Study Of Falling-film Flow Field, Heat Transfer And Evaporation On Horizontal Tubes

Falling film evaporation process has high heat transfer coefficient, small heat loss, high efficiency, stable operation and other significant advantages, and enjoys a large number of applications in modern industry. Falling film evaporation process involves complex two-phase heat and mass transfer problems. In-depth study of its heat transfer mechanism possesses significant meaning for improving the performance of falling film evaporation condensers. This paper carried out a research from the theoretical and experimental aspect.Falling film evaporation process involves the phase change of the working fluid. Since the latent heat of vaporization of the fluid is large, evaporation has an important influence on heat transfer. This paper analyzed the heat transfer processes of horizontal tube falling film evaporation.. A falling film evaporation model was proposed and verified by experiment. The experiment shows that the model reflects the law of falling film evaporation, and the falling evaporation rate increases along the flow direction of the liquid film. Under the experimental condition, When the film temperature is less than 50 ℃, the latent heat of vaporization of water at the ratio of heat exchange surface heat flux is less than 10%. When the film temperature is larger than 90 ℃, the ratio is larger than 48%. When the inlet temperature of falling film is increased from 20 ℃ to 60 ℃, the average falling film evaporation rate is increased by 8.5 times. When the spray density is decreased from 0.122 kg/(m·s) to 0.052 kg/(m·s), the falling film evaporation rate is increased by 6%. As the relative humidity increases, the evaporation latent heat decreased significantly; When the relative humidity is increased to 0.8, the heat transfer between the falling film and the tube is primarily under sensible way.This paper established the CFD model of horizontal tube falling film. The influence of spray density, temperature and heat flux for heat transfer was studied and verified using water as the working fluid. The study shows that under the whole experimental condition the heat transfer coefficient decreases along the flow direction of the horizontal tube surface. Within the scope of the 135 ~ 180 °, the heat transfer coefficient is increased by 4% with the increase of the angle. In the range of 120 ~ 150 ° the wave fluctuation is most dramatic and the dryout is easily triggered. There is an optimal value for the spray density to improve the overall heat transfer coefficient. under the experimental condition, when the spray density of 1 kg/(m·s) is reached, the influence of spray density for the overall heat transfer coefficient is neglectable. When the spray water inlet temperature is higher, the temperature difference between the tube surface and the water is smaller. Within the scope of the experiment, heat flux has little impact on overall heat transfer coefficient.