Fundamental studies of thermal transport and liquid-vapor phase change using microscale diagnostic techniques.

摘要

Fundamental studies of thermal transport and fluid flow in single-phase and two-phase systems are conducted using microscale diagnostic techniques. An infrared micro-particle image velocimetry technique is extended to the study of fluid flows in a low signal-to-noise ratio environment. Measurements of flow distribution in a microchannel heat sink are conducted with this technique. It was concluded that careful manifold design is critical for ensuring that uniform flow is supplied to the parallel microchannels.;The effects of surface roughness on nucleate pool boiling and flow boiling are studied. For pool boiling, a range of surface roughness from Ra = 0.038 mum to 10.0 mum are investigated. In water, the 10.0 mum surface results in approximately twice the heat transfer coefficients obtained with the 0.038 mum surface, at a fixed heat flux. The influence of surface roughness on the heat transfer coefficient is correlated as h ∼ (Ra) 0.1 for water and h ∼ (Ra) 0.2 for FC-77. Due to the differences in the surface-roughness exponent, no single nucleate pool boiling correlation in the literature offers satisfactory predictions for the entire range of experimental conditions under consideration.;Modifications to an existing pool boiling correlation for improved accuracy over a greater range of surface roughness were explored. An experimental database of almost 3000 data points was constructed from studies in the literature and the present work. A modified correlation was developed that includes the difference in behavior of nucleate boiling due to surface roughness effects in moderately wetting fluids such as water than for highly wetting fluids. The correlation was found to lead to improved predictions across the experimental database.;For flow boiling, the range of surface roughness under investigation is between 1.4 mum and 6.7 mum. At higher heat fluxes, 25 to 35% higher heat transfer coefficients are measured on the 6.7 mum roughness surface compared to the 1.4 mum surface. The ability of flow boiling correlations to accurately predict the heat transfer coefficients over a wide range of surface roughness was explored.;Laser-induced fluorescence thermography was demonstrated for studying the temperature field around a growing vapor bubble during nucleate flow boiling. The capabilities of the technique were explored and with further refinements, the technique shows promise in yielding important insights into the nucleate boiling heat transfer phenomenon.