Study of adiabatic length effect on the thermal performance of the wick-less straight heat pipe using RELAP5

The heat pipe is a device that can transfer heat very effectively and passively. Heat pipes are widely used as thermal management tools in various fields, including nuclear reactors. Although heat pipes can have various geometries and shapes, heat pipes consist of three parts: the evaporator, adiabatic, and condenser. The adiabatic will pass the working fluid in gas from the evaporator to the condenser and pass the working fluid in a liquid from the condenser to the evaporator. There are very limited studies on the effect of adiabatic length on thermal performance. Therefore, research has been carried out on the effect of adiabatic length for heat pipes that have a fixed evaporator and condenser. The research was conducted by numerical simulation on a straight wick-less heat pipe using the RELAP5 computer program. The heat pipe is made of a tube of 1-inch stainless steel, and the working fluid is water. The evaporator and condenser sections have the same length of 50 cm, while the adiabatic sections have variations of 10, 20, 30, 40, 50, 100, 150, and 200 cm. The heat pipe is placed on a heat source of atmospheric water at 80 °C as well as saturated steam with temperature variations from 80 to 120 °C. The heat sink is cooling water at a temperature of 29 °C. The study results show that the thermal performance of the heat pipe is proportional to the adiabatic length when the heat source temperature is much greater than 110 °C and vice versa when the heat source temperature is much lower than 110 °C. The adiabatic length does not affect the performance of the heat pipe when the heat source temperature is around 110°C. However, the increase or decrease in heat transfer capability is minimal, so it can be concluded that the adiabatic length has little effect on the thermal performance of the wick-less straight heat pipe.