Study On Liquid Film Flow And Heat Transfer Characteristics Of Vertical Wall Falling Film Cooling System

At present,energy consumption is increasing,and environmental pollution has seriously restricted the long-term development of human society.Exploring and developing efficient heat transfer technology is of great significance to social and economic development.The liquid film flow has the advantages of high heat transfer coefficient,high heat flux density,small flow rate,small temperature difference,low consumption,simple structure,etc.The enhanced heat transfer characteristics under the condition of high heat flux in industrial production can be optimized by using the excellent characteristics of liquid film flow.In order to effectively utilize the advantages of liquid film flow to enhance heat and mass transfer,the key point is to clarify the internal flow characteristics and heat and mass transfer mechanism of liquid film.Keep the liquid film flow stable so that it is uniformly covered on the surface of the heat source.If the liquid film breaks and the surface of the heat source appears speckle,the heat can not be consumed,which leads to the uneven heat distribution of the heat source.Therefore,it is necessary to study the flow process and heat transfer characteristics of falling film.In this paper,the steel containment in the passive safety system of nuclear power plant is taken as the research object,and the vertical wall falling film flow and heat transfer experimental platform is built.The experimental study on the flow and heat transfer of the liquid film outside the vertical wall was carried out.The thickness and temperature distribution of the liquid film outside the wall under different experimental conditions were measured.The effects of different experimental conditions on the film spreading and evaporation were obtained.The liquid film fluctuates with time,and the surface of the falling film exhibits solitary waves of various sizes.As Re increases,the liquid film fluctuates and the average liquid film thickness increases.Under the cooling of the falling film spray,the wall temperature drops rapidly and then reaches a stable state.Due to the difference of the location,the closer to the inlet of the spray liquid,the slower the change of wall temperature is.At the same location,the liquid film temperature is significantly lower than the wall surface temperature,and as the liquid film spreads,the temperature difference between the two becomes smaller.In addition,the numerical simulation of liquid film flow and heat transfer characteristics outside the vertical wall is carried out.Based on the VOF multiphase flow model,the gas-liquid surface evaporation heat transfer process is considered.From the point of view of the falling film spreading process,the spray liquid spreads freely along the wall surface under the combined action of gravity and surface tension,and gradually spreads into an inverted trapezoid with a wide width and a narrow width.As the Re number increases,the thickness and velocity distribution of liquid film increase,and the fluctuation is more obvious.The reverse air flow has an effect on the spreading of the liquid film,and the surface of the liquid film is hindered by the reverse air flow,and the spreading is hindered.As Re increases,the temperature of the liquid film decreases.As the wall temperature rises,the temperature of the liquid film rises.The reverse flow enhances the heat transfer between the liquid film and the wall surface,and the temperature of the liquid film rises.The increase of the wall tilt angle can enhance the heat transfer performance between the liquid film and the wall surface.As the wall tilt angle increases,the gravity component affecting the falling film spread increases gradually,and the liquid film temperature rises.