Enhanced Cooling for High Heat Flux Applications Using Droplet Impact and Optical Cavitation

Doctor of Philosophy, Graduate Program in Mechanical Engineering
University of California, Riverside, August 2015
Dr. Guillermo Aguilar, Chairperson

Demand for increased heat fluxes in high-power thermal management applications drives research into improved cooling techniques. Liquid and two-phase cooling methods provide cost-effective and potent means of heat extraction. Sprays of atomized liquid, in particular, deliver very high heat flux due to large cooling surface area, rapid evaporation, and continuous delivery of fresh cold liquid to the target. Sprays are difficult to model and study due to the inherent complexity of highly dynamic two-phase flows and the wide range of factors which influence the cooling  effects.  A detailed study  of the impact of liquid droplets, alone and in sequential trains, onto a variety of impact surfaces is presented, with examination of the fluid properties, impact characteristics, and environmental conditions that govern the dynamics of the liquid after impact and the overall cooling effect. The interaction of successive droplets in a train is found to dramatically influence heat transfer, depending on the frequency of impacts.

As a spray is used to cool a target surface, a liquid film will often develop. A thermal boundary  layer  develops  within  the  liquid  film,  reducing  the  effective  cooling  rate  by isolating hot liquid near the surface. Agitation will minimize the thermal boundary layer, restoring  the  heat  flux.  Optical  cavitation  presents  a  unique  method  of  non-intrusively breaking down the thermal boundary layer. By inducing cavitation within the layer, near the surface,  the  growth  and  collapse  of  the  bubble  will  draw  cool  liquid  from  outside  the boundary layer and deposit it near the surface. To achieve this effect, the optical and fluid conditions that contribute to cavitation are explored, in particular looking at the growth and collapse dynamics and the sequences of bubbles resulting from irradiation by a continuous wave  laser.  Novel  experimental  observations  of  asymmetric  collapse  phenomena  are reported, and among the first measurements of cavitation frequency for continuous wave laser irradiation are reported.