Analytical and experimental study of thin film evaporation in heat pipes

As the heat flux increases, the thin film evaporation occurring in the evaporator plays a key role in a heat pipe. A better understanding of heat transfer characteristics in the evaporating thin film region will lead to optimizing the thin film region and enhancing the evaporating heat transfer in the heat pipe. An analytical model describing thin film evaporation is developed including the effects of frictional shear stress, surface tension, curvature variation in the thin film interface and disjoining pressure. A unique experimental system was constructed to investigate the effect of heat flux on film thickness in the thin film evaporation region. It has been found that the thin film evaporation is highly affected with the superheat, operation temperature, liquid type, disjoining pressure and thermal conductivity of the liquid. The prediction shows that the interface temperature is not constant at the liquid-vapor interface in the evaporating thin film region. Increasing the thermal conductivity of working fluid leads to directly increasing the heat flux through the evaporating thin film region. The film thickness of interline region was first measured using the experimental setup established in the current investigation. As the input power increases, the film thickness at interline decreases, which is similar to the theoretical prediction. Using this new information on thin film evaporation, a mathematical model predicting the temperature drops occurring in a grooved heat pipe was developed.