The objective of this study is to experimentally and numerically determine the effect of powder sizes on the void fraction which subsequently affects the heat transfer characteristics of miniature sintered–wick heat pipe usually applied in the notebook computer. The powder size of the wick is a profound influence on the heat transfer performance as well as the economic aspect of the heat pipe. By using computer program to simulate the void fraction of the sintered wick made of two sizes of metal powder and accordingly the mixture fraction of such metal powder the optimum throughput heat transfer of the miniature heat pipe can be predicted. The inputs of the simulation program are size specification the thickness of copper powder and powder fraction. The heat transfer characteristics of the heat pipe will be determined by firstly obtaining the void fraction of such sintered wick. Then the heat transfer rate will be calculated from the void fraction. The optimum void fraction can be solved as the point at which the void fraction and heat rate are appropriate on the basis of economic consideration. In order to validate such calculation the experiments have been conducted with several kinds of metal powders. The wick employs the copper powder with range of diameter of 150 - 212 and 250 - 297 micron respectively by wick molding with sintering process. The miniature heat pipe with outside diameter of 6 mm and the total lengths of 200 mm was used. The wick thickness was fixed of 0.46 mm. Water was used as a working fluid. From the result the heat transfer performance at normal state of the heat pipe with powder fraction of 1 : 3 has found to be as high as 25 W which is quite closed to the predicted one.

This paper presents an establishment of a mathematical model to determine the optimal size of heat pipe (thermosyphon) heat exchanger for reduce the inlet air temperature in evaporative cooling system of greenhouse. The condition of a mathematical model is the total length of heat pipe is 1.5 m. A mathematical model will determine the optimal value of evaporator section and condenser section. The range of evaporator length is 0.5 – 1.4 m with an increment of 0.1 m and condenser length is in range of 0.1 – 1.0 m with an increment of 0.1 m. The inlet air temperature in evaporative cooling of greenhouse is depend on ambient temperature about 35 degree Celsius. While the cooling water temperature is 20 degree Celsius. Evaporative pad of evaporative cooling system is 2 m in width and 7 m in length. From the conditions above the optimize results from a mathematical model are as follows: evaporator section length is 0.9 m and condenser section length is 0.6 m moreover it was shown that investment net saving and maximum heat rate are 79.83 Thousand Baht 0.552 Million Baht and 15.6 kW respectively.