Tongxun Yi

Real-Time Prediction of Incipient Lean Blowout in Gas Turbine Combustors

The present paper addresses real-time prediction of incipient lean blowout in partially premixed, liquid-fueled gas turbine combustors. Near-lean-blowout combustion is characterized by the intensified, low-frequency combustion oscillations (typically, below 30 Hz). Two indices, namely, the normalized chemiluminescence root mean square and the normalized cumulative duration of lean blowout precursor events, are recommended for lean blowout prediction. Both indices are associated with the statistical characteristics of the flame structure, which changes from the normal distribution to the Rayleigh distribution at the approach of lean blowout. Both indices change little within a large range of equivalence ratios and start to shoot up only when lean blowout is approached. To use the two indices for lean blowout prediction, one needs to perform a detailed a priori lean blowout mapping undersimulated engine operating conditions. However, the mapping can be done without running the engines very close to lean blowout.

Combustion Instabilities and Control of a Multiswirl Atmospheric Combustor

Thermo-acoustic instability and lean blowout (LBO) are investigated experimentally in an atmospheric swirling combustor. Possible factors affecting combustion instability are identified. With less than 1.0% of the combustion air, monochromatic or continuous air forcing of the swirling shear layer and fuel line, can reduce pressure oscillation amplitude by up to 90% to 98.5%; Phase-shift air forcing of the flame can reduce pressure oscillation amplitude by 78%. When pressure pulsations are attenuated, simultaneous reduction of NOx is observed and the flat flame front changes into a conical shape. For LBO, with decreasing equivalence ratio, relatively intense oscillations emerge from smooth turbulent combustions, followed by a more stable and quieter state before blowout. The regions of intense heat release oscillations overlap those of high mean heat release. Depending on the swirling flow field, lifted flame or anchored flame is observed for LBO. LBO limit extends at higher air flow rate. A dump swirler with a lower velocity region near the dump plane substantially extends the LBO limit by 42.8%. High frequency air forcing of the fuel line helps maintaining stable combustion near LBO.

Characterization of Near-Blowout Combustion Oscillations in a Lean, Partially Premixed Gas Turbine Combustor

Lean blowout (LBO) is a major technical challenge for dry-low-emission (DLE) combustion in gas turbine engines. For aero engines, LBO is further exacerbated by a large turndown ratio, sometimes more than ten. Intensified, low-frequency combustion oscillations, possibly caused by local flame quenching and reignition, are usually observed prior to complete blowout. Compared with dynamic instability, these near-LBO combustion oscillations usually occur at much lower frequencies, and are of much less intensity. The current paper presents the LBO phenomenology observed in an atmospheric, turpentine-fueled, partially premixed gas turbine combustor, and characterizes it using spectral analysis, autocorrelation coefficients, phase portrait, fractal dimension, and the largest Lyapunov exponent. All these measures suggest the chaotic behavior of near-LBO dynamics. The fractal dimension is less than 3 for combustion near LBO, which may suggest the possibility of characterizing LBO using low-order models.

Mean flow regulation of a high frequency combustion control valve based on pulse width modulation and system identification

Strong combustion instabilities within gas turbine engines adverse combustion efficiency, shorten engine life cycles and may even cause hardware or structure failures. Fuel modulations, which introduce externally unsteady heat release rate perturbations out of phase with pressure oscillations, is an effective and practical approach for combustion instability control. A high frequency fuel valve capable of large fuel modulations (above 30% of mean flow) up to 750 Hz is presented in this paper. Fuel modulations are achieved by pushing fuel out of the valve cavity using a Terfenol-D rod that extends or contracts with external magnetic fields, and mean flow is controlled by a step motor using pulse width modulation. However, this valve suffers from significant variations of mean flow when starting fuel modulation. To follow the flow command and effectively reject strong interferences of fuel modulations on mean flow, a LQG pulse width modulation controller based on closed-loop system identification is developed, which achieves faster response than a traditional proportional derivative controller. With effective mean flow regulation, this fuel valve damps out strong pressure pulsations in an unstable swirling atmospheric combustor up to 23 dB.

Lean Blowout Features and Control in a Swirl-Stabilized, Partially Premixed Gas Turbine Combustor

*† Early prediction and effective extension of lean blowout (LBO) are desirable for both aero and stationary gas turbines. Due to the complexities of finite thermo-chemistry/turbulence interactions, fuel atomization, evaporation and air/fuel mixing, accurate theoretical prediction of LBO is usually difficult. This paper aims to predict the proximity to LBO based on the phenomenological observations of unique and universal near-LBO features, namely the intensified low frequency combustion oscillations and the increased heat release uniformity when approaching LBO. Several indices, such as the normalized chemiluminescence RMS, pressure RMS, time ratio of the LBO precursor events, modified kurtosis, and the two-point heat release ratio, are developed to quantify the LBO features. All these indices consistently increase when approaching LBO. A unified definition of the LBO proximity based on the online computation of these indices, and a conceptual control scheme capable of simultaneous LBO extension and emission reduction are presented. It is also shown that, in certain circumstances, active LBO control may have to combine with dynamic instability attenuation.

Dynamics and Control of a High Frequency Fuel Valve and its Application to Active Combustion Control

An active combustion control valve capable of high-frequency large-amplitude fuel modulations is presented. This valve utilizes a hydraulic piston-cylinder structure. Mean fuel flow rate is controlled by valve opening via a step motor; and fuel modulation is achieved via the extension and contraction of a Terfenol-D rod. Experiments and low-order modeling are performed to optimize valve performance. Finite compressibility of fuel within the valve is considered based on fuel modulus. A fuel with smaller modulus favors larger fuel modulations. Fuel modulations could be more than 40% of mean flow at 700 Hz. An adaptive controller is developed to track and regulate mean flow. Phase-shift fuel control achieves pressure attenuation up to 27 dB in an unstable atmospheric swirling combustor.

Effects of Chemical Kinetics and Heat Loss on Near-LBO Combustion Dynamics - Stability Analysis

*† Both the linearized and the nonlinear WSR (well-stirred-reactor) models are employed to examine the effects of chemical kinetics and heat loss on near-LBO (lean blowout) combustion dynamics. Please note that, in the present paper, the combustion dynamics simply refers to the temporal evolution of pressure or the heat release rate (quantified by OH* chemiluminescence), instead of the combustion instability. This study is mainly motivated by LBO prediction and control in DLE combustion systems. Eigenvalues analysis of the linearized third-order WSR models shows that, with decreasing equivalence ratios, a complex conjugate pair of eigenvalues emerges from three negative real ones, moves left towards the right half phase plane, and finally crosses the imaginary axis. A pair of complex eigenvalues corresponds to an oscillating mode, whose damping ratio consistently decreases at the approach of LBO. A lower preheat temperature, a higher percentage of incomplete combustion, and more heat loss exacerbate near-LBO combustion stability. The predicted near-LBO oscillating frequency is typically below 25 Hz, and decreases with incomplete combustion and increases with heat loss. Model predictions qualitatively and even quantitatively agree with the experiments. Numerical simulation of the normalized, nonlinear, unsteady WSR models is performed to examine the combustor’s responses to large external disturbances. “Triggered instability” is observed, i.e. the combustor may remain stable in the presence of small external disturbances, but may undergo subcritical bifurcations to flame quenching when the external disturbances exceed certain thresholds. A higher equivalence ratio, a higher preheat temperature, less heat loss, and a smaller percentage of incomplete combustion are very effective in strengthening the flame’s robustness to external disturbances. It is numerically demonstrated that that zero-mean small-amplitude fuel modulations, based on the modern feedback control principles, can be very useful in enhancing the lean combustion stability without exacerbating the overall emissions.

Real-Time Prediction of Incipient Lean Blowout in a Partially Premixed, Liquid-Fueled Gas Turbine Combustor

The present paper addresses real-time prediction of incipient lean blowout (LBO) in partially premixed, liquid-fueled gas turbine combustors. Near-LBO combustion is characterized by the “intensified” low-frequency combustion oscillations, typically below 30 Hz. Two indices, namely the normalized chemiluminescence RMS and the normalized cumulative duration of LBO precursor events, are recommended for LBO prediction. Both indices are associated with the statistical characteristics of the flame structure, which changes from the normal distribution to the Rayleigh distribution at the approach of LBO. Both indices change little within a large range of equivalence ratios and start to shoot up only when LBO is approached. To use the two indices for LBO prediction, one needs to perform a detailed a priori LBO mapping under simulated engine operating conditions. However, the mapping can be done without running the engines very close to LBO.Copyright

Online Predicting the Occurrence of Combustion Instability based on the Computation of Damping Ratios

*† This paper presents a method for online predicting the onset of combustion instability, based on the computation of damping ratios. It is well known that, by Galerkin projection of acoustic eigenmodes onto acoustic equations, combustion instability can be formulated as a set of coupled, second-order, nonlinear oscillators. Under stable combustion and during the initial phase of combustion instability, pressure oscillations are weak, thus heat release perturbations caused by acoustic oscillations can be linearized into functions of pressure and pressure changing rates. These linear terms can be assimilated into the stiff and damping terms of the oscillators, and the broadband, turbulent, background heat release oscillations can be considered as the input signal. In this way, the response of pressure to background heat release oscillations can be characterized by a closed-loop transfer function. By assuming the background heat release oscillations have constant amplitude nearby the resonant frequencies (usually a reasonable assumption), the pressure spectrum may be conceived as a scaled version of the Bode plot. A procedure similar to Discrete-Fourier-Transform, but capable of higher accuracy and more suitable for real time operation, is used for spectrum estimation. The damping ratios are figured out from the spectrum nearby the resonant peaks using a weighted-least-mean-square method. Pressure data measured at two unstable combustors are analyzed, and both show that the damping ratio decreases more than three times and reaches the global minimum before combustion oscillations develop into the nonlinear limit cycle stage. Nomenclature k a ~ : the k th sample of a real signal; i β : scaled amplitude of background heat release rate oscillations; 0 θ : initial phase, rad; φ : equivalence ratio; i

Characteristics and Control of an Unstable Liquid-Fueled Multi-Swirling Combustor

This paper reports our initial results on active control of combustion instability in a lean direct fuel injection combustor featured with multiple air swirls and distributed fuel injection. Fuel modulation is achieved by “pushing” fuel out of the valve cavity using a magnetostrictive rod that extends or contracts with unsteady currents going through its surrounding coil. To follow the flow commands and quickly reject exogenous disturbances on mean fuel flow rate, an LQG pulse-width modulation controller based on system identification models is developed. As a starting point, we tried a phase shift controller whose optimal control phase is directly obtained from system identification models. Pressure attenuation up to 20 dB is achieved within 150 ms using pressure feedback that achieves more pressure attenuation than optical fiber feedback. It is found that, during unstable combustions, pressure has a higher signal noise ratio than the optical fiber output mainly because the thermoacoustic system has a high gain around its resonant frequencies. Time-varying phase relationship exists between heat release and pressure. Partial blockage of the combustor exit considerably reduces combustion oscillation intensity and postpones the occurrence of unstable combustion to a higher equivalence ratio. This may be because the exit blockage increases the acoustic impedance at the combustor exit and modifies the reacting flow field. With higher preheat temperature, strong combustion oscillations may occur at a lower equivalence ratio. Pressure pulsations may exhibit hysterisis with equivalence ratio, especially at lower preheat temperature.Copyright