Experimental study of heat transfer enhancement in a falling film of R21 on an array of horizontal tubes with MAO coating

Abstract

The paper presents investigation results on heat transfer in falling films of refrigerant R21 on an array of horizontal tubes with a coating deposited by the method of microarc oxidation (MAO). Different MAO-coatings were applied on the surface of tubes of aluminum alloys AD31 and D16T with a diameter of 10 mm in silicate-alkaline, acidic, and phosphate electrolytes. Two coated test tubes were copper-plated additionally in a copper sulfate solution. The enhancement of heat transfer on the tubes with a porous coating depends in a complex manner on the parameters of MAO-coating surfaces defined by their treatment regimes. The highest 3-fold heat transfer enhancement in comparison with plain tube in the nucleate boiling regime was achieved on the surface of a tube made of alloy D16T with the MAO coating of 30-μm thickness, obtained in a silicate-alkaline electrolyte. Additional deposition of copper particles on the MAO-coatings leads to an enhancement of heat transfer by (20–30) % compared to plain tube. In the studied range of Reynolds number 600–1500, the effect of irrigation density on the heat transfer coefficients is not observed.