Development and experimental validation of simple methodologies for modelling of flow boiling in microchannels with large pressure drop

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Everything should be made as simple as possible but not simpler.'' Inspired by Albert Einstein's famous quote, this thesis proposes simple but accurate approaches to model the complex interplay between the various thermophysical phenomena occurring in flow boiling. The proposed methodologies incorporate the effect of flashing and local thermophysical properties on two-phase pressure drop evaluation, and hence on heat transfer prediction. The governing equations are derived in an elegant close form, consisting of a system of ODEs, and their simulation is computationally inexpensive. The new modelling approaches are validated by conducting experiments on a microchannel with a large pressure drop.Dissipating high amount of heat flux is an important issue of modern thermal management with the ever-increasing demands for high performance and miniaturization. Flow boiling in microchannel heat sinks has emerged as one of the most effective solutions for cooling high and ultrahigh heat flux devices such as high-performance computer chips, laser diodes and nuclear fusion and fission reactors. Design of these miniature devices requires a proper estimation of the two-phase heat transfer, which, in turn, necessitates an accurate prediction of pressure drop in flow boiling. Two-phase pressure drop in microchannels is relatively high as compared to conventional channels, due to their very small sizes and moderate mass fluxes, the latter being so in order to achieve reasonable heat transfer coefficients. Due to the large pressure gradient, the saturation temperature drops, and hence the effects of flashing and axial variation of thermophysical properties become significant. An extensive review is done on flow and heat transfer in microchannels, and the existing predictive methods are assessed using experimental data. The existing approach for modelling flow boiling pressure drop evaluates thermophysical properties at the system pressure and neglects the effect of flashing. To develop the flow boiling heat transfer coefficient, the existing methods assume a linear pressure profile to estimate local saturation temperature. These methods give fairly accurate predictions for small pressure drop cases, but not in cases where a relatively large pressure drop occurs. It indicates the need for improvements in predictive approaches for flow boiling pressure drop and heat transfer, especially for large pressure drop.
Supervisor: Manmohan Pandey