Experimental investigation on R134a free convection condensation outside the enhanced tubes with different structural micro-fins

Abstract

The free convection condensation heat transfer of R134a outside the horizontal copper tubes with micro-fin under heat fluxes varying between 4.2 and 60.2 kW m² and saturation temperatures of 35 °C, 40 °C and 45 °C was investigated experimentally in the paper. The difference in four micro-fin tube structure is mainly outer diameter, fin structure, fin number, fin height, etc. The experimental result shows an increase in the heat transfer coefficient is observed with a decrease in the heat transfer temperature difference and saturation temperature, under the same application requirement, the heat transfer temperature difference of smooth tube is close to 3.68∼10.83 times that of enhanced tubes. Meanwhile, refrigerant thermal resistance ratio surpassing 0.5 implies that, in the specific experimental conditions, refrigerant thermal resistance consistently serves as the predominant resistance. Most correlations always underestimate the heat transfer characteristics with a deviation of greater than 26.8 % through the comparative analysis between experimental data and theoretical calculation. Apart from Sreepathi et al. and Kumar et al. correlations, moreover, the prediction effect of three-dimensional micro-fin enhanced tube is inferior to that of two-dimensional micro-fin enhanced tube for the selected correlations. Therefore, a fresh correlation is introduced, incorporating insights from the comparison results presented and the relevant experimental data, it can predict R134a heat transfer coefficient with ±1.0 % mean deviation.