Theses and Dissertations from UMD
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Item PRESSURE-BASED PREDICTION OF SPRAY COOLING HEAT TRANSFER(2010) Abbasi, Bahman; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)One of the main challenges of spray cooling technology is the prediction of local and average heat flux on the heater surface. It has been suggested that spray cooling heat transfer depends on the local spray mass flux. However, in this work it is hypothesized and demonstrated that local single-phase and boiling heat transfer can be predicted within ±25% of the measured values from the local normal pressure produced by the spray. In the single-phase study, hollow cone, full cone, and flat fan sprays operated at three standoff distances, five spray pressures, and two nozzle orientations were used to identify the relation between impingement pressure and heat transfer coefficient. PF-5060, PAO-2, and PSF-3 were used as test fluids, resulting in Prandtl number variation between 12-76. A 7×7 mm2 micro-heater array consisting of 96 platinum resistance heaters operated at constant temperature was used to measure the local heat flux. A separate test rig was used to make impingement pressure measurements for the same geometry and spray pressure. The heat flux data were then compared with the corresponding impingement pressure data to develop a pressure-based correlation for single-phase spray cooling heat transfer. Hollow cone and full cone PF-5060 sprays at three subcooling levels were used for the two-phase heat transfer study. The conventional wisdom is that the temperature at which critical heat flux (CHF) is observed changes with the droplet impact velocity, droplet number density, and droplet size. However, the present measurements indicate that although the magnitude of CHF is strongly dependent on the spray characteristics, the temperature at which CHF occurs lies within a very narrow band (about ±5°C) for smooth flat surfaces. This was also observed from local measurements at various radial distances using hollow cone and full cone spray nozzles where the local mass flux varies dramatically. This observation along with liquid properties and subcooling were used to develop a correlation to predict local CHF for PF-5060 sprays. The single-phase and CHF correlations were combined to predict local spray cooling curve within ±25% of the measured valued over the sprays impingement zones.Item Investigation of Enhanced Surface Spray Cooling(2006-11-29) Silk, Eric A; Kiger, Ken; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Phase change technology is a science that is continually finding new applications, from passive refrigeration cycles to semiconductor cooling. The primary heat transfer techniques associated with phase change heat transfer are pool boiling, flow boiling, and spray cooling. Of these techniques, spray cooling is the least studied and the most recent to receive attention in the scientific community. Spray cooling is capable of removing large amounts of heat between the cooled surface and the liquid, with reported heat flux capabilities of up to 1000 W/cm2 for water. Many previous studies have emphasized heat flux as a function of spray parameters and test conditions. Enhanced spray cooling investigations to date have been limited to surface roughness studies. These studies concluded that surface tolerance (i.e. variations in machined surface finish) had an impact upon heat flux when using pressure atomized sprays. Analogous pool boiling studies with enhanced surfaces have shown heat flux enhancement. A spray cooling study using enhanced surfaces beyond the surface roughness range may display heat flux enhancement as well. In the present study, a group of extended and embedded surfaces (straight fins, cubic pin fins, pyramids, dimples and porous tunnels) have been investigated to determine the effects of enhanced surface structure on heat flux. The surface enhancements were machined on the top surface of copper heater blocks with a cross-sectional area of 2.0 cm2. Measurements were also obtained on a flat surface for baseline comparison purposes. Thermal performance data was obtained under saturated (pure fluid at 101 kPa), nominally degassed (chamber pressure of 41.4 kPa) and gassy conditions (chamber with N2 gas at 101 kPa). The study shows that both extended and embedded structures (beyond the surface roughness range) promote heat flux enhancement for both degassed and gassy spray cooling conditions. The study also shows that straight fins provide the best utilization of surface area added for heat transfer. An Energy conservation based CHF correlation for flat surface spray cooling was also developed. CHF predictions were compared against published and non-published studies by several researchers. Results for the correlations performance show an average mean error of ±17.6% with an accuracy of ±30% for 88% of the data set compared against.