Numerical Simulation Of Laminar Mixed Convection In A Liquid Film At The Bottom Of An Elliptical Tube

Liquid film flow exists in many systems engineering and technical equipment fields,such as falling-film evaporator,heat pipe radiator,heat dissipation of nuclear reactor protective shell,cooling of electronic chip,etc.The flow and heat transfer characteristics of condensing liquid film or evaporating liquid film are closely related to practical engineering problems and have been widely concerned by scholars at home and abroad.Under the joint action of buoyancy effect and surface tension,there exists complex mixed convection heat transfer process in liquid film.The analysis and study of mixed convection heat transfer in liquid film can provide theoretical basis for the development of system engineering and production and life.The three-dimensional physical model and mathematical model of the liquid film at the bottom of the horizontal elliptical tube are established in this project.Based on the finite volume method,the laminar mixed convection at the bottom of the elliptical tube subjected to the hot liquid film is numerically simulated by FLUENT to study its flow and heat transfer characteristics.The influence of buoyancy effect,Marangoni effect and thermal capillary effect on convective process was analyzed,and the distribution of temperature field and velocity field in liquid film under different working conditions was obtained.The influence mechanism of dimensionless numbers such as Ra number,Ma number and Re,free surface heat transfer coefficient,transverse ratio of pipeline section,liquid film thickness and other factors are discussed respectively.The main conclusions are as follows:First,the buoyancy effect has a certain inhibitory effect on Marangoni convection,buoyancy-Marangoni convection is more stable than pure Marangoni convection,and the coexistence of the two can enhance the heat transfer strength at the bottom of the liquid film.Second,When the liquid film at the bottom of the elliptic tube is in a macroscopic static state,the heat transfer intensity inside the liquid film is enhanced by increasing the temperature difference in the vertical direction,regardless of zero gravity or constant gravity.With the increase of temperature difference in horizontal direction,the heat transfer intensity inside the liquid film increases continuously in zero gravity,and first increases and then decreases in constant gravity.Third,when the liquid film at the bottom of the elliptical tube is in a macroscopic static state,the heat transfer intensity inside the liquid film increases first and then decreases with the increase of the transverse ratio of the pipeline section,and decreases all the time under constant gravity.With the increase of liquid film thickness,the change of heat transfer intensity inside the liquid film decreases in zero gravity and increases first and then decreases in constant gravity.Fourth,when the liquid film at the bottom of the elliptical tube is in a macroscopic flow state,the temperature difference in the vertical direction increases,and the inlet section of the fluid flow is slightly shortened and the heat transfer intensity of the fully developed section is obviously enhanced under zero gravity and constant gravity conditions.With the increase of liquid film velocity,the flow inlet section increases obviously,and the heat transfer intensity in the fully developed section increases.The length of the flow inlet section increases when the transverse ratio of the pipeline section increases,and the heat transfer intensity of the fully developed section increases at zero gravity,but increases first and then decreases at constant gravity.With the increase of liquid film thickness,the length of flow inlet section increases obviously,while the heat transfer intensity of fully developed section remains unchanged.With the increase of the heat transfer coefficient of the free surface,the length of the flow inlet section increases slightly,while the heat transfer intensity of the fully developed section increases slightly.