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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.

The evaporating thin film region is an extended meniscus beyond the apparent contact line at a liquid/solid interface. Thin film evaporation plays a key role in a highly efficient heat pipe. A detailed mathematical model predicting fluid flow and heat transfer through the thin film region is developed. The model considers the effects of inertial force, disjoining pressure, surface tension, and curvature. Utilizing the order analysis, the model is simplified and can be numerically solved for the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate in the evaporating thin film region. The prediction shows that while the inertial force can affect the thin film profile, interfacial temperature, meniscus radius, heat flux distribution, velocity distribution, and mass flow rate, in particular, near the non-evaporating region, the effect can be neglected. It is found that a maximum velocity, a maximum heat flux, and a maximum curvature exist for a given superheat, but the locations for these maximum values are different.friction factor, f ¼ 4s w 1 2 q l u 2 h lv latent heat of vaporization (J/kg) k thermal conductivity (W/m K) K curvature (m -1 ) _ m mass flow rate (kg/s) p pressure (N/m 2 ) P R reference pressure (N/ m) 2 q heat transfer (W) q¢¢ heat flux (W/ m 2 ) Re Reynolds number, Re ¼ q l ud l l t time (s) T temperature (K) u velocity in the x-direction (m/s) " u mean velocity in the x-direction (m/s) v velocity in the y-direction (m/s) x coordinate (m) y coordinate (m) d film thickness (m) d 0 non-evaporating film thickness (m) l viscosity (N s/m 2 ) q density (kg/m 3 ) r surface tension (N/m) s shear stress (N/ m 2 )

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)

An extensive numerical analysis of the temperature distribution and fluid flow in a heat sink currently being used for cooling desktop computers was conducted, and demonstrated that if the base of a heat sink was fabricated as a heat pipe instead of a solid material, the heat transfer performance could be significantly increased. It was shown that as the heat sink length increases, the effect of the thermal conductivity of the base on the heat transfer performance increases to be a predictable limit. As the thermal conductivity is increased, the heat transfer performance of heat sinks is enhanced, but cannot exceed this limit. When the thermal conductivity increases to 2,370 W/m-K, the heat transfer performance of the heat sinks will be very close to the heat transfer performance obtained assuming a base with infinite thermal conductivity. Further increases in the thermal conductivity would not significantly improve the heat transfer performance of the heat sinks.