Toward LES of an ignition sequence in a full helicopter combustor

Being able to ignite or reignite a gas turbine engine in a cold and rarified atmosphere is a critical issue for many manu-facturers. In the Vesta combustor of Turbomeca, 18 main burners are ignited by two pilot flames. The success of an ignition attempt depends on the completion of three steps. In a first step, a spark plug must ignite the fuel spray produced by each starting burner to generate the pilot flames. In a second step, these two flames must provide a sufficient amount of energy to initiate a stable combustion in the main burners located in their vicinity. Finally, the reactive zone must propagate from one main burner to its nearest neighbour until the whole chamber is ignited. These three steps are strongly influenced by the topology of the flow as well as the two-phase turbulent combustion phe-nomena. To represent properly the physics involved in such a complex and unsteady process, Large Eddy Simulation (LES) seems to be a powerful tool. This technique has been successfully applied to turbulent reactive flows (Selle et al.1) and recently extended to two-phase combustion in gas turbines (Pascaud et al.2). The numerical tool used in the present study couples an eulerian solver for the liquid spray with a LES solver for the gas flow. Thanks to the use of unstructured grids, it can be applied to complex geometries. This work focuses on the propagation of the flame from the starting burners to the whole chamber but not on the ignition of the starting burners themselves: the spray flame produced by the starting injector is simply replaced by a hot gases jet. First, two-phase flow calculations will be presented in which this jet ignites one isolated burner. Then, some results will be shown from a single-phase flow calculation done on the IBM supercomputer BlueGene/L- cited by TOP500 as the world’s fastest machine- where the use of 2048 parallel processors have enabled to start computing on the full combustor domain (i.e. 18 main injectors + 2 pilot flames). Report number: TR/CFD/05/7