Computational fluid dynamics analysis of ignition characteristics of a reversed flow combustion chamber with large eddy simulation approach

Ignition characteristic is one of the most important issues in a gas turbine combustion chamber design. In this study, two-phase reacting flow through a reversed flow combustion chamber is analyzed using Computational Fluid Dynamics (CFD) with Large Eddy Simulation (LES) approach. The combustion chamber analyzed belongs to a small scale turboprop / turboshaft engine (160-180 kW) that will be used in 2-5 seats commuter aircraft. Analyses have been carried out for sector and full annular (FAnn) geometry. CFD analyses were focused on simulation of ignition, flame propagation and source of potential instabilities. The computations were performed to identify the thermo-fluid and heat transfer characteristics for the combustion chamber. First of all, reacting simulations with Reynolds Averaged Navier-Stokes (RANS) method are done in order to have a reference case to LES computations. Due to the steady-state approach, diffusive nature of the RANS method and the isotropic turbulence assumption of the employed turbulence model (Realizable k-ε); it has been presumed that same thermo-flow field will be obtained with the RANS method at each combustor 1/12 sector. Therefore the RANS computations are carried out for one sector of the combustor in order to reduce the computational costs and have a faster convergence. During sector analysis, mesh, combustion model and two phase flow characteristics studies have been carried out. In order to determine the two-phase flow characteristic LISA (Linearized Instability Sheet Analysis) break-up and TAB break-up models have been used. Sandia Flame-D test data have been used for combustion model validation studies. After that, reacting RANS calculations have been performed for a single sector as well. By this means, a methodology could be established as a reference to the FAnn simulations with less computation time. Combustor basic performance parameters such as outlet temperature distribution, velocity distribution, Radial Temperature Distribution Factor (RTDF), Overall Temperature Distribution Factor (OTDF) and ignition parameters were predicted. Time dependent temperature field images were gathered during simulation and these images were used to make movies which enables to understand ignition and flame propagation behaviors of the combustion chamber. As a result, fuel injection and ignitor has started 3 ms in flow time and flame propagation is completed around 63 ms. RTDF and OTDF are found as %5 and %29 respectively.