Sungbae Park

Shear flow-driven combustion instability: Evidence, simulation, and modeling

Combustion instability arises mostly due to the coupling between heat-release dynamics and system acoustics. In these cases, the acoustic field acts as the resonator, while the heat-release dynamics, induced due to the impact of pressure or flow velocity perturbations on the equivalence ratio, flame length, residence time, shear layer, etc., supplies energy to support pressure oscillations. In this paper, we discuss another mechanism in which the resonator may not be the acoustic field: instead, it is an absolutely unstable shear layer mode acting as the source of sustained oscillations; which morph as large-scale eddies at a frequency different from acoustic modes. These perturb the flame motion and provide energy to the acoustic field, acting here as an amplifier to support pressure oscillations. We present evidence for the existence of this mechanism from several experiments, in which a pressure spectral peak can not be explained using acoustic analysis of the system, but instead matches the predicted absolute instability mode of the separated shear layer, and from numerical simulations. We also examine the impact of the mean velocity and temperature distribution on the frequency and growth rate to determine the dynamics leading to an absolute instability, and show that absolutely unstable modes are likely to arise at low-equivalence ratios. In some cases, they can also be present at near-stoichiometric conditions, that is, only low- and near-unity stoichiometry can support an absolutely unstable mode. We use numerical simulation to distinguish between different unstable modes in the separating shear layer, the layer mode and the wake mode, and derive a reduced-order model for the shear-driven instability using proper orthogonal decomposition analysis. Estimates of pressure amplitudes of fluid dynamic modes, when applied as forcing functions to the acoustic field, using this analysis compare favorably with experimental data.

Advanced Closed-Loop Control on an Atmospheric Gaseous Lean-Premixed Combustor

Active control of pressure oscillations has been successfully applied to a lean premixed prevaporized (LPP) combustion rig operating at atmospheric conditions. The design of the rig is based on the primary stage of the Rolls-Royce RB211-DLE industrial gas turbine. Control was achieved by modulating the fuel flow rate in response to a measured pressure signal. The feedback control is an adaptive, model-based self-tuning regulator (STR), which only requires the total time delay between actuation and response to achieve control. The STR algorithm achieves a reduction of up to 30 dB on the primary instability frequency. This performance was an improvement of 5-15 dB over an empirical control strategy (simple time-delay controller) specifically tuned to the same operating point. Initial robustness studies have shown that the STR retains control for a 20% change in frequency and a 23% change in air mass flow rate.

Optimal control of a swirl-stabilized spray combustor using system identification approach

An optimal controller using the system identification (SI) method was developed for a swirl-stabilized spray combustor operating between 30 and 114 kW. The efficacy of the controller was tested with two different nozzle configurations. The first consisted of a dual-feed nozzle whose primary fuel stream was utilized to sustain combustion, while the secondarystreamwasused for active control. The second configuration used a single-feed nozzle with two different swirling air streams. An LQG-LTR (linear quadratic Gaussian-loop transfer recovery) controller was designed using the SI-based model to determine the active control input, which was in turn used to modulate the secondary fuel stream. Using this controller, the thermoacoustic oscillations, which occurred under lean operating conditions, were reduced to the background noise level. A time-delay controller was also implemented for comparison purposes. The results showed that the LQG-LTR controller yielded an additional pressure reduction of 14 dB compared to the time-delay controller in both configurations. This improvement can be attributed to the added degrees of freedom of the LQG-LTR controller that allow an optimal shaping of the gain and phase of the controlled combustor over a range of frequencies in the neighborhood of the unstable mode. This leads to the extra reduction of the pressure amplitude at the unstable frequency while avoiding generation of secondary peaks.

Heat release dynamics modeling of kinetically controlled burning

We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) and one-step kinetics to describe the unsteady combustion process. The analysis incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations and is cast in the form of a first order filter. We are able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations, as function of the operating conditions, for example, the mean equivalence ratio and mass flow rate. The model predicts a 180° sudden shift in phase between imposed flow oscillations and resulting heat release between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability as the equivalence ratio is lowered or the average mass flow rate is increased. A number of experimental investigations, in which the inlet nozzles were choked to minimize equivalence ratio oscillations, are used to corroborate this conclusion. Next, the heat release-mass flow relationship is coupled with the acoustic field and is applied to predict instability conditions in high swirl combustion. The predictions agree qualitatively with the corresponding experimental observations.

Adaptive Combustion Instability Control with Saturation: Theory and Validation

The control of a class of combustion systems, susceptible to damage from self-excited combustion oscillations, is considered. The controller injects some fuel unsteadily into the burning region, thereby altering the heat release, in response to an input signal detecting the oscillation. An adaptive control design, called self-tuning regulator (STR), has recently been developed, which attempts to meet the apparently contradictory requirements of relying as little as possible on a particular combustion model while providing some guarantee that the controller will cause no harm. This paper focuses on an extension of the STR design, when, as a result of stringent emission requirements and to the danger of flame extension, the amount of fuel used for control is limited in amplitude. A Lyapunov stability analysis is used to prove the stability of the modified STR when the saturation constraint is imposed. Simulation and experimental results show that in the presence of a saturation constraint the self-excited oscillations are damped more rapidly with the modified STR than with the original STR.

Reduced order modeling of reacting shear flow

Thermoacoustic instability in premixed combustors occurs occasionally at multiple frequencies, especially in configurations where flames are stabilized on separating shear layers that form downstream of sudden expansions or bluff bodies. While some of these frequencies are related to the acoustic field, others appear to be related to shear flow instability phenomena. It is shown in this paper that shear flows can support self-sustained instabilities if they possess absolutely unstable modes. The associated frequencies are predicted using mean velocity profiles that resemble those observed in separating flows and for profiles obtained from numerical simulations, and are shown to match those derived from experimental and numerical investigations. It is also shown that the presence of density profiles compatible with premixed combustion can affect this frequency and can change the absolute instability mode into a convectively unstable mode thereby reducing the possibility of the generation of self-sustained oscillations. A qualitative prediction of the pressure amplitudes resulting from these shear layer modes is shown to be consistent with experimental measurements. The results from the stability analysis are combined with those using the Proper Orthogonal Decomposition (POD) method to yield a reduced-order model.

Adaptive Closed-Loop Control on an Atmospheric Gaseous Lean-Premixed Combustor

Active control of pressure oscillations has been successfully applied to a lean premixed prevapourised (LPP) combustion rig operating at atmospheric conditions. The design of the rig is based on the primary stage of the Rolls-Royce RB211-DLE industrial gas turbine. Control was achieved by modulating the fuel flow rate in response to a measured pressure signal. The feedback control is an adaptive, model-based self-tuning regulator (STR), which only requires the total time delay between actuation and response to achieve control. The STR algorithm achieves a reduction of up to 30 dB on the primary instability frequency. This performance was an improvement of 5–15 dB over an empirical control strategy (simple time-delay controller) specifically tuned to the same operating point. Initial robustness studies have shown that the STR retains control for a 20% change in frequency and a 23% change in air mass flow rate.Copyright

Adaptive low-order posi-cast control of a combustor test-rig model

Recently, an adaptive posi-cast controller has been developed for dynamic systems with large time-delays. In this paper, we evaluate its performance in the context of a 4 MW combustor test-rig model that mimics many of the dynamic characteristics of an actual engine including a significant time-delay. Using closed loop input-output data and system identification, a model of the test-rig was derived. Using this model, adaptive posi-cast controllers were designed and detailed numerical simulation studies were carried out. These studies include: (1) the closed-loop performance of the adaptive controller, (2) comparison of the adaptive controller with an empirical phase-shift controller, (3) robustness with respect to parametric uncertainties, (4) robustness with respect to unmodeled dynamics and uncertain delays, (5) performance in the presence of noise, and (6) effect of saturation constraints on the control input amplitude. These studies show that the adaptive posi-cast controllers decrease pressure oscillations much faster than the phase-shift controller without generating peak splitting. It also showed that the adaptive controller was capable of stabilizing the plant in the presence of 20% change in the resonant frequency, an order of magnitude change in the damping ratio, and unmodeled dynamics. The studies showed that the performance of the adaptive controller improved as the magnitude of the saturation constraints on the control input increased.

A model-based self-tuning controller for kinetically controlled combustion instability

Combustion instability is one of the distinct characteristics in continuous combustion processes such as gas turbines, ramjet engines and afterburners. This combustion instability often occurs near blow limit and high thermal out conditions and generates large amplitudes of heat release and pressure oscillations. Active control has been used to suppress combustion instability, but due to complex dynamics of the combustion processes, modeling of the combustion system has often been limited the application of the active control. Recently, we developed a combustion model which is applicable to kinetically controlled combustion instability. In this paper, we first examine the sensitivity of the model characteristics, which show that the model parameters change by 100% for a 10% change in the operating conditions. Next, we propose a self-tuning controller, and show that it effectively suppresses the pressure oscillations in the presence of changing operating conditions.

Heat release dynamics modeling for combustion instability analysis of kinetically controlled burning

We present a heat release dynamics model which utilizes a well-stirred reactor (WSR) model and one-step kinetics to describe the unsteady combustion process. The model incorporates the linearized mass and energy equations to describe the response of the reactor to external perturbations, and is cast in the form of a first order filter. The model is able to predict the phase between the mass flow rate oscillations and the resulting heat release fluctuations, as function of the operating conditions, e.g., the mean equivalence ratio and mean mass flow rate. The model predicts a sudden shift in phase in the region between the maximum reaction rate and the blow-out limit. We show that this phase change may trigger combustion instability. We use this novel model to predict combustion instability conditions in high swirl combustion, and demonstrate that these predictions agree qualitatively with experimental studies.