Effect of Filling Ratios and Adiabatic Length on Thermal Efficiency of Long Heat Pipe Filled with R-134a

The effect of R-134a filling ratio and adiabatic length on thermal efficiency of the long heat pipe are investigated. An experimental heat pipe is fabricated from the straight copper tube having an inner diameter of 11.3 mm. The evaporator and condenser length are 100 mm. The R-134a used as the working fluids filled in the heat pipe was tested. Filling ratios of the working fluid to heat pipe volume at 10, 15 and 20% and input heat flux rates of 1.97 to 9.87 kW/m2 were carried out in order to determine the thermal efficiency and optimum condition. The inlet cooling water temperature was kept constant at 20̊C. The adiabatic lengths of heat pipe for these experiments were 300, 500 and 700 mm (adiabatic length ratios 0.60, 0.71 and 0.77). The results show that the filling ratios have more significant effect on the thermal efficiency than adiabatic length. The optimum filling ratio of 15% and 5.92 kW/m2 of heat flux are found to be favorable at shorter adiabatic length.