Heat transfer and critical heat flux on a modified surface during boiling under conditions of natural convection

Heat transfer and critical heat flux on flat rectangular (16 x 24 mm) heat-releasing surfaces (HS) made of aluminum alloy D16 and modified by microarc oxidation (MAO) were investigated. Two types of coatings made using different electrolytes were studied: KOH and phosphates. Coatings had good adhesion to the HS and high dielectric strength. The thickness of coatings was 20 and 7 mu m, respectively. The studies were carried out using freon R21 under conditions of natural convection with horizontal orientation of the HS. For comparison, measurements were carried out on identical test sections without a modifying coating: the first section was polished; the second one was ground with a roughness of 20 mu m. It was shown that the heat transfer intensity on the modified HS was not lower than that on the ground HS with a roughness of 20 mu m, and it was almost twice as high as that on the polished HS. The values of the critical heat flux on the studied surfaces were almost the same. The use of these types of coatings provides electrical insulation of the heat-releasing element without compromising the efficiency of heat removal.