Fluted Helix Channel Coupling Effect Between Heat And Mass Transfer Of Rising Film Evaporation Along A Horizontal Tube

Liquid rising film can uniformly cover the surface of the fluted helix horizontal tube due to capillary-induced rewetting along with evaporation. Evaporation on the surface of the liquid film has a great influence on heat transfer of the liquid film. The main purpose of the present thesis is to study the coupling effect between heat and mass transfer on the surface of the liquid rising thin film evaporation.The liquid film temperature, moist air boundary layer temperature and vapor concentration profile were numerically computed. The formation process of the rising film formation on the fluted helix surface of a horizontal tube was experimentally investigated. The heat and mass transfer of the rising film evaporation are firstly coupled through the interface evaporation of the liquid rising film. The circumferential temperature of the tube wall was measured by 0.5mm armored thermocouples laid in the wall of the tube. Applying the tube wall temperature to the numerical computation, the liquid thin film temperature can be calculated. A numerical model of moist air boundary layer on the surface of the liquid film was established to obtain velocity, temperature and concentration profile. Experimental investigation on rising film evaporation heat transfer on the fluted helix surface of a horizontal tube is obtained. The circumferential film thickness, solution concentration and heat transfer coefficient were experimentally obtained and compared with theoretical results. Comparing the rising film heat transfer of water with NaCl solution as working fluid can provide practical support for the project applications.Research results indicate that the liquid film thickness and the temperature of the liquid rising film nearly replicate the results from numerical solution; due to capillary-induced rewetting, a fluted helix tube with small pitches is better for the formation of the liquid rising film and can enhanced heat transfer performance; NaCl solution can form the film easier and heat transfer performance of NaCl solution is better than that of water; the heat transfer coefficient increases with the rising of the solution concentration. Numerical results indicate that the moist air boundary layer thickness on the surface of the liquid film increases along the circumferential direction due to the effect of entrainment; when laminar flow, the heat transfer coefficient of boundary layer decreases with the rising of the boundary layer thickness; mass rate of evaporation increases on the surface of the liquid film along the circumferential direction because the liquid film surface temperature increases and the film thickness decreases; when the relative humidity of the air increases, evaporation of the liquid film is inhibited and then the heat transfer performance reduces. This study opens the way for enhancing heat and mass transfer of the liquid thin film.