Date of Award

2012

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical Engineering

First Advisor

Kizito, John

Abstract

Energy conversion devices produce large heat loads during operation and therefore efficient thermal management schemes for their optimal operation are needed. The goal of this work is to develop thermal management methods for high heat flux applications using two-phase spray cooling techniques. Specifically, thermal management method is developed to achieve higher heat fluxes in the range of 100-1000 W/cm2 . Based on the knowledge gaps identified in the literature, the specific objectives developed were to determine the effect of the test surface area scalability on critical heat flux (CHF), to determine the effect of surface modification of the test substrate on heat transfer performance, to determine the heat flux regimes in a spray cooling experiment and to determine the effect of liquid film thickness on heat transfer performance in spray cooling applications. A two-phase spray cooling thermal loop equipped with a high speed video imaging and data acquisition system was used to obtain the experimental data presented in this research work. The experimental result showed that modification of a smooth surface increased the heat transfer performance. A heat flux gain of about 130% and heat transfer coefficient enhancement of 8500 W/m2 -K were achieved with a modified surface over a smooth surface. It was determined that the modification of the surface in the form of wicking grooves increased the fluid wettability and spread on the surface of the test substrate. A phenomenon was observed at incipience of critical heat flux in spray cooling experiment akin to vapor film formation in nucleate pool boiling. Formation of a single bubble covering the whole heated surface was observed. It was identified that the single bubble covering the surface of the test substrate at critical heat flux decreased the surfaceto-fluid wettability due to the vapor core inside the single bubble. In addition, Novec 7000 (1-methoxyheptafluoropropane), a new working fluid, was identified as a prospect for cooling applications. Lower excess temperatures were obtained with Novec 7000 fluid when compared to water.

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