Background With the relatively recent emergence of non-petroleum-derived aviation gas turbine fuels, it was appropriate to review the complete list of jet-fuel specifications to assess whether they were sufficiently robust to ensure fit-forpurpose within the new paradigm. Although this has been an industry-wide endeavour, there were some particular research areas that were identified for special in-house attention by Sasol, as the world’s first commercial producer of approved and certified semi-synthetic and fully synthetic jet fuel. The project described in this report formed part of one of these research areas, which pertained to ignition and combustion stability in gas turbines and the role played by various fuel attributes and properties. The project was conducted at the Sasol Advance Fuels Laboratory based at the University of Cape Town. Objectives The project entailed the design and construction of a combustion test facility for conducting synthetic jet fuel research. The primary intended focus of the facility was the investigation of ignition and combustion stability behaviour of various test fuels, ranging from commercial jet fuel to single component model fuels. The scope of the project also included the design of both a basic homogeneous and a heterogeneous combustor which served to verify the facility’s suitability for investigating the influence of fuel chemistry and combustor inlet conditions on ignition and combustion stability limits. Test facility design Design criteria, such as the required test condition range, facility scale, cost and safety, were considered during the generation of design concepts and the selection of the final facility design. The facility design was approached as a number of integrated subsystem designs. The final facility design employed a single positive displacement blower that allowed testing to be conducted under both vacuum and pressurised combustor inlet conditions depending on the configuration of the flow control valves. An absolute pressure range of 70kPa to 150kPa was attainable over a temperature range of 263K to 340K. The fuel system allowed primary zone equivalence ratios of 0.3 to 1.5 over the full air mass flow range of 0.85kg/min to 18kg/min.. This allowed the study of both temperature and pressure influences on ignition and combustion stability limits. A homogeneous and a heterogeneous combustor were designed to allow the study of both fuel chemistry influences in isolation and in conjunction with mixing and evaporation effects. Test programme As the sign-off acceptance criterion for the commissioning of the test facility, a test programme was conducted with a small selection of single component model fuels and some petroleum-derived Jet A-1. These tests were used to provide not only proof of the facility’s capabilities, but also to confirm the sensitivity of the equipment to detect and measure the expected influence of autoignition chemistry on threshold combustion performance. Tests with single component model fuels were performed using the premixed homogeneous combustor to assess the measurement capability of the test facility in terms of the influence of autoignition chemistry on lean ignition and lean blowout behaviour. This was followed by tests with petroleum-derived Jet A-1 in the heterogeneous combustor to assess the temperature and pressure dependence of ignition and combustion stability behaviour. Finally in order to determine how the results obtained in the homogeneous combustor translated to a heterogeneous environment, the lean blowout behaviour of two single component model fuels were compared with that of petroleum-derived Jet A-1. Conclusions The test programme provided conclusive evidence of the successful commissioning of the test facility. The results of the pressure and temperature influence evaluation clearly illustrated the repeatability of test results and the suitability of the test facility and the heterogeneous combustor design for investigating the ignition and extinction behaviour of practical synthetic jet fuel alternatives. The results of the fuel autoignition chemistry evaluation, using both combustor designs, revealed evidence of the influence of fuel chemistry and physical property effects. These results were seen to validate the motivation for designing and constructing a facility that would enable further study of the influence of fuel chemistry on ignition and extinction behaviour, and its particular relevance to synthetic gas turbine fuel formulation.

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