Jerry Seitzman

Active Control of Lean Blowout for Turbine Engine Combustors

A complete, active control system has been developed to permit turbine engine combustors to operate safely closer to the lean-blowout (LBO) limit, even in the presence of disturbances. The system uses OH chemiluminescence and a threshold-based identification strategy to detect LBO precursor events. These nonperiodic events occur more frequently as the LBO limit is approached. When LBO precursors are detected, fuel entering the combustor is redistributed between a main flow and a small pilot, so as to increase the equivalence ratio near the stabilization region of the combustor. This moves the effective LBO limit to leaner mixtures, thus increasing the safety margin. The event-based control system was demonstrated in an atmospheric pressure, methane-air, swirl-stabilized, dump combustor. The NOx emissions from the piloted combustor were found to be lower than those from the unpiloted combustor operating at the same safety margin and same nominal velocity field. The controller minimizes the NOx at constant total power by keeping the pilot fuel fraction at the lowest value needed to limit the number of precursor events to an acceptable level.

NONLINEAR FLAME TRANSFER FUNCTION CHARACTERISTICS IN A SWIRL- STABILIZED COMBUSTOR

An understanding of the amplitude dependence of the flame response to acoustic excitation is required in order to predict and/or correlate combustion instability amplitudes. This paper describes an experimental investigation of the nonlinear response of a lean, premixed flame to imposed acoustic oscillations. Detailed measurements of the amplitude dependence of the flame response were obtained at approximately 100 test points, corresponding to different flow rates and forcing frequencies. It is observed that the nonlinear flame response can exhibit a variety of behaviors, both in the shape of the response curve and the forcing amplitude at which nonlinearity is first observed. The phase between the flow oscillation and heat release is also seen to have substantial amplitude dependence. The nonlinear flame dynamics appear to be governed by different mechanisms in different frequency and flowrate regimes. These mechanisms were investigated using phase-locked, two- dimensional OH Planar laser-induced fluorescence imaging. From these images, two mechanisms, vortex rollup and unsteady flame liftoff, are identified as important in the saturation of the flame’s response to large velocity oscillations. Both mechanisms appear to reduce the flame’s area and thus its response at these high levels of driving. DOI: 10.1115/1.2720545

FLAME AND FLOW TOPOLOGIES IN AN ANNULAR SWIRLING FLOW

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 eddy simulations (LES). Particular emphasis is given to the distinctly different flame and flow topologies that exist in these flows, and their sensitivity 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 importance of the centerbody shape, and its associated impact on the structure of the central recirculating flow, as differentiating between two different families of flame shapes.

Chemiluminescence Based Sensors for Turbine Engines

This work focuses on the use of naturally occurring optical emissions, specifically chemiluminescence, for sensing applications in active control and health monitoring of combustors. First, monitoring local equivalence ratio (φ), at the reaction zone, has been demonstrated using the ratio of CH to OH chemiluminescence. This ratio (CH*/OH*) increases monotonically with equivalence ratio, and the dependence on equivalence ratio has been shown to be a universal function for combustor configurations ranging from unconfined jet flames to swirl and dump stabilized combustors. There is essentially no difference between the CH*/OH* ratio for methane and natural (city) gas, but the ratio has a lower sensitivity to φ for n-heptane compared to methane or natural gas. The ratio was found to increase almost linearly with pressure for natural gas/methane combustion above 3 atm. Second, chemiluminescence emission from the combustor was used to detect precursor events to blowout, using a robust thresholding method. This method was shown to be successful in jet flames and swirl/dump stabilized combustors using premixed methane/air and nonpremixed Jet-A/air. This method gives the kind of information on proximity to blowout that can be used by an active control system to prevent lean blowout in low NOx turbine engine combustors. INTRODUCTION Both active control and health/performance monitoring systems for turbine engine combustors require knowledge of the state of the combustion processes within the combustor. For example, active control can be an efficient way to expand turbine engine combustor operating limits without loss of performance and safety. In the area of emissions control, it is known that NOx can be reduced by use of low fuel-air ratios in the flame region. However, operation under these conditions also makes the combustor prone to lean blowout (LBO) problems. Thus sensors that could give advance warning of LBO, in conjunction with an active control system, would *Graduate Research Assistant, Student Member AIAA † Undergraduate Research Assistant ‡ Associate Professor, Associate Fellow AIAA permit lower emissions operation. In a similar way, significant variations in local equivalence ratio in premixed or partially premixed combustors can lead to temperature nonuniformities that will increase NOx emissions and decrease the useful life of the turbine. Thus active control or engine health monitoring systems would be aided by sensors that could monitor fluctuations in local flame zone equivalence ratio inside a combustion chamber. In general, reliable and versatile sensors are required. For most engine applications, they must also provide measurements of conditions at locations away from the combustor wall, thus nonintrusive methods are preferred. In addition for active control systems that use state feedback, the sensor time response is also an important issue. Optical sensors offer the benefit of being able to gather data from extremely hostile environments (e.g., the combustion zone), and to do so over large regions of space. With the rapidly growing capability of these technologies for sensor hardware, there is an increased interest and need to develop data interpretation strategies that will allow optical flame emission data to be converted to meaningful combustor state information, such as heat release rate, proximity to LBO, and local flame zone equivalence ratio. There are a number of optical methods that can give information about the combustion process nonintrusively, e.g., optical emission, absorption, fluorescence and other spectroscopic methods. The focus of this work is on the simplest of all these techniques, viz., observing the naturally occurring, optical emissions from the combustor. While there are a number of sources for optical radiation from a combustor, the source most directly connected to the combustion reactions is chemiluminescence. This radiation is from high energy states of molecules (typically electronically excited states) that are produced by chemical reactions. Once produced, the excited molecules will transfer to lower energy states, in part by emitting light. This is known as chemiluminescence. Since the intensity of emission is proportional, in part, to the chemical production rate of the particular molecule, the chemiluminescence intensity can be related to (specific) chemical reaction rates. For this reason, chemiluminescence has been used previously as a rough measure of reaction rate and heat release rate. Thus chemiluminescence can

Evaluation of Chemiluminescence as a Combustion Diagnostic under Varying Operating Conditions

Modeling of OH*, CH* and CO2* chemiluminescence, using recently validated reaction mechanisms, is used to examine chemiluminescence sensing of heat release and equivalence ratio in lean, premixed methane and syngas (H2/CO) flames. The effect of pressure, reactant preheat, aerodynamic strain, fuel-air ratio and product recirculation on spatially integrated chemiluminescence signals are considered. In the syngas mixture studied (equal molar amounts of H2 and CO), heat release rate measurements with either OH* or CO2* chemiluminescence are predicted to exhibit significant sensitivity to equivalence ratio (Φ), pressure, reactant temperature and (to a lesser extent) aerodynamic strain. At high pressures, the chemiluminescence is practically independent of strain rate. Mixing of hot products and reactants before burning (e.g., EGR) also has little impact. OH* chemiluminescence is found to have some advantages for heat release sensing in turbulent premixed flames at near stoichiometric conditions. It has a lower strain dependence and is less sensitive to Φ variations than CO2* for near stoichiometric mixtures. For leaner conditions, heat release measurements employing CO2* may be advantageous, due to its lower dependence on Φ, pressure and preheat temperature. The ratio of CO2* to OH* chemiluminescence is not useful for equivalence ratio sensing with syngas fuels. For methane, OH*, CH* and CO2* can be used for heat release sensing, but all are also functions of Φ, pressure and reactant preheat. OH* and CO2* chemiluminescence are not significantly influenced by adiabatic product recirculation, while OH* and CH* are relatively insensitive to strain. Overall, CH* may be preferable for heat release sensing applications at elevated pressures and reactant temperatures such as those found in gas turbine combustors. For equivalence ratio sensing in lean methane combustion, the ratio of CH* to OH* chemiluminescence is useful, However, this ratio is highly dependent on the operating pressure and reactant temperature. For example, the ratio monotonically increases with Φ at atmospheric pressure, but monotonically decreases at high pressure. So the CH*/OH* ratio can be used for equivalence ratio sensing only at certain conditions in methane combustion. Finally thermal production of OH* in high pressure combustors, and CO2* background for “single” wavelength detection systems can be problematic.

OPTICAL AND ACOUSTIC SENSING OF LEAN BLOWOUT PRECURSORS

Approaches for identifying precursor events to lean blowout (LBO) in premixed or partially premixed combustors have been examined. This work characterizes the behavior of a premixed, swirl stabilized dump combustor near the lean blowout equivalence ratio limit using chemiluminescence (optical) and acoustic radiation emissions from the combustor. The results show that the transient behavior of the flame as lean blowout is approached can be characterized by short duration, localized extinction and reignition events. These events increase in frequency and duration as LBO is approached. Several methods based on signal thresholding, statistical analysis and frequency analysis are presented to transform the raw sensor output into a simple LBO proximity measure for use in an active control system. The thresholding approach utilizing the optical sensor provides the fastest time response. The sensor requirements for such systems are also presented, and it is shown that they can be met with practical devices.

LAMINAR FLAME SPEEDS OF SYNTHETIC GAS FUEL MIXTURES

Laminar flame speeds of H2 /CO/CO2 mixtures have been measured over a range of fuel compositions, lean equivalence ratios, and reactant preheat temperature (up to 700 K). The measurements are compared to numerical flame speed predictions based on two reaction mechanisms: GRI Mech 3.0 and a H2 /CO mechanism. For undiluted and nonpreheated mixtures, the current results agree with previous data and the numerical calculations over most of the range tested. The measured flame speeds increase as the H2 content of the fuel rises and for higher equivalence ratios. The most significant difference between the measurements and models is for high CO content fuel with the H2 /CO mechanism, and the high H2 content fuel at the leanest conditions with the GRI mechanism. For CO2 diluted fuels, measured flame speeds decrease as predicted. However, agreement between the measurements and predictions worsens with increasing CO2 dilution. Deviations as large as 40% are observed at lean equivalence ratios and 20% CO2 levels. For reactant preheat temperatures below ∼400K, the measured flame speeds generally match the calculated flame speeds within 10%. At higher preheat temperatures, however, the discrepancy between the measurements and the calculations increases, reaching levels of ∼30% at 700 K. The measured temperature dependence is closer to the predictions from GRI Mech 3.0 than from the H2 /CO mechanism.Copyright

Methane Oxycombustion for Low CO2 Cycles: Blowoff Measurements and Analysis

Increasing concerns about climate change have encouraged interest in zero-CO 2 emission hydrocarbon combustion techniques. In one approach, nitrogen is removed from the combustion air and replaced with another diluent, typically carbon dioxide or steam. In this way, formation of nitrogen oxides is prevented and the exhaust stream can be separated into concentrated CO 2 and water by a simple condensation process. The concentrated CO 2 stream can then be sequestered or used for enhanced oil recovery. Burning fuels in an O 2 /CO 2 diluent raises new combustion opportunities and challenges for both emissions and operability: this study focuses on the latter aspect. CH 4 /O 2 /CO 2 flames have slower chemical kinetics than methane-air flames and as such, flame stability is more problematic as they are easier to blow off. This issue was investigated experimentally by characterizing the stability boundaries of a swirl stabilized combustor. Near stoichiometric CO 2 and N 2 diluted methane/oxygen flames were considered and compared with lean methane/air flames. Numerical modeling of chemical kinetics was also performed to analyze the dependence of laminar flame speeds and extinction strain rates upon dilution by different species and to develop correlations for blowoff boundaries. Finally, blowoff trends at high pressure were extrapolated from atmospheric pressure data to simulate conditions closer to those of gas turbines.

Chemiluminescence Measurements and Modeling in Syngas, Methane and Jet-A Fueled Combustors

φ φ φ) and, for the syngas fuels, with reactant preheating and dilution. The model results for all three species are in good agreement with the experiments, except in the swirl-stabilized combustor near its lean blowout limit, where the assumptions embedded in the ideal 1-d, adiabatic flame model used are likely to fail. The models are then used to analyze chemiluminescence for equivalence ratio and heat release rate sensing in methane and Jet-A fuel combustion. The ratio of CH * to OH * emission is found be a good indicator of equivalence ratio for methane flames, increasing monotonically from φ~0.7 to at least 1.3; for Jet-A flames, however, the model shows the ratio is non-monotonic, with a minimum near φ=0.75. For all the fuels studied, all three chemiluminescence sources are less than ideal indicators of heat release if the equivalence ratio of the flame is also changing significantly, since the emission intensity normalized by the fuel consumption rate is dependent on φ. For syngas and methane systems, CO2 * chemiluminescence shows the least variation. For Jet-A, all three species have similar variations; the best choice for minimizing φ effects depends on the φ and temperature range of interest.

An Active Control System for LBO Margin Reduction in Turbine Engines

A complete, active control system has been developed to permit turbine engine-like combustors to operate safely closer to the lean blowout (LBO) limit, even in the presence of disturbances. The system uses OH chemiluminescence from the combustion process and a threshold based, event definition to detect LBO precursor events. These precursors appear random in time, and occur more frequently as the LBO limit is approached. When LBO precursors are detected, fuel entering the combustor is redistributed between a main flow and a small pilot, so as to increase the equivalence ratio near the stabilization region of the combustor. This moves the effective LBO limit to leaner mixtures, thus increasing the safety margin. The control system was demonstrated in an atmospheric pressure, methane-air, swirl-stabilized, dump combustor. The NOx emissions from the piloted combustor were found to be lower than from the unpiloted combustor operating at the same safety margin and nominal velocity field. The controller minimizes the NOx by reducing the pilot fuel fraction at constant total power setting until an unacceptable number of precursor events are observed. A set of control options for custom operation of the controller for a specific combustor are discussed.