Amy Lynch

A Detailed Investigation of Bluff Body Stabilized Flames

Abstract : Reduced Order Models (ROMs) and Computational Fluid Dynamics (CFD) codes are tools used to predict the extinction of flames behind bluff bodies. Accurate prediction of these models and codes is predicated on their validation with experimental data. This paper describes detailed experiments to obtain validation data for bluff body stabilized flames over a wide range of conditions. Included are non-reacting data from CFD and LDV, lean blowout and high speed images for three different flame holders. In our previous paper (Kiel 2006) it was asserted that the large vortices were a major driver of extinction. Those assertions are further supported here. It is concluded that the vortex dynamics and not geometry is the dominant mechanism for bluff body flame extinction. This conclusion is supported by the lean blowout data, by the high speed images and reference data from NACA.

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

Simultaneous 10-kHz PLIF and Chemiluminescence Imaging of OH Radicals in a Microwave Plasma-Enhanced Flame

This paper examines the structure of microwave (MW)-enhanced flames through 10-kHz imaging. High repetition rate laser diagnostic methods are used to simultaneously record 2-D images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma-enhanced flame. Collecting both OH planar laser-induced fluorescence and chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume-integrated excited state emission. A tunable, MW waveguide plasma source-operating at 2.45 GHz and delivering 90-130 W to the flowfield-ignites and sustains a CH4/air flame, whereas laser-induced fluorescence and chemiluminescence are acquired at a sustained framing rate of 10 kHz, using two intensified CMOS cameras and a synchronized laser. Multiple geometries and flames (premixed and nonpremixed) are studied by adjusting gas flow compositions and the plasma applicator nozzle components. A stoichiometric premixed flame configuration produces a divergent flame with large-scale fluctuations and vortex shedding into ambient air and is capable of feedstock flow velocities for combustion-to-plasma power ratios . Another arrangement produces plasma along the initial mixing layer of a nonpremixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with a little flamefront fluctuation. The high-speed diagnostics capture OH signals in cinematic sequences, providing new understanding of the plasma-assisted flame holding mechanism and allowing for the tracking of individual flow feature development.

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

Improved Correlation for Blowout of Bluff Body Stabilized Flames

Abstract : With the advent of high-speed diagnostics and computers, new observations concerning the extinction process have been made, with the most general conclusion being that the extinction process is a wake phenomenon, where the flame is highly strained and dominated by large vortices. In the present paper a new correlation for lean extinction is derived using a linear least-squares fit and more than 800 data points from historical and current experiments. Fits of various dimensionless parameters are made, but the best fit is that of a Damkoehler number with ignition delay as the chemical time scale, verifying many previous conclusions. Finally, it is concluded that flame-holder size--not shape--is the driving parameter that represents the flame-holder geometry.

Characterization of Bluff-Body-Flame Vortex Shedding Using Proper Orthogonal Decomposition

Flame stabilization has been of interest for many decades. Bluff-body flame stabilization has been incorporated in gas turbine engines as a means of secondary combustion in high-speed flows. The current work is focused on understanding vortex shedding and its contribution to both blow off and flame stability. Two modes of shedding, Kelvin-Helmoltz and Von-Karman, have been observed to play a major role in the stability and blow off of these bluff-body flames. Typically researchers have observed these modes visually but have been unable to quantify the effective contribution under various flow conditions. The present work is focused on the implementation of Proper Orthogonal Decomposition (POD) as a means of characterizing the energy and nature of these shedding modes as flames transition to acoustic instabilities and blow off. POD provides a new method of assessing the shedding mode and complements the pure visualization and vorticity calculations performed to date. POD is implemented on high-speed images of bluff-body flames at multiple equivalence ratios in an experimental test section. During this equivalence-ratio scan, the flame transitions to an acoustic instability. By incorporation of POD, the symmetric and asymmetric energy contributions through instability and blow off can be described.

The Influence of Stoichiometry and Flame-Holder Shape on Flame Dynamics and Acoustics (Preprint)

Abstract : Combustion instability manifests itself by the coupling of heat release and chamber acoustics. These instabilities can be present in any type of combustion system, including gas turbine engines, scramjet engines, and industrial furnaces and boilers. Much research has been conducted on the coupling of acoustics and heat release for lean-burning systems. Historically, models of these systems assume the flames to be short and the mean fields to be incompressible. Proposed here is a new approach to coupling dynamics. If the governing equations are considered to be compressible, then a relationship among acoustics, vorticity, and pressure can be derived. In this study the relationship among vortex shedding, flame dynamics, and acoustics is explored for a bluff-body-stabilized flame using high-speed flame images and high-speed pressure transducers. It is demonstrated that the flame radiates sound over a broad spectrum and that thermoacoustic coupling occurs when the flame sound radiation couples with one of the modes of the combustion chamber.

Characterization of Inverse Diffusion Flames by Planar Laser Induced Fluorescence of CO and OH

Two-color OH planar laser induced fluorescence (PLIF) thermometry and two-photon CO PLIF are used to characterize inverse diffusion flames. These flames are important tools to aid in understanding the secondary reaction zones in gas turbine engines that result from film-cooling air reacting with fuel-rich packets exiting from the combustor. For the experiments performed here, the exhaust from a well-stirred reactor is channeled to a test section where three different film-cooling geometries are used to create inverse diffusion flames. The two-color OH PLIF data shows the area with significant reactions as well as a two dimensional temperature field. The two-photon CO PLIF shows the consumption of the CO as a primary fuel in the secondary reaction zone.

Heating and Efficiency Comparison of a Fischer-Tropsch (FT) Fuel, JP-8+100, and Blends in a Three-Cup Combustor Sector

In order to realize alternative fueling for military and commercial use, industry guidelines be met. These aviation fueling requirements are outlined in MIL-DTL-83133F(2008) or ASTM D 7566-Annex standards and are classified as “drop-in” fuel replacements. This paper provides combustor performance data for synthetic-paraffinic-kerosene- (SPK-) type (Fisher-Tropsch (FT)) fuel and blends with JP8

Simultaneous planar OH imaging of microwave plasma enhanced combustion at 10 kHz

This study examines the structure of various microwave enhanced flames through 10 kHz imaging. High-speed laser diagnostic methods are implemented to simultaneously record two dimensional images of OH laser-induced fluorescence and chemiluminescence within an atmospheric plasma enhanced flame. A tunable, microwave waveguide plasma source ignites and sustains CH 4/air flame, while signal acquisition operates at a framing rate of 10 kHz, utilizing two intensified CMOS cameras and a synchronized laser. Resultant image sequences are temporally correlated between the two diagnostics, each with greater than 1000 continuous frames (100 ms time span). Multiple geometries and flames are studied by adjusting gas flow compositions and plasma applicating nozzle components, producing optically accessible premixed and non-premixed flames. The plasma source is powered by a continuous 2.45 GHz magnetron producing 360 W of power. Microwave power input to the nozzle is reduced to 90 to 130 W using stub tuners and a sliding short to control the reflected microwave power, isolated and absorbed by a circulator equipped with a dummy load. Collecting both OH planar laser-induced fluorescence with chemiluminescence allows for observation of OH radicals in the plane of the thin laser sheet as well as volume integrated excited state emission. One premixed flame geometry produces a divergent flame with large scale fluctuations and vortex shedding into ambient air, capable of stoichiometric feedstock flow velocities greater than 20 m/s for combustion-to-plasma power ratios greater than 10:1. Another arrangement produces plasma along the initial mixing layer of a non-premixed flame, yielding a thin cylindrical reaction zone of coincident chemiluminescence and fluorescence, periodically separating downstream. Replacing the fuel with rich premixed gases produces a narrow conical flame anchored by the circular plasma discharge with very little front fluctuation. The high-speed diagnostics successfully capture OH signals in cinematic sequences, allowing for tracking of individual flow feature development.