Heat transfer, void fraction and pressure drop during condensation inside inclined smooth and microfin tubes

Abstract

This paper presents experimental heat transfer and pressure drop measurements during the condensation of R134a at a mean saturation temperature of 40 °C inside a microfinned copper tube with an inner diameter of 8.92 mm and a helix angle of 14°. Experiments were conducted for mean qualities from 0.1 to 0.9 at different inclination angles ranging from −90° (vertically downwards) to +90° (vertically upwards), at mass fluxes of 200–600 kg/m2s. Heat transfer coefficients were calculated directly from measured data, while the frictional pressure drops were obtained from the experimental data using the Bhagwat and Ghajar void fraction correlation developed in 2014. Results were compared with those obtained from a similarly sized smooth tube having an inner diameter of 8.38 mm to obtain the relative microfin heat transfer enhancement and pressure penalty factors. For both tubes, it was found that the heat transfer coefficient increased significantly with the mean vapour quality and mass flux. For the microfin tube, the highest heat transfer coefficients were obtained at tube inclinations of between −15° and −5° (downward flows), while for the smooth tube, the highest heat transfer coefficients occurred between inclinations of between −30° and −15° (downward flows). The heat transfer enhancement factor for the microfin tube was between 0.98 and 2.38 depending on the inclination angle. For both tubes, it was found that higher frictional pressure drops occurred at higher mass fluxes. In most cases, higher vapour qualities produced higher frictional pressure drops depending on the flow pattern. The lowest frictional pressure drop occurred at either horizontal tube positions or vertical downward flow inclinations. Microfin pressure drop penalty factors ranged between approximately 0.8 and 4 depending on the mass flux, inclination and vapour quality.