MODELLING OF LAMINAR FILM REFLUX CONDENSATION IN AN INCLINED SMALL DIAMETER TUBE AND EXPERIMENTAL VERIFICATION

Abstract

A physical model for reflux condensation of pure saturated vapours in an inclined small diameter tube is presented. The condensate drains due to gravity as a smooth, laminar film tangentially to the tube wall. In the inclined tube the condensate accumulates in the lowermost part of the tube and flows gravity-controlled to the bottom end of the tube. Starting from a balance of forces for a differential control volume of the condensate film a differential equation for the local film thickness is derived. This differential equation is solved numerically by finite differences. When the local film thickness is known, the local heat transfer coefficient can be determined. The mean condensation heat transfer coefficient for the entire surface of the tube is calculated by integration of the local values. The results for the film thickness and the heat transfer are compared with experimental data obtained dining reflux condensation of refrigerant R134a in a tube with an inner diameter of 7 mm and a length of 500 mm. The film thickness measurements have been carried out with the ultrasonic pulse-echo technique at the lowermost point of the tube. It is found that the calculated values for the film thickness and the mean heat transfer coefficient are in good agreement with the experimental data. The deviation is less than 15%. The model predicts well the condensation process in the inclined tube.