Stanislav Kostka

Comparison of line-peak and line-scanning excitation in two-color laser-induced-fluorescence thermometry of OH

Two-line laser-induced-fluorescence (LIF) thermometry is commonly employed to generate instantaneous planar maps of temperature in unsteady flames. The use of line scanning to extract the ratio of integrated intensities is less common because it precludes instantaneous measurements. Recent advances in the energy output of high-speed, ultraviolet, optical parameter oscillators have made possible the rapid scanning of molecular rovibrational transitions and, hence, the potential to extract information on gas-phase temperatures. In the current study, two-line OH LIF thermometry is performed in a well-calibrated reacting flow for the purpose of comparing the relative accuracy of various line-pair selections from the literature and quantifying the differences between peak-intensity and spectrally integrated line ratios. Investigated are the effects of collisional quenching, laser absorption, and the integration width for partial scanning of closely spaced lines on the measured temperatures. Data from excitation scans are compared with theoretical line shapes, and experimentally derived temperatures are compared with numerical predictions that were previously validated using coherent anti-Stokes-Raman scattering. Ratios of four pairs of transitions in the A2Sigma+<--X2Pi (1,0) band of OH are collected in an atmospheric-pressure, near-adiabatic hydrogen-air flame over a wide range of equivalence ratios--from 0.4 to 1.4. It is observed that measured temperatures based on the ratio of Q1(14)/Q1(5) transition lines result in the best accuracy and that line scanning improves the measurement accuracy by as much as threefold at low-equivalence-ratio, low-temperature conditions. These results provide a comprehensive analysis of the procedures required to ensure accurate two-line LIF measurements in reacting flows over a wide range of conditions.

Laser-Induced Fluorescence Measurements of Product Penetration Within an Ultra-Compact-Combustor

Stanislav Kostka∗ Spectral Energies, LLC, Dayton, Ohio 45431 Richard D. Branam Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433 Michael W. Renfro University of Connecticut, Storrs, Connecticut 06269 Patrick J. Lakusta Air Force Institute of Technology, Wright-Patterson Air Force Base, Ohio 45433 James R. Gord U.S. Air Force Research Laboratory, Wright-Patterson Air Force Base, Ohio 45433 and Sukesh Roy∗∗ Spectral Energies, LLC, Dayton, Ohio 45431

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.

Blowoff dynamics of V-shaped bluff body stabilized, turbulent premixed flames in a practical scale rig

Near blowoff dynamics and blowoff characteristics of premixed flames stabilized by a triangular flame holder in the midspan of a rectangular duct were studied using high speed imaging at 500 fps and simultaneous PIV and OH PLIF. Near blowoff dynamics manifested by the onset of asymmetric vortex shedding and local extinction in the form of flame holes were observed. Observations are presented describing the final blowoff event and its precursor: asymmetric sinuous modes of flame motion. It has been hypothesized that partial or total extinction of flame in the shear layers is the major factor that determines the evolution of the asymmetric mode and the final blowoff event. This phenomenon is further evidenced by an observation of the presence of flame kernels within the recirculation zone which under stable conditions contain only combution products. Whether a flame can survive an almost total extinction is governed by the ability of a reacting wake during these times to reignite the extinguished shear layers. I. Introduction Ground-based and aero gas turbine engine applications routinely incorporate bluff body-stabilized flame holders for primary or secondary combustion in high speed flows. The bluff body is placed in the mid flow of a high-speed duct and produces a recirculating wake structure that allows combustion to stabilize and then propagate into the free stream. In stable conditions, the recirculating wake structure steadily entrains hot combustion products from the adjacent shear layers and carries them upstream to ignite cool reactants as they are mixed in the wake shear layers [1]. The difficulty in design and implementation of bluff-body combustors is a result of the small range of fuel/oxidizer mixing conditions that yield stable combustion for given airflow. Temporal or spatial changes in airflow or fuel flow frequently produce changes in the flame structure that can cause extinction or combustion instabilities. Therefore, for a particular design, the stability of these flames needs to be carefully characterized over the intended operating envelope to optimize the coordination of the subsystems that control air and fuel flow with the purposes of maximizing combustion efficiency, minimizing the need for relights during times of critical operation, and minimizing thermo-acoustic instabilities. In order to predict blow off early in the design stage of any combustor, the fundamental phenomena of blow off needs to be captured conceptually and analytically and used to optimize the hardware design. Investigations have been conducted for close to sixty years with the objectives of understanding the underlying

Transitional Blowoff Behavior of Wake-Stabilized Flames in Vitiated Flow

The flame holding and blowoff characteristics of bluff body stabilized premixed flames were studied in a rectangular duct with a triangular flame holder in the midspan of the duct cross section. Lean blowoff was first investigated at unvitiated and uniformly mixed flow conditions in order to characterize the baseline flame behavior and compare it with data from the literature. The lean blowoff margin was then characterized with upstream air vitiated at equivalence ratios of 0.15 and 0.33, without and with cooling, respectively. Further studies were performed with non-uniform fuel profiles (rich or lean in the center or asymmetric), thus imposing single and double fuel gradients near the flame holder. Fuel profiles were characterized using laser induced acetone fluorescence. Transitional blowoff behavior was documented with measurements of CH chemiluminescence emissions from the wake as well as high-speed imaging of flame dynamics. Simultaneous PIV and OH PLIF measurements were taken at transitional fuel-air ratios to capture flame edge and aerodynamic behavior as blowoff was approached to determine the driving mechanisms of final blow off. Post processing of the flame images and flow field revealed the interaction between the velocity field and flame sheet.

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.

OH-PLIF Calibration and Investigation within the Ultra Compact Combustor

The AFIT Combustion Optimization and Analysis Laser (COAL) labs modular design and state-of-the-art diagnostic systems make it a flexible and important facility for the analysis of combustion processes. The objectives of the current research are to install several enhancements in the lab, validate the laser diagnostic system, characterize the igniter for AFITs Ultra-Compact Combustor (UCC) sections, and perform a non-intrusive laser diagnostic, performance, and high-speed video analysis of a flat-cavity UCC section. Validation of the laser system was accomplished using OH Planar Laser-Induced Fluorescence (PLIF) in a laminar hydrogen-air flame produced by a Hencken burner. Results are compared to previous research to show improvements. Both ratios of intensities and excitation scans in the OH (A-X) (1-0) electronic transition system are used to measure temperature and species concentrations. Igniter characterization was accomplished using open-air flammability and flame height observations to select an anticipated operating condition. That condition was validated by attaching the igniter to the UCC section and observing its performance. An operating procedure is recommended. A PLIF flame location study using optically-clear quartz windows on the combustor was performed in the cavity-vane area. Performance measurements and high- speed video footage were also acquired in order to analyze the system. Results are compared to previous experimental and Computational Fluid Dynamics (CFD) research. Future work will include instantaneous two-color PLIF and other laser diagnostic studies of several different locations inside AFITs flat- and curved-cavity UCC sections.

High-speed imaging of OH radicals in flames using fiber-coupled UV-PLIF

A fiber-coupled, high-speed UV-PLIF system employing a long multimode silica fiber is developed for detection of OH in harsh combustion environments. Single-laser-shot, 10kHz, OH-PLIF imaging of unsteady flames is demonstrated.

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

In advanced gas turbine engines that feature very short combustor sections, an issue of fuel-rich gases interacting with downstream components exists. In all of these engines there are regions downstream of the primary combustion section that will require the use of film-cooling in the presence of incompletely reacted exhaust. This will lead to the possibility of additional combustion reactions resulting from the combination of unburnt fuel and oxygen-rich cooling films. Research has been accomplished to understand this secondary reaction process. This experimental film-cooling study expands the previous investigations by attempting to reduce or remove the negative effects that result from secondary combustion in the coolant film. An upstream row of holes was added to a row of previously tested shaped coolant holes to understand if the reactions could be mitigated at the downstream locations. Several combinations of cooling schemes were investigated and the heat flux downstream was measured. Planar Laser Induced Fluorescence (PLIF) was used to measure OH concentration in the combustion zones to understand where the reactions occurred. It was discovered that creating a full sheet of air upstream could effectively protect the downstream row from the negative impacts of the fuel-rich crossflow.© 2012 ASME