Viscous Dissipation Effects For Liquid Flow In Microchannels B. Xu, K.T. Ooi * , C. Mavriplis, and M.E. Zaghloul Institue for MEMS and VLSI Technologies The George Washington University, Washington, DC 20052, USA * School of Mechanical and Production Engineering Nanyang Technological University, Singapore 639798 E-mail: mbxu@seas.gwu.edu (B.Xu), mavripli@seas.gwu.edu (C.Mavriplis), ABSTRACT Different phenomena have been observed in various works indicating that the mechanisms of flow and heat transfer in microchannels are still not understood clearly. There is little experimental data and theoretical analysis in the literature to elucidate the mechanisms. It is reasonable to assume that, as the dimensions of flow channels approach the micro-level, viscous dissipation could be too significant to be neglected due to a high velocity gradient in the channel. Thus, deviations from predictions using conventional theory that neglects viscous dissipation could be expected. In this paper, the effects of viscous dissipation in microchannel flows are analyzed and examined theoretically. A criterion to draw the limit of the significance of the viscous dissipation effects in the microchannel flows is suggested based on the present analysis. Keyword: Viscous dissipation, microfluidics 1 INTRODUCTION With microfluidic systems becoming more attractive to biomedical and lab-on-chip systems, understanding the flow is becoming very important. Different phenomena have been observed in various works indicating that the mechanisms of flow and heat transfer in microchannels are still not understood clearly. Choi [ 1 ] experimentally studied the friction factors, and convective heat transfer coefficients for both the laminar and turbulent flow of nitrogen gas in microtubes, for the tube diameters ranging from 3 μm to 81 μ m. His experimental results indicate significant departures from the thermofluid correlations used for conventional-sized tubes. Pfahler et al. [ 2,3] presented an experimental investigation for gas and liquid flows in microchannels. In their studies, both gases and liquids, were used. They concluded that as the channel depth was decreased, there appeared to be a critical channel size where the general behavior of the experimental observations deviates from the predictions. Different phenomena for different fluids were also observed. For liquid flow in the smaller channels, they found that the friction constant was smaller than that predicted by the conventional theory. Peng et al. [ 4,5 ], and Wang and Peng [ 6 ] experimentally studied friction flow and heat transfer characteristics of water flowing through microchannels with different hydraulic diameters. They showed that the flow changes from the laminar to the transition regime at Reynolds numbers (Re) ranging from 200 to 700 for different channels; these values were lower than the conventional values. It was also found that for the transition to turbulent flow, the range of the transition zone, and heat transfer characteristics of both transition and laminar flow were unusual as compared to the conventional-sized flow situations. Yu et al. [ 7 ] experimentally and theoretically investigated the fluid flow and heat transfer in microtubes with dry nitrogen gas and water as working fluids. In their work, for both fluids, a reduction in the friction factor was observed for the laminar flow and a smaller reduction was observed in the transition and turbulent regimes. Adams et al. [ 8 ] performed an experimental investigation of single-phase forced convection in circular channels with diameter 0.76mm and 1.09mm and Re ranging from 3,200 to 23,000. Heat transfer coefficients and Nusselt numbers were found to be higher than these predicted by traditional theory and the trends were found to be in agreement with the data obtained by Yu et al. [ 7 ]. These differing phenomena observed in various research works indicate that the mechanisms of fluid flow and heat transfer through microchannels are not understood clearly. This is particularly true for liquid flows and heat transfer in microchannels. Tso et al. [ 9] discussed the single-phase convective heat transfer in microchannels by using the data extracted from the figures presented by Wang and Peng [ 6 ]. They explained that viscous dissipation would affect liquid flow at low Reynolds numbers in microchannels. It seems reasonable to say that, as the dimensions of the channels approach the micro-level, viscous dissipation could be too significant to be neglected due to the existence of the high velocity gradient. Therefore in the microchannel flow prediction, the commonly neglected viscous dissipation, may be too significant to ignore. In this paper, effects of viscous dissipation in microchannel flows are analyzed and examined theoretically. A criterion to draw the boundary of the significance of the viscous dissipation effects is also suggested based on the results of the present analysis.