Visualization and Evaporation Resistance Measurement for Groove-Wicked Flat-Plate Heat Pipes: Various Working Fluids and Powder-Groove Evaporator

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1 1th IHPS, Taipei, Taiwan, Nov. 6-9, 211 Visualization and Evaporation Resistance Measurement for Groove-Wicked Flat-Plate Heat Pipes: Various Working Fluids and Powder-Groove Evaporator Shwin-Chung Wong and Chung-Wei Chen Department of Power Mechanical Engineering National Tsing Hua University, Hsin-Chu 3, Taiwan, R.O.C address: ABSTRACT This work experimentally compared the evaporation characteristics of three working fluids: water, methanol and acetone, in groove-wicked flat-plate heat pipes. In addition, the performance enhancement by filling copper powders in the evaporation section was investigated. The parallel, U-shaped grooves have a width of.25 mm and a depth of.16 mm. Uniform heating was applied to the copper base plate near one end, and a cooling water jacket was connected at the other end. The evaporative resistance was calculated based on the difference of the plate temperature and the vapor temperature respectively under and above the center of the heated zone. With stepwise increase of heat load, the behavior of the working fluid in the grooves was visualized and the evaporative resistances were measured. For all the three fluids, longitudinal liquid recession with a steep-sloped liquid front can be visualized above a certain heat load. Behind the short liquid front is the accommodation region where the meniscus appeared to anchor on the top corners of the groove walls. With increasing heat load, the liquid fronts gradually left the heated zone, accompanied by increasing plate temperatures. Water exhibited the best performance with highest maximum heat loads and lowest evaporative resistances. However, the differences in the maximum heat loads for different fluids were much smaller than the differences in their figures of merit. No boiling was observed in the groove wick for all the three fluids. With the groove-powder evaporator, the evaporation performance was greatly improved for water. However, the improvements for methanol and acetone were not as significant as for water. Keywords: heat pipe, evaporation, evaporative resistance, groove wick, nucleate boiling 1. INTRODUCTION The groove wick is one of the common wicks used in heat pipes or vapor chambers. Under evaporation, the capillary characteristics of the grooves of different cross-sectional shapes have been investigated (Ma, 1996; Stroes, 1997; Anand, 22; Catton, 22; Nilson, 26). Theoretical models for groove-wick heat pipes have been able to include the formulation of thin-film evaporation (Jiao, 26; Do, 28). Visualization studies of groove wicks in operating vapor chambers or heat pipes have been performed (Lips, 28; Wong, 211a). Lips et al. (28) visualized the behavior of methanol in vapor chambers with a groove wick machined on a copper plate. Boiling was observed in the copper grooves for heat fluxes higher than 3 W/cm 2. In our recent work (Wong, 211a), combined visualization and evaporative resistance measurement for water in the evaporating groove wick was conducted for operating flat-plate heat pipes. Steep-sloped liquid fronts suddenly appeared above a threshold heat load at a certain position in the heated zone. Under thermally stable operation, independent longitudinal oscillations existed in most grooves, exhibiting a constantly varying zigzag front line. With incremented heat loads, these liquid fronts gradually receded out of the heated zone. Concurrently, the base plate temperatures and the evaporative resistances increased. Behind the short liquid front was the accommodation region where the meniscus appeared to anchor on the top corners of the groove walls. This observation verified Nilson et al s (26) theoretical prediction of a jump-like transition from the accommodation region to the corner-flow region. No boiling was observed for the evaporating water in the grooves. The present work continued our visualization experiments on the evaporation characteristics in operating groove-wicked flat-plate heat pipes (Wong, 211a). First, we tested two more working fluids: methanol and acetone. Although water has uniquely high latent heat and surface tension to result in outstanding thermal performance, its high freezing point and incompatibility with aluminum containers motivate the selection of different working fluids

2 (Lips, 28; Wong, 211b). Second, a composite powder-groove evaporator was also investigated. Composite wicks, such as powder-mesh wicks (Mochizuki, 26; Wong, 21) or powder-groove wicks (Chang, 27) utilize the high capillarity of powder wicks in the evaporator and avoid its low permeability in other regions. Tang et al. (21) compared the capillarities of the groove wick, the powder wick, and the groove-powder wick by their wetted heights by ethanol. The groove-powder wick had the highest capillarity and the groove wick the lowest. The reason why the groove-powder wick had a higher capillary than the powder wick was the presence of sharp corners at the powder-wall contacts. In the present work, the performance of powder-groove evaporator was also tested for the three working fluids. 2. EXPERIMENTAL METHODS The experimental methods followed our recent work for a groove wick (Wong, 211a). Fig. 1 shows the overall test setup. The heat pipe had a glass top wall for observation. The internal space of the heat pipe was mm 3. With the contact surfaces between different pieces sealed with o-rings, the whole structure, including the wicked copper base plate, the top glass window, and stainless-steel frames, was tightened with bolts. A selected amount of degassed deionized water was filled into the heat pipe right after it was evacuated down to a pressure of Torr. While the saturate charge to fill the grooves was.3 ml, the actual charge was.85 ml for water and.7 ml for methanol and acetone to compensate for the trapped liquid at the corners in the chamber. The 1.1 cm 1.1 cm heated surface of the heat pipe was connected to the uniform-temperature top surface (1. cm 1. cm) of a heating post via a layer of thermal grease. The heating post was carefully insulated except at its top end. The bottom of the other end of the heat pipe was cooled by a 5 mm-long cold plate with running water at 2. Cut-off trenches were made on the bottom side of the base plate around the heated area and at the boundary of the cooled zone, as shown in Fig. 2. These trenches reduced the plate thickness to 1 mm to suppress the lateral conduction through the base plate. A CCD camera equipped with a microscopic lens shot vertically downward through the observation window. Illumination was provided by several high-intensity LED lights. The heat load was increased stepwise. Data were taken under a thermally stable condition for each heat load, with all temperature variations less than.2 K within 5 min. Fig. 1 Test apparatus and thermocouple layout. trench thermocouple trench Fig. 2 Bottom view of the copper base showing trenches and thermocouple holes. Twelve K-type thermocouples were used to measure temperatures at selected positions with a resolution of.1. Among them, T1 and T2 were located in the heating post for the estimation of the total heat loads Q t. T3 to T5 measured the temperatures of the copper plate in the heated zone, with T4 at the center and T3 and T5 at a 4.5 mm distance from the center (Fig. 2). T6 to T9 were used to calculate the lateral conduction through the base plate, Q cond (Wong, 21; Wong, 211b). T1 indicated the base temperature under the center of the condensation section. The temperature measured shortly above the center of the evaporator was denoted T11, which reflected the vapor temperature leaving the evaporator center. The vapor temperature above the center of the condensation section was monitored by T12. The parallel capillary grooves were chemically etched on a 4 mm-thick C12 oxygenless copper base plate. Fig. 3 shows the top view of the grooved plate assembled in the heat pipe, with the positions of the heater, the cold plate and the trenches marked. The grooved area on the base plate was 115 mm 26 mm, with 65 grooves. Through the glass window, the whole grooved area can be visualized. The cross-sectional

3 dimensions of the grooves are illustrated in Fig. 4. The U-shaped grooves had a top width of.25 mm and a depth of.16 mm. The top width of the groove walls was.15 mm. To further create a powder-groove wick, fine irregular copper powders (diameters < 75 m) were filled in all the grooves covering a 24 mm length from the evaporator end. The wicked copper plate was then sintered in an 85 hydrogen/nitrogen atmosphere for 2 h. The top view of the sintered powder-groove wick is shown in Fig. 5. Fig. 3 Top view of the grooved plate in the heat pipe. The evaporative resistance R e was determined as R e = (T4 T11)/Q, [K/W] (1) where Q = Q t Q cond. The net heat flux was q = Q/A, with the heated area A = 1.21 cm 2. In this study, the ratios of Q cond /Q t were limited to 13 % for water and 2 % for methanol and acetone, before a complete dryout within the heated zone. In general, Q cond /Q t was higher for a larger evaporative resistance when more heat would travel through the base plate to the cooled zone. There were at least two reasons for high lateral conduction in groove-wick experiments. First, the cross-sectional size of the grooves was larger, leading to smaller thin film evaporation areas. Second, above certain threshold heat loads, dryout appeared and then expanded with increasing heat load, causing higher evaporative resistances. The uncertainty in the measurements of Q was 7 %, which was determined by the maximum uncertainty associated with the nonlinearity of the temperature readings T1-T2 and the comparison with the output of the DC power supply. The uncertainty in R e values was also 7 %, since the uncertainties primarily arose from those of Q, according to the definition of R e in Eq. 1. The thermophysical fluid properties of the three different working fluids at 3, as well as their figures of merit, are shown in Table 1. Fig. 4 Cross-section of the groove. Before each test, the wicked copper plate was carefully cleaned and then put in a high-temperature hydrogen/nitrogen environment to remove the surface oxide. This process ensured a good copper surface wettability. In this work, the copper surface wettability was quantified by the static contact angle of a sessile water drop on a smooth surface. For all the tests, the static contact angles of water on a smooth copper surface were controlled at Both methanol and acetone fully wetted the copper surface. powders in groove Fig. 5 Top view of the powder-groove wick. Table 1. Thermophysical properties of water, methanol and acetone at 3. ρ l (kg/ m 3 ) λ (kj/kg) σ (N/m) + Figure of merit N = ρ l λσ /µ l 3. RESULTS AND DISCUSSION 3.1 Visualization Results for Groove Wick µ l (cp) N(W/m 2 ) + water E11 methanol E1 acetone E1 Our previous experiments for water (Wong, 211) indicated that steep liquid fronts appeared in the evaporator above a threshold heat load. Before the front was a dry region, while behind was a wet accommodation region in which the meniscus suspended on the upper edges of the groove wall. Under thermally stable operation, the liquid fronts oscillated back and forth. The

4 forward motion (toward the evaporator end) was slower and the receding motion was faster. The forward motion was pushed by a pulse of the draining condensed water, and the receding motion resulted from the rapid evaporation by the hotter dry region. Independent liquid motions in different grooves led to a constantly varying zigzag front line. When the heat load was further increased stepwise, the fronts receded and gradually moved out of the heated zone. Similar behavior was observed here for methanol and acetone. Fig. 6 shows the liquid front images for the three working fluids. At the liquid front is a short region including one brightest spot in the middle and two slander and slanted bright side wings resulting from the refraction across a steep-sloped meniscus. Behind the fronts, the images of the wet region appear axially unchanged. Some subtle differences, however, were observed between the oscillating motions of different working fluids. Water exhibited the slowest oscillating motion and shortest oscillation distances, no more than a few times the channel width. Oscillation distances as long as ten times the channel width could be observed for methanol and acetone. During the receding motion of these cases, the suspended meniscus became more and more curved by evaporation over a long distance until it nearly vanished, and the front rapidly receded across the distance. It is noted that the dryout in the groove wick is a continuous process without a clear sign. For mesh or powder wicks, Q max can be clearly defined right before the occurrence of a local dryout in the evaporator, which is accompanied by a drastic increase in the evaporative resistance (Wong, 21). The issue of determining a criterion for Q max for groove wicks will be discussed in Section 3.2. adiabatic section heated zone water methanol acetone Fig. 6 Steep liquid fronts in grooves showing intensified reflection. x (mm) Fig. 7 illustrates the variations of the averaged liquid front positions with increasing heat load for representative methanol and acetone cases. In Fig. 7, the origin of the vertical coordinate x is acetone methanol evaporation zone adiabatic zone L= -4.5mm L= 4.5mm Fig. 7 Liquid front positions versus heat load Q for a methanol and an acetone case. located at the center of the heated zone, where thermocouple T4 was implanted. The longitudinal positions of T3 and T5, as well as the heated zone and adiabatic zone, are also marked. The methanol front appeared under 14 W at x = mm. With increased heat loads, the liquid front gradually receded out of the heated zone for Q > 18 W. The acetone front appeared under 9.5 W at x = -2 mm and moved out of the evaporation zone for Q > 14 W. In comparison, the heat loads for the water front to move out of the evaporation zone were Q > 22 W (Wong, 211a). The sequence of these heat load values is consistent with the sequence of the figures of merit of the three fluids, N w > N m > N a (Table 1). However, the differences in these heat loads are much less than those in their figures of merit. Our previous experiments for a multi-layer mesh wick (Wong, 211b) have verified the proportionality between Q max (the maximum heat load before local dryout occurring in the evaporator) and N, for water, methanol and acetone. The present groove-wick data indicate only qualitative, rather than quantitative, agreement. The weaker oscillating motion observed for water can be understood now. In the present groove-wick tests, the heat loads for water were only slightly larger. However, the heat of evaporation,, of water is much larger than those of methanol and acetone (Table 1). This implies smaller volume flow rates for water in the grooves. Furthermore, the viscosity of

5 water is higher. Consequently, the oscillating motion of water appeared weaker. In the present experiments, no boiling was observed in the groove wick for all three fluids. In our previous visualization studies for operating flat-plate heat pipes with meshed or powdered wicks, no boiling was observed for water. However, weak boiling was observed for acetone and methanol in a mesh wick (Wong, 211b). Lips et al. (28) observed boiling of methanol in a groove-wicked vapor chamber under pretty low heat flux (> 3 W/cm 2 ). 3.2 Temperatures and Evaporative Resistances of Groove Wick The base plate temperatures T3, T4, and T5 in the heated zone, as well as the vapor temperature T11, are plotted against Q in Figs. 8 and 9 for the above methanol and acetone case, respectively. The open symbols represent the situation that the liquid front has receded over the thermocouple. The temperatures of T3 and T4 were very close to each other but those of T5 are lower due to stronger lateral conduction therein. Along with the liquid front recession (Fig. 7), the base temperatures and the vapor temperatures increased continuously as a result of enlarging dryout region. The evaporative resistances, determined with Eq. 1, are shown against Q in Fig. 1 for the above methanol and acetone cases, along with a typical water case (Wong, 211a). Water exhibits the lowest minimum R e, and acetone presents the highest. The trend of increasing R e with increasing Q reflects the gradually receding liquid fronts which left with enlarging dryout regions.. T3,4,5 - Tv (K) methanol, groove wick T3-Tv T4-Tv T5-Tv Fig. 8 Differences of temperatures in the heated zone of base plate (T3, T4, T5) with vapor temperature T v above the center of heated zone (T11) versus heat load Q for a methanol case. Open symbols represent that the thermocouple has been exposed in the dryout region. T3,4,5 - Tv (K) Re (W/K) acetone, groove wick T3-Tv T4-Tv T5-Tv Fig. 9 Differences of temperatures in the heated zone of base plate (T3, T4, T5) with vapor temperature T v above the center of heated zone (T11) versus heat load Q for an acetone case. Open symbols represent that the thermocouple has been exposed in the dryout region actetone, groove wick methanol, groove wick water, groove wick Fig. 1 Comparison of R e versus Q between water, methanol and acetone cases. Concerning the criterion for Q max for the present groove-wick tests, there may be different selections. One could be the heat load when the liquid front just moves out of the heated zone. However, this criterion can only be applied to groove wicks because the liquid fronts are otherwise invisible. We may alternatively select an arbitrary common threshold R e to define the Q max s for different test cases. For example, a threshold R e of.2 K/W could be selected. This leads to Q max s of 2 W, 15 W, and 7 W for water, methanol and acetone, respectively, for the cases in Fig. 1. It is the criterion for Q max applied for both the groove wick and the groove-powder wick in the present study

6 . Re (W/K) Re (W/K) actetone, groove wick methanol, groove wick acetone, powder-groove wick methanol, powder-groove wick Powder-Groove Wick groove wick groove wick powder-groove wick Fig. 11 Comparison of R e versus Q between groove and powder-groove evaporators for water. Fig. 12 Comparison of R e versus Q between groove and powder-groove evaporators for methanol and acetone. Evaporation performance of the powder-groove wick was tested for the three fluids. In the present work, the powder-groove wick contained fine irregular powders in every groove for a 24 mm length of the evaporator end (Fig. 5). Fig. 11 indicates that the powder-groove wick yields significant improvements in the evaporation performance for water. The minimum value of R e was lowered to about.5 K/W, in comparison with.15 K/W for the groove wick. The former value of R e,min was comparable to those of sintered mesh and powder wicks (Wong, 21; 211b). In addition, higher heat loads were attained. Using a threshold R e of.2 K/W as the criterion, the Q max s increased from about W for the groove wick to 45 W for the powder-groove wick (Fig. 11). The increased Q max resulted from enhanced capillarity, and the reduced R e,min can be attributed to the thinner evaporating film sustained in the fine wick pores and the increased interline area for evaporation (Wong, 21; 211b). The results for methanol and acetone are compared in Fig. 12. For these two fluids, the performance improvement using the powder-groove wick is not so significant as for water in both Q max and R e,min. The data in Figs indicate an interesting fact that methanol and acetone operate more favorably in the groove wick rather than in the powder-groove wick. We also investigated whether boiling occurred in the groove-powder wick with the abundant nucleation sites on the irregular powders. Under sufficiently high heat loads, the liquid would recede into the wick in the heated zone and become difficult to observe. Hence, we examined at the location of x = 1 mm, which was out of the heated zone and immersed in the liquid. For water, there was no sufficient sign of boiling except for weak liquid motion which was more likely caused by the fluctuating liquid flow from the condenser. For methanol and acetone, strongly agitated liquid motion was observed to reflect nucleate boiling. 4. CONCLUSIONS The evaporation characteristics in operating flat-plate heat pipes wicked with U-shaped grooves were experimentally studied. Three different working fluids were investigated using a groove wick and a groove-powder wick. With stepwise increase of heat load, the behavior of the fluid in the grooves was visualized, and the evaporative resistances were measured under thermally stable situations. The following conclusions were reached. 1. For all the three fluids, steep-sloped liquid fronts suddenly appeared at a certain position in the heated zone above a certain threshold heat load. With incremented heat loads, the liquid fronts gradually receded out of the heated zone, and the base plate temperatures and the evaporative resistances increased concurrently 2. Under thermally stable operation, independent longitudinal oscillations existed in most grooves, exhibiting a constantly varying zigzag front line. Weaker oscillating motion and shorter oscillation distances were observed for water. 3. The evaporation performance of water in the groove wick was better than those of methanol and acetone to a limited extent. However, water presented much better performance in the powder-groove wick in both maximum heat load and evaporative resistance. 4. No boiling was observed in the present groove wick for all the three fluids; however, agitated

7 liquid motion reflected nucleate boiling of methanol and acetone in the groove-powder wick. ACKNOWLEDGEMENT This work was funded by National Science Council, ROC under Contract NSC E-7-29-MY2. REFERENCES [1] Anand, S., De, S., and DasGupta, S., Experimental and theoretical study of axial dryout point for evaporation from V-shaped microgrooves, Int. J. Heat Mass Transfer, Vol. 45, pp , 22. [2] Catton, I. and Stroes, G.R., A semi-analytical model to predict the capillary limit of heated inclined triangular capillary grooves, J. Heat Transfer, Vol. 124, pp , 22. [3] Chang, J.C.S.,. Liu, J.-K, Wang, C.-H., Huang, C.-Y., Pei, J., Wang, Y., and Feng, Y., Preliminary study of partially powder-sintered micro groove heat pipes in notebook cooling application, 14 th International Heat Pipe Conference, Florianopolis, Brazil, Apr , 27. [4] Do, K.H., Kim, S.J., and Garimella, S.V., A mathematical model for analyzing the thermal characteristics of a flat micro heat pipe with a grooved wick, Int. J. Heat Mass Transfer, Vol. 51, pp , 28. [5] Jiao, A.J., Ma, H.B., and Critser, J.K., Evaporation heat transfer characteristics of a grooved heat pipe with micro-trapezoidal grooves, Int. J. Heat Mass Transfer, Vol. 5, pp , 27. [6] Lips, S., Lefévre, F., and Bonjour, J., Nucleate boiling in a flat grooved heat pipe, Int. J. Thermal Sci., Vol. 48, pp , 28. [7] Ma, H.B. and Peterson, G. P., Experimental investigation of the maximum heat transport in triangular grooves, J. Heat Transfer, Vol. 118, pp , [8] Mochizuki, M., Nguyen, T., Saito, Y., Horiuchi, Y., Mashiko, K., Sataphan, T., and Kawahara, Y., Latest vapor chamber technology for computer, The 8 th International Heat Pipe Symposium, September, 26, Japan. [9] Nilson, R.H., Tchikanda, S.W., Griffiths, S.K., and Martinez, M.J., Steady evaporating flow in rectangular microchannels, Int. J. Heat Mass Transfer, Vol. 49, pp , 26. [1] Stroes, G.R. and Catton, I., An experimental study of the capillary performance of triangular versus sinusoidal channels, J. Heat Transfer, Vol. 119, pp , [11] Tang, Y., Deng, D., Lu, L., Pan, M., and Wang, Q., Experimental investigation on capillary force of composite wick structure by IR thermal imaging camera, Exp. Thermal and Fluid Sc., Vol. 34, pp , 21. [12] Wong, S.-C. and Chen C.-W., Visualization and evaporation resistance measurement for groove-wicked flat-plate heat pipes, ASME 9 th Int. Conference on Nanochannels, Microchannels, and Minichannels, June 19-22, 211, Edmonton, Canada. [13] Wong, S.-C., Lin, Y.-C., and Liou, J.-H., Visualization and evaporation resistance measurement in heat pipes charged with water, methanol or acetone, Int. J. Thermal Sci., 211, accepted. [14] Wong, S.-C., Liou, J.-H., and Chang, C.-W., Evaporative resistance measurement and visualization for sintered copper-powder evaporator in operating flat-plate heat pipes, Int. J. Heat Mass Transfer, Vol. 53, pp ,