Combined effects of filling ratio and wick surface coating on thermal performance of cylindrical heat pipes

Abstract

Growing technological developments to improve the thermal efficiency of heat pipes resulted in many innovative techniques. The use of nanofluids with superior thermophysical properties compared to conventional fluids is one such technique. However, the service life of nanofluid driven heat pipes is affected by agglomeration and stability issues, demanding an alternate solution. Boiling heat transfer rate of a heat pipe depends on the availability of working fluid at the evaporator, which is decided by the wick wettability and filling ratio. Wick wettability is improved with a hydrophilic coating using sol-gel dip coating process. Coating improves wick capillarity at the expense of wick permeability. Hence, an accurate balance of wick coating thickness and evaporator filling ratio are essential for the enhanced performance. The present work aims to demonstrate the benefits derived from optimal wick coating over these issues posed by nanofluids and to provide a better understanding on device level thermal performance. Thermal performance of coated mesh wick cylindrical heat pipe is experimentally studied for various coating thicknesses, evaporator filling ratios and inclinations. The existence of an optimum evaporator filling ratio for each coating thickness in boosting the limits of heat transfer is established. For the optimized values of inclination, coating thickness and evaporator filling ratio, a maximum reduction of 38.49% in thermal resistance and an improvement of 25% in thermal efficiency are obtained at 150 W heat input. The results from the repeatability test confirms coating stability.