Numerical simulations of the particle-laden gas–solid flow in horizontal circular pipes have been used to identify the role of particle collision coefficients in flow regimes within it. A four-way coupling Euler–Lagrangian approach was employed, using direct numerical simulations of the gas phase and Lagrangian particle tracking to account for the drag, gravitational and lift forces, together with particle–wall and inter-particle interactions. The influences on the flow of the mass loading ratio (Φm) and of the coefficients of restitution for collisions both between particles and the wall (ep−w) and between particles (ep−p) are assessed by examining the fluid and particle velocities, particle concentration distribution, turbulence kinetic energy, static pressure, inter-phase transferred momentum, and the secondary flow motions of both the fluid and particle phases. Three dominant flow regimes that include three sub-regimes based on their secondary flow patterns are identified, the transition between which depends on the combination of Φm, ep−w, and ep−p. Additionally, the quantitative dependence of these transitions on these three parameters is also reported for a series of Stokes and Froude numbers.
In this study, large eddy simulations (LES) of turbulent coflow jets are performed and designed to investigate the effects of the jet-to-coflow velocity ratio, Vr, on jet characteristics. A fully developed turbulent pipe flow at Re=10,000, based on the bulk velocity and pipe diameter, is employed as the jet outlet in this work. A comparison between laminar and turbulent jets is performed against the experimental results of a jet produced by a fully developed turbulent pipe flow. For the coflow jet, simulations with different jet-to-coflow velocity ratios (Vr = 3, 6, 12, and ∞) are performed to investigate the turbulence intensities and the decay of the centerline velocity of the jet. The results give two constant decay rates: Ku≈0.144 for single-phase jets and Ku≈0.133 for particle-laden jets. With a decrease in Vr (i.e., a higher coflow velocity), the results show a higher peak value and a larger droop rate for turbulence intensities. This study is then extended to investigate particle distribution under a two-way coupling regime, using a Lagrangian framework. The particle velocity and distribution along the jet centerline, and the particle clustering and radial probability distribution in the jet downstream domain are analyzed with the same coflow jet parameters. The particles tend to move faster and distribute preferentially in the center region with a decrease in Vr, which agrees with the increasing turbulence intensities along the jet centerline in the present work.
Numerical simulations have been conducted to identify the dominant mechanism responsible for driving secondary flow motions in horizontal particle-laden pipe flows, based on an analysis of the forces acting on each phase. A four-way coupling Euler–Lagrangian approach was employed, using direct numerical simulations for the gas phase and Lagrangian particle tracking to account for the drag, gravitational and lift forces, together with the interactions that occur for both particle–wall and inter-particle collisions. The four different flow regimes, which had been identified previously as depending on various combinations of flow parameters and are characterised by the secondary flow structures of both the fluid and particle phases, were identified via varying the mass loading alone from
$\varPhi _m=0.4$
to
$\varPhi _m=1.8$
. The distribution of the divergence of Reynolds stresses was used to help characterise the classes of the secondary fluid flow. This shows that secondary fluid flows of both the first and second kinds can either exist separately or co-exist in such flows. The forces exerted on the fluid phase by the pressure gradient and fluid–particle interactions were examined qualitatively and quantitatively to identify their contribution to the secondary fluid flow motions. A similar study was also applied to the drag, lift and gravitational forces exerted on the particle phase for the secondary particle flow motions. These were found to explain the secondary flows of both the fluid and particle phases with regard to both the flow direction and magnitude, together with the interaction between the two phases.
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