The present study concerns a particle-laden, swirling flow through a pipe expansion. A gas-particle flow enters the test section through a center tube, and a swirling air stream enters through a coaxial annulus. The swirl number based on the total inflow is 0.47. Numerical predictions of the gas flow were performed using a finite-volume approach for solving the time-averaged Navier-Stokes equations. The predicted mean velocity profiles showed good agreement with experimental results when using the standard k-ε turbulence model. The turbulent kinetic energy of the gas phase, however, is considerably underpredicted by this turbulence model, especially in the initial mixing region of the two jets. The particle dispersion characteristics in this complex flow were studied by using the Lagrangian method for particle tracking and considering the particle size distribution. The influence of the particle phase onto the fluid flow was neglected in the present stage, since only low particle loadings were considered. The particle mean velocities were again predicted reasonably well and differences between experiment and simulation were only found in the velocity fluctuations, which is partly the result of the underpredicted turbulent kinetic energy of the gas phase. The most sensitive parameter for validating the quality of numerical simulations for particle dispersion is the development of the particle mean number diameter which showed reasonable agreement with the experiments, except for the core region of the central recirculation bubble. This, however, is attributed again to the predicted low turbulent kinetic energy of the gas phase.
SUMMARYPredictions were performed for two different confined swirling flows with internal recirculation zones. The convection terms in the elliptic governing equations were discretized using three different finite differencing schemes : hybrid, quadratic upwind interpolation and skew upwind differencing. For each flow case, calculations were carried out with these schemes and successively refined grids were employed. For the turbulent flow case the k-& turbulence model was used. The predicted cases were a laminar swirling flow investigated by Bornstein and Escudier, and a turbulent low-swirl case studied by Roback and Johnson. In both cases an internal recirculation zone was present. The laminar case is well predicted when account is taken of the estimated radial velocity component at the chosen inlet plane. The quadratic upwind interpolation and skew upwind schemes predict the main features of the internal recirculation zone also with a coarse grid. The turbulent case is well predicted with the coarse as well as the finer grids, the skew upwind and quadratic upwind interpolation schemes yielding results very close to the measurements. It is concluded that the skew upwind scheme reaches grid independence slightly before the quadratic upwind scheme, both considerably earlier than the hybrid scheme.
KEY WORDS Confined flow Swirling flow k--E Model predictions
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