Enhancement of capillary and thermal performance of grooved copper heat pipe by gradient wettability surface

The flow of working liquid from condensing section to evaporative section, driven by capillary pressure in grooved microchannels, plays a dominate role in heat transfer for a grooved heat pipe. To enhance the capillary flow and thermal performance, gradient wettability surfaces inside grooved heat pipes were fabricated by the alkali assistant oxidation technique. The tests of thermal performance demonstrated a maximum decrease of 92.6% in thermal resistance by fabricating a wettability gradient (CA = 20 degrees-85 degrees) in the adiabatic section inner surface of a grooved heat pipe compared with the untreated copper heat pipe. A mathematical model with nth power function of wetting gradient profile was proposed to predict quantitatively the liquid flow velocity in rectangular microchannels inside the grooved heat pipe driven by capillary force and an extra driving force due to the surface wettability gradient. Both experimental and theoretical results show that the relative error between the predicted increase of flow velocity and experimental enhancement of thermal performance due to the gradient wettability in grooved heat pipes is less than 17.5%, indicating rationality of the proposed model. The model prediction further demonstrates an optimal n order within 0.5-1 for the power function of the wetting gradient profiles. (C) 2016 Published by Elsevier Ltd.