Yedidia Neumeier

The Role of Unmixedness and Chemical Kinetics in Driving Combustion Instabilities in Lean Premixed Combustors

Abstract This paper presents the results of a study of the potential causes of frequently observed combustion instabilities in low NOx gas turbines (LNGT) that burn gaseous fuels in a premixed mode. The study was motivated by indications that such systems are highly sensitive to equivalence ratio perturbations. An unsteady well-stirred reactor model was developed and used to determine the magnitude of the reaction rate and heat release oscillations produced by periodic flow rate, temperature or equivalence ratio perturbations in the combustors inlet flow at different mean equivalence ratios. This study shows that the magnitudes of the reaction rate and heat release oscillations produced by these perturbations remains practically unchanged, decreases, and significantly (i.e., by a factor of 5-100) increases, respectively, as the equivalence ratio decreases. These results strongly suggest that equivalence ratio perturbations, which are an indication of reactants unmixedness, playa key role in the driving o...

Nonlinear pressure-heat release transfer function measurements in a premixed combustor

This paper describes an experimental study of the response of a premixed combustion process to imposed pressure oscillations. These data were obtained to improve current understanding of the nonlinear phenomenon in unstable combustors that play an important role in their limit-cycle behavior. They were obtained by forcing oscillations in the combustor at discrete frequencies and measuring the resulting pressure and global CH* radical chemiluminescence oscillations. These data suggest that the nonlinear relationship between pressure and heat-release oscillations, as opposed to nonlinear gas dynamic processes, play an important role in saturating the amplitude of self-excited oscillations in premixed combustors. Specifically, it was found that the ratio of the magnitude of the heat-release response to pressure perturbations decreased at large amplitudes of driving: that is, that gain saturation plays a role in the combustors nonlinear dynamics. The role of amplitude dependence of the phase between the pressure and heat-release fluctuations is less clear from the data, however, because it was found to be nearly independent and moderately dependent on the drive amplitude at different driving frequencies. Nonlinear interactions between a natural combustor mode (at 167 Hz) and those due to the imposed driving (at 157, 190, and 235 Hz) were also observed. Specifically, we observed a steady decrease in amplitude of the natural mode oscillations with increasing amplitude of forcing, even though they were at separate frequencies. This behavior appears to be due to frequency locking of the natural mode oscillations, a well known nonlinear oscillatory phenomenon. The amplitudes at which these nonlinear interactions began to be evident were lower than that where nonlinearities in the pressure-CH* amplitude relation were observed.

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.

Forced Response of a Swirling, Premixed Flame to Flow Disturbances

This paper presents measurements of the nonlinear response of a premixed flame to harmonic oscillations. These measurements were obtained to improve understanding of nonlinear flame dynamics in unstable combustors. Simultaneous measurements of CH ∗ and OH ∗ chemiluminescence, pressure, and velocity were obtained over a range of forcing amplitudes and frequencies. These data show that the flame chemiluminescence response to imposed oscillations saturates at pressure and velocity amplitudes on the order of p � /p0 ∼ 0.02 and u � /u0 ∼ 0.3. The value of the fluctuating to the mean chemiluminescence signal at saturation varied with equivalence ratio. The phase between the chemiluminescence and acoustic signal also exhibits substantial amplitude dependence, even at driving levels where their amplitude ratios are nearly linear. In contrast, the pressure-velocity relationship remains linear with constant phase over the entire amplitude range. Thus, these results suggest that the nonlinear dynamics of premixed combustors are controlled by the acoustics-heat-release relationship, as opposed to gas dynamical processes. The mechanism for saturation of the flame response appears to be caused by nonlinear interactions between the flow forcing and the parametric flame instability, possibly through their impact on the fluctuating flame position. This parametric instability occurs at half the forcing frequency and is caused by the oscillating flow acceleration in the presence of the density jump at the flame. To our knowledge, this observation of the parametric instability is the first in a turbulent, swirl-stabilized flame. Interactions between the nonlinear heat release and linear combustor acoustics were characterized by sweeping the driving frequency through a resonant combustor frequency. With increasing driving amplitude, the resulting frequency response curves began to saturate in amplitude and bend over toward lower frequencies. Similar to the classical nonlinear Duffing oscillator, this bending of the frequency response curves manifests itself as a subcritical bifurcation in combustor response, whose gain and phase exhibited jumps and hysteresis. The bifurcation structure was measured in detail over a range of conditions and shown to have a two-dimensional dependence upon both amplitude and frequency.

Active control of combustion instabilities using real time identification of unstable combustor modes

The theoretical foundation and performance of an active control system that rapidly characterizes and damps combustor instabilities whose characteristics are not known in advance are presented. The heart of this control system is an observer that uses, for example, a measured combustor pressure to determine the frequencies and amplitudes of the combustor modes in real time. This information is input into a controller that provides each mode with a gain and a phase shift prior to sending a signal to an actuator that injects an oscillating fuel stream into the combustor. The paper uses numerical simulations to demonstrate the capabilities of the developed controller. First, it is shown that the developed observer can determine the frequencies and amplitudes of oscillating modes without having any prior knowledge about their characteristics. Next, it is shown that this controller can identify and control instabilities involving several modes in linearly unstable systems. Finally, the ability of the developed controller to rapidly damp a nonlinear, shock-wave like, instability in a rocket combustor by controlling the fundamental mode and its harmonics is demonstrated.

Development and Demonstration of a Stability Management System for Gas Turbine Engines

Rig and engine test processes and in-flight operation and safety for modem gas turbine engines can be greatly improved with the development of accurate on-line measurement to gauge the aerodynamic stability level for fans and compressors. This paper describes the development and application of a robust real-time algorithm for gauging fan/ compressor aerodynamic stability level using over-the-rotor dynamic pressure sensors. This real-time scheme computes a correlation measure through signal multiplication and integration. The algorithm uses the existing speed signal from the engine control for cycle synchronization. The algorithm is simple and is implemented on a portable computer to facilitate rapid real-time implementation on different experimental platforms as demonstrated both on a full-scale high-speed compressor rig and on an advanced aircraft engine. In the multistage advanced compressor rig test, the compressor was moved toward stall at constant speed by closing a discharge valve. The stability management system was able to detect an impending stall and trigger opening of the valve so as to avoid compressor surge. In the full-scale engine test, the engine was configured with a one-per-revolution distortion screen and transients were run with a significant amount of fuel enrichment to facilitate stall. Test data from a series of continuous rapid transients run in the engine test showed that in all cases, the stability management system was able to detect an impending stall and manipulated the enrichment part of the fuel schedule to provide stall-free transients.

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.

Demonstration of Active Control of Combustion Instabilities on a Full-Scale Gas Turbine Combustor

This paper presents the results of an investigation of active control of combustion instabilities in a natural gas, high-pressure, full-scale gas turbine combustor that was retrofitted with an Active Control System (ACS). The combustor test rig simulates the geometry, inlet airflow distribution, and pressurization of a can-type combustor that exhibits dynamic flame instabilities at some off-design operating conditions. Two essential features of the investigated ACS are 1) a real-time mode observer that identified the frequencies, amplitudes and phases of the dominant modes in the pressure signal and 2) a fast response servo valve that can modulate a large portion of the gaseous fuel. Two active control configurations were studied. In the first configuration, the actuator was mounted on one of two premixed fuel stages, and in the second configuration it was mounted on the inlet to the stabilizing diffusion stage. In both configurations, the ACS damped combustion instabilities, attenuating the dominant mode by up to 15dB and reducing the overall broadband noise by 30-40%. NOx emissions were also reduced by approximately 10% when control was applied. Finally, this study demonstrated the importance of having a fast multiple-mode observer when dealing with complex combustion processes with inherently large time delays.Copyright

A procedure for real-time mode decomposition, observation, and prediction for active control of combustion instabilities

This paper presents and examines the behavior of a nonlinear observer. The observer is a system whose time evolution is described by a set of nonlinear differential equations. The input to the observer is a time dependent signal, assumed equal to a sum of N sinusoids, each described by a frequency, amplitude, and phase that do not change with time. An N-mode observer is designed for a nominal input composed of N modes. The observers task is to track these three characteristics for each component mode of the input. When the actual input to the observer is indeed composed of N modes, then the frequencies, amplitudes, and phases of these modes correspond to an equilibrium point of the observer. Numerical computer simulations, supported by analytical arguments, strongly suggest that this equilibrium point is asymptotically stable and has a rather large domain of attraction. Such simulations also suggest that the observer is robust, namely, it is able to sufficiently identify the characteristics of N dominant modes when forced with an M-mode input, when M is greater than N. This is the case most likely to be encountered in practice. From the point of view of combustion instabilities, the usefulness of the observer rests on the premise that the unstable pressure oscillations in a combustion chamber be quasi-steady and have a discrete spectral content. In such a case, an N-mode observer with a large enough N could, in principle, track the frequencies, amplitudes, and phases of the individual unstable modes in real-time. The information obtained during this process can be used to cancel any time delays in the actuators, and form, in accordance with Rayleighs criterion, the secondary feedback fuel flow for optimal damping of the instability. A preliminary version of this observer has performed this task extremely well in numerous experiments with an unstable combustor.

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.