Heat transfer characteristics of obliquely dispensed evaporating falling films on an elliptic tube

Abstract

This study examines the evaporation heat transfer characteristics of obliquely dispensed falling films over a horizontal elliptic tube. The theoretical model applied the volume-of-fluid (VOF) method to simulate the spreading development of falling liquid films for resolving the distributions of velocity, temperature, volume fraction in conjunction with the convective heat transfer coefficient. Specifically, a user defined function (UDF) was formulated to treat the evaporative effect at the liquid film surface to the environment. The predicted heat transfer coefficients and film thickness distributions around the circular and elliptic tube surfaces were validated by the experimental data from the available literatures. Based on the test results via comparing different flow models, the SST k-ω model provides the best capabilities in terms of the better prediction accuracy and less calculation time. CFD simulations were then extended using the validated computational model to explore the effects of inlet liquid mass flow rate and obliquely dispensed angle on the liquid film distribution and heat transfer characteristics over the horizontal elliptic tube. At a low liquid flow rate of 0.093 kg/m-s, the liquid films failed to fully enfold the tube surface with the appearance of dry areas, causing the substantial decline of heat transfer effectiveness. The condition at a high liquid flow rate of 0.186 kg/m-s achieved the average heat transfer coefficient up to 4.18 kW/m2 K, indicating good thermal performance similar to literature reports. Reasonable average heat transfer coefficients can be attained for the variation of dispensed angle within 30°.