Study On Liquid Film Thickness And Heat Transfer Characteristics Of Horizontal Falling Film In Semi Elliptical Tube

Falling-film evaporation heat transfer device with high efficiency,by the compact structure,small temperature difference,the liquid fills the other characteristics have been less widely used.At present,horizontal falling-film evaporators are mostly horizontal tubes.Although the Ying-Ying area is large,in many cases,it can't meet the needs of energy-saving production and some special situations.How to reduce the shape resistance and the flow resistance while strengthening Heat transfer is the focus of the research at home and abroad,so it is of great significance to develop and study the high efficiency and low resistance heat transfer enhancement element with energy saving.The upper half of the semi-elliptical semi-elliptical tube or semi-elliptical tube,instead of the traditional horizontal circular heat exchange tube,the first can both increase the cross-sectional area of the heat transfer area,but also reduces the eddy current on the fluid shape resistance The heat transfer is more stable;secondly,the external liquid flow in the circumferential direction is slightly faster than the circular tube,and a more uniform and thinner liquid film is obtained;in addition,the semielliptical tube in the lower half of the heat transfer tube can accelerate the fusion between droplets,Wall speed of liquid discharge,compared with the thin liquid film tube,help to improve the heat transfer efficiency,thereby enhancing the evaporation efficiency.In this paper,through the experimental research and numerical simulation of two aspects of a detailed analysis.The experimental platform of falling film tube was set up and different tube types(round tube,semi-elliptical tube and oval tube)were completed by digital image processing technology respectively.Different spray volumes(0.013 L / min ~ 0.21 L / min)Height ?(5 mm ~ 35 mm);different tube spacing S(5 mm ~ 30 mm)of the liquid film changes.At the same time,the heat transfer characteristics of the semi-elliptical horizontal falling film tube were studied by numerical simulation.The flow field,temperature field distribution,velocity field distribution,mean local heat transfer coefficient and average Nusselt number of liquid film under different half-shaft ratios E = 1,3,5,7,Optimal structural parameters of elliptical horizontal falling film tube.The CFD simulation combined with orthogonal experimental design was used to determine the operating parameters(evaporation temperature,spray volume,heat transfer temperature difference,fabric height,tube wall roughness)of the optimal semi-elliptical drop tube.For the follow-up design and development of new semi-elliptical heat exchanger laid the theoretical foundation.The experimental results show that:(1)The gas-liquid interfacial line of the falling film process and the film thickness of the falling film tube vary with the circumferential angle of the tube wall through the experiment of digital image processing technology.The experimental value of the tube is compared with the theoretical value,the error is small,and the goodness of fit is better.Image digitization technology can be applied to film thickness research.(2)Under the same working conditions,the experimentally compared the changes of the liquid film thickness of the circular tube,the elliptical tube and the semi-elliptical tube with the variation of the circumferential angle of the tube wall and the change of the average liquid film thickness show that the thick oval tube,Semi-elliptical tube thinnest distribution.(3)With the increase of spray volume,the thickness of liquid film in semi-elliptical tube firstly increases and then decreases.With the increase of fabric height,the thickness of liquid film in semi-elliptical tube becomes thinner gradually.With the increase of tube spacing,Elliptic tube liquid film thickness becomes thinner.(4)In the experiment,the spraying amount of the semi-elliptical tube with the section circumference of 79 mm and the long-short axis ratio of 2.1 is about 0.14 L / min,the height H of the liquid cloth is less than 15 mm and the tube spacing S is less than 20 mm.The best stability and circumferential angle between 0 ° ~ 180 °,the liquid is not easy to accumulate,is conducive to full heat transfer,improve heat transfer efficiency.The simulation results show that:(1)The simulation is carried out under the same working conditions with different ratios of length,length and half-axis.The change of thickness,velocity field,temperature field and heat transfer characteristic are analyzed respectively.3-7 have relatively optimal heat transfer characteristics between the structure can effectively improve the liquid flow tube outer membrane,the film thickness decreases,the membrane flow faster.(2)Compare the experimental data to verify the feasibility of the simulation,and test the grid independence.The difference between the experimental data and the simulated data is 2.7%.The simulation of this model is feasible.The number of grids has little effect on the simulation results.(3)Combining the numerical simulation and orthogonal experimental design,the optimal operation parameters of the semi-elliptical horizontal falling film tube were studied.The results showed that the order of influencing factors was evaporation temperature,spray volume,heat transfer temperature difference,cloth height> Wall roughness.(4)For the semi-elliptical single tube with the optimal semi-axial ratio E = 5,the optimum level is spray volume 2 m / s,fabric height 15 mm,heat transfer temperature 10 ? and evaporation temperature 40 ?.Roughness 1.5.