Sudipta Saha

Date of Award

Spring 2019

Document Type

Open Access Thesis

Department

Mechanical Engineering

First Advisor

Tanvir Farouk

Abstract

This thesis work describes a numerical study on the effect of evaporative cooling on the augmentation of forced convective cooling. In recent years, on-demand phase change boosted cooling has drawn major interest where convective heat transfer is augmented/aided by evaporative heat and mass transport processes. This dual mode (convection and evaporation) cooling method is envisioned to drastically enhance the heat transfer coefficient where conventional convective cooling has already reached its maximum value and furthermore dry cooling is still a desired objective. A multi- dimensional mathematical model has been developed to conduct simulations over a range of operating parameters to obtain insight into the ‘hybrid’ system where phase change process and convection both contribute to the heat transfer process. The system being modeled consists of a thin liquid water film that undergoes evaporation as a result of being exposed to a prescribed heat flux and laminar convective flow condition. The mathematical model utilized comprises of coupled conservation equations of mass, species, momentum and energy for the convection-evaporation domain (gaseous), and only mass and energy conservation being resolved in the liquid film domain, together with a moving mesh to resolve the receding liquid film. Predictions from the simulations indicate that in comparison to pure forced convection cooling, under convective- evaporative conditions the overall heat transfer coefficient is increased by a factor of ~ 5, where evaporation alone contributes to 80% - 90% of the overall performance. For a fixed heat flux, an increase in Reynolds number was found to increase the heat transfer coefficient and vice versa for film thickness. It has been found that overall heat transfer coefficient can be enhanced by making the film thinner, since the conducting resistance across the liquid film diminishes as the film thickness is reduced. A critical film thickness has been identified beyond which the conductive resistance becomes dominant and starts to attenuate the thermal performance. Spatiotemporally averaged interface temperature has been found to be increasing with the increase of film thickness, as an evidence of suppressed cooling. A critical Reynolds number is identified beyond which no significant increase in overall heat transfer coefficient is observed. Furthermore, surface enhancement studies have been conducted with view to assessing its effect on overall thermal performance of convective-evaporative dual mode heat transfer system. To accomplish that, bottom surface of the evaporating liquid has been modified by introducing circular grooves to promote mixing. A set of parametric study based on enhanced surface structures predicts that surface modification results in a significant reduction in thermal resistance across the liquid film.

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Rights

© 2019, Sudipta Saha

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