A two-dimensional model is presented to predict the overall heat transfer capability for a sintered wick structure. The model considers the absence of bulk fluid at the top surface of the wick, heat conduction resistance through the wick, capillary limitation, and the onset of nucleate boiling. The numerical results show that thin film evaporation occurring only at the top surface of a wick plays an important role in the enhancement of evaporating heat transfer and depends on the thin film evaporation, the particle size, the porosity, and the wick structure thickness. By decreasing the average particle radius, the evaporation heat transfer coefficient can be enhanced. Additionally, there exists an optimum characteristic thickness for maximum heat removal. The maximum superheat allowable for thin film evaporation at the top surface of a wick is presented to be a function of the particle radius, wick porosity, wick structure thickness, and effective thermal conductivity. In order to verify the theoretical analysis, an experimental system was established, and a comparison with the theoretical prediction conducted. Results of the investigation will assist in optimizing the heat transfer performance of sintered porous media in heat pipes and better understanding of thin film evaporation.

A mathematical model for predicting the oscillating motion in a pulsating heat pipe is presented. The model considers the thermal energy from the temperature difference between the evaporator and condenser as the driving force for the oscillating motion, which will overcome both the frictional force and the force due to the deformation of compressible bubbles. The results show that the oscillating motion depends on the temperature difference between the condensing section and evaporating section, the working fluid, the operating temperature, the dimensions, and the filled liquid ratio. The results of this investigation will assist in the development of miniature pulsating heat pipes capable of operating at increased power levels. List of symbols Across-sectional area (m 2 ) D diameter (m) f friction factor, dimensionless g gravitational acceleration (m/s 2 ) h fg latent heat of vaporization (kJ/kg)Greek alphabets l viscosity (N s/m 2 ) q density (kg/m 3 ) s time (s) s s shear stress (N/m 2 ) U filled liquid ratio, i.e., the liquid volume divided by the total volume x frequency (rad/s)

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