Statistical Evaluation of Ignition Phenomena in Turbojet Engines

Abstract

Turbojet engines must demonstrate their ability to be ignited on high altitude airfields or reignited in flight. These extreme conditions require the ignition of the engine in a large field of aerodynamic and thermodynamic conditions. Usually, a Mach-Altitude domain in which combustion can be established is used to represent the ignition performances of the aircraft engine. Nowadays, the prediction of this domain during the design process is mostly based on empirical rules. On the contrary, for the steady regimes, thanks to the use of computer aided design (CAD) and computational fluid dynamics (CFD) the combustor design is mainly done on the workstation. Finally the validation of the ignition configuration and the determination of the ignition domain demand an expensive experimental program. In an industrial context, it is therefore very important to include the ignition system design in the CAD process to reduce the costs of the experimental testing of the ability of a turbojet engine to be ignited. This requires the elaboration of a well tuned CFD model that is able to simulate the ignition phenomena properly. The aim of this study was therefore to gather experimental data on ignition to get a database for statistical evaluation of the ignition phenomena. A sample of 150 experiments at same operating conditions was performed. The experimental data were analyzed regarding the number of sparks needed to ignite and the ignition delay time. Further on, the probability that a certain spark will lead to ignition was computed. The paper discusses how the sample was statistically treated and the ignition probability was computed. Further the results on ignition probability are presented. The results of this study will be used to validate the ignition module of the AVBP LES CFD-Code at CERFACS in future works.