Abstract:
Experimental data are reported for condensation of R134a in an 8.38 mm inner diameter smooth tube in
inclined orientations with a mass flux of 200 kg/m2 s. Under these conditions, the flow is stratified and
there is an optimum inclination angle, which leads to the highest heat transfer coefficient. There is a need
for a model to better understand and predict the flow behaviour. In this paper, the state of the art of existing
models of stratified two-phase flows in inclined tubes is presented, whereafter a new mechanistic
model is proposed. The liquid–vapour distribution in the tube is determined by taking into account
the gravitational and the capillary forces. The comparison between the experimental data and the model
prediction showed a good agreement in terms of heat transfer coefficients and pressure drops. The effect
of the interface curvature on the heat transfer coefficient has been quantified and has been found to be
significant. The optimum inclination angle is due to a balance between an increase of the void fraction
and an increase in the falling liquid film thickness when the tube is inclined downwards. The effect of
the mass flux and the vapour quality on the optimum inclination angle has also been studied.
