Simulation And Experimental Validation Of Condensing Droplet Behaviors On Fin Surfaces In Heat Exchangers Under Wet Conditions

Fin-and-tube heat exchangers are widely used in a variety of applications in the air-conditioning, refrigeration and process industries. The dominant thermal resistance for fin and tube heat exchangers is generally on the air side, and the key point for improving the performance of the fin-tube heat exchanger is to enhance the air side performance.Under the real operation conditions of the room air conditioner, the fluid on the air side of fin-tube heat exchanger is moist air, and water vapor in the moist air condenses onto the fin surface when the fin surface temperature is lower than the dew point temperature. The condensates form small water droplets on fin surface, and the small water droplets grow up to greater ones and move down under the coupling effect of gravity, surface tension and air flow. During the condensing process, the droplet formation, growth and movement on the fin surface may influence each other, resulting in the complicated influence of droplets on the heat and mass transfer characteristics of moist air. In order to quantitatively know the influence of the retained droplets and further to improve the air side performance of fin-and-tube heat exchanger, a model for simulating the condensing droplet behaviors on the fin surface is needed, including the droplet formation, growth and movement. The purpose of the present study is to propose a numerical model for simulating the simultaneous droplet behaviors(formation, growth and movement). Moreover, the influences of fin structure and hydrophilic coating on the droplet behaviors as well as on the heat and mass transfer characteristics are investgated numerically and the model validation is also performed.The numerical model for predicting the droplet movement characteristics was developed, and the influences of the fin surface characteristics and the air drag force on the droplet movement characteristics were investigated numerically. The droplet movement on the fin surface is governed by the coupling effect of gravitational force, air drag force and wall adhesion force, and the wall adhesion force is controlled by the contact angles along the triple contact line of droplet. The shapes as well as the contact angles of the droplets are varying with the gravitational force and air drag force. The deviation angle was proposed to evaluate the degree of droplet deformation, and the the contact angles were predicted based on the modification of existing model by reflecting the effect of the gravitational force and air drag force as the deviation angle. The continuity and momentum equations for two-phase flow on the fin surface were developed. The momentum source term caused by wall adhesion force, with the contact angles of droplet being specified, was employed in the control equations. And the influences of receding contact angles and air velocities were investigated based on the proposed model.The numerical model for predicting the droplet formation and growth characteristics was developed, and the influences of operation conditions on the heat and mass transfer characteristics during the droplet formation and growth processes were investigated numerically. In the model, the mass transfer rates during droplet formation and growth processes were introduced into the control equations as source terms. The mass transfer rate for droplet formation was calculated by the heterogeneous nucleation rate and critical nucleation radius of droplet, and the nucleation rate was obtained by the heterogeneous nucleation free energy with the plate fin surface as the nucleation substrate. The mass transfer rate for droplet growth was predicted based on the species conservation of water vapor on phase interface between the droplet and the moist air. And the influences of inlet air velocities and relative humidity on the heat and mass transfer characteristics were investigated based on the proposed model.The numerical model for predicting the influences of fin structures and surface modification on the condensing droplet behaviors was developed. The influences of fin structures including the wave and slit on the droplet shape and contact angles had been analyzed, and the calculation method of contact angles at the triple contact line of the droplet was developed based on the Young-Laplace equation. The momentum source term with the contact angles being specified was introduced into the control equation to reflect the influences of the wave and slit on the droplet movement characteristics. The influences of hydrophilic coating on the droplet formation and shape had been analyzed, and the calculation method of mass transfer rate for the droplet formation was developed based on the species conservation of water vapor. The mass source term for the mass transfer rate druring the droplet formation process was introduced into the control equation to reflect the influences of hydrophilic coating on the droplet formation characteristics.The visualization experiments were performed to validate the proposed models of condensing droplet behaviors on the fin surface under wet conditions. The validation results of droplet movement model have shown that, the average deviation between simulation results and experimental data for the contact angles of droplets with different volume is 1.2% and the maximum deviation is 1.8%; the average deviation for the velocity of droplet with fixed volume is 8.1% and the maximum deviation is 10.0%. The validation results of droplet formation and growth model have shown that, the predicted droplet behaviors obtained by the proposed model agree well with the visulization images of the condensing process captured from the front view of the fin surface; the predicted results of mass transfer j factor obtained by the model can describe 89% of the experimental data within the deviation limit of ± 25% and the mean deviation is 17.3%; the predicted results of heat transfer j factor can describe 94% of the experimental data within the deviation limit of ±20% and the mean deviation is 11.8%. The validation results indicate that the proposed models can predict the condensing droplet behaviors well.