David L. Blunck

Mid-IR hyperspectral imaging of laminar flames for 2-D scalar values.

This work presents a new emission-based measurement which permits quantification of two-dimensional scalar distributions in laminar flames. A Michelson-based Fourier-transform spectrometer coupled to a mid-infrared camera (1.5 μm to 5.5 μm) obtained 256 × 128pixel hyperspectral flame images at high spectral (δν̃ = 0.75cm(−1)) and spatial (0.52 mm) resolutions. The measurements revealed line and band emission from H2O, CO2, and CO. Measurements were collected from a well-characterized partially-premixed ethylene (C2H4) flame produced on a Hencken burner at equivalence ratios, Φ, of 0.8, 0.9, 1.1, and 1.3. After describing the instrument and novel calibration methodology, analysis of the flames is presented. A single-layer, line-by-line radiative transfer model is used to retrieve path-averaged temperature, H2O, CO2 and CO column densities from emission spectra between 2.3 μm to 5.1 μm. The radiative transfer model uses line intensities from the latest HITEMP and CDSD-4000 spectroscopic databases. For the Φ = 1.1 flame, the spectrally estimated temperature for a single pixel 10 mm above burner center was T = (2318 ± 19)K, and agrees favorably with recently reported laser absorption measurements, T = (2348 ± 115)K, and a NASA CEA equilibrium calculation, T = 2389K. Near the base of the flame, absolute concentrations can be estimated, and H2O, CO2, and CO concentrations of (12.5 ± 1.7) %, (10.1 ± 1.0) %, and (3.8 ± 0.3) %, respectively, compared favorably with the corresponding CEA values of 12.8%, 9.9% and 4.1%. Spectrally-estimated temperatures and concentrations at the other equivalence ratios were in similar agreement with measurements and equilibrium calculations. 2-D temperature and species column density maps underscore the Φ-dependent chemical composition of the flames. The reported uncertainties are 95% confidence intervals and include both statistical fit errors and the propagation of systematic calibration errors using a Monte Carlo approach. Systematic errors could warrant a factor of two increase in reported uncertainties. This work helps to establish IFTS as a valuable combustion diagnostic tool.

Imaging Fourier-transform spectrometer measurements of a turbulent nonpremixed jet flame

This work presents recent measurements of a CH4/H2/N2 turbulent nonpremixed jet flame using an imaging Fourier-transform spectrometer (IFTS). Spatially resolved (128×192 pixels, 0.72  mm/pixel) mean radiance spectra were collected between 1800  cm(-1)≤ν˜≤4500  cm(-1) (2.22  μm≤λ≤5.55  μm) at moderate spectral resolution (δν=16  cm(-1), δλ=20  nm) spanning the visible flame. Higher spectral-resolution measurements (δν=0.25  cm(-1), δλ=0.3  nm) were also captured on a smaller window (8×192) at 20, 40, and 60 diameters above the jet exit and reveal the rotational fine structure associated with various vibrational transitions in CH4, CO2, CO, and H2O. These new imaging measurements compare favorably with existing spectra acquired at select flame locations, demonstrating the capability of IFTS for turbulent combustion studies.

Experimental and Computational Studies of an Ultra-Compact Combustor

Reducing the weight and decreasing pressure losses of aviation gas turbine engines improves the thrust-to-weight ratio and improves efficiency. In ultra-compact combustors (UCCs), engine length is reduced and pressure losses are decreased by merging a combustor with adjacent components using a systems engineering approach. High-pressure turbine inlet vanes can be placed in a combustor to form a UCC. Eliminating the compressor outlet guide vanes (OGVs) and maintaining swirl through the diffuser can result in further reduction in engine length and weight. Cycle analysis indicates that a 2.4% improvement in engine weight and a 0.8% increase in thrust-specific fuel consumption are possible when a UCC is used. Experiments and analysis were performed in an effort to understand key physical and chemical processes within a trapped-vortex UCC. Experiments were performed using a combustor operating at pressures in the range of 520–1030 kPa (75–150 psi) and inlet temperature of 480–620 K (865–1120 °R). The primary reaction zone is in a single trapped-vortex cavity where the equivalence ratio was varied from 0.7 to 1.8. Combustion efficiencies and NOx emissions were measured and exit temperature profiles obtained, for various air loadings, cavity equivalence ratios, and configurations with and without turbine inlet vanes. A combined diffuser-flameholder (CDF) was used in configurations without vanes to study the interaction of cavity and core flows. Higher combustion efficiency was achieved when the forward-to-aft momentum ratios of the air jets in the cavity were near unity or higher. Discrete jets of air immediately above the cavity result in the highest combustion efficiency. The air jets reinforce the vortex structure within the cavity, as confirmed through coherent structure velocimetry of high-speed images. A more uniform temperature profile was observed at the combustor exit when a CDF is used instead of vanes. This is the result of increased mass transport along the face of the flame holder. Emission indices of NOx were between 3.5 and 6.5 g/kgfuel for all test conditions. Ultra-compact combustors (with a single cavity) can be run with higher air loadings than those employed in previous testing with a trapped-vortex combustor (two cavities) with similar combustion efficiencies being maintained. The results of this study suggest that the length of combustors and adjacent components can be reduced by employing a systems level approach.Copyright

Turbulent Radiation Statistics of Exhaust Plumes Exiting from a Subsonic Axisymmetric Nozzle

near the tip of the potential core and downstream. Axial and radial variation in radiation intensity fluctuations is similar to those reported for flames. Autocorrelation coefficients of the radiation intensity are approximated reasonably well by exponential curves. Integral time and length scales increase monotonically downstream of the core region and are consistent with Taylor’s hypothesis. The break frequency and slope of the normalized power spectral density function are comparable to those reported for turbulent jet flames. These findings suggest that reacting flows can be used to predict trends in turbulent radiation properties of exhaust plumes.

Influence of Turbulent Fluctuations on the Radiation Intensity Emitted from Exhaust Plumes

Measured and computedmean and fluctuating radiation intensities are reported for a subsonic exhaust plume and used to characterize the influence of turbulent fluctuations on mean radiation properties. Narrowband radiation intensity measurements were acquired using an infrared camera fitted with a narrowband filter (4:34 0:1 m). Unsteady three-dimensional calculations were used to estimate both time-dependent and mean temperature and partial pressure values of the exhaust species in the flow. From these scalar values the mean and root mean square of the radiation intensity were calculated using a narrowband radiation model. Axial distributions of the calculated intensities based on the time-dependent quantities show trends similar to thosemeasured, but consistently overpredict the intensity by 40%. Intensity distributions based on mean scalars were in better quantitative agreement, but decay more rapidly downstream than experimental data. Turbulent fluctuations have little effect on the mean radiation intensity near the nozzle exit, but they become increasingly more important downstream in the flow. The influence of turbulent fluctuations on mean intensity values increases with radial distance from the centerline. This trend correlates to increases in normalized fluctuations in the temperature and radiation intensity.

Trajectory, Development, and Temperature of Spark Kernels Exiting into Quiescent Air

Measurements of the trajectory, temporal development, and temperature of spark kernels are needed for understanding the ignition process in spark ignition and gas turbine engines. Motivated by this, an infrared camera was used to obtain narrowband radiation intensity measurements of spark kernels exiting into quiescent air. An inverse deconvolution technique was implemented to estimate the temperature and sensible energy of the kernels. This technique is evaluated by a sensitivity analysis and comparisons to measurements of a well characterized flame. Infrared images show that the kernels develop into a toroidal shape after exiting from the igniter. The statistical distribution of the spark kernel trajectory is symmetric. Buoyancy forces are negligible. Regions of high and low radiation intensity are observed in the kernels, indicating temperature gradients within the gases. The radiation intensity emitted by the kernels decreases by more than an order of magnitude after exiting the igniter. Average temperature values decrease by less than 30% over 2 cm of the spark trajectory. Over that same distance the sensible energy of the kernels decreases by 80%.

Experimental Characterization of the Reaction Zone in an Ultra-Compact Combustor

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

Study of Narrowband Radiation Intensity Measurements from Subsonic Exhaust Plumes

Narrowband radiation intensitymeasurements were obtained of exhaust plumes exiting from a converging nozzle with varying Reynolds numbers (2:4–6:1 10), Mach numbers (0.4–1.0), and temperatures and species compositions corresponding to low fuel-to-air equivalence ratios (0.17–0.28). The plumeswere generated by burning jet A with dry air flowing through a laboratory gas turbine combustor. Radiation intensity measurements were acquired using an infrared camera fitted with a narrowband filter (4:34 0:1 m). The narrowband radiation intensity leaving adiametric path exhibited apower dependence (2.8) on the equivalence ratio. Smaller changes in the intensity were observed for plumes with near-identical equivalence ratios but varying exit velocities. These smaller changes are attributed to differences in the sensible and kinetic energy and the changes in the mixing rates with ambient air. The magnitude of the radiation intensities emanating from chordlike paths normalized by that emitted from the diametric path (at an identical distance from the nozzle exit) plotted as a function of the distance between these paths show approximately self-similar profiles for some conditions. A departure from the self-similar profiles is observed for conditionswhere thewidth of the infrared images contracts significantly. For the test conditions studied, the intensity emanating from the diametric paths first decayed linearly with increasing distance from the nozzle exit and then decayed exponentially with an inflexion point near the end of the plume core.

Impact of an Upstream Film-Cooling Row on Mitigation of Secondary Combustion in a Fuel Rich Environment

In advanced gas turbine engines that feature very short combustor sections, an issue of fuel-rich gases interacting with the downstream turbine components can exist. Specifically, in combustors with high fuel-to-air ratios, there are regions downstream of the primary combustion section that will require the use of film-cooling in the presence of incompletely reacted exhaust. Additional combustion reactions resulting from the combination of unburnt fuel and oxygen-rich cooling films can cause significant damage to the turbine. Research has been accomplished to understand this secondary reaction process. This experimental film-cooling study expands the previous investigations by attempting to reduce or mitigate the increase in heat flux that results from secondary combustion in the coolant film. Two different upstream cooling schemes were used to attempt to protect a downstream fan-shaped cooling row. The heat flux downstream was measured and compared between ejection with air compared to nitrogen in the form of a heat flux augmentation. Planar Laser Induced Fluorescence (PLIF) was used to measure relative OH concentration in the combustion zones to understand where the reactions occurred. A double row of staggered normal holes was unsuccessful at reducing the downstream heat load. The coolant separated from the surface generating a high mixing regime and allowed the hot unreacted gases to penetrate underneath the jets. Conversely, an upstream slot row was able to generate a spanwise film of coolant that buffered the reactive gases off the surface. Essentially no secondary reactions were observed aft of the shaped coolant hole ejection with the protective slot upstream. A slight increase in heat transfer was attributed to the elevated freestream temperature resulting from reactions above the slot coolant. Creating this full sheet of coolant will be a key toward future designs attempting to control secondary reactions in the turbine.

Reacting Flows in Ultra-Compact Combustors with Combined-Diffuser Flameholder

In ultra-compact combustors, the length of the system is reduced by integrating the turbine turning vanes within the combustor and the primary zone is in a cavity recessed from the core flow. To understand the combustion characteristics of an ultra-compact combustor, the turning vanes were removed and a combined-diffuser flameholder was integrated upstream of a trapped-vortex combustor cavity. Two variants of the combined-diffuser flameholder are compared. In configuration A, the row of passages nearest the trapped-vortex combustor is opened, whereas for configuration B, an additional row of flow passages vertically adjacent is also open. Grid-independent three-dimensional simulations of these geometries are performed following a steady, multiphase, Reynolds-averaged Navier–Stokes, C-progress variable/flamelet approach with source terms to model effusion cooling. Nondispersive infrared analyzers were used to measure the concentrations of CO2, O2, and CO at the exit of the test section using three sets of ...