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A numerical optimization study is performed on a small-scale high-swirl cavity-stabilized combustor. A parametric geometry is created in CAD software that is coupled with meshing software. The latter automatically transfers meshes and boundary conditions to the solver, which is coupled with a post-processing tool. Steady, incompressible three-dimensional simulations are performed using a multi-phase Realizable k-ϵ Reynolds-averaged Navier-Stokes (RANS) approach with the non-adiabatic flamelet progress variable (FPV). There are nine input parameters based on geometrical control variables. There are five output parameters, viz., pattern factor (PF), RMS of the profile factor deviation, averaged exit temperature, averaged exit swirl angle, and total pressure loss. An iterative design of experiments (DOE) with a recursive Latin Hypercube Sampling (LHS) is performed to filter the most important input parameters. The five major input parameters are found with Spearman’s order-rank correlation and R2 coefficient of determination. The five input parameters are used for the adaptive multiple objective (AMO) optimization. The AMO algorithm provided a candidate design point with the lowest weighted objective function. This design point was verified through CFD simulation. The combined filtering and optimization procedures improve the baseline design point in terms of pattern and profile factor. The former halved from that of the baseline design point whereas the latter turned from an outer peak to a center peak profile, closely mimicking an ideal profile. The exit swirl angle favorably increased 25%. The averaged exit temperature and the total pressure losses remained nearly unchanged from the baseline design point.

The capability of current turbomachinery-based engines limit the obtainable altitude and flight Mach number of modern aircraft. Maturing hypersonic technologies such as ram and scramjets allow greatly increased flight velocity but are not able to power themselves from the ground and, thus, rely on lift aircraft. Combined-cycle engines incorporate turbomachinery and ramjet technologies to allow both high and low flight Mach numbers. These high Mach capable systems are typically flown using specialty fuels such as JP-7 that are more applicable to hypersonic applications than conventional gas-turbine fuels such as JP-8/Jet A or JP-5. Potential issues exist, however, when operating a combined cycle that employs a legacy main combustor since these platforms were not designed for operation with these fuels. Of particular concern is the re-ignition performance of the turbomachinery core. The relight of the gas-turbine combustor occurs at high altitudes and relatively high vehicle speeds, yielding low pressure and temperature in the combustor as well as high combustor-dome velocities. All of these conditions are unfavorable for ignition. Additionally, heavy fuels require more energy for atomization and vaporization, which increases the probability that ignition will become a problem in a turbine-based combined-cycle (TBCC) system. Successful demonstration of legacy main-combustor technologies in hypersonic combined-cycle applications will eliminate the need for costly design of new main-burner technology. The literature does not provide information on the effects of running specialty fuels such as JP-7 and JP-10 in burners with conventional aerodynamic features. To fill that gap, a three-cup sector of a conventional fighter-class swirl-stabilized combustor configured to provide optical access through the sidewall was used in the study. Two test conditions representative of varying flight Mach number and altitude are evaluated. For each flight condition, the combustor pressure drop is varied to characterize ignition as a function of burner inlet velocity. Data are correlated by loading parameter and equivalence ratio at ignition, and at each test point the conditions are varied until ignition cannot be achieved.

One of the challenges of gas turbine combustor research is to accurately measure and model air mass flow rates through complex air injection schemes. Accurate measurements and computations of air mass flow rates are necessary for determining air and fuel distributions, which influence a range of combustor operation and performance characteristics. Experimental and computational studies were performed on a representative gas turbine combustor swirler. The swirler geometry consists of four component flows: two co-rotating annular axial swirls, one radial swirl, and cooling on the periphery of the swirler. The purpose of this study is to compare measured and computed air mass flow rates in a realistic swirler with a complex geometry and to quantify the magnitude of the interaction effects between air passages. The measurements of the air mass flow rates were performed using a calibrated air flow stand. The computations were performed using commercial Computational Fluid Dynamics (CFD) tools. Comparisons between measured and computed air mass flow rates show good agreement for the individual and total flow configurations. Significant interaction effects among the swirling flows are observed when all of the air passages are open. The radial swirl mass flow rate decreases by 2.7% and the outer axial swirl mass flow rate increases by 3.8% when the individual component flow configuration is compared to the total flow configuration. The computed mass flow rates demonstrate that the interactions among the swirl flows create a significant change in mass flow distribution within the swirler.

This article describes an investigation of flame shapes and flow configurations in a premixed, swirl-stabilized dump combustor. High swirl, annular nozzle flows of this nature enable a variety of different flame configurations and heat release distributions with their associated flow fields. These differences are significant, since each of these configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle lifetime, and liner heating. These different configurations arise because multiple flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. We present results from high-speed luminosity imaging, particle image velocimetry (PIV), and OH-planar laser induced fluorescence (PLIF) to illustrate time-averaged and instantaneous flame shapes and flow fields associated with the different configuration “families.” Selected cases are compared with large edd...

A mid-infrared fiber-coupled laser system constructed around three time-division-multiplexed quantum-cascade lasers capable of measuring the absorption spectra of CO, CO2, and N2O at 100 kHz over a wide range of operating pressures and temperatures is demonstrated. This system is first demonstrated in a laboratory burner and then used to measure temperature, pressure, and concentrations of CO, CO2, and N2O as a function of time in a detonated mixture of N2O and C3H8. Both fuel-rich and fuel-lean detonation cases are outlined. High-temperature fluctuations during the blowdown are observed. Concentrations of CO are shown to decrease with time for fuel-lean conditions and increase for fuel-rich conditions.

Volatile fuel costs have initiated a global sprint for technologies that will increase fuel efficiency in gas turbine engines, which continue to be a primary propulsion system for commercial and military aircraft. The Ultra-Compact Combustor (UCC) is an advanced gas turbine combustor which integrates the high pressure turbine inlet guide vanes into the combustor. In comparison to conventional combustor systems, the UCC has the potential to shorten engine length, decrease engine weight, and reduce pressure losses entering the high pressure turbine rotor, all of which contribute subtantially to improving engine efficiency. The UCC has been studied in two forms in the literature – the Trapped-Vortex concept (TV) and the High-g concept (HG); the current study focuses on the latter. The HG-UCC utilizes a rectangular cavity wrapped around the outside circumference of the combustor where fuel and air are injected in a manner that generates a highly swirling flow within the circumferential cavity. The flow within the cavity experiences a centripetal acceleration (“g-load”) which varies with tangential velocity and radius from the combustor axis. This “high-g” effect has the potential to increase flame speeds and volumetric heating rates, as well as enhance fuel air mixing and spreading providing the potential to reduce fuel injector count without compromising performance. Experiments were performed on the HG-UCC combustor at higher operating pressures, temperatures, and equivalence ratios than previous high-g experiments conducted in a similar uncooled inter-turbine burner (ITB). Results show that the HG-UCC has high combustion efficiencies for the entire range of data considered. The effects of pressure and temperature, cavity driver angle, centripetal acceleration (g-load), and equivalence ratio are studied for their impacts on combustion efficiency and nitrogen oxide (NOx) emissions. An important result of this work is the construction of NOx correlations for the HG-UCC. Test data performance results and correlations are compared to previous trapped vortex and high-g UCC experiments.

During the past fiscal year, researchers with the Air Force Research Laboratorys Combustion Branch made substantial progress in numerous areas including: 1) ultra-compact combustors, 2) inter-turbine burner concepts, 3) bluff-body stabilized turbulent flames, 4) well-stirred reactors for chemical kinetics, and 5) detonation-stabilized turbulent flames.Lean blowout data was collected on propane and jet fuel bluff-body stabilized flames and was combined with data taken from past literature to create a database of over 1,100 data points. The best correlation for the overall dataset included pressure, temperature, the ratio of flameholder diameter to lip velocity, oxygen level, and the hydrogen-to-carbon ratio of the fuel as factors. The R-squared value of this correlation was 0.873. The exponents on the factors indicated that high pressure and temperature lowered the equivalence ratio at lean blowout, as did high levels of oxygen. The correlation for the ignition delay dataset with pressure, temperature, and the ratio of flameholder diameter to lip velocity as factors had an R-squared value of 0.942. This relatively small, uniform dataset correlates very well. When ignition delay time replaces pressure and temperature, the data continues to correlate well, with an R-squared value of 0.918. For this limited dataset, the ignition delay time is an adequate representation of the chemical timescale.

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.

Significant benefits can be obtained with respect to engine thrust-to-weight ratio and specific fuel consumption if the length, weight, and pressure drop of the combustor can be reduced. The ultra-compact combustor (UCC) has the potential to aid the realization of these benefits by integrating neighboring components such as the compressor exit diffuser and the turbine inlet guide vanes (IGV) within the combustor using a systems-level engineering approach. The UCC presented here utilizes a trapped-vortex cavity. This combustor design has been shown to exhibit larger turn-down ratios, higher flame stability, shorter flame lengths, and acceptable NOx emissions when compared to conventional richburn, quick-quench, lean-burn combustors. The axial distance required to complete combustion within the mainstream dictates a minimum combustor length for obtaining acceptable levels of combustion efficiency. Hence, characterization of the reaction zone within a UCC is required to optimize the length. In this study OH* chemiluminescence imaging is used to assess the characteristics of the reaction zone via windows in the side and top of the combustor. CO, NOx, and total hydrocarbon (THC) emissions indices obtained with gas-sample probes at the exit of the combustor as well as computed combustion efficiencies are provided as a reference for the OH* chemiluminescence. Configurations with no turning vanes (CDF and CDF-2), with standard vanes (CDF-2SV), and with radialvane-cavity (RVC) vanes (CDF-2RV) were used. The first study shows that the CDF-2 configuration has similar combustion efficiencies compared to that of the previously studied CDF configuration between 0.6 1.1 but has a higher peak OH* intensity and higher window exit intensity than that of the CDF configuration. The second study shows that the addition of standard vanes to the UCC decreases the peak and exit OH* intensities and lowers the exit temperature peak to 30% height, while the addition of the RVC vane tends to increase the peak and exit OH* intensities and raise the exit temperature peak to 50% height. Combustion efficiencies are similar for the CDF-2, CDF-2SV, and CDF-2RV configurations up to = 1.1. Combustion efficiency remains above 99% for the CDF2SV configuration up to = 2.0. The third study shows that OH* intensity increases

Two different test sections (ducts), with optical ports for optical fibers, were mounted to the exhaust end of two rotating detonation engines (RDE’s). The test sections were used to obtain velocity, temperature and water concentration from the exhaust flow of the RDE. These measurements were obtained with time-division-multiplexed tunable diode laser absorption spectroscopy (TDM-TDLAS). The spectroscopic system used to take these measurements used two diode lasers to scan across spectroscopic absorption features of H2O near 1.3 μm (≈ 7,435 to 7,442 cm−1 and 7,465 to 7,471 cm−1). These features were analyzed to obtain the temperature and H2O concentration as well as the velocity of the exhaust of the RDE. The combustion of C2H4 from a Hencken burner was also studied to provide a check for the temperature measurements. It was found that after the RDE reached a steady state condition the temperature and H2O concentration remained relatively constant with standard deviations of 50 K and 0.08 moles/m respectively. The combustion products of the RDE were also found to have an average velocity of 364 m/s with velocity fluctuations of approximately 110 m/s.

A variety of different flame configurations and heat release distributions, with their associated flow fields, can exist in high swirl, annular flows. Each of these different configurations, in turn, has different thermoacoustic sensitivities and influences on combustor emissions, nozzle life, and liner heating. These different configurations arise because at least three flame stabilization locations are present, associated with the inner and outer shear layers of the annulus, and the stagnation point of the vortex breakdown region. This paper discusses the flame and flow topologies that exist in these flows. These results illustrate the importance of the sensitivity of flame configurations to geometric (such as centerbody size and shape, combustor diameter, exhaust contraction) and operational (e.g., bulkhead temperature, preheat temperature, fuel air ratio) parameters. We particularly emphasize the centerbody shape as differentiating between two different families of flame shapes. Results are shown illustrating the time averaged and instantaneous flame shape and flow fields, using high speed PIV, OH-PLIF, and luminosity imaging.

A number of high-speed experimental techniques provide the opportunity to collect spatio-temporal information about the evolution of the flowfield and reactions in turbulent flames. This is particularly important for understanding static and dynamic stability in combustion devices that require bluff bodies to anchor the flame. In such cases, KelvinHelmholtz and Von Karman shedding modes have been known to affect flame stabilization and blow-off, leading to an interest in studying the coherent structure dynamics with high data bandwidth and advanced data mining techniques. One of the primary goals of this work is to employ the continuous wavelet transform (CWT) to characterize and quantify the nature of shedding in bluff body stabilized flames. Planar laser-induced fluorescence (PLIF) experimental data have been collected under different flow rates and equivalence ratios for a V-gutter flame holder, and coherent structures are analyzed using single- and multi-point (cross) wavelet transforms. This enables evaluation not only of the dominant modes within the flow, but also the time-dependence of these modes as a function of location within the flame. These methods are used to evaluate the potential for utilizing advanced numerical tools for extracting information about coherent structure dynamics in complex reacting flows.

Optical measurement techniques are required to determine flow-field parameters for the development and evaluation of advanced combustion systems. Parameters include but are not limited to velocity, temperature, and species concentrations. There are many challenges to implementing diagnostic techniques in practical combustion rigs, including temperature/pressure effects, vibration, flow perturbation, limited optical access, as well as optical engineering and alignment. Current implementation options discussed here include incorporating in-situ launching assemblies and integrating fiber-based methods. First, a novel laser launching system is described for high-temperature, high-pressure combustion applications in the High-Pressure Combustion Research Facility (HPCRF) at Wright-Patterson AFB. This insitu, optical-based assembly is placed upstream in the plenum section of a combustor sector rig and used to launch double-pulsed laser light for particle-image velocimetry (PIV). Design and implementation challenges will be discussed and representative data shown. In addition to launching capability, this assembly provides the means for light collection. Optical designs will be discussed for coupling scattered light or fluorescence photons from sheet-based techniques such as PIV or planar laser-induced fluorescence (PLIF) to double-frame, high-speed, intensified cameras. Investigations into the use of this assembly for PLIF in reacting flows will be presented. Second, fiber-based approaches involving a novel imaging fiber and associated purged, watercooled probe for high-speed imaging of practical combusting flows in the HPCRF with CMOSbased cameras will be discussed. Together these two approaches represent a powerful methodology for bringing the advantages of nonintrusive, optical combustion diagnostics to practical hardware in large-scale combustion test facilities.

High-repetition-rate hyperspectral absorption spectroscopy was used to monitor gas temperature, pressure, velocity, and H2O mole fraction in a research-grade pulseddetonation engine (PDE) at the Air Force Research Laboratory. The hyperspectral source that was used is termed the TDM 3-FDML because it consists of three time-division multiplexed (TDM) Fourier-domain modelocked (FDML) lasers. This source monitors sufficient spectral information in the H2O absorption spectrum near 1350 nm to permit temperature measurements over the wide range of conditions encountered throughout the pulsed-detonation engine cycle. Absorption-feature Doppler velocimetry is accomplished using a novel counter-propagating beam approach designed to minimize common-mode flow noise.

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.

Hyperspectral absorption spectroscopy is being used to monitor gas temperature, velocity, pressure, and H(2)O mole fraction in a research-grade pulsed-detonation combustor (PDC) at the Air Force Research Laboratory. The hyperspectral source employed is termed the TDM 3-FDML because it consists of three time-division-multiplexed (TDM) Fourier-domain mode-locked (FDML) lasers. This optical-fiber-based source monitors sufficient spectral information in the H(2)O absorption spectrum near 1350 nm to permit measurements over the wide range of conditions encountered throughout the PDC cycle. Doppler velocimetry based on absorption features is accomplished using a counterpropagating beam approach that is designed to minimize common-mode flow noise. The PDC in this study is operated in two configurations: one in which the combustion tube exhausts directly to the ambient environment and another in which it feeds an automotive-style turbocharger to assess the performance of a detonation-driven turbine. Because the enthalpy flow [kilojoule/second] is important in assessing the performance of the PDC in various configurations, it is calculated from the measured gas properties.