The authors examine three different types of plasma discharges in their ability to stabilize a lifted jet diffusion flame in coflow. The three discharges include a single-electrode corona discharge, an asymmetric dielectric-barrier discharge (DBD), and a repetitive ultrashort-pulsed discharge. The degree of nonequilibrium of this pulsed discharge is found to be higher than that for the DBD. Furthermore, this pulsed discharge causes the most significant improvement in the flame stability. The optimal placement of the discharge electrodes is investigated, and it is found that there is a close relation between this placement and the emission spectra, suggesting use of the emission spectra as a possible indicator of fuel/air mixture fraction. The optimal placement is mapped into mixture-fraction space by use of a fully premixed flame experiment of known mixture fraction. The result shows that the mixture fraction, which corresponds to the optimal placement, is much leaner than that of a conventional lifted jet flame

Plasma-Discharge Stabilization of Jet Diffusion Flames

The purpose of this paper is to monitor the dynamic cross-sectional behavior of swirling flames using the tunable diode laser absorption spectroscopy (TDLAS) tomographic system. The newly developed online and highly spatially resolved imaging system based on TDLAS tomography was employed to monitor and reveal directly the reaction process in the swirling flame by reconstructing the 2-D distributions of temperature and H2O concentration over a cross section of the flame. The system was demonstrated to be capable of capturing the temperature distribution accurately and inferring the thermal expansion over the cross section of interest in the swirling flame generated by a model swirl injector operating in partially premixed combustion mode. As the equivalence ratio was decreased, thermal oscillations extracted from the real-time 2-D reconstructions were used to infer the instability of the swirling flame. Furthermore, the developed system was applied to capture dynamically the process of blowout of the swirling flame, illustrating that the system can provide firsthand and reliable visual data to help prevent the flame from lean blowout (LBO). This paper reports the first experimental observation of the dynamic cross-sectional behavior of the swirling flame, enabled by the high spatial and temporal resolutions provided by the TDLAS tomographic imaging system. The developed system can help better understand the LBO mechanism so as to improve the performance of low-emission gas turbine combustors.

/pdf/online-cross-sectional-monitoring-of-a-swirling-flame-using-1n60lgb053.pdf

Online Cross-Sectional Monitoring of a Swirling Flame Using TDLAS Tomography

** † ‡ Thermoacoustic instability and lean blowout (LBO) are monitored in propane/air flames in a swirl-stabilized combustor using a novel tunable diode laser (TDL) sensor for gas temperature using wavelength-scanned laser absorption of two neighboring water vapor transitions near 1.4 μm. Although the gas composition and temperature are not uniform along the sensor line-of-sight, temperature oscillations and fluctuations are clearly identified in the FFT power spectra of the time-resolved data. Detailed experiments are conducted to optimize the position of the sensor line-of-sight in the flame for thermoacoustic instability and LBO sensing. The output of the sensor is used to suppress the thermoacoustic oscillations in a phase-delay feedback control system, and acoustic modulation of the intake air flow suppresses the temperature oscillations by more than 7dB. The low-frequency temperature fluctuations measured by the TDL sensor increase sharply as the flame approaches LBO. The intensity of these low-frequency fluctuations is used to detect the proximity to LBO, and can be used as a control variable for active LBO suppression without knowing the LBO fuel/air ratio limit. The TDL sensor results are compared with traditional pressure and emission sensors, and the TDL sensor is found to offer several advantages including better spatial resolution and insensitivity to background acoustic noise and flame emissions.

Sensing and Control of Combustion Instabilities in Swirl-Stabilized Combustors Using Diode-Laser Absorption

This paper develops a novel strategy for prediction of lean blowout in gas turbine combustors based on symbolic analysis of time series data from optical sensors, where the range of instantaneous data is partitioned into a finite number of cells and a symbol is assigned to each cell. Depending on the cell to which a data point belongs, a symbolic valueisassignedtothedatapoint.Thus,thesetoftimeseriesdataisconvertedtoasymbolstring.The(estimated)state probability vector is computed based on the number of occurrence of each symbol over a given time span. For the purpose of detecting lean blowout in gas turbine combustors, the state probability vector obtained at a condition sufficientlyawayfromleanblowout(referencestate)isconsideredasthereferencevector.Thedeviationofthecurrent state vector from the reference state vector is used as an anomaly measure for early detection of lean blowout. The results showed that the rate of change of the anomaly measure with equivalence ratio changed significantly as the systemapproachedleanblowout.Thischangeinslopeofthecurvewasobservedapproximatelyatasimilarproximity to lean blowout for different operating conditions and, hence, could be used as an early lean blowout precursor. The actual location of the change of slope depended primarily on the choice of reference state. This technique performed satisfactorily over a wide range of premixing.

/pdf/lean-blow-out-prediction-in-gas-turbine-combustors-using-7ojnuqg8e1.pdf

Lean Blow-Out Prediction in Gas Turbine Combustors Using Symbolic Time Series Analysis

Lean methane flame stability in a premixed generic swirl burner: Isothermal flow and atmospheric combustion characterization

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.

/pdf/an-active-control-system-for-lbo-margin-reduction-in-turbine-cd55abbiyv.pdf

An Active Control System for LBO Margin Reduction in Turbine Engines

§** †† This paper describes a method for detecting and preventing lean blow out (LBO) in a premixed, swirl stabilized combustor. The acoustic signal is filtered to detect localized extinction ‘events’, which increase in frequency as the flame equivalence ratio, φ, approaches the LBO limit, φ φ φ φLBO. As the flame becomes leaner, φLBO can be effectively shifted to lower equivalence ratios by redirecting a fraction of the total fuel into a central, premixed pilot. This actuation can increase the equivalence ratio in the stabilization region at a constant power setting and counter the flame lift associated with LBO. The resulting control system maintains lean flame stability in the presence of flow fluctuations. The combustor can therefore operate with an improved safety margin at lower temperatures and less NOx (nitrous oxides) emissions.

/pdf/acoustic-sensing-and-mitigation-of-lean-blow-out-in-premixed-mnmcce03fu.pdf

Acoustic Sensing and Mitigation of Lean Blow Out in Premixed Flames

This paper describes a method for efficiently detecting and preventing lean blow out (LBO) in a premixed, swirl stabilized combustor. The acoustic signal is processed to determine the real time LBO probability. This requires detection of localized extinction ‘events’ and rapid calculation of event frequency. As LBO probability increases, a proportional derivative controller actuates valves to redirect a fraction of the total fuel into a central, premixed pilot. The actuation increases the equivalence ratio in the stabilization region at a constant power setting and counters the flame lift associated with LBO. It is shown that the developed controller can prevent LBO even during rapid transients in equivalence ratio, φ.Copyright © 2005 by ASME

Acoustic Based Rapid Blowout Mitigation in a Swirl Stabilized Combustor

Input shaping is an effective method for reducing oscillatory motion in linear systems. Many physical systems, however, exhibit discontinuous dynamics, such as saturation, rate limiting, backlash, and dead-zone. These hard nonlinearities can degrade the vibration reducing properties of shaped signals. This paper investigates the detrimental effects of dead-zone on a class of input-shaped commands. A mitigation strategy is proposed for reducing these detrimental effects when the value of the deadzone can be estimated. The robustness of this mitigation approach to uncertainties in the dead-zone width is also determined. Theoretical developments are experimentally verified using an industrial 10-ton bridge crane.

/pdf/analysis-and-mitigation-of-dead-zone-effects-on-systems-4ryyi304cy.pdf

Shashvat Prakash

Papers

An Active Control System for LBO Margin Reduction in Turbine Engines

Acoustic Sensing and Mitigation of Lean Blow Out in Premixed Flames

Acoustic Based Rapid Blowout Mitigation in a Swirl Stabilized Combustor

Analysis and mitigation of dead-zone effects on systems using two-impulse zv input shaping

Investigation of Mode Shift Dynamics of Lean, Premixed Flames