Falling film evaporation on horizontal tubes with smooth and structured surfaces

Abstract

<p>The present study includes the development of a model for falling film evaporation on a horizontal plain tube and extensive experimental tests for tubes with plain and commercial structured surfaces in a spray evaporator. The model defines heat transfer in three distinct regions: the jet impingement region, the thermal developing region, and the fully developed region. The heat transfer coefficient in the thermal developing region was estimated by considering one-dimensional transient heat conduction across the film. The developed heat transfer coefficient was calculated by solving Nusselt's problem for film evaporation. Compared with experimental data, the present model predicted data in good agreement with experiment;Heat transfer within a test cylinder heated by a cartridge heater was analyzed. Spray evaporation tests using electrically heated test sections with smooth, GEWA-T, Thermoexcel-E, and High Flux surfaces were conducted to investigate the effects of liquid supply mode, surface structure, surface aging, surface subcooling, heat flux, film flowrate, liquid feed height, and rate of heat flux change. Complementary pool boiling experiments were also conducted;The falling film evaporation provides heat transfer coefficients higher than the natural convection that characterizes pool boiling at low superheats. The falling film evaporation data for the structured surfaces merge with the respective pool boiling curves at high heat flux. High Flux and Thermoexcel-E surfaces are characterized by incipient boiling at low superheats and high boiling coefficients, realized by internal thin film evaporation. The first-stage nucleate boiling on the Thermoexcel-E surface before normal boiling is described for the first time. GEWA-T surfaces primarily enhance the convective heat transfer through extended surface and surface tension effects. In both falling film evaporation and pool boiling on Thermoexcel-E, a pre-dried surface presents higher normal heat transfer coefficients than those from a preboiled surface. Also, slowly increasing the heat flux results in normal coefficients higher than those obtained with step changes in power. Film flowrate and liquid feed height have small effects on non-boiling convection. The effects vanish when boiling is dominant. Also it was found that fouling is a serious problem with the High Flux porous surface when boiling water.</p>