Study On Internal Flow Mechanism Of Air-lift System

With the release of the “2017 the Belt and Road” report about the development of channel dredging industry,the process of urbanization in china has been accelerated,and the domestic garbage and construction waste have been deposited seriously in urban rivers,which not only affect the lives of the residents around the rivers but increased the risks of urban safety and health.Therefore,the dredging of rivers and lakes has become an urgent problem to be solved at present.As one of the effective tools for lifting water or solid materials,airlift dredging system is widely used in river and lake dredging and other engineering fields because of its simple structure,low maintenance cost and less environmental pollution.The successful application of this system is of strategic significance for reducing natural disasters,promoting port development and improving urban environment.The jet-airlift dredging system have been discussed in this paper,the variation law of gas-liquid-solid three-phase flow characteristics with structural parameters and operation parameters in the system is deeply analyzed.A small airlift system designed by us was used to the experimental study on the effects of gas flow rate,submergence ratio,gas inlet mode(jet and radial)and solid supply on the flow characteristics and lifting performance of mixed fluid,in which 10 mm rice ceramic particles be used as the lifting target,meanwhile,the structure characteristics of the internal flow field in each stage are studied based on the high-speed camera technology and image processing.By considering the influence of gas characteristics on solids motion in mixed fluid,the solid velocity model is established based on the theory of three-phase flow and the momentum theorem,in which the disadvantages of conventional models has been resolved effectively,these disadvantages are including more empirical formulas and limited to a single gas state,through the above analysis,the effect of structure parameters and operation parameters on the solid motion is obtained.The gas velocity model in gas-liquid-solid three-phase flow is established based on the gas velocity model in gas-liquid two-phase flow and by combining the three-phase flow theory and the bubble dynamics theory.By comparing with the former,thekey factors that the effect of liquid-solid phase on gas movement in three-phase flow are obtained.At the same time,by considering the momentum theorem and the non-linear variation characteristics of the gas along the riser axis,the one dimensional governing equation of the mixed fluid is established,and the relationships between the velocity of each phase in gas-liquid-solid three-phase flow is obtained.The results show that under the different solid supply conditions,the variation trend of liquid flow rate is similar to that of the gas flow rate,which it is increases first and then decreases.At the condition of higher gas flow rate,the liquid velocity of sufficient solid supply is obviously higher than that of the owe-solid supply,moreover,with the increase of submergence ratio,the difference between them has become more and more obvious.With the increase of gas velocity,the solid velocity is higher than that in the owe-supply conditions when enough solids are supplied.At higher gas velocity,the decrease of the former is lower than that of the latter with the increase of submergence ratios.Under the condition of low gas velocity and low submergence ratio,the effect of jet inlet and radial inlet on the lifting of liquid phase and solid phase is relatively small.While under the condition of high gas velocity and high submergence ratio,the lifting performance of the jet air-inlet way is better than that of the radial air-inlet way.The mixed fluid in the riser has different flow patterns at different gas velocity stages,the bubbles concentration is decrease with the increase of the lifting distance under the lower gas velocity,and the bubble size increased gradually,in the meantime,the effect of bubbles on solids increases gradually,the circulating rise of solids between wall and core under the action of gas-liquid phase.The vibration effect of the mixed fluid is increased at the higher gas velocity stage and its concentration decreased gradually,in addition to,the liquid phase and solid phase are move along the pipe wall in the lifting process,the gas-liquid mixture are produced different action mode to the solids under the different flow patterns,this phenomenon is especially prominent in bubble churn flow.