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.
A Doctoral Thesis. Submitted in partial fulfilment of the requirements for the award of Doctor of Philosophy of Loughborough University.