A study of pool boiling of a dielectric liquid (PF 5060) from single-layered enhanced structures was conducted. The parameters investigated were the heat flux, the width of the microchannels and the microchannel pitch. The boiling performance of the enhanced structures increases with increase in channel width and decrease in channel pitch. Simple single line curve fits are provided as a practical way of predicting the data over the entire nucleate boiling regime. The influence of confinement on the thermal performance of the enhanced structures was also assessed. The main parameter investigated was the top space (0 mm { 13 mm). High-speed visualization was used as a tool . For the total confinement ( = 0 mm), the heat transfer performance of the enhanced structures was found to depend weakly on the channel width. For >0 mm, the enhancement observed for plain surfaces in the low heat fluxes regime is not present for the present enhanced structure. The maximum heat flux for a prescribed 85 oC surface temperature limit increased with the increase of the top spacing, similar to the plain surfaces case. Two characteristic regimes of pool boiling have been identified and described: isolated flattened bubbles regime and coalesced bubbles regime. A semi-analytical predictive model applicable to pool boiling under confinement is developed. The model requires a limited number of empirical constants and is capable of predicting the experimental heat flux within 30%.

Thesis(M.S.)--Auburn University, 2006. / Abstract. Vita. Includes bibliographic references.

An experimental and numerical investigation was conducted to study boiling and condensation - the two most important phenomena occurring in a dual-chamber thermosyphon. Boiling experiments were carried out using water at sub-atmospheric pressures of 9.7, 15 and 21 kPa with a three-dimensional porous boiling enhancement structure integrated in the evaporator. Sub-atmospheric pressure boiling achieved heat fluxes in excess of 100 W/cm2 with negligible incipience superheat, for wall temperatures below 85 oC. Reduced pressures resulted in reduction of heat transfer coefficient with decrease in saturation pressure. The boiling enhancement structure showed considerable heat transfer enhancement compared to boiling from plain surface. 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 parametric study showed that a reduction in liquid level of water increased the CHF for boiling with plain surfaces. For boiling with enhanced structures, the liquid level for optimum heat transfer increased with increasing height of the enhanced structure. A numerical model was developed to study condensation of water in horizontal rectangular microchannels of hydraulic diameters 150-375 µm. 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. Rectangular microchannels achieved significantly higher heat transfer coefficient compared to a circular channel of similar hydraulic diameter. Increasing the inlet mass flow rate resulted in a higher heat transfer coefficient. Increasing the inlet temperature difference between wall and vapor led to a thicker film and a gradually decreasing heat transfer coefficient. Increasing the channel dimensions led 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 thermal performance of thermosyphons, and development of a analytical model of condensation in rectangular microchannels, which identified the system parameters that affects the flow and thermal performance during condensation.