Investigation on the thermal performance of flat-plate heat pipes with various working fluids under different inclination angles

Abstract

As one promising high-efficiency equipment, flat-plate heat pipes show an important influence on the thermal management of energy systems. This research experimentally investigates the effects of working fluids (acetone, ethanol, and 1.0 wt% Al2O3 water-based nanofluid), liquid filling ratios (30%, 45%, 60%, and 80%), and inclination angles (0°, 30°, 60°, and 90°) on thermal resistance and equivalent heat transfer coefficient of flat-plate heat pipes under input powers of 15 W, 30 W, 45 W, and 60 W. Results indicate that the flat-plate heat pipe with filling ratio of 60% has the minimum thermal resistance of 2.50 °C/W. Compared with filling ratios of 30%, 45%, and 80%, thermal resistance for 60% filling ratio decreases by 42.8%, 27.8%, and 50.7%. The thermal resistances for inclination angles of 0°, 30°, and 90° increase by 88.9%, 2.97%, and 11.8% compared to that with an inclination angle of 60°. The equivalent heat transfer coefficient of the flat-plate heat pipe increases by 90.8% when the inclination angle increases from 0° to 60° due to an effect of gravity. The heat transfer coefficient of the flat-plate heat pipe decreases by 7.2% when the inclination angle increases from 60° to 90°. The thermal resistance of the flat-plate heat pipe using Al2O3 nanofluid as the working medium decreases by 15.2% and 58.7% compared to those with acetone and ethanol. It is observed that partial dryout occurs when the input power is above 30 W. The evaporator with 5 cm length shows the best heat transfer performance of the flat-plate heat pipe.