The Numerical Simulation And Research Of Gravity Falling Film Outside The Horizontal Tube

The heat and mass transfer in falling-film of process liquid, with minimal temperaturedifference, has been accepted to have the ability of enhancing the efficiency of the heatexchanger. Hence, it has been widely used in sea water desalination, chemical system,refrigeration, et al. However, too little liquid supply may give rise to a drastic reductionin heat exchange performance; on the other hand, liquid overfeeding to avoid such acondition may cause water film thicker and heat resistance bigger and leads to thedecreasing of the overall heat transfer performance. Therefore, the thickness of the film isone key factor which influences the heat and mass process. Experimental measurement offilm thickness at different annular angles is influenced by numerous factors which makeit difficult to measure the thickness completely and correctly. So it is very important toinvestigate the characters of the falling film flowing around the horizontal tube by themethod of numerical simulation with a role of “numerical experiment”.In this study, the lattice model with two dimensional for numerical simulation of thefalling film flowing around the horizontal tube were established, and through the study ofparameters in CFD software Fluent, using the volume of fluid method for theliquid-vapor interface model, falling film flowing around the horizontal tube weresimulated. For the sake of examining the influence of tube diameter, liquid flow rate andannular angle on the film flowing, the simulation conditions took five tube diameters(12,25,38,57and90) and five spray densities (0.1,0.15,0.2,0.25and0.3kg/(m·s)),25various working conditions in total. In the procedure of simulation, theflow rate and two phase diagram of gas-liquid at seven annular angles (15o,30o,60o,90o,120o,150oand165o) were observed at any time. The output data was saved every200iteration steps (corresponding to0.01second) after the film flowed steadily; theoutput data of3000steps (0.15second) in sum were saved and400phase diagrams wereobtained. In order to probe the surface of two-phase flow, the phase diagrams were treated in software Tecplot by wild scheme, the film thickness at seven different annularangles were read out in the spot whose fraction of liquid phase volume were0.5.Meanwhile, the flow rate of7annular angles at25condi-tions was got from175phasediagrams (7×25). Through the treatment of the data, the equation of the film thicknesswith three kinds of factors (tube diameter, spray diameter and annular angle) wasregressed and so did the film flow velocity with them. According to the data andequations, the flow behavior of falling film was induced as following:(l) The water film and its thickness, velocity and flow rate fluctuates irregularly all thetime; the fluctuation abates slightly with the increase of tube diameter; the film thicknessattends to distribute along annular angle uniformly.(2) The water film goes to thinner with the decrease of spray density, and when thespray density is too small, some spot of the tube wall surface will be dry up.(3) The average film thickness is positively correlated with the flow rate, and isnegatively correlated with the film velocity.(4) When the tube diameter and spray rate are invariant, the thickness of film first turnsfrom thick to thin, then to thick, and the velocity first increase then decrease during theflowing process from the top of the tube to the bottom.(5) When the spray rate is kept constant, the film thickness in the areas of15o~165ogoes to thinner with the increase of the spray diameter.(6) The average thickness in0o~15oand165o~180ois much thicker than in15o~165o,the reason are that water flow changes the direction suddenly at the top and strikestogether from two side of the tube at the bottom.The spray density in water film evaporation air cooling technology is smaller than infalling-film technology of process liquid. For the sake of examining the influence of tubediameter, liquid flow rate and annular angle on the film flowing, the simulationconditions took five tube diameters (12,24,32,38and57) and five spray densities(0.02,0.026,0.032,0.038and0.044kg/(m·s)),25various working conditions in total. Inthe procedure of simulation, the flow rate and two phase diagram of gas-liquid at seven annular angles (15o,30o,60o,90o,120o,150oand165o) were observed at any time. Themethod of reading data and processing is the same as larger spray density describedabove. And the laws of film are also the same as larger spray density after concluding thedata of2800flow and2800film thickness, except that the wave amplitude of film issmaller. Furthermore, the specialty of “film wriggle” reveals in falling film at smallspray density condition.