comparative thermal resistances at various heat fluxes Figure 3 collects the total thermal resistances detected from the plate-type LHP at various heater powers. The suffixes CP and fan for the symbols shown in Fig. 3 respectively quote for the water cooling and airflow cooling conditions. This set of results are typical for the test conditions with the evaporator and condenser positioned on the same horizontal plane or when the condenser is allocated above the evaporator on the same vertical plane. As seen in Fig. 3, All the tested Rt consistently decrease with the increase of heater power for both vertical and horizontal orientations. At the lower heat fluxes, the impact of heater power is greater than those at the higher heater powers. With the low heater powers after the start at about 8-12W, the wick and vapor grooves are submerged under water for both horizontal and vertical orientations. The mensci over the interface between wicks are not completely formed. The start of the LHP is mainly triggered by the pulsating boiling from the liquid in the vapor grooves with the minor assistance from the wick capillary force. Even if the external heat flux keeps feeding into the evaporator, the absorbed energy in the LHP is transmitted into the boiling latent heat to supply the thermal energy for vapor bubble growth. Once the vapor bubble have grown and detached the heating surface, the thermal system releases and convects the enthalpy through the evaporator boundary at once. During this process after a complete cycle, the surface temperatures over the heat source exhibit the high-to-low cyclic variation. In this regard, the percentage of high temperature over a cyclic period is longer than that with the low temperature. At the same time, the LHP system undergoes the temperature oscillations with the larger amplitudes. R t( /W ) 4.5 4.0 3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 50 100 150 200 250 Q (W) Rt(F1-CP-0) Rt(F1-fan-0) Rt(F1-CP-90) Rt(F1-fan-90) Fig.3 Variations of Rt for the tested LHP against Q at CP and Fan cooling conditions with horizontal and vertical orientations. (F1 stands for filling ratio of 52.9%) This experimental test rig consists of the loop heat pipe (LHP), the simulated heat source, the water bath at constant temperature, water pump, cooling fan, electrical power supply and the data acquisition system. Fig. 1 depicts (a) the water-cooled plate-type LHP and (b) the plate-type LHP cooled by the forced convective airflow. Following the increase of heat flux form the evaporator after which the vapors from the vapor grooves are motivated toward CC, the wick structure is fully functioned. If the cooling duty supplied from the condenser is sufficient, the Rt of the LHP is stabilized at the lower value, Fig. 3. As an illustrative example, the present LHP-F1 module is stabilized at Rt = 0.187 with water cooling and Rt = 0.2 with airflow cooling. 3.1.2 Impact of filling ratio on thermal resistance The results compared in Fig. 4 show that the total thermal resistances of the present LHP increase with the decrease of filling ratio from the F3 to F1 conditions for the test conditions examined. At the heater power of 250W with water cooling, the total thermal resistances are 0.187, 0.199 and 0.227 at the filling ratios of 52.9% (F1), 48.5% (F2) and 44.2% (F3) respectively. Relative to the total thermal resistance at the filling ratio of 52.9% (F1) condition, the total thermal resistances at the filling ratios of 48.5% (F2) and 44.2% (F3) increase 6.4% and 21.4% respectively. With airflow cooling conditions at Q=250W, the total thermal resistances are 0.206, 0.214 and 0.246 at the filling ratios of 52.9% (F1), 48.5% (F2) and 44.2% (F3) respectively. The total thermal - 99 -

resistances at the filling ratios of 48.5% (F2) and 44.2% (F3) are raised 4.1% and 19.6% from the Rt reference obtained at the filling ratio of 52.9% (F1). With the lower filling ratio, the current LHP is not suitable to operate at eth high power ratings. Under the low filling ratio and high heater power, the wetting condition for the wick in the evaporator is partially dried, leading to the increased Rt. vertical and horizontal orientations seen in Fig. 5 are not substantial. R t( /W ) 0.5 0.4 0.3 Rt(F1-CP-0) Rt(F1-CP-90) Rt(F1-fan-0) Rt(F1-fan-90) 0.6 0.2 R t( /W ) 0.5 0.4 0.3 0.2 0.1 50 100 150 200 250 Q (W) Rt(F1-CP-0) Rt(F2-CP-0) Rt(F3-CP-0) Rt(F1-fan-0) Rt(F2-fan-0) Rt(F3-fan-0) Fig. 4 Variations of Rt against Q with three filling ratios at horizontal orientation. (F1, F2, F3 respectively stand for filling ratios of 52.9, 48.5 and 44.2%) 3.1.2 Impact of orientation angle on thermal resistance Figure 5 depicts the impact of orientation angle on the performances of thermal resistance by comparing the various Rt versus Q curves obtained with five orientation angles. With water cooling conditions, the Rt obtained from the vertical orientation are lower than the horizontal counterparts in the range of 20%~27%. With the airflow cooling conditions, the Rt obtained from the vertical orientation are lower than those obtained from the horizontal counterparts in the range of 15.5% ~ 18.4%. With the vertical orientation, the gravity provides the additional pressure head to motivate the circulation of working fluid, while the capillary forces from the wick structures are the only available driven potential for coolant circulation when the LHP is horizontal. In addition, the vertical orientation can also assist to distribute the liquid fluids in the CC to keep wetting CC. Due to the combined beneficial effects by gravity; the total thermal resistances of the present LHP at the vertical orientation conditions are consistently lower than the horizontal counterparts. Nevertheless, due to the inherent heat pipe characteristics with insensitive influences by orientation, the differential thermal resistances between the 0.1 50 100 150 200 250 Q (W) Fig. 5 Impact of orientation angle on Rt performances. 3.2 Starting and temperature oscillation performances Due to the relatively large heat leak from the capillary wick, which is a specific feature for such plate-type LHP, the heat leakage into CC will promote vapor bubble generation or increase the vapor pressure in CC. Consequentially, the returned liquid flow is undermined which can even lead to dry-out. As a result, the starting characteristic plays an important role for the performance of the plate-type LHP. This study utilizes the hybrid wick structure to thermally reduce the heat leak by using the wick with low thermal conductivity. The starting tests were performed at the heater powers in the range of 10~250W (heat fluxes 1.6~Wcm-2) with three filling ratios of 52.9% (F1), 48.5% (F2) and 44.2% (F3) under horizontal and vertical orientations. The test results confirm the favorable starting performance for the present LHP. Based on the test results, the starting process for the present plate-type LHP is successively constituted by four steps. At the first step, water fills the vapor grooves. When the minimum starting heater power is supplied to the evaporator, the liquid in these grooves is heated to trigger the pulsating boiling mode. At this initial starting period, the elongated period of vapor-bubble initiation, growth and detachment takes place as an intermittent process. As the grooves are filled with liquid, the menisci are not yet fully formulated. As a result, the pressure difference for the LHP at the initial stage is not sufficient to push the entire liquid working fluid into CC from the locations where the vapor prevails over during the normal operating conditions. As a result, the temperature oscillations with large amplitudes exceeding 100C are observed along with the - 100 -

intermittent boiling activities in the evaporator. Stage (2): When the liquid in the vapor grooves are fully vaporized and pushed away, the steam or vapor bubbles will push the liquid from the vapor line toward the condenser. During this stage, the amplitudes of temperature oscillations is moderated from those in the first stage and fall to range less than 50C. Stage (3): The evaporation pressure accumulated in the evaporator is sufficient to drive the liquid from the vapor grooves, vapor line and in a portion of the condenser to CC. At this moment, the condenser becomes fully functioned. The thermal resistance of the system is less affected by varying the heat load. The temperature oscillations detected from the LHP system is mainly caused by the pressure waves generated in the condenser due to the two-phase instabilities. The amplitudes of temperature oscillation during this stage is less than 10C. Stage (4): Along with the further increased heater power, the heat leak from the system is accordingly increased. The intermittent vapor bubbles can be generated in CC. This incurs the pulsating pressure waves at the exits of CC and liquid line at which the local temperature oscillations are observed. Nevertheless, these pressure wave oscillations remain as localized effects so that the temperature oscillations are not noticeably reordered from the other thermocouples but only detected by the thermocouples at the two locations. Having successfully started the plate-type LHP, two possible final statuses can be reached. One status is characterized by the periodical temperature waves with the temperature oscillations at the same frequencies at the various locations along the LHP. The other status reaches the stable operating conditions with the temperatures remain at the stable conditions. Based on the experimental results, the plate-type copper-water LHP could develop the oscillating conditions with the heater powers in the range of 8~50W for both vertical and horizontal orientations at various filling ratios. With the heater power above than 50W, the present LHP always operate at the stable conditions. 3.2.1 Impact of filling ratio on starting performance Figures 6 and 7 compare the temporal wall temperature variations at horizontal orientation with the filling ratios of 52.9% (F1), 48.5% (F2) and 44.2% (F3) for Q = 20 and 50W respectively. When the LHP reaches the quasi-steady condition, the amplitudes of temperature oscillation on the heater source reach maximum with the filling ratio of 52.9% (F1). With the filling ratios of 48.5% (F2) and 44.2% (F3), the amplitudes of wall temperature oscillations are relatively moderated. With the higher filling ratio, the driven pressure-potential required to drive the coolant circulation in the present LHP is relative large, leading to the high evaporation pressure and temperature. Meanwhile, at the same heater power, the lesser vapor space at the higher filling ratio increases the vapor pressure in the LHP system. As a result, with the same orientation angle and heater power, the starting time for the LHP with the higher filling ratio takes longer than those with the lesser filling ratios. 70 65 60 55 50 45 0 200 0 600 800 1000 t (s) Fig. 6 Temporal temperature variations on the surface of heat source at 20W with three filling ratios. 55 50 45 F1 F2 F3 0 150 300 450 600 750 900 t (s) Fig. 7 Temporal temperature variations on the surface of heat source at 50W with three filling ratios. 3.2.2 Impact of orientation angle on starting performance As seen in Fig. 8, with fixed heater power, the oscillating amplitude and period of temperature oscillations on the surface of heat source for the vertical orientation are larger than their horizontal counterparts. Nevertheless, for the vertical orientation, the power range within which the wavy-like wall temperature oscillations prevail is F1 F2 F3-101 -

less than the power range with wavy temperature oscillations for the horizontal orientation. With the vertical and horizontal orientations, the ranges of heater power within which the wavy-like wall temperature oscillations proceed are respectively 8~30W and 8-50W. The relative positions between the condenser and evaporator alter the vapor-liquid distributions in the LHP system and affect the liquid level in the liquid accumulator, which effects in combination affect the operation of the present LHP. In general, the liquid-vapor distribution has the direct influences on the growth or collision of vapor bubbles in the liquid accumulator which determine the pattern of temperature oscillations through the system. Nevertheless, either the constant period of variable period temperature oscillations, the present system is never diverged to the scenarios with increasing temperature oscillations. With horizontal orientation, the present LHP system show the periodical temperature oscillation with constant amplitudes; while the vertical system exhibits the temperature oscillations with variable periods. 60 50 0 200 0 600 800 1000 t (s) angle=0 angle=90 Fig. 8 Temporal variations of wall temperature on the surface of heat source with vertical and horizontal orientations at 20W. 3.3 Streamwise wall temperature distributions along LHP Figure 9 depicts the streamwise wall temperature distributions along the horizontal LHP with water and airflow cooling conditions at three filling ratios for heater power of 250W. With airflow cooling, the wall temperatures are about C higher than the water-cooling counterparts. The differential temperatures due to various filling ratios from F1~F3 are also about C. With horizontal orientation at the heater power of 250W, the surface temperatures on the heat source are 71.890C, 75.C, 81.50C and 75.4C 79.880C, 83.980C for the water cooling and airflow cooling conditions, respectively. The similar results are found when the orientation is vertical. As the heater power increases, the temperature levels along the liquid line are 15~300C less than those along the vapor line. Such sub-cooling conditions over the condenser section can not only fulfill the sub-cooling requirements, but also prevent the returned liquid flow due to the bubble generation in the liquid line when it is heated. At the F1 filling ratio, the surface temperature of the heat source is controlled at the level less than 750C for all the test conditions; which ensures the cooling capability for the increased heater power from the electronic chipsets in the foreseen future. 80 60 20 0 F1-Fan-0 F2-Fan-0 F3-Fan-0 F1-CP-0 F2-CP-0 F3-CP-0 Th Te Tv,in Tv,out Tc1 Tl,in Tl,out Tcc Figure 9 Streamwise wall temperature distributions along the horizontal LHP with water and airflow cooling conditions at three filling ratios and Q=250W. 4. CONCLUSIONS 1. Along with the increased heater power, orientation angle and/or the filling ratio in the tested ranges, the total thermal resistance of the present LHP is reduced. The starting temperature of this LHP is increased with the increase of filling ratio. At the higher hater power, the starting temperature for the present LHP is accordingly reduced. 2. In the heater powers of 8~50W, various degrees of temperature oscillations develop along the present LHP. With the heater power above than 50W, the present LHP remains at stable operating conditions. 3. With both horizontal and vertical conditions, the starting temperatures for the present LHP are increased as the filling ratio increases. At the stable operating condition, the increased filling ratio reduces the total thermal resistance. 4. At the same orientation angle and heater power, the amplitudes of the temperature oscillations for this LHP are increased as X - 102 -

the filling ratio increases. Along with the increased filling ratio, the range of heater power, within which the temperature oscillations occurs, is accordingly reduced, leading to the reduced power band for temperature oscillations. 5. At the same filling ratio and heater power, the vertical LHP exhibits the lesser degrees of wall temperature oscillations, except at the specific initiation stage during which the larger degrees of temperature oscillations take place. NOMENCLATURE ID=internal diameter(mm) Q = heater power (W) R t = the total thermal resistance(w/ ) T = the surface temperature of the heat source( ) h T = temperature of heat sink( ) s Systems, Denver, CO, USA, paper # 1999-01-2007, 1999. [7] Wan zhon-xin,liu wei, ZhangLiang,Ming Ting-zhen, Heat Transfer Investigation of Miniature Flat Heat Loop Pipe Evaporator for Heat Dissipation of Electronic Apparatus,Chinese Journal Of Electron Devices,Vol.30(6),pp.2197-2200&2204,20 07 [8] Dongxing Gai, Wei Liu, Zhichun Liu, Jinguo Yang, Temperature Oscillation of mlhp with Flat Evaporator, Heat Transfer Research, vol.(4):pp.321-332, 2009. [9] S.Becker, S.Vershinin,V.Sartre,E. Laurien, J.Bonjour and Yu.F. Maydanik, Steady state operation of a copper-water LHP with a flat-oval evaporator, Applied Thermal Engineering,vol.31(5),pp.686-695, 2010. ACKNOWLEDGEMENT The work was supported by the foundation of Asia Vital Components Co.,Ltd. REFERENCES [1] Maydanik Yu F. Loop heat pipes. Applied Thermal Engineering, 25(5-6),pp.635-657, 2005. [2] McGlen R. J., Jachuck R, Lin S. Integrated thermal management techniques for high power electronic devices. Applied Thermal Engineering, vol. 24(8-9),pp.1143-1156,2004. [3] D. Reay and P. Kew, Cooling of electronic components, in Heat Pipes,5th ed. Oxford, U.K., Elsevier, 2006, pp. 319 339. [4] V.G. Pastukhov, Y.F. Maydanik, Active coolers based on copper-water LHPs for desktop PC, Applied Thermal Engineering,vol. 29 (14-15), pp. 31-3143,2009 [5] Yury F. Maydanik, Sergey V. Vershinin, Vladimir G. Pastukhov. Loop heat pipes for cooling systems of servers, IEEE Transactions on Components and Packaging Technologies,vol.33 [6] Ku J., Operating characteristics of loop heat pipes, Proceedings of the 29th International Conference on Environmental - 103 -