The influence of the inclination angle on void fraction and heat transfer during condensation inside a smooth tube

Abstract

The void fraction of condensation flow is an important parameter in determining the heat transfer and pressure drop characteristics of two-phase flow. In the past, studies involving void fraction, as well as heat transfer, have focused on horizontal and vertical flows. The current study measured void fractions during condensation and determined heat transfer coefficients for a full range of tube inclinations ranging from vertical downwards to vertical upwards at a condensation temperature of 40 ᵒC. The void fractions were measured using capacitive void fraction sensors. The measurements were taken on an 8.38 mm inner diameter smooth tube with 200 W of induced condensation heat transfer at mass fluxes ranging from 100 – 400 kg/m2.s and vapour qualities ranging from 10 – 90%. Alongside the void fractions, heat transfer measurements were also obtained for the full range of tube inclinations and compared with the void fraction measurements. The experimental data set generated consisted of 340 data points. At each data point, void fraction and heat transfer coefficients were measured. It was found that at combinations of low mass fluxes and vapour qualities, the void fraction and heat transfer coefficients were significantly affected by changes in inclination angle. Maximum values of heat transfer tended to coincide with downward inclinations where void fraction values decreased at low mass fluxes. Minimum heat transfer values tended to coincide with minimum void fraction measurements at upward inclinations for low mass fluxes. Predictions with established heat transfer, void fraction and flow pattern methods were also considered. For inclinations other than horizontal, the results were predicted less satisfactorily for low mass flux and vapour quality conditions. At high mass flux and vapour quality conditions, the void fraction and heat transfer coefficients tended to be independent of the inclination angle. At some intermediate mass flux and vapour quality conditions, the void fraction and heat transfer coefficients were observed to be independent of the inclination angle despite significant changes in the prevailing flow patterns. A sensitivity analysis revealed that the void fraction measurements needed to be very accurate to enable their use for heat transfer prediction purposes.