| The gas-solid two-phase turbulent opposed jet flow is widely used in industry.However,the flow characteristics of this kind of flow are not fully understanded until now.The existing research focused on the macroscopic flow characteristics and less concerned the multiscale phenomenons such as the structures of the gas stagnation plane,the complex flow structures in the transient flow field and particle movement behavior.In this paper,direct numerical simulation(DNS)and large eddy simulation(LES)are used to study the two-phase flow characteristics in turbulent opposed jet flow through the gas stagnation plane scale,turbulence scale,single particle scale and mescoscopic scale of particle cloud.Results can extend the basic understanding of the flow chararcteristics of this kind of flow.Firstly,the six-order finite difference method is used to directly solve the weakly compressible isothermal Navier-Stokes equations,the direct numerical simulations for the low and moderate Reynolds numer(Re ? 4100)turbulent opposed jet flow is conducted.The bifurcation phenomenon in a closely spaced opposed jet flow is identified.The influence of the nozzle disturbance on the instability of the impinging plane for the moderate nozzle separation(L/d = 12)opposed jet flow is studied.It is found that the radial jet in the impinging stream is more unstable than the axial jet,and the asymmetry disturbance on the nozzles can cause the impinging plane to to oscillate.At the higher frequency disturbed excitation,the impinging plane will appear a significant low frequency quasi-periodic oscillation phenomenon.The DNS results of the moderate Reynolds number(Re = 4100)turbulent opposed jet flow are analyzed by the proper orthogonal decomposition method.Results show that the unstable impinging plane is mainly composed of axial oscillation and radial deflection,in which the axial oscillation produces high-intensity turbulence in the impinging zone.In this paper,the Lagrangian impinging plane in the impinging zone is extracted by the finite Lyapunov exponent method,and the process of the unstable motion of the impinging plane is explained from the perspective of the fluid parcel.By applying high-precision filter and sub-grid scale model to the direct numerical simulation,a large eddy simulation method based on high-order finite difference is developed,and the high Reynold number(Re = 13500)turbulent opposed jet flow of medium(L/d = 12)and large nozzle separation(L/d= 30)are simulated.In the large nozzle separation flow,the turbulence is fully developed due to the axial jet,and the flow structures are less affected by the impact of the collision of two jets.The characteristic scales of the flow structures and the amplitude and period of the impinging plane are larger than the medium spacing turbulent opposed jet flow.Based on the large eddy simulation of the gas turbulent opposed jet flow,the Lagrangian point source method is used to track the particle trajectory and the backward force of the particles to the fluid is considered.The collision between the particles is dealed with the deterministic hard-sphere collision model.The motion characteristics of particles with different inertia(8<St<180)and different particle volume fraction(2E-5<?v<4.8E-3)are studied.It is found that the inertia of particles is the main factor affecting the maximum distance of the particles penetrating into the reverse jet.The interspersed movement of the bilateral particles in the impinging zone results in a smaller average particle velocity and a larger particle fluctuation velocity.The small inertial particles(St = 8)are more easily affected by the gas flow structure,and the particle concentration distribution is intermittent.Particle collision suppresses the penetration motion of the particles,resulting in an increase in the particle concentration in the impingign zone and a larger particle velocity gradient.The strong particle collision makes the particle distribution in the impinging zone tend to be isotropic and convert part of the axial fluctuation velocity into radial flucutation velocity.Unstable stagnation plane will cause the particles to oscillate similar to the gas-phase stagnation plane,when the particle inertia is smaller or the particle collision effect is stronger,the oscillation phenomenon of particles in the impinging zone is stronger.The presence of particles will weaken the amplitude and oscillation period of the impact surface,and the weakening effect increases with the increase of the particle loading.The turbulent kinetic energy decreases at the medium nozzle seperation,but at the large nozzle seperation,the decreasing of particle velocity is slower than the gas velocity,the gas is accelerated by the particle so that the gas velocity gradient becomes larger and the turbulent kinetic energy of the gas phase is also enhanced.Finally,based on the particle data simulated by the Lagrangian method,the mescoscopic velocity and random unrelated stress of particle cloud are obtained,results indicate the particle collision has a great effect on the random unrelated stress.Hence,a new random unrelated stress model considering the particle collision is proposed for the mesoscopic euler formalism.The results show that in the cases of different gas-phase Reynolds numbers,nozzle seperations and different particle inertia conditions,the effect of new model is better than the existing models whether or not considering the particle collision. |
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