Date of Graduation
Statler College of Engineering and Mineral Resources
Mechanical and Aerospace Engineering
John M. Kuhlman.
Spray cooling is a topic of current interest for its ability to uniformly remove high levels of waste heat from densely packed microelectronics. It has demonstrated the ability to achieve very high heat fluxes, up to 500 W/cm2 with water as the coolant, making it an attractive active thermal management tool.;Full Computational Fluid Dynamic (CFD) simulations of spray cooling are infeasible due to the complexity of the spray (drops fluxes of 106 drops/cm2-sec) and heater surface physics requiring impractical resources. Thus a Monte-Carlo (MC) spray cooling simulation model based on empirical data is under development to serve as a cost effective design tool. The initial MC model shows promise, but it lacks additional physics necessary to predict accurate heat fluxes based on nozzle conditions and heated surface geometry.;This work reports spray and single drop experiments with the goal of computing the volume beneath a droplet impact cavity (the sub-cavity volume) created by a single impinging droplet on an initial liquid layer. A Phase Doppler Particle Analyzer (PDPA) was utilized to characterize a spray of interest in terms of integrated global Weber, Reynolds, and Froude numbers for varying flow conditions. Results showed that the spray droplet diameters decreased and velocities increased with increasing nozzle gage pressure. A relevant test plan for the single drop experiments has been created from the measured PDPA spray profiles combined with residual spray film thickness measurements from literature resulting in: 140≤We≤1,000, 1,200≤ Re≤3,300, and 0.2≤h0*≤1.0. Froude numbers were not able to be matched for the current single drop experiments (spray: 32,800≤Fr≤275,000).;Liquid film thicknesses under the cavity formed by a single droplet have been measured versus radius and time via a non-contact optical thickness sensor for the selected range of dimensionless numbers (We, Re, and h0*). Sub-cavity radius histories have also been analyzed utilizing high-speed imagery techniques to create the cavity thickness traverse profiles. Time dependent sub-cavity volumes have been computed by integrating these subcavity liquid film thicknesses versus radius at various times. It is found that higher We and lower h0* result in a more radially uniform sub-cavity surface contour versus time, except for thinner liquid film regions which are observed near the outer bottom cavity radius. The subcavity volume was found to be nearly constant for a majority of the cavity lifetime and increased with We and h0*. These results will be incorporated into the MC model to improve its predictive capability in future work.;In addition, splashed droplet diameters and velocities have been extracted from PDPA data for a spray impinging normal to a smooth surface. It was found that the splashed droplets had sizes which were similar to the impinging spray droplets, and had velocities that never exceeded 3 m/s. The splashed droplet results have a negligible contribution to cavity formations due to their low Weber number. This splashing data has been detailed for future implementation into the MC model in terms of mass conservation in the liquid film.
Hillen, Nicholas Lee, "Droplet Impact Sub-cavity Histories and PDPA Spray Experiments for Spray Cooling Modeling" (2013). Graduate Theses, Dissertations, and Problem Reports. 3387.