Experimental Study On Velocity Distribution Of Falling Liquid Film Outside Horizontal Tube

The falling film evaporator outside the horizontal tube has many outstanding advantages such as high heat transfer coefficient,low energy consumption,and small heat exchange temperature difference which has been applied in many fields.During the heat transfer process,the falling motion of the fluid between the tubes and the flow of the liquid film on the surface of the tube interact with each other,which together determine the heat transfer effect of the liquid film outside the tube.In order to understand the relationship between liquid film flow and heat transfer outside the tubes,the experimental analysis of the characteristics of fluid falling motion and liquid film velocity distribution between horizontal tubes are operated.The experimental results show that the tangential velocity and axial velocity of liquid film outside the tube increase with the increase of Reynolds number and tube spacing,and decrease with the increase of tube diameter.In the sheet mode,the tangential velocity of the liquid film decreases in the range of 0?-90?and 120?-180?along the circumferential angle,and increases in the range of 90?-120?.The tangential velocity reaches the maximum at 0?and the minimum at 180?which is close to 0.In the column state,the tangential velocity distribution of liquid film is the same as that of curtain flow when dimensionless axial distance L~*?0.3,and when 0.3<L~*?1.The tangential velocity of liquid film decreases in the range of 0?-30?and 90?-120?,and increases in the range of 30?-90?and 120?-180?.Along the axis of the horizontal tube,the tangential velocity of the liquid film decreases with the increase of the distance,in which the decreasing range decreases with the increase of the circumferential angle in the column mode.The axial velocity of liquid film increases with the increase of Reynolds number under the same flow regime and is the highest in the jet-sheet modes under different flow modes.The axial velocity of the liquid film along the axial direction presents a certain periodic variation and the fluctuation amplitude of the liquid film axial velocity increases with the increase of the Reynolds number.The axial velocity of liquid film reaches the maximum in the impinging zone and reaches the minimum in the middle of two adjacent liquid columns in the jet modes.It is found that the fluid descent velocity between the tubes increases first and then decreases rapidly in the case of columnar flow and sheet modes in the study of the characteristics of the velocity between horizontal tubes.The height Hs of the stagnant affected zone is defined by the vertical distance between the position of the measuring point where the maximum speed is obtained and the horizontal pipe below.The results show that the height of stagnation influence zone increases with the increase of Reynolds number and the flow pattern has a greater influence on the stagnation effect.The greater the spray height is,the more obvious the stagnation effect is at the same modes.Meanwhile it is found that the flow inside the liquid column is more complex,and the horizontal velocity in the same liquid column changes with the change of position.