Effect of GTL-Like Jet Fuel Composition on GT Engine Altitude Ignition Performance: Part I—Combustor Operability

Abstract

The current fuel used in aviation turbines is kerosene, and is tightly controlled to a well defined specification. The past 50 years of simultaneous development between the aviation turbine and kerosene jet fuel has led to the fuel specification. The design of the combustion system has also been developed with this fuel chemistry and specification. In the past 5 years, there has been a ground swell of interest in alternative fuels for aviation, where the fuels can be made from a variety of feedstocks and processes. The chemistry and composition of species within future alternative fuels will change from the current kerosene jet fuel specifications; therefore research has been carried out looking at the effects of some of the fundamental component species that will be found in potential future fuels. The gas turbine combustion ignition and stability characteristics were studied while fuelled by a series of gas-to-liquid (GTL) Synthetic Paraffinic Kerosene (SPK)-type fuels by measurement of the successful ignition and flame stability regimes at realistic altitude temperatures and pressures. The combustor under test was a multi-sector representation of an advanced gas turbine combustor and fuel injector. Tests were conducted on the Rolls-Royce plc TRL3 (Technology Readiness Level) sub-atmospheric altitude ignition facility in Derby, UK. The facility was operated at simulated altitude conditions of 6 and 8 psi combustor inlet pressure with corresponding air and fuel temperatures to represent combustor conditions following flame-out during high altitude cruise. The GTL SPK-type fuels were selected to generate a pseudo-Design of Experiments (DoE) matrix in which the iso- to normal-paraffin ratio, cyclic paraffin content, and carbon number range were varied to isolate the effects of each. Tests were conducted at combinations of air mass flow rate and fuel-air ratio necessary to map the regimes of successful ignition and flame stability. All fuels indicated little or no deterioration to the weak boundary of the ignition regime, nor the weak extinction limits, within the scatter of the experimental method. Evidence was found that a commercial GTL SPK, as well as one of the DoE blends, may have greater ignition performance at simulated altitude conditions. Further testing at higher TRL levels is recommended to confirm this finding. The test programme was supported by DLR, German Aerospace Centre, through high-speed diagnostic imaging of the ignition process, including OH* and CH* chemi-luminescence measurements, which is the subject of a separate complementary paper.