Characteristic Time Scale Modelling of Gas Turbine Combustor Ignition Limits at Sub Idle Conditions

Abstract This paper presents a study of aerospace gas turbine combustor operability performance to capture the lean boundaries of ignition at sub idle conditions. Presented in previous literature, the characteristic time scale method of ignition modelling (CTM) captures the time scales of the droplet evaporation and the chemical reaction processes and limits these against the aerodynamic time scale available for the droplet ignition to occur. Minimum ignition energy predictions can then be found. In the present study, the method was applied to experimental datasets taken over a range of pressures and temperatures within the sub idle operating regime, typically seen in a combustor during ignition at altitude. High-speed imaging of the initial 1.0ms of ignition was undertaken to examine the effect of inlet pressure and temperature on expansion of an unfueled spark, and the impact of fuel on the initial growth of this kernel to assess model input assumptions. Pressure was found to impact the expansion of the unfueled kernel, and the presence of fuel to impact the heat release within the kernel at time scales of under 0.1ms. The predicted changes in ignition boundaries with variation of inlet pressure and temperature were evaluated against experimental ignition loop datasets. The model was found to give a good prediction of the mass flow and FAR ignition limits for the inlet air pressures and temperatures tested. The evaporation time scale was demonstrated to dominate ignition when the pressure drop across the test section was low. This was driven by poor atomization of fuel at reduced mass flow rates through the FSN. As the test section pressure drop was increased, the chemical reaction processes were seen to dominate, due to reduced droplet size driving a reduction in evaporation time scales.