Control of nucleate boiling with micro-machined surface features.

This thesis discusses the production and use of laser-machined boiling grids that provide controlled nucleate boiling and enhanced heat transfer characteristics for application primarily to IC engine cooling systems. The surface features of heated plates are known to have a significant effect on nucleate boiling heat transfer and bubble growth dynamics. Nucleate boiling starts from discrete bubbles that form on surface imperfections, such as cavities or scratches. The gas or vapours trapped in these imperfections serve as nuclei for the bubbles. After inception, the bubbles grow to a certain size and depart from the surface. The bubble departure process significantly increases heat transfer rates compared to pure convection. In this work, special heated surfaces were manufactured by laser machining cavities into polished aluminium plates. This was accomplished with an Nd:YAG laser system, which allowed drilling of cavities of a known diameter. The size range of cavities was 25 to 300 micrometers. The resulting nucleate pool boiling was analysed using a high-speed imaging system comprising an infrared laser and high resolution CCD camera. This system was operated up to a 2 kHz frame rate and digital image processing allowed bubbles to be analysed statistically in terms of departure diameter, departure frequency, growth rate, shape and velocity. Data were obtained for heat fluxes up to 150 kW.m'2. Bubble measurements were obtained working with water at atmospheric pressure. The surface cavity diameters were selected to control the temperature at which vapour bubbles started to grow on the surface. The selected size and spacing of the cavities was also explored to provide optimal heat transfer. Insights into the interaction and interseeding mechanism were obtained. The research has demonstrated that nucleate boiling can be controlled by optimally sized and spaced laser-machined cavities in heated metal surfaces.