Experimental Study On Condensation Heat Transfer Characteristics And Enhanced Heat Transfer Of Steam With Non-condensable Gas Outside Vertical Tubes

Steam condensation is common in various engineering fields,but when steam contains non-condensable gases,condensation heat transfer coefficient decreases sharply.Because the condensation of non-condensable gas vapor involves complex phase change heat transfer process,gas phase mass transfer process,condensate form and its changes process,its mechanism research has not been generally explained.Exploring the condensation heat transfer mechanism and enhancing the heat and mass transfer direction of steam containing non-condensable gas has been a hot topic for scholars.In this paper,the air-vapor mixed gas is used for the condensation of vertical smooth tubes and strengthened tubes.The process of heat and mass transfer and the mechanism of enhancing heat and mass transfer of steam containing non-condensable gas were studied by means of experimental research.The main research contents and conclusions of the research obtained are as follows:(1)The essence of the condensation heat transfer process containing non-condensable gas vapor is the heat transfer process controlled by the gas phase diffusion mass transfer.The vapor passes through the diffusion layer from the main body region to the condensation wall through the diffusion layer having a high concentration of non-condensable gas near the condensation wall surface,and the heat exchange process includes convection heat exchange of the mixed gas,vapor condensation heat transfer,and conduction of the condensate.The condensate shedding scouring creates forced convection disturbance on the gas phase diffusion layer,thins the gas phase heat transfer mass transfer diffusion boundary layer,improves the temperature gradient and concentration gradient in the gas phase boundary layer,and reduces the velocity vector and the temperature gradient vector and the concentration gradient vector.The synergy angle enables the velocity field to cooperate with the temperature gradient field and the concentration gradient field to enhance heat transfer.(2)The experimental study of condensation heat transfer under different conditions of natural convection were carried out by constructing a visualized experimental platform for steam condensation of non-condensable gas outside the vertical heat exchange pipes.The image of the condensate on the outer wall of the vertical stainless steel smooth tube was captured by a high-speed camera.Visual observation shows that there are four kinds of condensate forms on the outer wall surface of the vertical stainless steel smooth pipe:the coexistence of the bead stream,the coexistence of the bead film,the stable film shape,and the mixed shape.The form and dynamic process of the condensate have different effects on the mass transfer and heat transfer of the gas diffusion layer.The more frequent the condensate sheds and scours,the better the mass transfer heat transfer effect.The condensation heat transfer characteristics were studied by experiments under different non-condensable gas mass content,subcooling and pressure conditions,and the total condensation surface heat transfer coefficient gradually decreased with the increase of non-condensable gas content.The overall trend of the total condensation heat transfer coefficient of increases firstly with the increase of the subcooling degree of the wall.After reaching the maximum value,it decreases with the increase of the subcooling.Based on the influencing factors of condensation heat transfer characteristics,a new experimental correlation of non-condensable gas vapor condensation experiments for vertical tubes was proposed.(3)Based on the experiments of stainless steel smooth pipes and brass light pipe,the stainless steel surface infiltration layer enhanced tubes and the brass surface infiltration layer strengthening tube were respectively subjected to the condensation test of non-condensable gas vapor under the same conditions as the smooth pipes.There are two types of condensate on the outer wall surface of the enhanced tubes:the coexistence of the bead stream and the bead shape;Compared with the dynamic change process of the smooth pipe,the enhanced tubes sheds more frequently,the range is larger,the speed is faster,and the amount of condensed liquid is larger;The form and dynamic process of the condensate on the surface of the enhanced tube promotes the mass transfer and heat transfer process of the gas-phase diffusion layer.Under the same steam partial pressure and non-condensable gas mass content,the experimental data of the enhanced tube is compared with the smooth tube.The heat transfer effect of the enhanced tube is better than that of the smooth tube.When the mass content of the non-condensable gas is 30%,the heat transfer coefficient of the stainless steel enhanced tube is 1.3～3.0 times that of the stainless steel smooth tube,and the heat transfer coefficient of the brass enhanced tube is 1.1～1.9 of the brass smooth tube.