Study On The Gas-liquid Phase Flow Field Outside The Cap Space Of CTST

The combined trapezoid spray tray(CTST),as a new type of tray colum, has the cap with special trapezoid tridimensional structure, making gas-liquid two-phase flow and mass transfer extend from the tray to the most tray space to change the traditional planar mass transfer mode. Its application has achieved the effect of enhancing the capacity, increasing the efficiency and saving the energy to a great extent and has gained good economic benefits. The previous researchers have established modeling and analog computation for the two-phase flow of the lift segment and spray segment in the combined trapezoid spray tray cap and achieved the liquid film flow features and droplet size distribution rules, but they have not carried on research on gas-liquid two-phase flow of the space outside the cap of CTST. In addition, they have insufficient research on size distribution of this part of CTST. Therefore, this thesis carries out the main research on the gaseous phase distribution and droplet size distribution outside the cap space of CTST to provide the theory basis for the tray’s improvement in the application.Firstly the mathematical model of gaseous phase distribution and droplet size distribution outside the cap space was established in this paper. For the condition that discrete droplet is entrained by gas, in the model, we use particle trajectory model to stimulate gas-liquid two-phase flow condition. In the model, we consider the drag force of high-speed gas to liquid, gravity suffered by droplets and virtual mass force. Turbulent diffusion of droplets is computed by stochastic trajectory model. The established two-phase flow model in this article has the following features: ① have made detailed analysis to gaseous phase distribution and droplet size distribution in the space outside the cap on the combined trapezoid spray tray and established the mathematical model that can correctly describe gaseous phase distribution and droplet size distribution to increase the numerical analog’s accuracy; ② Because of the special structure of the spray cap on the combined trapezoid spray tray, model sealing of gaseous phase distribution is adopted the RNG k-ε turbulence model; Droplet size distribution is adopted the stochastic trajectory model for analog computation. The establishment of this model more correctly describes CTST’s flow conditions outside the cap; ③ For the droplet size distribution, we consider the collision, crush and coalescence of the droplet to make the analog result more accurately depict the state of the droplet; ④ Make analysis of the commonly used numerical computation method and finally decide the basic approach to solve the governing equation is the finite volume method; We select the second order upwind for difference scheme and use the SIMPLE algorithm for speed-pressure association algorithm.Forthermore the experimental study was conducted on the gas-liquid phase flow field outside the cap space of CTST. The experiment was conducted on the gaseous phase distribution and droplet size distribution in the space outside the cap by using the hot film anemometry measure system and the high speed photography technology. A research was also conducted on the injection performance of CTST such as spray-hole gas speed, liquid flow, spray taper angle, droplet distribution density, droplet speed distribution density and liquid film speed. Then the results were analyzed to get the variation of spray-hole gas speed with kinetic energy factor of the hole and the high and low intervals of droplet speed distribution, i.e. [5°,20°] and [40°,50°].Thirdly the Computational Fluid Dynamics(CFD) software FLUENT was used to conduct numerical modeling and the change regularity of gas distribution and droplet size distribution was obtained. In the theoretical calculation, the numerical simulation of each step has been introduced. It included the turbulence model, computing method, array of difference, and mesh generation which was simulated choose. And the model presented well on the gas flow condition outside the cap of CTST. The influence factors to combined trapezoid spray tray droplet size distribution can be reduced to: gas speed, supernatant liquid height above the tray, type of the cap and the physical property of the liquid. In addition, the research on the droplet size distribution shows that the drops which diameter no more than 2 mm occupied a higher percentage among the drops under the spray condition of the combined trapezoid spray tray and the droplet size distribution is more appropriate to the upper limit lognormal distribution function.The comparison between the experimental study and the simulation value is performed afterwards and the experiment verification was accomplished finally. By the study, one can get that the experimental value of the gas flow condition outsiade the cap of CTST is consistent with the simulation value, which proved the correctness of the theoretical model. The study also makes a research on the other relevant injection performance and the change rule of gas speed on the spray-hole along with the kinetic energy factor on the tray hole was gained. And the conclusion that the main influencing factors to liquid film speed are spray-hole gas speed and the liquid film position on the spray-hole was also reached.