| Cyclone separator is an important equipment in gas-solid separation process,widely used in petrochemical industry,environmental protection,coal power generation,metal smelting and other industries.Although its internal geometry structure has no moving parts inside,the swirling flow inside the cyclone is unstable which affects the distribution of the flow field.Moreover,the inside flow field distribution predominantly affects the gas-solid separation performance.Thus,the study on the flow instability in the cyclone has a far-reaching significance for engineering practices.Currently,scholars have done a lot of research on the flow field in the cyclone separator,which plays an important role in the application of the cyclone separator.But a limited amount of papers on the flow instability inside the cyclone can be found,and the physics of vortexes inside cyclone is not perfectly studied.In addition,previous studies use tangential velocity to speculate the structure of the vortex when the flow field is analyzed.It is not clear about the motion pattern and development of the vortex in the flow field space inside the cyclone separator.In this article,the flow instability inside the cyclone is studied by means of numerical simulation and experimental investigation.The research results are as follows:(1)Dynamic hot-wire analyzer(DHWA)was used to measure the flow field inside the cyclone separator.The transient flow field and dynamic characteristics were analyzed.Fast fourier transformation(FFT)method was applied to analyze the spectrum of the experimental data of z=-300 mm and z=-900 mm.Experimental data show that in the radial direction of z=-300 mm section,the main frequency is mainly around 49.50 Hz.At the z=-900 mm section,the main frequency is 40.79 Hz in r=0-40 mm,while the main frequency is 49.50 Hz in r=40-70 mm.In addition,the spectrum analysis was carried out on the transition region(r=37-46 mm).The measurement points have two main frequencies,that is,the phenomenon of "double main frequencies" has taken place.It is worth noting that the second main frequency is not the multiple frequency of the first main frequency.(2)On the basis of experimental analysis,numerical simulation was performed to further study the flow characteristics,turbulence pulsation structure and turbulence intensity distribution in cyclone separator.The spectrum diagram corresponding to instantaneous velocity was obtained by FFT method.The numerical simulation results were in good agreement with the experimental data.The distribution of tangential velocity presents obvious non-axial symmetry.The deviation from the vortex core center to the geometric center of each section is not the same.However,it does not change linearly with the increase of cyclone height,but has certain fluctuation.Near the bottom of the exhaust pipe,the vortex core moves fast and the activity area is concentrated.Through the analysis of amplitude,RMS and TSI,it is found that the flow instability increases gradually when the moving fluid enters the cyclone separator from the inlet section.After reaching a certain degree,the flow instability gradually decreases until it becomes relatively stable.(3)By analyzing the main frequency distribution of all monitoring points,it was found that the main frequency in the cyclone separator is neither single main frequency distribution nor gradually decreasing main frequency distribution,but has three specific main frequency values,which are 49.5 Hz,36.5 Hz and 8.5 Hz,respectively.With the movement of the fluid,the flow also develops from a single vortex motion dominated by a main frequency to a double vortex motion dominated by two different main frequencies.The energy is transferred from a vortex with higher frequency(whose amplitude decreases gradually)to a vortex with lower frequency(whose amplitude increases gradually).In this process,the motion frequency of the vortex is "self-sustaining",which is reflected in the fact that the motion frequency of the vortex remains basically unchanged until the vortex disappears and the energy dissipation is completed.Therefore,the motion frequency of the vortex in the whole space presents a quantum distribution.In addition,the influence of operation parameters and geometric parameters on the flow instability is analyzed.The results show that the cylinder length has no effect on the first motion frequency when the external conditions does not change.In the cyclone separator,the phenomenon of "double main frequencies" and the generation of double vortex motion require certain external conditions,such as the length of the cylone reaching a certain value.Under the conditions of different inlet velocities,the motion frequency of the vortex in the cyclone separator changes with different inlet velocity,and increases with the increase of inlet velocity.The position of the first boundary and the second boundary in the cyclone separator is almost unchanged,indicating that the development of the vortex has little relation with the inlet velocity.The vortex motion changes in the inner swirl region,while the outer swirl region develops stably.Because of the turbulent diffusion,the whole flow field is finally changed.(4)The instability of the swirling flow is the apparent feature of the moving fluid,while the internal essential feature is the motion of the vortex.The Q-criterion is utilized to identify the vortex structure,and then a new three-dimensional iso-vortex surface is made.According to the new iso-vortex surface,the motion form of the vortex is more intuitive and specific.From the perspective of vortex,the flow field distribution and energy transformation in cyclone separator are analyzed.The influence of vortex on the inner space of cyclone separator is quantified by iso-vortex surface,vortex lines,vorticity and vortex core center,which improves the understanding of inner space vortex.In addition,the appropriate geometric structure and operating conditions are conducive to improve the balance of vortex structure,enhance the flow stability of cyclone,and improve the separation efficiency. |
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