Studies of Thermal Bubble Pump in a Microchannel Loop

Transcription

1 Studies of Thermal Bubble Pump in a Microchannel Loop Tzong-Shyng Leu, Yan-Hao Liu National Cheng Kung Univeristy, Tainan, Taiwan,701, R.O.C Tel : x63638, Fax: , tsleu@mail.ncku.edu.tw 10th IHPS, Taipei, Taiwan, Nov. 6-9, 2011 ABSTRACT In this paper, a novel MEMS-fabricated thermal bubble pump is proposed. By using thermal capillary effect in a microchannel loop, bubbles are designed to move only in one direction of the loop. In the experiments, microscopic flow visualization is used to study thermal bubble phenomena including nucleation, growth, and movement in the microchannel loop. By driving the heater with a single square-wave signal, bubble nucleates and grows asymmetrically above the heater within a microchannel loop. During heating period, bubble grows only in one direction of microchannel loop. As soon as the heating pulse is turned off, an interesting phenomenon is found. Instead of bubble collapse, bubble moves toward growing direction. Continuous bubble grows and moves periodically when square-wave pulsing signals are applied, Thermal bubble pumping phenomena are achieved successfully in the experiments. The experimental results also found the thermal management along the microchannel loop is the key control parameter. By adjusting background temperature near the heater and the channel loop, thermal bubble pulsing pump phenomena can be sustained. Detailed mechanism for pumping force will be discussed in this paper. Keywords: thermal capillaryr, Microchannel Loop, Thermal Bubble Pump 1. INTRODUCTION In the past few years, microfabrication has emerged as a promising technology for miniaturizing and integrating heat pipe systems. Many heat pipes in microscale applications utilize capillary effects to induce net flow and obtain heat spreading or exchange goals. For examples, the micro capillary pumped loop (MCPL) developed by A. Hoelke [1] was built from a similar concept to the micro loop heat pipe [2,3]. Since then, micro capillary pumped loop was developed by Laura Meyer et al. [4]. All these days, studies for micro capillary pumped loop have not been explored completely. A micro-cooler may exploit a microscale phase change or single-phase heat transfer to dissipate heat from electronic packages, including miniature heat pipes [5,6]. Regardless of the configuration, an effective micro-cooler will have to be able to fully control the motion of fluids throughout the system. The combination of microscale heat transfer and fluid dynamics along with high surface-to-volume ratios makes the development of an efficient micro-cooler challenging. The conceptual design and fabrication of a micro-cpl was the first addressed by Kirshberg et al. [7]. The initial design involved a completely passive three-port micro-cpl, and is described schematically in their work. Herein, micro-devices could support extremely large gradients of pressure due to the small size. Although, the MCPL system developed by authors [8] starts up successfully and depriming phenomenon does not occur until heat flux reaches W/cm 2. Thus, it can be regarded as an effective cooling device which meets current needs for electronic cooling. In this study, the novel idea of thermal bubble pump with the use of thermal capillary effects becomes very appealing. With this goal in mind, we investigated the feasibility of pumping fluid in micron scale channels loop with the use of thermal bubble pump. A thermal capillary driven pumping system for moving discrete bubble within microfabricated channel loop has been developed. As fluidic channel dimensions decrease, surface tension forces increase relative to the gravitational, viscous, and inertial forces. The thermal bubble pumping mechanism is first tested using microfabricated flow devices constructed from PDMS mcirochannel and micro heater on a glass substrate. Next, a numerical analysis is presented to understand the use of this mechanism in bubble pump systems. 2. THERMOCAPILLARY PUMPING THEORY A discrete bubble within a fluidic channel contains two menisci (Figure 1), each with an associated radius of curvature. The curvature of the meniscus produces a pressure difference at the interface called capillary pressure that can be defined by the following form of the Young-Laplace Equation:

2 Gσ cosθ Pc = Pv Pl = (1) d In this equation, P c is the capillary pressure (see Probstein 1989 [9]), P v is the interface pressure on the vapor side, P l is the interface pressure on the liquid side, θ is the contact angle (see de Gennes 1985 [10] and Dussan 1979 [11] for reviews) between the liquid and solid phases, σ is the liquid tension, d is the channel diameter height., and G is a constant specific to channel geometry G=4 for circular, G=2 for slit-like, and G=2[1+height/width] for square or rectangular. Although Eq. (1) was derived from a static force balance, the balance is a reasonable approximation for moving bubbles if the static contact angle is replaced by the dynamic contact angle (Rose and Heins, 1962 [12]) and inertial and shear forces are negligible compared to surface tension forces. Although a pressure difference exists between the liquid and vapor phases across each interface, as shown in Figure 1, the bubble pressures between the ends are balanced (P IR -P IA =0) and the bubble remains stationary. By manipulating the surface tension on one side of a bubble, a pressure difference can be generated for bubble motion. This pressure difference can be produced by heating one bubble/liquid interface because surface tension decreases linearly with meniscus temperature Jasper, 1972 [13]. σ = a bt (2) where a and b are positive empirical constants. We refer to this heat-induced, pressure-driven flow as thermocapillary pumping (TCP). The appeal of a heat induced mechanism lies in the simplicity of its application within a microfabricated device. Furthermore, the device requires no moving parts, is self-contained, requires no open electrodes, and is applicable to discrete bubbles. Bubble motion by TCP occurs by cooling the receding interface or heating the advancing interface of a bubble. A pressure difference across the length of the bubble is subsequently induced by the change in capillary pressure at the cooling or heating end. This induced pressure difference between the bubble ends from the thermally induced surface tension differences Pc is Gσ R cosθ R Gσ A cosθ A Pc = PIR PIA = ( ) ( ) (3) d d The subscript A refers to the advancing meniscus of a moving bubble, while R refers to the receding interface in Figure 1; P IR and P IA are the pressures at the receding and advancing interface of the bubble, respectively, and Note that Eq. (3) describes the capillary pressure difference produced in either a hydrophilic or hydrophobic system. Thermal bubble pump eliminates the need for an external pressure source, thus yielding simple, self-contained devices. Furthermore, thermal bubble pump is compatible with any channel surface that produces a curved liquid interface including silicon, glass, quartz, and various polymers. Figure 1. Thermocapillary pumping of a bubble The equation for the driven pressure for a bubble movement in Eq.(3) will be valid by experiments. We have tested this theory by measuring thermal bubble velocity in actual microfabricated devices by using IPA liquid. Those results are described in the following sections. 3. BUBBLE PUMP DESIGN AND FABRICATION Fig. 2 shows the schematics for the thermal bubble pump with dimension in micron. The main components of the thermal bubble pump device are the micro heater on a glass substrate and PDMS-fabricated microchannel loop with depth of 50 micron. The micro heater is fabricated by sputtering TaAl thin film metal material. The heating area was triangular shape with based 100 micron and height 150 micron. The heating power can be obtained by the formula P=V 2 /R, where V is the voltage input and R is the resistance of the micro heater. The width of channel loop is 100 micron with inlet and outlet. About 150 micron upstream of micro heater tip, there is a narrow channel region with width of 35 micron. It is used as a check valve to prevent bubble growth in the upstream direction. The thermal bubble pump system was fabricated using standard MEMS technology. Figure 3 shows the thermal bubble pump device with a ruler and zoom-in image for the micro heater and check valve

3 A TE cooler Inlet Check valve X cooler Figure 2 Schematic sketch of thermal bubble pump TFE tube y B B` Heater x Outlet Origin point,h 0 Channel Outlet Heater A` (heater-off) time t c. Thermal bubble nucleated on the microheater grows in both upstream and downstream directions. The growth of upstream interface of thermal bubble is restricted and stopped before the check valve. Then, the thermal bubble will continue to grow in downstream direction only during the heater period, as shown in Figure 4(a). After heating time, the bubble grows to a certain size. By placing a thermalelectric (TE) cooler beneath of the thermal bubble pump chip and close to micro heater region (as shown in Figure 2), one side of thermal bubble pump chip is cooled. A temperature gradient between two ends of the thermal bubble is generatd, as shown in Figure 4(b). Due to thermal capillary effect, the unbalanced capillary pressure force between two interfaces causes the bubble moving (Figure 4(c)). Bubble, as well the liquid in the channel loop, is driven by the thermal capillary force toward downstream direction. (a) Check valve Microchannel Heater (b) Figure 3 (a) Microfabricated thermal bubble pump device and (b) the microscopic image near micro heater and the check valve. 4. BUBBLE PUMP OPERATION PRINCIPLE Figure 4 shows the A-A cross sectional sketch of Figure 2 that demonstrates the operation principle of thermal bubble pump. By input a pulse signal into micro heater with voltage V=4 volt, heating (heater-on) time t H, and cooling Figure 4 The operation principle of thermal bubble pump. (a) Due to check valve, thermal bubble growth downstream only during the heating-period. (b) Thermalelectric cooler is used to generate temperature gradient between two ends of the thermal bubble. (c) thermal bubble movement due to thermal capillary effect 5. EXPERIMENTAL RESULTS Figure 5 shows one of the experimental results during thermal bubble pump operation. By inputing a pulse signal into micro heater with voltage V=4 volt, heating (heater-on) time t H =4 sec, and cooling (heater-off) time t c =16 sec. Figure 5(a) shows the thermal bubble at the end of heating time t=4sec. A thermal bubble nucleated on the micro heater grows in downstream direction only during the heating period (0~4 sec), as shown in Figure 5(a). A TE cooler with cooling temperature of 16 o C placed at a location upstream of the micro heater with a distance of

4 1000 m (Xcooler=-1000 m in Figure 2). Temperature gradient between two ends of the thermal bubble is formed. Bubble driven by thermal capillary force moves toward downstream direction from t=6~20 sec, as shown in Figure 5(b)~5(e). (d) (a) (e) Figure 5 The sequence images during thermal bubble pump operation. (a) t=4sec (b) t=6sec (c) t=12sec (d) t=16sec (e) t=20sec (b) (c) To study the bubble movement, one uses microscopic image sequence during thermal bubble pump operation and identifies bubble center locations (X B ) in the x-coordinate of Figure 2. Figure 6 shows the results of bubble center location (X B ) for different TE cooler locations (X cooler ). In Figure 6, the bubble center location moves toward downstream during heating time period (0~4 sec) with different moving speed. For TE cooler locations at downstream of micro heater (X cooler >0), the bubble the bubble center moving speed is slower than the cases of TE cooler locations at upstream of micro heater (X cooler <0). After heating pulse is off, the bubble center moves backward from t=4~6 sec in all cases. For t=6~20 sec, one can find bubble center actually moves backward for the case of X cooler = 2160 m. If TE cooler location changes between <X cooler < 1160 m, the bubble center starts to move toward downstream with a increasing speed. One can find the fastest bubble center movement occurs for the case of X cooler =-1000 m. For the cases of Xcooler= m and m, the bubble center

5 does not move substantially from t=6~20 sec. From the analysis of bubble center location, one can conclude that the thermal capillary effect will be maximum for the case of X cooler = m. the channel wall. Figure 7 The temperature difference ( T B ) between two interfaces of thermal bubble for different TE cooler locations. Figure 6 The movement of bubble center location (X B ) for different TE cooler setup distances (X cooler ) To answer why the maximum thermal capillary effect happens for the case of X cooler = m, numerical simulation is performed to find out the temperature field. By using image sequence from the experimental results, the temperature difference ( T B =T h -Tc as defined in Figure 4(b) ) between advancing (T h ) and receding (T c ) interfaces is identified for different TE cooler locations. Figure 7 shows the temperature difference ( T B ) for different TE cooler locations. The temperature difference ( T B ) shows no difference for TE cooler location at X cooler = 2160 m which explains the bubble center will not move forward in Figure 6. For X cooler = 0 and m cases, the positive temperature difference ( T B ) are found to explain the bubble movement due to thermal capillary effect. However, the maximum temperature difference ( T B ) does not happen at X cooler = m case, but Xcooler= m case in Figure 7. One suspects that the bubble boiling heat transfer did not include in the current numerical simulation. Detailed modeling has to be performed for the actual temperature field on 6. CONCLUSIONS In this paper, thermal bubble pump by using thermal capillary driven force is demonstrated and studied. The experimental results found the thermal management along the microchannel loop is the key control parameter. By using TE cooler location to adjust background temperature field near the heater, thermal bubble pump phenomena can be sustained and maximum pumping performance happens at X cooler = m. Numerical simulation is carried out to study the thermal capillary effect of bubble pump. Some cases agree well with experimental results, However, the maximum temperature difference ( T B ) does not appear for X cooler = m case, but X cooler = m case More detailed temperature fields for thermal bubble pump have to be studied in the future. NOMENCLATURE d the channel diameter height., and G is a constant specific to channel geometry G a constant specific to channel geometry P c the capillary pressure P IR the pressures at the receding interface of the bubble, P IA the pressures at the advancing interface of the bubble, P v interface pressure on the vapor side, P l the interface pressure on the liquid side, X B bubble center location X cooler TE cooler location Pc The induced pressure difference

6 between the bubble ends from the thermally capillary effect T B temperature difference between receding and advancing interfaces. θ contact angle between the liquid and solid phases, σ liquid surface tension coefficient Subscript A R refers to the advancing meniscus of a moving bubble, refers to the receding interface ACKNOWLEDGEMENT The authors would like to thank the National Science Council of the Republic of China, Taiwan for financially supporting this research under Contract No. NSC E MY2. REFERENCES [1] Hoelke, A. et al., Analysis of the Heat Transfer Capacity of a Micromachined Loop Heat Pipe, ASME, 3, pp (1999). [2] Lauru, J. M. and Phinney, L. M., Optimization Study of a Silicon-Carbide Micro-Capillary Pumped Loop, International Electronic Packaging Technical Conference and Exhibition, Maui, Hawaii, USA (2003). [3] Dickey, J. T. and Peterson, G. P., Experimental and Analytical Investigation of a Capillary Pumped Loop, Journal of Thermophysics and Heat Transfer, 8, pp (1994). [4] Meyer, L., A Silicon-Carbide Micro-Capillary Pumped Loop for Cooling High Power Devices, 19 th IEEE Semi-Therm Symposium, pp (2003). [5] Cotter, T. P., Principles and Prospects for Micro Heat Pipe, 5th International Heat Pipe Conference, Tsukuba, Japan, pp (1984). [6] Kirshberg, J., Yerkes, K., Trebotich, D. and Lipmann, D., Cooling Effect of a MEMS Based Micro Capillary Pumped Loop for Chip-Level Temperature Control, ASME MEMS-2 (2000). [7] Pettigrew, K., Kirshberg, J., Yerkes, K., Trebotich, D. and Liepmann, D., Performance of MEMS Based Micro Capillary Pumped Loop for Chip-Level Temperature Control, MEMS: The 14th IEEE International Conference on Micro Electro Mechanical Systems, Switzerland (2001). [8] Wang, C. T., Leu, T. S. and Lai, T. M., Micro Capillary Pumped Loop System for a Cooling High Power Device, Experimental Thermal and Fluid Science, 32, pp (2008). [9] Probstein, R. F., Physiochemical Hydrodynamics: An Introduction, Butterworths, Boston, MA, p [10] de Gennes, P. G., Wetting: Statics and Dynamics, Re. Mod. Phys.,57, [11] Dussan, E. B., On the Spreading of Liquids on Solid Surfaces: Static and Dynamic Contact Angles, Ann. Re. Fluid Mech., 11, 371, [12] Rose, W., and R. W. Heins, Moving Interfaces and Contact Angle Rate Dependency, J. Colloid Sci., 17, [13] Jasper, J. J., The Surface Tension of Pure Liquid Compounds, J. Phys. Chem. Ref. Data, 1, 841 _