Numerical Simulation Of Falling Film Flow Outside Horizontal Elliptical Tubes

Horizontal tube falling film evaporation has great significance in improving the heat exchanger performance and saving the manufacturing and maintenance cost for its high coefficient of heat transfer under small temperature difference and liquid flow rate. But the technique did not widely applied in heat transfer systems because horizontal tube falling film evaporator has high requirements of falling film flow and film thickness distribution, the flow and the film distribution has a direct influence on the performance of the heat exchanger and is affected by liquid distributor, tube diameter, tube arrangement and many other parameters. The research of falling film heat transfer characteristic and the design of different parameters is not accurate and complete so the study of horizontal tube falling film flow and film distribution has an important significance.A horizontal falling film theory model is set up to research the falling film flow and the film thickness distribution. VOF method is used to track the gas-liquid surface and the distribution of falling film velocity, pressure, and falling film thickness is obtained and compared with the experimental results to check the correctness of the calculation model. By changing the liquid Reynolds number, tube pitch, tube diameter, tube circumferential angle and some other parameters, the falling film flow and film thickness distribution of different circular tubes as well as the elliptical tubes under the same condition is obtained. The feasibility of elliptical tubes in falling film evaporators is considered.The results show that through the comparison of horizontal tube falling film flow experimental and numerical results. Although there are certain differences in film thickness value, the film thickness distribution variation trend of numerical results fits well with the experimental results and can show the characteristic of horizontal falling film flow and film thickness distribution. In the range of tube circumferential angle from0°to180°,the falling film thickness decreases first and then increases.The film thickness increases with the liquid Reynolds number increases, decreases with the increasing of tube pitch. The fluctuation of the film thickness increases with the increasing of tube circumferential angle and at the region of tube circumferential angle of145°the film fluctuated most and dryout region is easily appeared. The falling film thickness distribution of a horizontal elliptical tube is similar with the circular tube and the film is more uniform, the influence of the elliptical tube is not linear and exists a optimal ellipticity. The falling film liquid spreads faster on the horizontal elliptical tube and enters developed region than a circular tube. At the bottom of the elliptical tube the liquid is more difficult to gather and the film is more uniform. The wetting rate of falling film tube increases with the increasing of liquid Reynolds number and at the bottom of the tube the liquid starts to separate from the tube and dryout region appears, the dryout region decreases with the tube ellipticity increasing.