Heat transfer and fluid flow in microchannels

Fluid flow and heat transfer characteristics in microchannels of different cross-sections; parallel plate, cylindrical and trapezoidal microchannels were studied. The trapezoidal microchannels were etched in silicon and glass by photolithographic techniques. The cylindrical microchannels of fused silica and stainless steel were readily available. Channels with depths of 18 μm to 300 μm were studied. The study was divided into three parts viz. theoretical modeling, numerical simulation and experimentation. Electrokinetic effects such as the effects of electrical double layer (EDL) at the solid-liquid interface and surface roughness effects were considered. An experimental apparatus was constructed and a procedure devised to measure the flow rate, pressure drop, temperatures and electrokinetic parameters like streaming potential, streaming current, and conductivity of the working fluid. Great care was taken so that the measurements were accurate and repeatable.; For steady state laminar flow and heat transfer in microchannels, mathematical models were developed that consider the effects of electrical double layer and surface roughness at the microchannel walls. The non-linear, 2-D, Poisson-Boltzmann equation that describes the potential distribution at the solid liquid interface was solved numerically and results were compared with a linear approximate solution that overestimates the potential distribution for higher values of zeta potential. Effects of the EDL field at the solid-liquid interface, surface roughness at the microchannel walls and the channel size, on the velocity distribution, streaming potential, apparent viscosity, temperature distribution and heat transfer characteristics are discussed.; The experimental results indicate significant departure in flow characteristics from the predictions of the Navier-Stokes equations, referred to as conventional theory. The difference between the experimental results and theoretical predictions decreases as the hydraulic diameter increases. For higher hydraulic diameters, the experimental results are in rough agreement with the predictions of Navier-Stokes equations. For the same volume flow rate, experimentally measured pressure gradients are significantly higher than conventional theory predictions. Therefore, the friction factor and apparent viscosity are higher. The results also indicate material dependence of the flow behavior. The observed effects are attributed to either to an early transition from laminar flow to turbulent flow or to the surface effects; electrokinetic and surface roughness effects.; For parallel plate microchannels, the electrokinetic effects explain the observed differences. For cylindrical and trapezoidal silicon microchannels, experimentally measured pressure drop is significantly higher than conventional theory prediction; with and without electrokinetic effects. For cylindrical microchannels, the electrokinetic effects were not measured as one material was conducting while as for the fused silica the diameter of microchannels was greater than 50 μm. For such large microchannels the EDL effects are negligible as shown by theory. For trapezoidal microchannels, flow rate, pressure drop, temperature and electrokinetic parameters were measured for three different electrolyte concentrations. It was found that the electrokinetic effects are negligible for trapezoidal microchannels having hydraulic diameters greater than 50 μm, and the higher-pressure requirement is because of surface roughness. The roughness-viscosity model developed for cylindrical and trapezoidal microchannels explains the higher-pressure requirement as measured experimentally. The predictions of the roughness viscosity model agree well with the experimental data. The heat transfer characteristics are similar to as obtained by various other researchers. The measured Nusselt numbers were lower than the conventional theoretical predictions but agree well with the modified con...