Forced-convection condensation heat-transfer on horizontal integral-fin tubes including effects of liquid retention

Abstract

Accurate and repeatable heat-transfer data are reported for forced-convection filmwise condensation of steam and ethylene glycol flowing vertically downward over two single, horizontal instrumented integral-fin tubes and one plain tube. Vapour-side, heat-transfer coefficients were obtained by direct measurement of the tube wall temperature using specially manufactured, instrumented tubes with thermocouples embedded in the tube walls. Both tubes had fin height of 1.6 mm and fin root diameter of 12.7 mm, with fin thickness and spacing of 0.3 mm and 0.6 mm, respectively for the first tube and 0.5 mm and 1.0 mm respectively for the second. Tests were performed at atmospheric pressure for steam with nominal vapour velocities from 2.4 m/s to 10.5 m/s and at three pressures below atmospheric with nominal vapour velocities from 8.4 m/s to 57 m/s for steam and 13 m/s to 82 m/s for ethylene glycol. The data show that both the finned tubes provide an increase in heat flux at the same vapour-side temperature difference with increasing vapour velocity. Visual observations were made and photographs obtained of the condensate retention angle at each combination of vapour velocity and pressure. It was observed that the curvature of the meniscus was distorted by the increase in vapour velocity and in many cases, the extent of condensate flooding changed compared to its value in the quiescent vapour case. In parallel, experiments involving simulated condensation on finned tubes were conducted using horizontal finned tubes in a vertical wind tunnel. Condensate was simulated by liquid (water, ethylene glycol and R-113) supplied to the tube via small holes between the fins along the top of the tube. Downward air velocities up to 24 m/s were used and retention angles were determined from still photograph. Eight tubes with a diameter at the fin root of 12.7 mm were tested. Five tubes of which had fin height of 0.8 mm and spacing between fins of 0.5 mm, 0.75 mm, 1.0 mm, 1.25 mm and 1.5 mm and three tubes had fin height 1.6 mm with fin spacings 0.6 mm, 1.0 mm and 1.5 mm. The results were repeatable on different days and suggested, for all tubes and fluids, that the retention angle asymptotically approached a value around 80o to 85o (from either lower or higher values at zero vapour velocity) with increase in air velocity. Good agreement was found with observations taken during the condensation experiments.