| Vapor condensation heat transfer at low pressure is widely used in the industrial applications,such as power generation,petro industry and heat pipe.It is of great importance to enhance the condensation heat transfer performance for efficient use of energy.However,the decrease of vapor pressure leads to the lift of thermal resistance of liquid-vapor interface and the hysteresis of droplet departure,and thus greatly degrades the heat transfer performance as the total thermal resistance increases.Furthermore,with the increase of the content of noncondensable gas,the mass transfer resistance at the liquid-vapor interface increases and the dominant thermal resistance changes from the liquid phase to the liquid-vapor interface,which also degrades the heat transfer performance.So,the key strategy to enhance the condensation heat transfer performance is the enhancement of mass transfer in the vapor phase and the decrease of the thermal resistance of vapor-liquid interface.For the enhancement of pure steam condensation heat transfer at low pressure,the hydrophobic surface and hydrophobichydrophilic hybrid surface are adopted to enhance mass transfer in the vapor phase by the regulation of droplet dynamic behavior and increase the frequency of droplet departure based on the solid-liquid interfacial effect.For the enhancement of heat transfer of mixed vapor condensation with large content of noncondensable gas,new annular fin structure is proposed for the optimization of the flow field of vapor phase,which enhances mass transfer in the vapor phase.The main research contents and results are as follows:(1)Complete hydrophobic tubes and hydrophobic-hydrophilic hybrid tubes were prepared using PFA coating in order to manipulate the droplet behavior to enhance the mass transfer at the vapor-liquid interface.An experimental system was designed and set up to conduct the condensation experiments on horizontally oriented tube.The influence of condensate dynamics on the condensation heat transfer is analyzed.The results show that under low steam pressure,the condensation on the bare copper-nickel alloy tube is filmwise.With the decrease of steam pressure and increase of surface subcooling,the hysteresis and film thickness of condensate increases.The fluctuation of vapor-liquid interface decreases and thus the decrease of the driving force of mass transfer and degradation of heat transfer performance.On the PFA-coated hydrophobic tube,dropwise condensation is achieved.Comparing to the condensation on hydrophilic surface,the droplet sweeping-caused disturbance of vapor is intensified.The vaporliquid thermal resistance decreases and thus leads to the enhancement of condensation heat transfer.On the hydrophobic-hydrophilic hybrid tube,dropwise and filmwise condensation coexist.Droplets can be self-propelled from the hydrophobic zone to the hydrophilic zone.The suction effect of droplets on one hand accelerates the liquid removal and surface renewal,on the other hand the instantaneous vacuum enhances vapor mixing,and thus further enhances the condensation heat transfer performance compared to the hydrophobic surface.(2)Based on the mechanism of vortex generators(VGs)on the fin surface,the arrangement of triangular VGs on fin surfaces are designed and optimized.This proposed configuration is proposed to introduce the fluid into the near-wake of the tube without evoking extra vortexes so that the stagnant wake can be attenuated,which is believed vital to the mass and heat transfer in mixed vapor condensation with large non-condensable gas concentration for finned tube exchanger.The influence of the design on the fluid flow and heat transfer characteristics is investigated numerically with the k-epsilon model.It is found that wake regions behind the circular tubes with VGs are diminished and the mixing between hot and cold fluid becomes more intensive.In the presence of VGs,the secondary flow is remarkably enhanced compared to the cases without VGs,resulting in larger number of vortices.Furthermore,VGs also play an important role in intensifying the turbulence and reducing the boundary layer.The value of Se and Nu in the case with VGs witnesses an increment by 45.8%-57.9% and 37.7%-42.2% compared to those without VGs,respectively.The results also show that the heat transfer performance is enhanced by 8.2%-77% as the aspect ratio of the triangular vortex generator decreases from 3 to 1. |
