Abstract:
The in-house code FLAMENCO is developed to simulate the mixing process in
a Dry Low NOx GTU Combustor. The physical approach is defined to model
3D-unsteady, compressible, multi-species flows where turbulence plays a major role.
For this purpose, Large Eddy-Simulation is applied in conjunction with high-order
schemes and stable formulation of volume fraction advection. Regarding the numerical
structure, FLAMENCO is a Finite-Volume Godunov-type algorithm equipped with
5th and 2nd Order non-oscillatory reconstruction in space and 2nd Order, 4-Stages
Explicit Runge-Kutta scheme for integration in time. From a mathematical point
of view, the multi-species approach is governed by the 5-Equation Transport Model
and is thermodynamically defined by iso-baric and perfect gas considerations, which
prevent pressure oscillations. Finally, an HLLC approximate Riemann solver computes
convective fluxes and 2nd Order centred differences accounts for dissipation terms.
Previous research with an old version of FLAMENCO failed due to low dissipation in the
jet injector tube. This issue stems from local energy being uncontrollably introduced in
this small region, leading to unphysical pressure values. It was found that the combination
of reflecting inflow, small cells and high gradients is responsible for acoustic wave
reflection and amplification. To overcome this problem, a number of modifications
including boundary conditions (Partially Non-reflecting, Non-reflecting and Nozzle-type
subsonic inflows and outflows), an Adaptive Reconstruction Scheme, a more dissipative
reconstruction scheme (5th Order WENO) and grid changes (only in jet injector) have
been introduced. As a result, local energy generation and evacuation become balanced
within physical boundaries, providing stable conditions in the whole domain.
Initially, extensive validation of the new numerical approach is conducted through
contrasted test cases such as Stationary and Moving Contact Wave, Shock Tube Problem,
Kelvin-Helmholtz Instability and 2D-3D Explosion Problems. In the same way, strategies
intended to overcome the low dissipation problem are analysed in a representative
configuration. After the validation process, several simulations involving coarse and
fine grids and different reconstruction schemes are run in the Dry Low NOx GTU
Combustor. Finally, results are compared with experimental data, showing really good
accuracy for 5th Order schemes, which is specially surprising in the coarse grid. In
this way, highly turbulent, heterogeneous structures such as Vortex Breakdown, Central
Recirculation Zone, Precessing Vortex Core and Secondary Vortices are very well
captured, demonstrating the suitability of the mixing model to deal with highly turbulent
flows where critical shear layers and high mixing ratios coexist in confined domains.