Optimization of Phase Change Heat Transfer in Biporous Media

As the heat transfer demands placed on small electronics devices increase, the demand for efficient evaporators for heat pipes and spreaders will increase in kind. Sintered copper porous media have found many uses in the electronics cooling industry as they effectively transfer energy while maintaining low heater side temperatures. Evaporator wicks of this type transfer heat through sensible and latent heat as the liquid evaporates. A biporous wick is particularly effective for this application as there are two distinct size distributions of pores; small pores to provide ample capillary pressure in order to drive flow through the wick and large pores to provide high permeability for escaping vapor. This dissertation is focused on the methods by which one can enhance and optimize the performance of biporous material. This includes changes to the geometry and a predictive model which can be used to predict dryout phenomenon in wicks. The experimental work consists of investigations carried out by the author on a variety of different wicks. These wicks' construction and purposes are detailed. Measurements of their peak effective heat transfer coefficients are presented and used as a basis for determining the most effective geometries in terms of heat transfer. Furthermore, the physics and motivations behind their geometries are also detailed.The modeling proposed in this work was inspired by the work by Kovalev [18], which used a pore size distribution in order to determine the most probable pore size at a given position. The model distinguishes phases by choosing a "cutoff" pore size, above which all pores were assumed to be filled with vapor and below which they are filled with liquid. For a given wick thickness and working fluid, this 1-D model predicts a temperature difference across the wick for a given input heat flux as well as other thermophysical properties. The modeling detailed in this work is compared to experimental data collected on biporous evaporators at UCLA for validation. The correlation of thermophysical properties such as phase permeabilities and a volumetric heat transfer coefficient to the pore size distribution are also explained. The experimental and simulation based methods discussed in this thesis are used as a basis to optimize sintered particle, biporous evaporators. Using the information gained in this thesis research effort, a methodology by which one could optimize a biporous wick for particular applications is explained as well as suggestions of future work needed to extend the foundations laid in this effort