Numerical Study On The Condensation Characteristics Of Steam With Non-condensable Gas On Different Vertical Profiled Surfaces

The phenomenon of steam condensation with non-condensable gas is widespread in various heat exchangers of several industrial processes.For the steam condensation processes,the presence of non-condensable gas will significantly reduce the heat transfer efficiency.However,the effect of non-condensable gas on steam condensation is not yet fully understood,and with different non-condensable gas contents of the condensation systems,the heat and mass transfer laws of the condensation process are different.In this paper,the numerical calculation method was used to calculate the steam condensation process outside different vertical profiled surfaces when the content of the non-condensable gas is 0.05～0.45,the established wall condensation model was compiled by UDF in Fluent software.The main results obtained are as follows:In this paper,combined with the Wall Condensation Model(WCM)and the Volume of Fluid(VOF)Model,a numerical model suitable for calculating the condensation process of steam with non-condensate gas on the surface of different vertical structures with the consideration of liquid film thermal resistance is established,and the change of non-condensable gas layer and liquid film layer in the condensation process can be calculated at the same time during the condensation process.Then,the vertical plate was selected as the research object,the condensation heat transfer coefficients calculated by established condensation model and both the experimental and theoretical calculation results under the same conditions obtained in literature were compared.The overall deviation was less than 20%,which proves that this established model is reliable.Firstly,the established wall condensation model was used to calculate the condensation process of the steam with non-condensation gas under different working conditions outside the vertical wall.It was found that the high-concentration non-condensable gas layer formed on the surface of the liquid film in the early stage of condensation process will gradually diffuse back to the mainstream under the effect of the concentration gradient.Eventually the concentration of non-condensable gas layer tends to be stable.The liquid film formed on the wall surface will show various dynamic behaviors such as fluctuation,sliding and shedding during the condensation process,which will affect the mass transfer on the phase interface.When the bulk concentration of non-condensable gas is below 0.15,the ratio of condensation heat transfer to total heat transfer on surface is significantly higher than 90%,and the ratio of liquid film thermal resistance to total thermal resistance is above 20%.This ratio is greatly affected by the thickness of the liquid film,thus ignoring the thermal resistance of the condensate film will bring a large error to the calculation results.Secondly,based on the calculated results of condensation process on vertical plate,the established condensation model was used to numerically simulate the steam condensation process on the arc-shaped corrugated plate with non-condensable gas.It was found that with the increase of the mixed gas flow rate,the originally uniformly distributed high-concentration non-condensable gas layer in the corrugated plate valley will produce vortex opposite to the liquid film flow direction,which will increase the thickness of the liquid film and waken the heat transfer process in the valley.Generally,as the inlet velocity of the mixed gas increases,the heat transfer at the peak is strengthened and the heat transfer at the valley is weakened.The thickness of the liquid film is affected by the inlet flow rate of the mixed gas and the subcooling of wall surface.The overall thickness of liquid film is thicker in the trough and thinner at the peak.And through calculation,the heat transfer capacity of the arc-shaped corrugated plate is increased by about 25%compared with the vertical plate of the same length.Numerical simulation results show that the condensation heat flux on the wall changes with the change of the wall structure,but the overall change range is not large.According to the research results of vertical plate and corrugated plate,the weak link in the heat transfer process is mainly the thermal resistance of non-condensable gas layer.Therefore,a new zigzag plate was proposed to disturb the non-condensable gas layer and thin the liquid film to reduce the thermal resistance thus to enhance the heat transfer rates during the condensation process.The trough of the zigzag plate is shifted downward to lengthen the leeward section and shorten the windward section,so as to bring greater airflow impact and improve the convective heat transfer.At the same time,the sawtooth structure can cause periodic fracture of the condensate film.By analyzing the condensation heat flux,convective heat flux and liquid film thickness on different surfaces under the same working conditions,it was found that the proposed zigzag plate can effectively disturb the non-condensable gas layer on the wall and reduce the overall thermal resistance of the liquid film.A comprehensive comparison found that the heat transfer capacity of the zigzag plate increased by about 85%compared to the vertical plate and increased by about 68%compared to the corrugated plate.Therefore,this model and conclusions obtained in this simulation have great significance to the mechanism research and application of the heat and mass transfer problems of steam condensation with non-condensable gas.