Keywords: microchannel, shear driven, pressure driven, numerical simulation.

This review paper discusses the numerical simulation of the flow in microchannels, along with the associated thermal transport. Of particular interest is the frequently encountered circumstance where the flow is driven by an imposed pressure and by moving surfaces that impart shear to the fluid. The shear itself generates a pressure differential in addition to the imposed pressure and, thus, affects the resulting flow and heat transfer. Practical processes where such flows arise include the thermal treatment of moving wires and fibers, as well as microscale devices during cooling, coating and drawing. One particularly important circumstance is the optical fiber coating process, where the moving surface enters a reservoir of coating fluid through a microchannel and similarly exits through another microchannel. The flow and thermal transport in these microchannels is driven by shear and pressure and influence the resulting coating very substantially.

The basic considerations in the numerical simulation of such microchannel flows are discussed in detail. The flow that arises and the menisci that are observed at the inlet and outlet regions of the two microchannels are discussed in terms of numerical and experimental results. The heat transfer effects are strongly influenced by the development of the flow and the existence of large viscous dissipation in fluids with high viscosity, such as polymers. Visualization has been used for obtaining a basic understanding of these flows. However, numerical modeling is needed for a study of the flow and heat transfer characteristics. The pressure rise in the microchannel for narrowing flow domains, such as those employed in dies, and a comparison with imposed pressures is another important aspect and is discussed. It is found that, in many practical problems, the shear generates much higher pressures than the typically imposed pressures and, thus, the flow is largely dominated by the shear effects due to the moving surfaces.

The basic approach to obtaining accurate simulation results on this complicated problem is outlined. Transformations are used to simplify the computational domain and different regions are employed to characterize the local behavior, particularly near the moving surface and in the entrance and exit microchannels. The drawing of optical fibers involves glass flow to form a single or multiple microchannels through a narrowing region, known as the neck-down region. For typical optical fibers, the process leads to a fiber of core diameter around 50 µm and outer diameter 125 µm, whereas hollow fibers have a central core of 40-80 µm. It is important to understand the basic mechanisms that arise as the fluid flow goes through a converging region, finally reaching a diameter of order 125 µm. Again, numerical simulation is needed to model this process which involves complex domains, combined transport mechanisms, free surface flows and strong material property variations. The numerical model is outlined and the approach to address these complexities is discussed.

Similarly, cooling of electronic systems often involves pressure-driven microchannel flows for enhanced heat removal. However, conjugate transport arises due to the conduction in the substrate that forms the microchannel. The flow in long microchannels, in the slip-flow region, is best simulated by parallel computing because of the intensive computational effort needed. A hybrid parallel-serial approach is outlined to efficiently obtain accurate results. The modeling of the conjugate transport and its effect on the flow and heat transfer are discussed. Comparisons between experimental and numerical results show fairly good agreement, indicating the validity of the model. A brief discussion of the current status in this area and future needs is also given.

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