Numerical analysis of enhanced heat-mass transfer in evaporator for condensing dehumidification in solar greenhouses

Abstract: An evaporator is serving as a core heat exchange equipment in condensing dehumidification system. Its heat exchange performance has posed a significant impact on the control effect of the wet environment in the solar greenhouse. In this study, delta winglet vortex generators (VGs) were introduced into the evaporator in order to enhance the comprehensive heat-mass transfer, and a heat-mass transfer model was established in the plain plate fin-and-tube evaporator. The maximum relative errors of the average Nusselt number and the flow resistance coefficient of the proposed model for the grid system were within 5%, and the numerical calculation method was verified by the experimental data. In this paper, three-dimensional dynamic simulation of the dehumidification process on the air side of evaporators was mainly based on VOF (Volume of Fluid) model and unsteady state calculation method. The formation and distribution features of the condensate were characterized by pressure, temperature, velocity, vorticity and liquid volume fraction field distribution on the air-middle interface, while, the heat-mass transfer and enhancement effects of evaporators were also analyzed by using the characteristic parameters and evaluation indexes under different dehumidification conditions (the inlet air velocity, uin=1-4m/s, inlet air relative humidity, RHin=50%-80%). The results showed that the average Nusselt number and the flow resistance coefficient on the air side of the plain plate fin-and-tube evaporator with VGs increased significantly as the flow velocity increased, whereas, as the relative humidity increased, the average Nusselt number on the air side increased obviously, but the increase of the flow resistance coefficient was small. In the evaporator, the VGs can remarkably improve the heat transfer, but deteriorate the flow resistance, where the average Nusselt number and the flow resistance coefficient on the air side increased by an average of 25.45% and 51.70%, respectively. Moreover, the mass transfer on the air side of the evaporator with VGs was significantly enhanced with the increase of the inlet air velocity and air relative humidity. The VGS can increase the dehumidification mass by an average of 50.79%, thereby to improve the dehumidification capacity of the evaporator with VGs. The amount of condensate increased around the first to fourth row of tubes in sequence. The main condensate on the fin surfaces downstream of VGS was distributed along the outer boundary of the longitudinal vortexes, where the shapes of the boundary were stripe and ellipsoid that affected by VGS. The VGs can also effectively reduce the droplet size of the condensate and the shedding radius, thereby to improve the drainage capacity on air side. Furthermore, the VGs can generally increase the efficiency of fins with an average increase of 2.00%. Therefore, the enhanced heat and mass transfer that introduced by delta winglet vortex generators can significantly improve the comprehensive heat-mass transfer performances of evaporators with an average increase of 9.61% under most dehumidification conditions, except uin≤1 m/s and RHin≥70%, while the 2 m/s was recommended as the preferable inlet air velocity under RHin≥70%. This finding can provide a reference on the structural optimization of promising evaporators for condensing dehumidification systems in solar greenhouse.