SUMMARY

A parametric investigation of boiling of water was performed in a dual-chamber thermosyphon to study the effects of sub-atmospheric pressure, boiling structure geometry and liquid-fill volume on thermal performance of the thermosyphon. Sub-atmospheric pressure boiling achieved heat fluxes in excess of 100 W/cm2 with negligible incipience superheat, while keeping boiling surface temperatures below 85 oC. Reduced pressures resulted in reduction of heat transfer coefficient with decrease in saturation pressure. The boiling enhancement structures showed considerable heat transfer enhancement compared to boiling from plain surface and also increased the critical heat flux (CHF). Increased height of the structure decreased the heat transfer coefficient and suggested the existence of an optimum structure height for a particular saturation pressure. A reduction in liquid level increased the CHF for boiling with plain surfaces. In case of enhanced structures, the results suggested the existence of an optimum liquid level for maximum heat transfer, which increased with increase in the height of the enhanced structure. A numerical model has been developed to study condensation in horizontal rectangular microchannels. The model incorporated surface tension, axial pressure gradient, liquid film curvature, liquid film thermal resistance, gravity and interfacial shear stress, and implemented successive solution of mass, momentum and energy balance equations for both liquid and vapor phases. The study is done for rectangular channels of hydraulic diameter, Dh = 150-375 µm and mass flow rates of 70-110 kg/m2-s with a fixed wall temperature. Simulation showed that significantly higher heat transfer coefficient can be achieved from a rectangular microchannel compared to a circular channel of similar hydraulic diameter. Increasing the inlet mass flow rate leads to higher heat transfer coefficient, while increasing the inlet temperature difference between wall and vapor also leads to a thicker film and a gradually decreasing heat transfer coefficient. Increasing the channel dimensions leads to higher heat transfer coefficient, with a reduction in the vapor pressure drop along the axial direction of the channel. The unique contributions of the study are: extending the knowledge base and contributing unique results on the effects of sub-atmospheric pressures, enhancement structure geometry and liquid-fill volume on the thermal performance of thermosyphons; development of a unique model for condensation in rectangular microchannels and identifying the system parameters that affects the flow and thermal performance during condensation.