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

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 degrees, 30 degrees, 60 degrees, and 90 degrees) 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 degrees 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 degrees, 30 degrees, and 90 degrees increase by 88.9%, 2.97%, and 11.8% compared to that with an inclination angle of 60 degrees. The equivalent heat transfer coefficient of the flat-plate heat pipe increases by 90.8% when the inclination angle increases from 0 degrees to 60 degrees 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 degrees to 90 degrees. 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.