Measurements and semi-empirical correlation for condensate retention on horizontal integral-fin tubes: Effect of vapour velocity

Abstract

Present study is based on simulated condensation using three fluids (water, ethylene glycol and R-141b) on integral-fin tubes, horizontally oriented in a vertical wind tunnel. Eight tubes of different dimensions were used with fixed fin root diameter and internal diameter of tubes which were kept 12.7 mm and 8.0 mm respectively. Fluid, from a tank mounted above the tube at considerable height flows through the small drilled holes in between the fins. Each tube was tested with a constant flow rate of fluid i.e. the flow rate was adjusted to a minimum value that was just sufficient to produce a smooth film of condensate. Air was used to simulate vapour with a variable velocity from 0 to 19 m/s. A hot wire anemometer was mounted just above the tube into the wind tunnel to measure the air velocity. Observation windows were available on both sides of test section to visualize the condensate retention on integral-fin tubes. Two digital cameras were mounted on both sides of test section to capture the photographs of retention angle on tubes. Condensate retention showed identical behavior on both sides of tube over the tested range of vapour Reynolds number. Retention angle is found to be a strong function of vapour Reynolds number, as retention angle increases with increasing vapour Reynolds number for fluids which have higher value of surface tension to density ratio, however, retention angle decreases with increasing vapour Reynolds number for fluids which have lower value of surface tension to density ratio (i.e. refrigerants). A semi-empirical correlation accounting for the effect of vapour velocity on retention angle is developed that predicts most of the present and earlier experimental data to within ±15% (for water and ethylene glycol) and to within ±5% (for refrigerants).