Numerical Simulation Of Liquid Film Flow And Heat Transfer Characteristics On Vertical Wall

Falling liquid film flow is a forced motion under the action of gravity,which exists in all aspects of our production and life.Because of its advantages of small power consumption,simple structure and high heat and mass transfer coefficient,it is widely used in many traditional industrial fields,such as energy,chemical industry,aerospace,refrigeration and electronics.It can be said that falling film flow is a great engineering significance.In this paper,the liquid film flow on the vertical wall is mainly studied by the computational fluid dynamics method,and the two-dimensional simulation is carried out under the normal temperature and heating conditions by FLUENT software.The flow state of the liquid film and the change of the instant film thickness are analyzed.The analysis was performed on fluctuations in wall shear and surface velocity in the flow direction.And the surface temperature and heat transfer coefficient of the liquid film under constant heat flux density are also studied.The flow state of the liquid film is also basically stable after the flow is stable,and there is no obvious change with time.In the flow direction,the liquid film flow state under different Reynolds numbers is similar,and it can be divided into three regions according to the flow state: the entrance zone,the development zone and the stable zone.The average liquid film thickness increases with increasing Reynolds number and decreases with increasing fluctuation amplitude.For the same fluctuation region,the greater the Reynolds number,the greater the frequency of liquid film fluctuations.The frequency of liquid film fluctuation in the three areas is shown as entrance area>development area> stable area.Along the flow direction,the average value of the wall shear force is gradually decreasing,but the fluctuation range of the instantaneous value increases.The fluctuation frequency of the wall shear force is the largest in the entrance area,followed by the development area and the smallest in the stability area.The waveform of the instantaneous fluctuation of the surface velocity is very similar to that of the liquid film.The average velocity and velocity fluctuation frequency are also the largest in the entrance area and the smallest in the stable area.There is no obvious phase difference between the surface speed and the liquid film.The velocity distribution in the membrane of the entrance area is approximately parabolic,and the maximum velocity appears on the surface of the liquid membrane.In the development zone and the stable zone,beforeand after the peak,the maximum velocity in the film appears on the surface of the liquid film.At the peak position,the maximum velocity in the film appears below the phase interface.Under the condition of constant heat flux density,the temperature of the liquid film surface as a whole gradually increases in the direction of flow,but it does not increase linearly.The temperature fluctuates with the fluctuation of the liquid film.In the inlet area of the flow,the temperature fluctuation of the surface of the liquid film is very small,and it increases approximately linearly.As the liquid film flows downward and enters the development section and the stable section,the fluctuation of the surface temperature increases.And for every complete wave: the peak value of the liquid film thickness corresponds to the lowest value of the surface temperature,and the bottom value of the liquid film thickness corresponds to the highest value of the surface temperature.Except that the fluctuation trend is opposite,other changes of the instantaneous liquid film surface temperature are consistent with the law of liquid film fluctuation.In the flow direction,the amplitude of the temperature rise of the liquid film surface decreases with the increase of the Re number,and increases with the increase of the heat flux density.The change trend of the heat transfer coefficient under different Reynolds numbers is similar,which shows that the heat transfer coefficient at the entrance is the largest,then decreases rapidly,and finally decreases gradually.When the heat flux is fixed,as the Reynolds number increases,the heat transfer coefficient gradually increases.When determining the physical properties,the change in heat flux density has little effect on the heat transfer coefficient.