Investigation On The Mechanism Of Steam Condensation Heat And Mass Transfer With Non-Condensable Gas On Vertical Plate

Condensation with non-condensable gas exists in many industries,such as air-conditioning applications,power plants,desalination,heat recovering.It is well known that non-condensable gas in vapor significantly decreases heat transfer rate.Therefore,a fundamental understanding of the characteristics and mechamism of condensation heat and mass transfer with non-condensable gases and discussing the influencing factors is instructive and necessary.A systemic and deep study on the mechamism of condensation heat transfer with the vapor-air mixture on a vertical wall was peformed through theoretical analysis,numerical simulation and experimental research.First of all,the condensation modes were analyzed and the droplet stability condition was built up based on gravity,shear force and surface tension.The stability of the condensate film subjected to the shear force was discussed.The gas phase velocity field and concentration gradient field in the diffusion layer were analyzed using the field synergy mechanism.The analysis of droplet stability conditions demonstrates that the droplet departure diameter on the vertical wall increases with the increase of the advancing contact angle or the decrease of the receding contact angle when the air-vapor mixture is still.When the advancing contact angle,receding contact angle and distance from the upper edge of the wall are predetermined,the departure diameter decreases with the increase of the mixture velocity.When advancing contact angle is constant,the departure diameter increases with the distance from the upper edge of the wall.The condensate morphology and dynamic behavior can be changed by changing the contact angle,mixture velocity and distance from the upper edge of the wall.The analysis of velocity field and concentration gradient field in the diffusion layer based on the principle of the field synergy mechanism shows that the best synergy of the velccity field and concentration gradient field formed by the normal velocity which is generated from the condensate dynamic behavior can reduce the thermal resistance in gas diffusion layer,which is the key to enhance the condensation heat transfer process with non-condensable gas.Secondly,the species transport model and VOF model were adopted to simulate and analyze the condensation heat transfer characteristics of the water vapor on the isothermal vertical wall with air in which the mass fraction is 5%-50%,and the law of the non-condensable gas concentration field,velocity field and local heat transfe coefficient distribution under such factors as wall temperature,air mass fraction and steam velocity were sudied.The results of numerical analysis show that the concentration of non-condensable gas rises sharply near the wall,and the concentration of non-condensable gas rises from zero and then drops to the mass fraction of non-condensable gas in the main body of mixed steam.The simulation results match well with the fact that the heat transfer process consisted of condensate film and gas diffusion layer.The normal velocity component on the wall increased with the increase of air mass fraction,which has an obvious enhancement effect on the heat transfer.The enhancement in heat transfer process slows down the decrease of heat transfer coefficient due to the increase of non-condensable gas mass fraction.The tangential velocity near the wall surface increases from zero to the mainstream speed,which has no obvious direct relation with the condensation heat transfer coefficient.It was proven that the increase of velocity component with small Angle between concentration gradient and temperature gradient is beneficial to the enhancement of mass transfer heat transfer process.Thirdly,based on the theory of diffusion layer,a generalized model of the condensation heat transfer with non-condensable gas on a vertical wall with simplifications and assumptions has been established to analyze the film waviness,suction effect and mixed convection.Theoretical calculation showed that the ratio of the thermal resistance of the diffusion layer to the total thermal resistance increased gradually with the increase of non-condensable gas,and the thermal resistance in diffusion layer plays an important role in the condensation heat transfer.The fluence of film waviness is considered by introducing the Reynolds number of film.The film waviness has great influence on the convection heat transfer coefficient of liquid film layer.With the increase of wa,The ratio of film convective heat transfer coefficient(hfilm)to the film convective heat transfer coefficient without considering the influence of film waviness increased from 1.22 to 1.53.With the increase of the mass fraction of the non-condensable gas,the influence of the waviness increases gradually.When the mass fraction of the non-condensable gas is low,its enhanced transfer effect is obvious.With the increase of the mass fraction of the non-condensable gas,the effect of the suction on the condensation heat transfer coefficient of the diffusion layer is gradually reduced.The mass transfer coefficient and mass transfer driving force were introduced into the relation between the mass transfer driving force and concentration difference to esbablish the relationship between the concentration gradient and condensate rate.Studies on the suction effect and mixed convection show that the continue condensation of water vapor near the interface results in advection and suction effects which accelerate the normal velocity;the mass transfer driving force produced by concentration difference enhances the convective heat and mass transfer in the diffusion layer.Lastly,a visual experimental system condensation with non-condendable gas was designed and constructed to investigate the law of the condensation modes and the growth of the droplet,and the influence of non-condensable gas mass fraction,surface cooling,air-vapor mixture pressure and inlet velocity on condensation heat transfer.A high-speed digital camera was employed to capture real-time images of water vapor condensation in non-condensable gas under different conditions in the process of condensation heat transfer.Based on the experimental data,the generalized condensation heat transfer coefficient correlation under mixed convection conditions was obstained with the least square method.The experimental results showed that the condensation modes varied with surface subcooling and air concentration and four condensation modes were observed in the experiments including drop,drop-streak,film,and streamlet.Under identical air concentrations,the condensation mode varied from the fine drop and film mode to the larger drop-streak mode.The heat transfer coefficient declined greatly under a lower air mass fraction(5-10%),while it declined slightly under a higher air mass fraction(30-50%),which matched well with the results of the simulation.With the increase of surface cooling,the process of the condensation was accelerated resulting in the increase of non-condensable gas mass fraction and subsequently of the thermal resistance in diffusion layer.When the increase of thermal resistance exceeds the dynamic increase caused by the increase of surface cooling,the condensation heat transfer coefficient decreased;otherwise,the heat transfer coefficient increased.The heat transfer coefficients increased with the increase of the pressue and inlet velocity.However,with the increase of the mass fraction of the non-condensable gas,the enhanced heat transfer effect caused by the increase of steam velocity was reduced.After a comparative verification,the calculation error between the experimental correlations and related literature correlations is within 20%,therefore,it can provide theoretical basis for the engineering design and development of condensation heat exchange equipment with non-condensable gas.