Study On The Liquid Film Flow Characteristics During The Falling Film Outside The Horizontal Curved Tube

Horizontal tube falling film evaporator is widely used in seawater desalination,petrochemical industry,refrigeration and other engineering fields because of its high heat transfer coefficient,small heat transfer temperature difference and compact structure.Horizontal falling film evaporator heat exchanger tubes use circular tubes,special-shaped tubes,etc.,but the flow of liquid film outside the tube is poor,liquid film is easy to accumulate at the top and bottom of the tube,resulting in the increase of liquid film thickness,greatly reducing the heat transfer efficiency.Elliptical tube and egg-shaped tube can reduce the thickness of liquid film outside the tube,but when the liquid film falls on each heat exchanger tube,liquid film accumulation will occur on the top and bottom of the tube.Aiming at the same problem between circular tube and special-shaped tube,this paper presents a heat exchanger tube with special-shaped cross section(curved tube).The specific shape of the cross section is related to two characteristic parameters,alpha angle and convex arc number n.Its design idea is to reduce the number of liquid film accumulation on each heat exchanger tube and enlarge the area of thin liquid film distribution.In this paper,the distribution of liquid film thickness outside curved pipes and the uniformity of flow field in pipes are studied,the main research contents include:(1)The physical model of falling film flow outside a circular tube and a curved tube is established.The distribution function of liquid film thickness outside circular and curved pipes is derived by solving the basic governing equations.The theoretical calculation results show that the distribution of liquid film thickness outside curved pipes with 60 degrees of alpha angle is thinner than that of curved pipes with 45 degrees and 50 degrees of alpha angle.The average thickness of liquid film outside curved pipes decreases with the increase of n and tends to an extreme value.When n=2,the average liquid film thickness outside the curved tube is 12.2%smaller than that of the circular tube.Therefore,the curved tube with a 60 degree angle is selected as the optimal tube shape for numerical simulation.(2)The VOF model of two-phase flow was used to simulate the flow process of liquid film outside a horizontal curved tube and a circular tube.The effects of spray density,liquid distribution height and cross-section shape influencing factor n on the thickness distribution and velocity of liquid film were studied.The results show that:a)the distribution of liquid film thickness outside curved tube is similar to that of circular tube,and it decreases first and then increases along the X-axis direction.The thickness of liquid film outside the tube increases with the spray density and decreases with the height of liquid distribution.b)The concave arc wall of curved surface tube can effectively guide the liquid film flow distribution,which makes the liquid film flow speed increase smoothly and the liquid film thickness become thinner and evenly distributed.c)Under the same working conditions,the average thickness of the liquid film outside the curved tube decreases with the increase of n,which is consistent with the theoretical calculation.When?=0.15kg/(m·s),the average outer liquid film thickness of curved tube(n=2)is 12.78%smaller than that of circular tube.When ?=0.22kg/(m.s),the average thickness of outer liquid film of curved tube(n=2)is 13.82%smaller than that of circular tube,the average thickness of outer liquid film of curved tube(n=3)is 7.72%smaller than that of curved tube(n=2),and the average thickness of outer liquid film of curved tube(n=4)is 5.15%smaller than that of curved tube(n=3).(3)Numerical simulation of flow field in curved tube using Reynolds time-averaged method,The results show that the fluid produces a large reflux zone near the inlet of the tube,accounting for 29%of the falling film length,and the velocity distribution in the upstream and middle of the tube is not uniform.After the non-uniformly arranged guide plate is installed,the reflux zone is reduced to 4.95%of the falling film zone length,and the velocity distribution of the flow field is more uniform.