Simulation Of Heat And Mass Transfer For Dehumidifying Of Moist Air Flow Over The Cold Fin Surface

Dehumidifying process is very common in the evaporating conditions of finnedtube heat exchangers which are widely used. So it is very significance to study the heatand mass transfer characteristic of fin and tube heat exchanger for fin designing andoptimization and improvement of the air side performance. On one hand, the waterretention has a great impact on the pressure drop and heat transfer of the air side; on theother hand, the water retention influence the mass transfer because the mass transfermechanism is totally different, that depends on whether the mass transfer happens onwater droplet surface or not. But up to now, there is no further study on the influence ofthe water retention on heat and mass transfer characteristic. In order to study themechanism of air side heat and mass transfer and clear the influence of water retentionon heat and mass transfer characteristic, a numerical model considering water retentionhas be established.This thesis has simulated the heat and mass transfer characteristic for fin-tube heatexchanger considering water retention and studied the air side parameters. The mainworks and conclusions are reviewed as follows:(1) A water droplet moving model was established based on force balance analysis,and in order to validate the accuracy of the model, an experiment setup was designedand built. The minimum contact angle and velocity of water droplet moving is capturedin the experiment, which compared with simulation results. The maximum deviationsare within±15%for the values of both minimum contact angle and velocity of waterdroplet moving, which show that the accuracy of the model.(2) The numerical simulation model had also been used to analyze the relations ofcritical water droplets volume and fin surface modification, which shows that thecritical water droplets volume varies only with the maximum contact angle andminimum contact angle. It is concluded that the critical volume increased with theadvancing contact Angle increased, or with the receding contact Angle decreased.(3) Based on the water droplet motion model, a numerical model is established forsimulating the simultaneous heat and mass transfer in the fin-and-tube heat exchangersunder dehumidifying conditions. Not only the new model has accounted for the masstransfer between the water vapor in the moist air and the condensed water liquid butalso the direct condensation model of water vapor is provided which brings thenumerical simulation model a step closer to the actual simultaneous heat and masstransfer in the wet conditions. The comparisons between the numerical simulation results and the existing experimental data in the open literatures have shown that thepredictions of convective heat transfer coefficients and mass transfer coefficients formoist air given by numerical model agree well with the experimental data. Themaximum deviations are within±15%.(4) The numerical simulation model based on water retention had been used toanalyze the relations of three different fin efficiencies under dehumidifying conditionsas well as the effects of fin pitch, tube rows, inlet air velocity and relative humidity.Moreover, the simultaneous heat and mass transfer analogy had been studied based onthe newly developed numerical model. The key parameters that will affect the Lewisfactor have been identified and the Lewis factor has been fitted into a correlation usingthe inlet relative humidity and Reynolds number as the variables according to thenumerical simulation results.