EXPERIMENTAL INVESTIGATION OF LOW HEAT FLUX WICKED HEAT PIPES PERFORMANCE WITH DIFFERENT WORKING FLUIDS AND TILT ANGLES

This experimental investigation focused on wicked heat pipe under different operating conditions. A single-layer 150-mesh wicked heat pipe was used in the experiments. The heat pipes were made of copper tubes with a diameter of 15 mm and a total length of 450 mm. The used working fluids were pure water, methanol, ethanol, acetone, and R134a. The working fluid charge rate was 50% of the evaporator volume. The power input from the evaporator surface was selected from five different values between 1.12 kW/m<sup>2</sup> and 5.58 kW/m<sup>2</sup>, which can be considered as relatively low. The heat carried by the heat pipe was transferred to the water circulating around the condenser section. The heat pipes were operated at inclination angles of 0°, 30°, 60°, and 90°, and the performance was assessed based on the thermal input power and tilt angle. Over the range studied, the best performance in terms of heat pipe surface temperature distributions was achieved with R134a. Heat pipe performance was improved with increasing tilt angle. Increasing the tilt angle also increased the boiling heat transfer coefficient. The gradual increase in heat flux was observed to result in an average increase of 10-15°C in surface temperatures in the evaporator section compared to the previous stage. The heat transfer coefficient by boiling increased with the increase in the tilt angle and heat input power. This increase was 67.54% for R134a at a heat input of 1.12 kW/m<sup>2</sup> and a tilt angle of 90°. The increase amounted to 214.43% for 5.58 kW/m<sup>2</sup> heat input.