Study On Spatial-temporal Instability And Analysis Of Dynamic Mode Of Swirling Flow In A Gas Cyclone

Gas cyclone is a typical gas-solid separation device,which is widely used in environmental protection,chemical industry,and other fields.Although the structure of the gas cyclone is simple,the strong swirling turbulent flow inside it is complex and is characterized by the instability of the swirling flow,which has an adverse effect on cyclone performance.In recent years,researchers have carried out a lot of research on the characteristics of the gas-solid flow in gas cyclones,which has greatly enriched the understanding of the instability of swirling flow.Our previous study has found that the flow in a gas cyclone actually exhibits eccentric swirling,which is the superposition of the spatial asymmetry(vortex eccentricity)and the temporal instability(precession vortex core).However,the coupling of the vortex eccentricity and the precession vortex core makes it difficult to analyze the impact of swirling instability on cyclone performance.Therefore,an in-depth understanding of the spatial-temporal instability characteristics,influencing factors,and mechanisms of the swirling flow is of great significance for the structural optimization and performance improvement of gas cyclones,and can also provide new ideas for the studies of similar swirling flow in nature and engineering.Based on the above background,this study conducts an in-depth study on the instability of the gas-solid flow and the energy loss in a gas cyclone.In view of the high temporal and spatial resolution requirements of the data for the instability analysis,the Computational Fluid Dynamics(CFD)method was adopted to obtain the flow field data,with experiments to verify the numerical model.At the same time,aiming at the problem that the pressure drop can only represent the total energy loss but cannot reflect the distribution of energy loss inside the equipment,a mean mechanical energy loss model was introduced to calculate the energy loss rate inside the gas cyclone to quantify the influence of the instability on energy loss.The model verification results show the reliability of the CFD model and the energy loss model,laying a solid foundation for the follow-up research.On the basis of understanding the temporal and spatial instability of the swirling flow,the Dynamic Mode Decomposition(DMD)method was used to conduct a modal analysis of the flow field of the gas cyclone,deeply exploring the relationship between the dominant modes and the precession vortex core,thereby contributing to a new understanding of the swirling instability.The specific research contents and conclusions are as follows:Firstly,the basic characteristics of the gas-solid flow and the energy loss were analyzed using Large Eddy Simulation(LES)and Lagrangian Particle Tracking(LPT)method.The results show that the gas flow has significant spatial asymmetry,and the time-averaged flow field can retain this main characteristic.Increasing the particle loading ratio will gradually reduce the swirl intensity,but the asymmetry of the flow field still exists.The pressure drop and energy loss of the gas flow show quasi-periodic fluctuations with time,reflecting the temporal instability of the swirling flow.As the particle loading ratio increases,the fluctuation of the pressure drop and the proportion of the turbulent dissipation both increase,indicating an increase in flow instability.Thus,it can be seen that the numerical simulation successfully identified the spatial asymmetry and temporal instability of the swirling flow.Secondly,symmetrical double and quadruple inlet gas cyclones were designed to clarify the characteristics,mechanism,and impact on energy loss of the vortex eccentricity.The results show that vortex eccentricity causes anomalies in velocities after coordinate system conversion,resulting in the appearance of negative tangential velocity regions and alternating positive and negative radial velocities.Symmetrical multi-inlet design can effectively improve the vortex eccentricity,but cannot suppress the precession vortex core.The improvement of the vortex eccentricity can enhance the swirl intensity and flow stability,while also causing an increase in energy loss.However,improving vortex eccentricity by increasing the number of inlets can lead to an increase in the energy loss of the inlet,and instead,suppress the enhancement of swirl intensity in the separation space.Therefore,in this study,the symmetric double inlet gas cyclone is optimal.Thirdly,by adjusting the Reynolds number and the swirl number,the characteristics and influencing factors of the precessing vortex core and its influence on the energy loss were studied.The results show that the spectral analysis of x,y velocity or vortex core center tracking data can identify the frequency of the precessing vortex core.From the analysis of the maximum deviation,the frequency of the precessing vortex core,and the radial distribution of the Strouhal number,it can be concluded that the Reynolds number has no effect on the precessing vortex core,while the swirl number has a significant effect.When the Reynolds number increases,the Euler number and the proportion of energy loss do not change significantly;while the swirl number increases,the Euler number decreases,but the proportion of energy loss remains unchanged.It is speculated that the impact of precession vortices on energy loss is mainly reflected in causing fluctuations in pressure drop and energy loss,but has little impact on the amount of energy loss.Finally,the DMD implementation process was constructed and the reliability of the method was verified in a three-dimensional case of the flow past a circular cylinder.Then,it was used for processing of the snapshots of the gas cyclone to explore the characteristics of the dominant modes.The results show that the dominant modes of the gas cyclone include time-averaged mode(1st mode),stable double helix mode(2nd&3rd mode),weakly attenuated double helix mode(4th&5th mode),and vortex breakdown mode(36th&37th mode).Except for the time averaged mode,the other dominant mode frequencies are closely related to the frequency of the precession vortex core.The energy ratio of each mode verifies the inference that the precession vortex has little effect on the amount of energy loss.The results of flow field reconstruction show that the basic DMD method can effectively capture large-scale coherent structures such as precession vortex core,but its ability to analyze nonlinear phenomena such as turbulence is limited.