An experimental and analytical investigation of the transient characteristics of flat plate heat pipes

Analytical models for predicting the transient performance of a flat plate heat pipe for both the startup and shutdown operations are developed in the present work. These models can be utilized for both startup and shutdown operations. The presented models can simulate the thermal performance of a flat plate heat pipe in cyclical startup and shutdown operations as well as other practical operation scenarios. The results indicate that the thermal diffusivity and the thickness of the wall and the wick dominate the penetration time. Increasing the effective thermal diffusivity would decrease the penetration time. The results show that the heat transfer coefficient has a substantial effect on the heat pipe time constant. The results also show that the time constant for the startup operation is very close to that for the shutdown operation. Furthermore, the time for a specified flat plate heat pipe to reach steady state depends on the heat transfer coefficient and is not affected by the magnitude of the input heat flux level. Finally, it is found that the temperature difference within the heat pipe walls is small and that the wick in the evaporator section creates the largest thermal resistance while the wick in the condenser section has a significant contribution to the total thermal resistance.; This work also presents an experimental investigation of the thermal performance of a flat plate heat pipe during startup and shutdown operations. The effect of input power and the heat transfer coefficient on the thermal performance of the heat pipe is investigated. It is found that the maximum temperature rise increases linearly with input heat flux. For smaller values of the convective heat transfer coefficient, increasing heat transfer coefficient results in a decrease in the maximum temperature rise, while for larger heat transfer coefficients, increasing heat transfer coefficient would have a relatively insignificant effect on the maximum temperature rise. It is also found that the maximum temperature difference within the heat pipe mainly depends on the power input while variations in the heat transfer coefficient does not have a significant effect on it. The heat transfer coefficient strongly affects the time it takes to reach steady state while input power has only slight effect on it. Empirical correlations for the maximum temperature rise, the maximum temperature difference, and heat pipe time constant are obtained.; The analytical results for the maximum outside surface temperature rise, maximum temperature difference, heat flux, and time constants are compared with the experimental results and are found to be in very good agreement.