The Flow And Mixing Characteristics Of Gas-Liquid Two Phases In Ejectors

During the chemical engineering processes such as hydrogenation, chlorination, sulfonation and tail-gas absorption, it is an important mode of production for the gas-liquid two phases to flow, mix and transfer mass. Ejectors have many advantages over other traditional static mixers such as compacter design, easier to manufacture, lower cost and more secure operation especially their higher mixing effect and mass transfer coefficients. Therefore, ejectors become more developed and potential multi-phase mixers and reactors. Many reports have researched the gas-liquid flow and mixing in ejectors, but the results are different and had individual scope of application due to the different ejector geometries, study methods and the species of fluids. Moreover, the systematic research on the gas-liquid flow in the ejector is seldom reported. So based on the advanced techniques of particle imaging velocity, planar laser induced fluorescence, computational fluid dynamics simulation and its coupling with population balance model, conventional chemical experiments, the primary issue of gas liquid flow patterns and their influential factors were discussed at first and then the systematic study on the effect of operation conditions and geometric parameters on the flow and mixing characteristics of gas-liquid in up-flow ejectors. At last, the principles of design and scaled-up for the ejector were analyzed via the similarity theory of fluid dynamics. The results were obtained as follows.(1) When the experimental systems of water-air and water-dioxide were selected to research the flow patterns of gas-liquid in the up flow ejector with and without swirl using PLIF technology, gas species have little effect on the flow patterns of gas-liquid because the difference of two gases'densities are quite little and so the Reynolds numbers for the two gases are also little different. However the swirl body has great effect on flow pattern. In absence of swirl, the jet flow was observed in the lower part of ejector. If the swirl was added, the gas and liquid started to mix at the suction chamber and the liquid jet didn't exist any more. (2) The Reynolds number at nozzle ReN, gas Reynolds number at the inlet of mixing tube ReG and gas to liquid flow rate ratio G/L have effects on the flow patterns. If ReN=2.36×10~4-1.27×10~5, when G/L≤0.2 the bubble flow exists. When G/L>0.2, the jet flow does. If the flow pattern is the jet flow, the liquid jet length depends on the ReN, ReG and G/L: when the ReN constant, the liquid jet length increases to the maximum at first and then decreases and at last stabilizes at some value. The maximum liquid jet lengths at different ReN are different which increases at first and then decreases with the ReN increasing. When ReN=5.84×10~4, the maximum liquid jet length is the maximum. In addition, the final stable length at different ReN is also different and it decreases with the ReN increasing. After the disintegrate of the liquid jet, the gas-liquid flow forms three patterns of bubble flow, cloudy flow and block flow. When ReG<5.0×10~2,the flow is bubble flow in the diffuser of the ejector. When ReG >4.5×10~3 , the flow is block flow in the diffuser of the ejector. When ReG =5.0×10~2~4.5×10~3时,the flow along the axial direction is jet flow, cloudy flow and bubble flow. In the ejector several flow patterns can coexist and also just occur one pattern such as bubble flow or jet flow.(3) When the bubble flow is present in the ejector, the mixing effect of gas and liquid is best. The bubble flow velocity and size distribution were discussed by the use of PIV measurement and CFD simulation. In the absence of swirl the directions of the average bubble velocity rotates a bit but the majority parallel to the axial line or the wall. When the swirl presents, the average flow field shows that the degree of upward motion of the rotation increases.(4) The nozzle Reynolds number, the gas Reynolds number (mixing tube inlet, no swirl) and the mixture Reynolds number (when there is swirl), and G/L affects the specific distribution of bubbles in bubble flow. When there is no swirl in the ejector, the gas tends to disperse into uniform bubbles in the liquid. At the same ReN, the bubble size increases with ReG increasing. At the same ReG, the larger the nozzle Reynolds number, the smaller the bubble size. In the presence of swirl, the bubble distributions are different from those in the absence of swirl. The bubbles are inclined to gather around the axial line asymmetrically and form"bubble chain". If ReN is the same and the mixture Reynolds number increases with the gas flow rate increasing, the bubble chain becomes border. When the gas flow rate is unchangeable and ReN and ReM become larger, the bubble chain becomes thinner.(5) Under the same operating conditions, the specific surface area for the ejector without swirl is larger than that with swirl at lower gas flow rate. When the gas flow rate increases, the differences of the two decrease. The simulations of the bubble size distribution in the ejector with the coupling of PBM and CFD shows that the numerical simulation results obtained coincide with the experimental results in the bubble distribution more or less. However, the simulated value is higher than the experimental one.(6) Experiments based on the reaction of NaOH and carbon dioxide were conducted to further discuss the gas and liquid mixing characteristics under different operating conditions by measuring the conversion efficiency of the rapid reaction. The conclusions coincide with the above discussions. If there is no swirl and ReN are the same, the mixing effect reduces with the G/L increasing. At the same G/L, the gas and liquid mixes better at larger ReN. When there is swirl and ReN are certain, the mixing effect of gas-liquid increases along with the G/L increasing. If G/L is constant, the mixing is better at higher ReN.(7) The relationship between CO2 volumetric concentration and the conversion rate of NaOH shows that the flow patterns play a major role on the reaction and mixing rather than the concentration of reactants. At lower G/L, the mixing is better without swirl than that with swirl under all ReN conditions. With the increasing gas-liquid ratio, the differences gradually decrease between two ejectors with and without swirl until G/L<1.4. When G/L=1.4, the mixing in the ejector with swirl is slightly better than that without swirl.(8) The effects of geometric parameters (diffuser angle, mixing tube length, distance and area ratio between the nozzle outlet and mixing tube inlet, area ratio of gas-liquid inlet) and operating conditions such as nozzle velocity on the pressure drop, air entrainment and jet coefficient were predicted in detail. The results show that when the diffuser angle increases, the pressure drop first increases and then decreases. And the air entrainment rate and jet coefficient increases. In the scope of simulations, the corresponding diffuser angle to the maximum pressure drop is about 3o. If Changes in the mixing tube length is considered only, with its increasing the pressure drop first increases and then decreases. At about 3.0 aspect ratio of the mixing tube the maximum pressure drop occurs. The air entertainment rate and jet coefficient decreases with the increasing mixing tube length. When the distance from nozzle to mixing tube increases, the pressure drop, air entertainment rate and jet coefficient all increases firstly and then decreases. The maximum pressure drop and air entertainment rate both corresponds to the distance to nozzle diameter ratio of about 3.0. When nozzle to mixing tube area ratio increases, the pressure drop reduces. When the area ratio reduces to 2.0 or so, small changes is in pressure drop. With the area ratio increases, the air entertainment rate and jet coefficient first increase and the decrease. When the ratio is about 3.0, the air entertainment is largest. Changes in the diameter of gas inlet to change the two-phase area ratio of imports, along with the area ratio increases, the pressure drop, air entertainment and jet coefficient all increase. When nozzle velocity is different while the geometry is unchangeable, with the increased nozzle velocity, the pressure drop increases and there is a significant linear increasing in air entertainment rate. While the jet coefficient first increases and then decreases. It can be seen that for a particular ejector, there is a nozzle velocity which corresponds to the optimized ejector performance.(9) Using the similarity theory and CFD simulation the rules of the amplification of the ejector were discussed simply. It comes to the conclusion that there is some numbers for the scaled-up of the ejector based on the mechanics similarity and single-value similarity which are area ratio, flow ratio, Reynolds number and Euler number. CFD simulation results show that when the movements meet similarity, the gas-liquid flow and mixing in the similar geometries is the same.