Dale T. Shouse

Ultra-Compact Combustors for Advanced Gas Turbine Engines

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.Copyright

Emissions Reduction Technologies for Military Gas Turbine Engines

Future military gas turbine engines will have higher performance than current engines, resulting in increased compressor and combustor exit temperatures, combustor pressures, and fuel-air ratios with wider operating limits. These combustor characteristics suggest undesirable exhaust emission levels of nitrogen oxides and smoke at maximum power and higher carbon monoxide and unburned hydrocarbons at low power. To control emission levels while improving performance, durability and cost, requires major advances in combustor technology. Current emissions control approaches as applied to conventional swirl-stabilized combustors include rich- and lean-burn strategies, together with staged combustion. These approaches, even in fully developed form, may not be sufficient to satisfy the projected design requirements. Unconventional combustor configurations may become necessary. Different engine cycles other than the standard Brayton cycle may also be used for special applications in order to avoid the use of excessive combustion temperatures. The paper presents an overview of the currently utilized emissions control approaches, comparing their performances and likely potential for meeting future requirements. Experimental results are presented for two non-conventional combustor configurations that have shown promise for advanced engine applications. A brief discussion is offered on cycle changes that could result in lower peak temperatures while maintaining advanced performance.

Performance Assessment of a Prototype Trapped Vortex Combustor Concept for Gas Turbine Application

GE Aircraft Engines and the Air Force Research Laboratory have been jointly developing a novel combustor technology concept for potential application in gas turbine engines. This novel combustor concept is known as the Trapped Vortex Combustor (TVC). The GE and AFRL team began work on the design of a prototypical TVC test rig in 1996. This effort represents the extension of earlier AFRL research with the TVC [1,2]. This work led to the fabrication of a 30.5 cm wide rectangular sector test rig capable of operation at inlet pressures up to 20.5 atmospheres, inlet temperatures up to 900 K, and to stoichiometric discharge conditions. Testing of the rectangular sector rig was initiated in mid year 1998. The performance evaluation performed on the test rig covered all aspects of gas turbine combustor performance and operability including ground start ignition, lean blowout, altitude re-light, emissions, combustion efficiency, exit gas temperature profile, and structural metal temperatures. Test rig operating conditions provided simulations of current commercial and military aircraft gas turbine engine cycles as well as some advanced engine cycles, with JP-8 type fuel. Data was also obtained at selected operating conditions for the LM2500 marine Navy duty cycle using DL-1 type fuel. The prototype rig has been operated for a total of approximately 300 run hours. 60 hours of run time at pressures exceeding 13.6 atmospheres and temperatures exceeding 675 K. 12 hours of run time at pressures exceeding 15.3 atmospheres, temperatures exceeding 780 K. Over 700 data points were obtained. The assessment of the demonstrated performance revealed the prototype TVC test rig had exceeded all initial expectations. Demonstrated ignition, blow out, and altitude re-light were up to 50% improved over current technology conventional swirl stabilized combustors. NOx emissions were in the range from 40% to 60% of the 1996 ICAO standard. Combustion efficiency at or above 99% was maintained over a 40% wider operating range than a conventional combustor. The performance and operability achieved with this prototype test rig has clearly demonstrated the validity and potential performance payoffs of the TVC concept. This paper will summarize the TVC rectangular sector test rig configurations evaluated as part of this test program, and the performance and operability achieved.Copyright

Aviation Fueling: A Cleaner, Greener Approach

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 CO2 emissions 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.

Experimental and Computational Study of Trapped Vortex Combustor Sector Rig with High-Speed Diffuser Flow

The Trapped Vortex Combustor (TVC) potentially offers numerous operational advantages over current production gas turbine engine combustors. These include lower weight, lower pollutant emissions, effective flame stabilization, high combustion efficiency, excellent high altitude relight capability, and operation in the lean burn or RQL modes of combustion. The present work describes the operational principles of the TVC, and extends diffuser velocities toward choked flow and provides system performance data. Performance data include EINOx results for various fuel-air ratios and combustor residence times, combustion efficiency as a function of combustor residence time, and combustor lean blow-out (LBO) performance. Computational fluid dynamics (CFD) simulations using liquid spray droplet evaporation and combustion modeling are performed and related to flow structures observed in photographs of the combustor. The CFD results are used to understand the aerodynamics and combustion features under different fueling conditions. Performance data acquired to date are favorable compared to conventional gas turbine combustors. Further testing over a wider range of fuel-air ratios, fuel flow splits, and pressure ratios is in progress to explore the TVC performance. In addition, alternate configurations for the upstream pressure feed, including bi-pass diffusion schemes, as well as variations on the fuel injection patterns, are currently in test and evaluation phases.

Exploration of Compact Combustors for Reheat Cycle Aero Engine Applications

An aero 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 geared fan for an ultra-high bypass ratio transport aircraft, or to drive an alternator for large amounts of electrical power extraction. Conventional gas turbines engines cannot drive ultra-large diameter fans without causing excessively high turbine temperatures, and cannot meet high power extraction demands without a loss of engine thrust. Reducing the size of the combustion system is key to make use of a NCT gas turbine cycle. Ultra-compact combustor (UCC) concepts are being explored experimentally. These systems use high swirl in a circumferential cavity about the engine centerline to enhance reaction rates via high cavity g-loading on the order of 3000 g’s. Any increase in reaction rate can be exploited to reduce 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 present experimental data of the Ultra-Compact Combustor (UCC) performance in vitiated flow. Vitiation levels were varied from 12–20% oxygen levels to simulate exhaust from the high pressure turbine (HPT). Experimental results from the ITB at atmospheric pressure indicate that the combustion system operates at 97–99% combustion efficiency over a wide range of operating conditions burning JP-8 +100 fuel. Flame lengths were extremely short, at about 50% of those seen in conventional systems. A wide range of operation is possible with lean blowout fuel-air ratio limits at 25–50% below the value of current systems. These results are significant because the ITB only requires a small (300°F) temperature rise for optimal power extraction, leading to operation of the ITB at near-lean-blowout limits of conventional combustor designs. This data lays the foundation for the design space required for future engine designs.Copyright

Computational Parametric Study of Fuel Distribution in an Experimental Trapped Vortex Combustor Sector Rig

Numerical simulations are performed to predict the flow properties in a liquid spray droplets fueled Trapped Vortex Combustor (TVC) sector rig. The quantities studied include aerodynamics, pressure drop, spray droplets trajectories, evaporation, mixing and combustion, and combustor exit temperature distributions. Previous numerical simulations of this TVC configuration have identified basic flow patterns and performance characteristics, and were generally in good agreement with experimental data. In the current effort, more detailed investigations were performed to understand the sensitivity of the TVC combustor to variations in the liquid fuel injection parameters. The computational model is described, including combustor geometry, boundary conditions for all combustion and cooling air injections, and spray droplets inlet conditions. A key finding is that liquid fuel injection boundary conditions for different types of downstream flows (cavity, high velocity cross flow) require different treatments, even though similar fuel injectors are used. This is evident in the large differences observed in the combustor exit plane pattern factor due to only minor differences in the fueling schemes. Combustor exit temperature profile strongly affects the design for turbine durability. With small changes in the temperature distribution, design modifications for the first turbine vane cooling schemes are required.Copyright

Operability and Efficiency Performance of Ultra-Compact, High Gravity (g) Combustor Concepts

Abstract : This paper presents a parametric design study of the Ultra-Compact Combustor (UCC), a novel design based on trapped-vortex combustor (TVC) work that uses high swirl in a circumferential cavity to enhance reaction rates via high cavity g-loading on the order of 3000 gs. Increase in reaction rates translates to a reduced combustor volume. Three combustor geometric features were varied during experiments which included (1) high-g cavity flame-holding method, (2) high-g cavity to main airflow transport method, and (3) fuel injection method. Experimental results are presented for these combustor configurations and results have shown promise for advanced engine applications. Lean blowout fuel-air ratio limits at 25-50% the value of current systems were demonstrated. Combustion efficiency was measured over a wide range of UCC operating conditions. This data begins to build the design space required for future engine designs that may use these novel, compact, high-g combustion systems.

Temperature Measurements in a Gas-Turbine-Combustor Sector Rig Using Swept-Wavelength Absorption Spectroscopy

Gas-temperature measurements in the combustion zone of a high-pressure gas-turbine-combustor sector rig were made with a Fourier-domain mode-locked laser using wavelength-agile absorption-spectroscopy techniques. These measurements are among the first employing broadband high-resolution absorption spectroscopy in gas-turbine-engine environments. Compared with previous measurements in reciprocating engines and shock tubes, signal contamination from thermal emission was stronger in this combustor rig; methods for managing emission during experimental planning and postprocessing are discussed. H 2 O spectra spanning 1330―1380 nm (which includes the ν 1 + ν 3 and 2ν 1 overtone bands) are presented along with a method for calculating gas temperatures from the spectra. The resulting temperatures are reported for a variety of combustor conditions. These tests show promise for simple gas-turbine sensors and potential for more detailed experiments involving tomographic reconstruction or multispecies concentration measurements.

Application of time-division-multiplexed lasers for measurements of gas temperature and CH4 and H2O concentrations at 30 kHz in a high-pressure combustor.

Two time-division-multiplexed (TDM) sources based on fiber Bragg gratings were applied to monitor gas temperature, H(2)O mole fraction, and CH(4) mole fraction using line-of-sight absorption spectroscopy in a practical high-pressure gas turbine combustor test article. Collectively, the two sources cycle through 14 wavelengths in the 1329-1667 nm range every 33 μs. Although it is based on absorption spectroscopy, this sensing technology is fundamentally different from typical diode-laser-based absorption sensors and has many advantages. Specifically, the TDM lasers allow efficient, flexible acquisition of discrete-wavelength information over a wide spectral range at very high speeds (typically 30 kHz) and thereby provide a multiplicity of precise data at high speeds. For the present gas turbine application, the TDM source wavelengths were chosen using simulated temperature-difference spectra. This approach is used to select TDM wavelengths that are near the optimum values for precise temperature and species-concentration measurements. The application of TDM lasers for other measurements in high-pressure, turbulent reacting flows and for two-dimensional tomographic reconstruction of the temperature and species-concentration fields is also forecast.