2011 13 th Electronics Packaging Technology Conference Enhanced Heat Transfer and Reduced Pressure Drop Using Stepped Fin Microchannels K. Balasubramanian 1,2 , P. S. Lee 1 , L. W. Jin 1 , S. K. Chou 1 , C. J. Teo 1 , S. Gao 2 1 Department of Mechanical Engineering, National University of Singapore 9 Engineering Drive 1, Singapore 117576 Email: mpelps@nus.edu.sg karthikb@nus.edu.sg 2 Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research) 11, Science Park Road, Science Park II, Singapore, 117685 Abstract Experiments on flow boiling were conducted in straight and stepped fin microchannels. The test vehicles were made from copper with a footprint area of 25mm x 25mm. The microchannels were formed by wire cut Electro Discharge Machining process and have surface roughness (Ra) of about 2.0 µm. Tests were performed on channels having nominal width of 300 µm and a nominal aspect ratio of 4 over different mass velocity range and inlet temperature of 90°C. It was observed that the two phase pressure drop across the stepped fin microchannel heat sink was significantly lower as compared to its straight counterpart. Moreover the pressure drop and wall temperature fluctuations were seen reduced in the stepped fin microchannel heat sink. It was also noted that the stepped fin microchannel heat sink had a better heat transfer performance than the straight microchannel heat sink, under similar operating conditions. This phenomenon in stepped fin microchannel heat sink is explained based on its improved flow boiling stability that reduces the pressure drop oscillations, temperature oscillations and hence partial dry out, by allowing the bubbles to expand span wise and hence flow downstream with less resistance. Introduction In spite of having the most versatile features in heat transfer performance, microchannel flow boiling suffers heavy pressure drop penalty and flow instabilities at higher heat fluxes, which degrades their reliability. Inception of flow boiling instabilities occurs when bubble growth is confined by the microchannels in span wise direction. Hence the bubbles tend to expand stream wise causing fluctuations in the flow. At extreme cases these fluctuations lead to flow reversal and partial dry out. Several measures had been undertaken to mitigate or minimize these instabilities [1-5]. Mukherjee and Kandlikar [6], based on their numerical study, proposed that channels with increasing cross-sectional area could be adopted to promote unidirectional growth of the vapor plugs and prevent reversed flow. Also, few efforts have been undertaken to experimentally explore the channels with increasing cross- sectional area. Lee et al [7] conducted experiments and identified that if the channel expands at the downstream, the flow instability was also reduced effectively. . They concluded that this is due to the utilization of the surface tension force, which is inversely proportional to the radius of curvature of a bubble meniscus. Since the expanded channel has a larger cross- sectional area at its downstream, the local bubble meniscus will have a large radius of curvature and result in the smaller surface tension force. The surface tension force difference of an elongated bubble between the upstream end of microchannel and the downstream end of expanded channel pushes this bubble toward the downstream exit. As a result, the flow becomes more stable. They established an instability parameter R for the general application for evaporative microchannels and validated it with experiments.They also observed that, the increased width of the microchannel at the downstream end reduces the local velocity of two-phase flow, so the pressure drop in expanding microchannels is also reduced. However, no results on their heat transfer performance were presented. Lee and Pan [8] compared the boiling heat transfer and two-phase flow of water in single shallow uniform-cross-section microchannel and a diverging one with a diverging angle of 0.183°. They concluded that the diverging microchannel presented a better heat transfer than that of the uniform-cross-section one, primarily due to more stable two-phase flow in the diverging microchannel. They also observed that the for the same mass flow rate, the diverging microchannel presented a higher single-phase flow pressure drop, while the two-phase flow in both cases showed approximately the same pressure drop for boiling at the same heat flux. However they did not investigate this effect on multiple channels having a common inlet and outlet plenums. Qu et al. [9] conducted flow boiling experiments in a heat sink containing an array of staggered square micro-pin-fins having a 200 x 200 µm2 pin cross-section by a 670 µm pin height. Three inlet temperatures of 30, 60 and 90 °C, and six maximum mass velocities for each inlet temperature, ranging from 183 to 420 kg/m2s, were tested. They concluded that two-phase micro-pin-fin heat sinks were able to provide better flow stability than their micro-channel counterparts. This is because the interconnecting nature of flow passages in micro- pin-fin arrays promotes a more stable two-phase flow. In the previous study Balasubramanian et al. [10] conducted experiments in straight and expanding microchannels with similar dimensions and operating conditions. The expanding channels were formed with the removal of fins at selected location from the straight microchannel design, instead of using a diverging channel. In this benchmarking study, it was observed that the two-phase pressure drop across the expanding microchannel heat sink was significantly lower as compared to its straight counterpart. The pressure drop and wall temperature fluctuations were seen reduced in the expanding microchannel heat sink. It was also noted that the expanding microchannel heat sink had a better heat transfer performance than the straight microchannel heat sink, under similar operating conditions. This phenomenon in expanding microchannel heat sink, which was observed in spite of it 978-1-4577-1982-0/11/$26.00 ©2011 IEEE 653