HEAT TRANSFER ENHANCEMENT DURING DOWNWARD LAMINAR FLOW CONDENSATION OF R134A IN VERTICAL SMOOTH AND MICROFIN TUBES

Abstract

This paper presents an experimental comparison of the laminar film condensation heat transfer coefficients of R134a in vertical smooth and micro-fin tubes having inner diameters of 7 mm and lengths of 500 mm. Condensation experiments were performed at a mass flux of 29 kg m(-2) s(-1). The pressures were between 0.8 and 0.9 MPa. The original smooth tube heat transfer model was modified by a well-known friction factor to account for the heat transfer enhancement effects due to the presence of micro-fins on the internal wall surface during annular flow regime conditions. Alterations of the local heat transfer coefficient, and condensation rate along the tube length during downward condensing film were determined, considering the effects of the temperature difference between the saturation temperature and the inner wall temperature of the test tubes, and the condensing temperature on these items. The results show that the interfacial shear stress is found to have significance for the laminar condensation heat transfer of R134a under the given conditions due to its better predictive performance than the classical solution neglecting the interfacial shear stress effect. A comparison of the condensation heat transfer coefficients was also done according to the condensing pressures. New empirical correlations of the condensation heat transfer coefficient belonging to the test tubes are proposed for practical applications.