Numerical investigation of the non-reacting unsteady flow behind a disk stabilized burner with large blockage

Description

This work deals with numerical simulations of annular jets, with particular emphasis onhigh blockage ratio jets. Annular jets are of practical interest because of their axisymmetric geometry and their strong recirculating flow due to flow separation. When combustion is included, recirculation guarantees high levels of mixing, leading to stable flames and reduced pollutants emission.The investigation is performed using numerical simulations, as annular jets at high blockage ratios have never been studied numerically before.Moreover, neither three-dimensional simulations nor unsteady simulations have ever been performed before on annular jets, the nature of which can be characterized by intense mixing, recirculation and vortex shedding.Therefore, the results of this work serve as a theoretical basis for the design of high blockage/high recirculation axisymmetric bluff body gas burners. The technique used for the simulations is the solution of the steady and unsteady Reynolds Averaged Navier-Stokes (RANS) equations with the flow solver CFX-TASCflow. The axisymmetricsteady simulations at several blockage ratios show that the predictions of the recirculation zone length is accurate at low blockage ratios. In the high blockage ratio range the simulations are inaccurate, as the flow is asymmetric. The velocity fluctuations obtained with a Reynolds Stress model are significantly below the measured fluctuations.Flow asymmetry at the high blockage ratio is observed with LDA measurements and threedimensional steady simulations. The asymmetry develops after the jet nozzle and is characterized by a preferential direction from one part of the annular jet to the other. The stagnation point is dislocated and shifted from the symmetry axis. Asymmetric flows coming out from symmetric geometries and boundary conditions has been already investigated by other researchers and are possible solutions of the non-linear problem expressed by the Navier-Stokes equations.The unsteady RANS simulations are generally able to capture the large vortex dynamicsand the associated velocity fluctuations. So, the total fluctuations can be computed from the coherent or deterministic (large eddy) fluctuations and the modelled (small eddy) fluctuations.Our three-dimensional unsteady simulations of the high blockage annular jet are performed using different approaches for the modeled velocity fluctuations. The approach with the Standard k-ε model shows damping of the coherent fluctuations, due to the excessive dissipation introduced by the turbulent viscosity. Instead, the no-model approach and the approach with a Reynolds Stress model present stable velocity oscillations.The time averaged solution of the three-dimensional unsteady simulations is asymmetric,with the same features obtained in the three-dimensional steady simulations. This indicates that the asymmetry persists also if large vortex fluctuations are introduced.When compared to the experimental results, both the no-model approach and the approach with a Reynolds Stress model show good agreement for the velocity fluctuations. The two approaches result in more accurate values of the fluctuations than a steady computation, which ignores the large scale unsteadiness of the flow. So, the contribution of the coherent fluctuations is crucial. Moreover, both approaches reveal an oscillation frequency which is of the same order of magnitude of the frequency previously measured in the combusting flow. This frequency is associated to the periodic movement of large vortex structures, e.g., vortex shedding and convection.In general, there is no superior performance of the approach with the Reynolds Stress model when compared to the no-model approach. This latter approach is a kind of LES, as the small scale dissipative effects are reproduced through numerical dissipation. Inside the recirculation zone the no-model approach give very good levels of fluctuations already with a medium resolution grid. The approach with the turbulence model behaves slightly better in the regions where the high frequency fluctuation contribution is larger and the grid is locally coarse (e.g., the downstream region).With a numerical study it is observed, that break of symmetry of high blockage ratio annular jets is preceded by oscillations in the near wall region. These perturbations propagate to the axial stagnation point and cause symmetry breaking. Indeed, both the imbalance of pressure and inertia force at the stagnation point, and the small thickness of the jet, represent an unstable condition for symmetry. Therefore, a small perturbation is able to distort the jet sufficiently to lose symmetry.