A 2D model is built on the package of FLUENT to study the effects of radial aspect ratio(R/W),length-to-width ratio(L/W),strain rate(SR),and buoyancy(Ri=Gr/Re2) on the validation of the simplified 1D model.In the present 2D model,the methane/air homogeneous reaction mechanism of Peters and the methane/air/platinum heterogeneous reaction mechanism of Deutschmann are applied.By comparison between the 1D and 2D numerical results,it is found that the validation of 1D model is highly related with the catalytic stagnation reactor configuration.For length-to-width ratio L/W = 1 configuration,1D laminar model is applicable when the radial aspect ratio R/W > 0.4.For R/W = 0.6,the reactor exhibited 1D characteristics when L/W < 1.Compared with the temperature and species profiles,the velocity distribution along the axis is more sensitive to the change of radial aspect ratio and length-to-width ratio.With increasing of the strain rate,the flame front goes closer to the catalytic wall surface and the difference between the 1D and 2D results decreases.For a valid 1D simulation,it is recommended that the strain rate should be greater than 20 s-1.The effects of natural convection can be neglected when Ri < 5.
Large eddy simulations(LES) were performed to study the non-reacting flow fields of a Cambridge swirl burner. The dynamic Smagorinsky eddy viscosity model is used as the sub-grid scale turbulence model. Comparisons of experimental data show that the LES results are capable of predicting mean and root-mean-square velocity profiles. The LES results show that the annular swirling flow has a minor impact on the formation of the bluff-body recirculation zone. The vortex structures near the shear layers, visualized by the iso-surface of Q-criterion, display ring structures in non-swirling flow and helical structures in swirling flow near the burner exit. Spectral analysis was employed to predict the occurrence of flow oscillations induced by vortex shedding and precessing vortex core(PVC). In order to extract accurately the unsteady large-scale structures in swirling flow, a three-dimensional proper orthogonal decomposition(POD) method was developed to reconstruct turbulent fluctuating velocity fields. POD analysis reveals that flow fields contain co-existing helical and toroidal shaped coherent structures. The helical structure associated with the PVC is the most energetic dynamic flow structure. The latter toroidal structure associated with vortex shedding has lower energy content which indicates that it is a secondary structure.
The present study aims at the investigation of the effects of turbulence-chemistry interaction on combustion instabilities using a probability density function(PDF) method.The instantaneous quantities in the flow field were decomposed into the Favre-averaged variables and the stochastic fluctuations,which were calculated by unsteady Reynolds averaged Navier-Stokes(U-RANS) equations and the PDF model,respectively.A joint fluctuating velocityfrequency-composition PDF was used.The governing equations are solved by a consistent hybrid finite volume/MonteCarlo algorithm on triangular unstructured meshes.A nonreacting flow behind a triangular-shaped bluff body flame stabilizer in a rectilinear combustor was simulated by the present method.The results demonstrate the capability of the present method to capture the large-scale coherent structures.The triple decomposition was performed,by dividing the coherent Favre-averaged velocity into time-averaged value and periodical coherent part,to analyze the coherent and incoherent contributions to Reynolds stresses.A simple modification to the coefficients in the turbulent frequency model will help to improve the simulation results.Unsteady flow fields were depicted by streamlines and vorticity contours.Moreover,the association between turbulence production and vorticity saddle points is illustrated.
