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The characteristic time model for ignition is used to correlate the spark ignition limits of fuel sprays in gas turbine combustors. The correlations are based on ignition data from several fuels in the JP-4 to JP-8 specification range and six gas turbine combustors. In the model, the criterion for ignition is that the spark energy must heat up an initial volume such that the heat release rate within that volume is greater than the loss rate. Heat generation is limited first by droplet evaporation and then the rate of hydrocarbon oxidation; heat loss is controlled by the rate of mixing (eddy diffusion). Conceptually, the rates are expressed as characteristic times in order to illustrate the relative importance of each process in the ignition event. Ignition limit correlations are given for each combustor as well as a single correlation of 322 data for four engines combined.

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Jet A fuel, solvent-refined coal II (SRC-II) middle distillate, and a 50:50 volume percent blend of Jet A and SRC-II middle distillate were burned in laboratory combustion rigs to study the effects of fuel properties on combustion performance. Centerline soot concentration, flame radiation intensity, and lean blowoff measurements were made in combustors simulating conventional and prechamber geometry gas turbine combustors. A special ignition rig was used to obtain minimum ignition energies. The centerline soot concentrations and flame radiation intensities for the SRC-II-middle distillate and the blend were very similar and were higher than those for Jet A. Characteristic time models correlating lean blowoff and minimum ignition energies indicated that the chemical properties of the fuels appeared to have only minor effects on these performance areas.