Current gas turbine design practice for military and commercial aircraft is becoming marginal to meet the increasingly severe, yet conflicting requirements for reduced fuel burn, engine weight, and exhaust emissions, while achieving enhanced parts durability. The engine combustion system can be a key enabler in achieving future design goals. In both conventional single-stage and staged combustors, attention is focused on incremental fluid dynamic changes to enhance fuel-air mixing. In addition, dramatic materials development appears necessary to permit the increased operating temperatures that will ensue from advanced Brayton cycles. This paper describes a revolutionary combustion system that is far from incremental, but appears to offer the potential for continuing advances in engine performance. A gas turbine engine has been proposed that uses a near-constant-temperature (NCT) cycle and an Inter-Turbine Burner (ITB) to provide large amounts of power extraction from the low-pressure turbine. This level of energy is achieved with a modest temperature rise across the ITB. The additional energy can be used to power a large fan for an ultra-high bypass ratio transport aircraft, or to drive an alternator for large amounts of electrical power extraction. Conventional gas turbine engines cannot drive ultra-large diameter fans without the use of excessive turbine temperatures, or a substantial number of turbine stages. In addition, these conventional systems cannot meet high power extraction demands without a loss of engine thrust. The objective is to demonstrate an Ultra-Compact Combustor (UCC) that can be used as a main burner or an ITB that does not impact engine thrust-to-weight, pollutant emissions, or overall system performance. Concepts for an Ultra-Compact Combustor (UCC) are being explored experimentally. This system uses high swirl in a circumferential cavity to enhance reaction rates via high cavity g-loading on the order of 3000 g’s. Increase in reaction rates translates to a reduced combustor volume. The UCC design integrates compressor and turbine features which will enable a shorter and potentially less complex gas turbine engine. This paper will describe different variations of the UCC design where both the fuel injection method, turbine vane design, and the fuel injection angles are varied in the UCC. Experimental results from the UCC at atmospheric pressure indicate that the combustion system operates at 95–99% combustion efficiency over a wide range of operating conditions burning JP-8 +100 fuels. Axial flame lengths were extremely short, at about 50% those of conventional systems.

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Projected growth of aviation depends on fueling where specific needs must be met. Safety is paramount, and along with political, social, environmental, and legacy transport systems requirements, alternate aviation fueling becomes an opportunity of enormous proportions. Biofuels—sourced from halophytes, algae, cyanobacteria, and “weeds” using wastelands, waste water, and seawater—have the capacity to be drop-in fuel replacements for petroleum fuels. Biojet fuels from such sources solve the aviation CO2emissions issue and do not compete with food or freshwater needs. They are not detrimental to the social or environmental fabric and use the existing fuels infrastructure. Cost and sustainable supply remain the major impediments to alternate fuels. Halophytes are the near-term solution to biomass/biofuels capacity at reasonable costs; they simply involve more farming, at usual farming costs. Biofuels represent a win-win approach, proffering as they do—at least the ones we are studying—massive capacity, climate neutral-to-some sequestration, and ultimately, reasonable costs.