Large-Eddy Simulation of air-fuel mixing in Dry Low NOx GTU Combustor

摘要

The in-house code FLAMENCO is developed to simulate the mixing process ina Dry Low NOx GTU Combustor. The physical approach is defined to model3D-unsteady, compressible, multi-species flows where turbulence plays a major role.For this purpose, Large Eddy-Simulation is applied in conjunction with high-orderschemes and stable formulation of volume fraction advection. Regarding the numericalstructure, FLAMENCO is a Finite-Volume Godunov-type algorithm equipped with5th and 2nd Order non-oscillatory reconstruction in space and 2nd Order, 4-StagesExplicit Runge-Kutta scheme for integration in time. From a mathematical pointof view, the multi-species approach is governed by the 5-Equation Transport Modeland is thermodynamically defined by iso-baric and perfect gas considerations, whichprevent pressure oscillations. Finally, an HLLC approximate Riemann solver computesconvective fluxes and 2nd Order centred differences accounts for dissipation terms.Previous research with an old version of FLAMENCO failed due to low dissipation in thejet injector tube. This issue stems from local energy being uncontrollably introduced inthis small region, leading to unphysical pressure values. It was found that the combinationof reflecting inflow, small cells and high gradients is responsible for acoustic wavereflection and amplification. To overcome this problem, a number of modificationsincluding boundary conditions (Partially Non-reflecting, Non-reflecting and Nozzle-typesubsonic inflows and outflows), an Adaptive Reconstruction Scheme, a more dissipativereconstruction scheme (5th Order WENO) and grid changes (only in jet injector) havebeen introduced. As a result, local energy generation and evacuation become balancedwithin physical boundaries, providing stable conditions in the whole domain.Initially, extensive validation of the new numerical approach is conducted throughcontrasted test cases such as Stationary and Moving Contact Wave, Shock Tube Problem,Kelvin-Helmholtz Instability and 2D-3D Explosion Problems. In the same way, strategiesintended to overcome the low dissipation problem are analysed in a representativeconfiguration. After the validation process, several simulations involving coarse andfine grids and different reconstruction schemes are run in the Dry Low NOx GTUCombustor. Finally, results are compared with experimental data, showing really goodaccuracy for 5th Order schemes, which is specially surprising in the coarse grid. Inthis way, highly turbulent, heterogeneous structures such as Vortex Breakdown, CentralRecirculation Zone, Precessing Vortex Core and Secondary Vortices are very wellcaptured, demonstrating the suitability of the mixing model to deal with highly turbulentflows where critical shear layers and high mixing ratios coexist in confined domains.