DEVELOPMENT OF PLASMA SPRAY-COATED TUBES FOR COMPACT EVAPORATOR

Heat transfer enhancement is required to achieve compactness and reduced temperature approach in heat exchangers. In the present study, heat transfer surfaces coated by radio frequency plasma-spraying have been examined with respect to their boiling heat transfer characteristics. Several different coatings from copper and Inconel 625 have been applied to the outside of copper tubes with 18 mm OD. The heat transfer performance of the horizontal tubes was evaluated in pool boiling experiments with R134a for heat fluxes between 1.0-100 kW/m² and reduced pressures from 0.032 to 0.141, which corresponds to saturation temperatures from -20 to +20°C. Significant heat transfer enhancement in comparison to a smooth tube surface was obtained over the full range of experimental conditions. For heat fluxes below 10 kW/m², the Inconel coatings produced higher enhancement ratios than the copper coatings, despite their lower thermal conductivity of 9.8 W/(m·K) in comparison to 400 W/(m·K). The highest enhancement ratio of 23.5 was obtained at a saturation temperature of 20 °C and a heat flux of 1.6 kW/m². For higher heat fluxes, the heat transfer enhancement for the Inconel coatings decreased due to the thermal resistance of the coating layer, while for the copper coatings a constant enhancement was observed.