Fin Model was used to predict the saturated vapor temperature in the
trapezoid grooved heat pipe. This model not only discussed the numerical
analysis of heat transfer mechanism in the evaporator, it also developed the equations to relate the inventory of the working fluid and heat transfer coefficient of the evaporator. The result showed the inventory as well as the working fluid thermodynamic properties would affect the temperature of the evaporator. The working fluids were pure water, ethanol and ammonia. The error was within 13% through the comparison of the experimental data and the theoretical model.