Analysis of heat transfer performance of cross-flow fin-tube heat exchangers under dry and wet conditions

Abstract

A three-dimensional analysis procedure for the detailed phenomenon in a fin-tube heat exchanger has been developed and applied to predict the heat/mass transfer characteristics of the wave-fin heat exchangers. The continuity, Navier-Stokes and energy equations together with the species equation for the air-vapor mixture are solved in a coupled manner, so that the inter-dependence between the temperature and the humidity can be properly taken into account, by using the SIMPLE-type finite volume method. The numerical procedure has been thoroughly validated against the measured data for a wave-fin and tube heat exchanger under both dry and wet conditions. The calculations have been carried out for two fin configurations, i.e., wave fin and slit fin, for various frontal velocity and humidity conditions. The detailed heat/mass-transfer characteristics are captured successfully. These are seen to be closely related to the flow filed. For the given fan power, the optimal number of rows of the heat exchanger is identified. A series of calculation reveals that the existing correlations in the literature considerably underestimate the fin efficiency especially for multi-row heat exchangers. The inaccuracy in the fin efficiency approximation leads to noticeable error in estimating the heat-transfer coefficient in the data-reduction process in the experimental studies. An improved formula is proposed in the thesis by taking into account of the fin pitch, frontal velocity, and the number of rows, which have not been considered before. The proposed equation is found to estimate 94 % of the computed fin efficiency of plate fins within ±2 % and 100 % within ±5 % in the range of practical operating conditions and fin geometries. As reported in earlier studies, the fin efficiency is nearly independent of relative humidity at the inlet if the humidity is higher than 35 %. In practical applications, such as air-conditioning or refrigeration, the present method can be useful in det...