Using LES to predict ignition sequences and ignition probability of turbulent two-phase flames

Abstract

Ignition and altitude reignition are critical issues for aeronautical combustion chambers. The success of the ignition phase of a combustor depends on multiple factors, from the characteristics of the ignitor to the spray droplet size or the level of turbulence at the ignition site. The optimal location of the ignitor or the potential of ignition success of a given energy source are therefore parameters of primary importance in the design and the certification of combustion chambers. To study ignition, series of experiments are usually performed but they are costly especially when multiple spark locations must be tested. For the same reason, current unsteady simulations are useful but do not give reliable results, and require a lot of simulations if different locations are to be tested, which brings the CPU cost to unreasonable values. Alternatives are hence needed and are the objective of this contribution. It is proposed here to derive a local ignition criterion, giving the probability of ignition from the knowledge of instantaneous non-reacting two-phase (air and fuel) flow data. This model is based on criteria for the phases of a successful ignition process, from the first kernel formation to the early flame propagation towards the injector. Comparison with experimental data on an aeronautical chamber shows qualitative agreement, indicating that the proposed ignition criterion, coupled to a Large Eddy Simulation of the stationary evaporating two-phase non-reacting flow, can be used to optimize the ignitor location and power.