Numerical Simulation Of Laminar Mixed Convection In A Circular Tube

Liquid film flow exists widely in the heat transfer process of engineering practice.Whenforced convection,convection caused by buoyancy,Marangoni convection caused by surface tension and thermocapillary convection exist simultaneously in the liquid film,the form of convection is called laminar mixed convection.Laminar mixed convection exists in many fields such as refrigeration,energy and chemical industry.In this paper,laminar mixed convection of liquid film in a circular tube is studied by taking the bottom of the tube heated at the bottom and the top of the tube as the free surface as the model and water as the working medium.The basic flow pattern and flow mechanism inside the liquid film were analyzed to explore the influence of Re,Ra,Ma and other criteria and free surface heat transfer coefficient h on fluid flow and heat transfer.In addition,the fluid flow and heat transfer in the liquid film were simulated without considering the Marangoni effect and the thermal capillary effect,and the two effects were compared to highlight the important role of Marangoni convection and thermal capillary convection in the actual heat transfer process.In this paper,ICEM software is used to divide grids,and FLUENT software based on finite volume method is used for simulation calculation,the research results have certain theoretical reference value for engineering design.The research contents of this paper are mainly divided into the following two parts:Firstly,the flow and heat transfer of liquid film at macroscopic quiescence are studied.At this time,there is no forced convection of the fluid,but there are Marangoni convection,thermocapillary convection and convection caused by buoyancy inside the liquid film.Through simulation analysis,the following conclusions are drawn:(1)the flow patterns of different vertical sections are the same when the fluid is at macro rest,forming a complete flow coil from left to right;(2)the free surface heat transfer coefficient h has little influence on the vertical section flow pattern and fluid temperature distribution,but with the increase of h,the heat transfer at the bottom wall surface increases to some extent;(3)when Marangoni effect and thermal capillary effect are considered,the changes of Ma number and Ra have little influence on the flow pattern of vertical section.However,with the increase of Ma number and Ra together,the fluid temperature increases significantly as a whole,and the heat transfer at the bottom wall increases significantly;(4)when Marangoni effect and thermal capillary effect are not considered,the basic flow patterns of vertical section change,and flow patterns of different sections are different.With the increase of Ra number,the number offlow rolls of vertical section increases,fluid temperature increases as a whole,and heat transfer at the bottom wall increases significantly.Secondly,the flow and heat transfer of liquid film in macroscopic flow are studied.At this time,forced convection exists in the fluid,which is combined with the Marangoni convection caused by surface tension,thermocapillary convection,and convection caused by buoyancy to form laminar mixed convection.Through simulation analysis,the following conclusions are drawn:(1)when the liquid film flows,the vertical section flow pattern passes through three stages,from incomplete flow coil to complete flow coil.The formation of complete flow coil marks the full development of flow;(2)the increase of Re number will cause the fluid to reach the position of full development and lie significantly behind the macroscopic flow direction,with little influence on the fluid temperature distribution and slight increase in heat transfer at the bottom wall;(3)the increase of heat transfer coefficient h on the free surface will cause the fluid to move backward slightly when it reaches the position of full development,which has little influence on the temperature distribution of the fluid and slight increase in heat transfer on the bottom wall;(4)when Marangoni effect and thermal capillary effect are considered,the increase of Ma number and Ra will cause the fluid to move forward in a small way at the position of full development,and the fluid temperature will increase significantly as a whole,and the heat transfer at the bottom wall will increase greatly;(5)when Marangoni effect and thermal capillary effect are not taken into account,the basic flow pattern of vertical section is different from that when these two effects are taken into consideration.At this time,the fluid moves forward at the position of full development with the increase of Ra number,the fluid temperature increases as a whole,and the heat transfer at the bottom wall surface increases substantially.