Active Control of Combustion Instabilities in a Matrix Burner Using Primary and Pilot Fuel and Acoustic Forcing

2010 ◽  
Author(s):  
Dieter Bohn ◽  
Nils Ohlendorf ◽  
Frank Weidner ◽  
James F. Willie
Keyword(s):  
Heat Release ◽  
Mass Flow ◽  
Gas Turbines ◽  
Open Loop ◽  
Nox Emissions ◽  
Combustion Instabilities ◽  
Loop Control ◽  
First Case ◽  
Pilot Fuel ◽  
Acoustic Forcing

Lean premixed flames applied in modern gas turbines leads to reduce NOx emissions, but at the same time they are more susceptible to combustion instabilities than diffusion flames. These oscillations cause pressure fluctuations with high amplitudes and unacceptable noise as well as the risk of component or even engine failure. They can lead to pockets of fuel being formed in the mixing chamber and to bad mixing, which leads to increase in emissions. This paper reports the successful decoupling of the pressure and heat release inside the combustion chamber of a matrix burner using two actuation techniques. This led to the successful attenuation of the dominant instability modes occurring inside the combustor of the matrix burner. In the first case, acoustic forcing was used to decouple the pressure and the heat release inside the combustor. This was achieved by using a loudspeaker to modulate the primary air mass flow. This was followed by using acoustic forcing in CFD to decouple the pressure and heat release inside the combustor. For the action of the loudspeaker, sinusoidal forcing was used to mimic the modulation action of the diaphragm of the loudspeaker. In the second case, a fast gaseous “on-off” injector was used to modulate the primary fuel mass flow. After this, pilot fuel modulation was used to stabilize the flame. The control law governing the primary and pilot fuel modulation is discussed in details. The effect of open loop control on NOx emissions in the burner is also reported and discussed.

Active Control of Combustion Instability Using Symmetric and Asymmetric Premix Fuel Modulation

2007 ◽  
Author(s):  
Daniel Guyot ◽  
Christian Oliver Paschereit
Keyword(s):  
Heat Release ◽  
Closed Loop ◽  
Modulation Frequency ◽  
Combustion Instability ◽  
Open Loop ◽  
Closed Loop Control ◽  
Nox Emissions ◽  
Loop Control ◽  
Asymmetric Modulation ◽  
Control Schemes

Active instability control was applied to an atmospheric swirl-stabilized premixed combustor using open loop and closed loop control schemes. Actuation was realised by two on-off valves allowing for symmetric and asymmetric modulation of the premix fuel flow while maintaining constant time averaged overall fuel mass flow. Pressure and heat release fluctuations in the combustor as well as NOx, CO and CO2 emissions in the exhaust were recorded. In the open loop circuit the heat release response of the flame was first investigated during stable combustion. For symmetric fuel modulation the dominant frequency in the heat release response was the modulation frequency, while for asymmetric modulation it was its first harmonic. In stable open loop control a reduction of NOx emissions due to fuel modulation of up to 19% was recorded. In the closed loop mode phase-shift control was applied while triggering the valves at the dominant oscillation frequency as well as at its second subharmonic. Both, open and closed loop control schemes were able to successfully control a low-frequency combustion instability, while showing only a small increase in NOx emissions compared to, for example, secondary fuel modulation. Using premixed open loop fuel modulation, attenuation was best when modulating the fuel at frequencies different from the dominant instability frequency and its subharmonic. The performance of asymmetric fuel modulation was generally slightly better than for symmetric modulation in terms of suppression levels as well as emissions. Suppression of the instability’s pressure rms level of up to 15.7 dB was recorded.

Measurement and Analysis of Flame Transfer Function in a Sector Combustor Under High Pressure Conditions

2003 ◽  
Author(s):  
W. S. Cheung ◽  
G. J. M. Sims ◽  
R. W. Copplestone ◽  
J. R. Tilston ◽  
C. W. Wilson ◽  
...  
Keyword(s):  
High Pressure ◽  
Transfer Function ◽  
Heat Release ◽  
Gas Turbine ◽  
Mass Flow ◽  
Atmospheric Pressure ◽  
Acoustic Waves ◽  
Gas Turbines ◽  
Transfer Functions ◽  
Unsteady Pressure

Lean premixed prevaporised (LPP) combustion can reduce NOx emissions from gas turbines, but often leads to combustion instability. A flame transfer function describes the change in the rate of heat release in response to perturbations in the inlet flow as a function of frequency. It is a quantitative assessment of the susceptibility of combustion to disturbances. The resulting fluctuations will in turn generate more acoustic waves and in some situations self-sustained oscillations can result. Flame transfer functions for LPP combustion are poorly understood at present but are crucial for predicting combustion oscillations. This paper describes an experiment designed to measure the flame transfer function of a simple combustor incorporating realistic components. Tests were conducted initially on this combustor at atmospheric pressure (1.2 bar and 550 K) to make an early demonstration of the combustion system. The test rig consisted of a plenum chamber with an inline siren, followed by a single LPP premixer/duct and a combustion chamber with a silencer to prevent natural instabilities. The siren was used to induce variable frequency pressure/acoustic signals into the air approaching the combustor. Both unsteady pressure and heat release measurements were undertaken. There was good coherence between the pressure and heat release signals. At each test frequency, two unsteady pressure measurements in the plenum were used to calculate the acoustic waves in this chamber and hence estimate the mass-flow perturbation at the fuel injection point inside the LPP duct. The flame transfer function relating the heat release perturbation to this mass flow was found as a function of frequency. The same combustor hardware and associated instrumentation were then used for the high pressure (15 bar and 800 K) tests. Flame transfer function measurements were taken at three combustion conditions that simulated the staging point conditions (Idle, Approach and Take-off) of a large turbofan gas turbine. There was good coherence between pressure and heat release signals at Idle, indicating a close relationship between acoustic and heat release processes. Problems were encountered at high frequencies for the Approach and Take-off conditions, but the flame transfer function for the Idle case had very good qualitative agreement with the atmospheric-pressure tests. The flame transfer functions calculated here could be used directly for predicting combustion oscillations in gas turbine using the same LPP duct at the same operating conditions. More importantly they can guide work to produce a general analytical model.

scholarly journals Forced synchronization of periodic and aperiodic thermoacoustic oscillations: lock-in, bifurcations and open-loop control

2018 ◽  
Vol 838 ◽  
pp. 690-714 ◽  
Author(s):  
Karthik Kashinath ◽  
Larry K. B. Li ◽  
Matthew P. Juniper
Keyword(s):  
Periodic System ◽  
Systems Approach ◽  
Numerical Study ◽  
Dominant Frequency ◽  
Open Loop ◽  
Response Amplitude ◽  
Loop Control ◽  
Acoustic Forcing ◽  
Lock In ◽  
Thermoacoustic Oscillations

Synchronization is a universal concept in nonlinear science but has received little attention in thermoacoustics. In this numerical study, we take a dynamical systems approach to investigating the influence of harmonic acoustic forcing on three different types of self-excited thermoacoustic oscillations: periodic, quasi-periodic and chaotic. When the periodic system is forced, we find that: (i) at low forcing amplitudes, it responds at both the forcing frequency and the natural (self-excited) frequency, as well as at their linear combinations, indicating quasi-periodicity; (ii) above a critical forcing amplitude, the system locks in to the forcing; (iii) the bifurcations leading up to lock-in and the critical forcing amplitude required for lock-in depend on the proximity of the forcing frequency to the natural frequency; (iv) the response amplitude at lock-in may be larger or smaller than that of the unforced system and the system can exhibit hysteresis and the jump phenomenon owing to a cusp catastrophe; and (v) at forcing amplitudes above lock-in, the oscillations can become unstable and transition to chaos, or switch between different stable attractors depending on the forcing amplitude. When the quasi-periodic system is forced at a frequency equal to one of the two characteristic frequencies of the torus attractor, we find that lock-in occurs via a saddle-node bifurcation with frequency pulling. When the chaotic system is forced at a frequency close to the dominant frequency of its strange attractor, we find that it is possible to destroy chaos and establish stable periodic oscillations. These results show that the open-loop application of harmonic acoustic forcing can be an effective strategy for controlling periodic or aperiodic thermoacoustic oscillations. In some cases, we find that such forcing can reduce the response amplitude by up to 90 %, making it a viable way to weaken thermoacoustic oscillations.

Suppression of Instabilities in Gaseous Fuel High-Pressure Combustor Using Non-Coherent Oscillatory Fuel Injection

2003 ◽  
Author(s):  
D. Shcherbik ◽  
E. Lubarsky ◽  
Y. Neumeier ◽  
B. T. Zinn ◽  
K. McManus ◽  
...  
Keyword(s):  
High Pressure ◽  
Active Control ◽  
Fuel Injection ◽  
Injection Rate ◽  
Open Loop ◽  
Combustion Instabilities ◽  
Open Loop Control ◽  
Pressure Oscillations ◽  
Loop Control ◽  
Fuel Injection Rate

This paper describes the application of active, open loop, control in effective damping of severe combustion instabilities in a high pressure (i.e., around 520 psi) gas turbine combustor simulator. Active control was applied by harmonic modulation of the fuel injection rate into the combustor. The open-loop active control system consisted of a pressure sensor and a fast response actuating valve. To determine the dependence of the performance of the active control system upon the frequency, the fuel injection modulation frequency was varied between 300 and 420 Hz while the frequency of instability was around 375 Hz. These tests showed that the amplitude of the combustor pressure oscillations strongly depended upon the frequency of the open loop control. In fact, the amplitude of the combustor pressure oscillations varied ten fold over the range of investigated frequencies, indicating that applying the investigated open loop control approach at the appropriate frequency could effectively damp detrimental combustion instabilities. This was confirmed in subsequent tests in which initiation of open loop modulation of the fuel injection rate at a non resonant frequency of 300Hz during unstable operation with peak to peak instability amplitude of 114 psi and a frequency of 375Hz suppressed the instability to a level of 12 psi within approximately 0.2 sec (i.e., 75 periods). Analysis of the time dependence of the spectra of the pressure oscillations during suppression of the instability strongly suggested that the open loop fuel injection rate modulation effectively damped the instability by “breaking up” (or preventing the establishment of) the feedback loop between the reaction rate and combustor oscillations that drove the instability.

Thermoacoustic Analysis of a Full Annular Lean Burn Aero-Engine Combustor

2013 ◽  
Author(s):  
Antonio Andreini ◽  
Bruno Facchini ◽  
Andrea Giusti ◽  
Ignazio Vitale ◽  
Fabio Turrini
Keyword(s):  
Heat Release ◽  
Gas Turbines ◽  
Three Dimensional ◽  
Thermal Fluctuations ◽  
Injection System ◽  
Combustion Instabilities ◽  
Lean Burn ◽  
Acoustic Effect ◽  
Lean Combustion ◽  
Critical Issues

In order to reduce NOx emissions, modern gas turbines are often equipped with lean burn combustion systems, where the engine operates near the lean blow-out limits. One of the most critical issues of lean combustion technology is the onset of combustion instabilities related to a coupling between pressure oscillations and thermal fluctuations excited by the unsteady heat release. In this work a thermoacoustic analysis of a full annular combustor developed by AVIO is discussed. The system is equipped with an advanced PERM (Partially Evaporating and Rapid Mixing) injection system based on a piloted lean burn spray flame generated by a pre-filming atomizer. Combustor walls are based on multi-perforated liners to control metal temperature: these devices are also recognized as very effective sound absorbers, thus in innovative lean combustors they could represent a good means both for wall cooling and damping combustion instabilities. The performed analysis is based on the resolution of the eigenvalue problem related to an inhomogeneous wave equation which includes a source term representing heat release fluctuations (the so called Flame Transfer Function, FTF) in the flame region using a three-dimensional FEM code. A model representing the entire combustor was assembled including all the acoustically relevant geometrical features. In particular, the acoustic effect of multi-perforated liners was introduced by modeling the corresponding surfaces with an equivalent internal impedance. Different simulations with and without the presence of the flame were performed analyzing the influence of the multi-perforated liners. Furthermore, different modeling approaches for the FTF were examined and compared with each other. Comparisons with available experimental data showed a good agreement in terms of resonant frequencies in the case of passive simulations. On the other hand, when the presence of the flame is considered, comparisons with experiments showed the inadequacy of FTFs commonly used for premixed combustion and thus the necessity of an improved FTF, more suitable for liquid fueled gas turbines where the evaporation process could play an important role in the flame heat release fluctuations.

Influence of the Interaction of Equivalence Ratio and Mass Flow Fluctuations on Flame Dynamics

2005 ◽  
Author(s):  
M. P. Auer ◽  
C. Hirsch ◽  
T. Sattelmayer
Keyword(s):  
Heat Release ◽  
Active Control ◽  
Mass Flow ◽  
Equivalence Ratio ◽  
Feedback Loop ◽  
Combustion Instabilities ◽  
Flame Dynamics ◽  
Secondary Fuel ◽  
Flow Oscillations ◽  
Mass Flows

Modern lean combustion systems are often prone to combustion instabilities — an interaction of acoustic waves, fluid dynamics and heat release oscillations. Mass flow oscillations are one important part of the feedback loop of combustion instabilities. Therefore, modulated mass flows of fuel or/and combustion air are main objectives in many studies on combustion instabilities and their active control (AIC, Active Instability Control). Flame response and flame transfer matrices are often determined by excitation of combustion air at various frequencies by sirens or loudspeakers. For the purpose of active control modulated secondary fuel is usually injected to the mass flow to dampen heat release fluctuations of the flame in order to de-couple the thermoacoustic feedback loop. This paper demonstrates the influence of modulated mass flows on the flame dynamics in an atmospheric test rig with a natural gas fired swirl burner. In the investigated cases the modulation of combustion air also result in equivalence ratio fluctuations due to choked main fuel injection. This combination has a tremendous effect on the flame dynamics. A model was developed to describe the interaction of equivalence ratio fluctuations and total mass flow oscillations and their influence on combustion instabilities. In experiments these equivalence ratio fluctuations were generated by injecting modulated secondary fuel. The derived model provides a deep insight into the driving mechanisms of combustion instabilities.

Application of Silicon Carbide Photodiode Flame Temperature Sensors in an Active Combustion Pattern Factor Control System

2009 ◽  
Author(s):  
Carl A. Palmer ◽  
Royce L. Abel ◽  
Peter Sandvik
Keyword(s):  
Silicon Carbide ◽  
Gas Turbines ◽  
Fault Tolerant ◽  
Flame Temperature ◽  
Diagnostic System ◽  
Open Loop ◽  
Bulk Temperature ◽  
Loop Control ◽  
Engine Model ◽  
Fuel Flow

This paper describes the development and initial application studies for an active combustion pattern factor controller (APFC) for gas turbines. The system is based around use of a novel silicon carbide (SiC) optical ultraviolet (UV) dual diode flame temperature sensor (FTS) developed by General Electric’s Global Research Center and GE Energy. The APFC system determines combustion flame temperatures, validates the values, and integrates an assessment of signal and combustion hardware health to determine how to trim the fuel flow to individual fuel nozzles. Key aspects of the system include: • Determination of each flame’s bulk temperature using the FTS. • Assessment of the reliability of the flame temperature data and physical combustion hardware health through analysis of the high frequency output of the sensor. • Validation of the flame temperature signal using a data-driven approach (model based validation - MBV). • Fusion of sensor ‘health indices’ into the APFC to alter the trim control signal based on the health (or ‘believability’) of each sensor and fuel nozzle/combustor. • Fault-tolerant peak/valley detection and control module that selects individual fuel valves to target for reducing pattern factor, while simultaneously balancing the overall fuel flow. The authors demonstrated feasibility of the approach by performing simulations using a quasi-2D T700 turbine engine model. Tests were run on the simulated platform with no faults, simulated sensor faults, and on a system with underlying combustion hardware issues. The final APFC system would be applicable for aviation, naval and land-based commercial gas turbines, and can be used in closed-loop control or adapted as an open-loop advisory / diagnostic system.

Improvement of Selective Catalytic Reduction System Performance in Combined Cycle Power Plants

2003 ◽  
Author(s):  
Anatoly Sobolevskiy ◽  
Tom Czapleski ◽  
Richard Murray
Keyword(s):  
Selective Catalytic Reduction ◽  
Mass Flow ◽  
System Performance ◽  
Gas Turbines ◽  
Power Plants ◽  
Catalytic Reduction ◽  
Active Material ◽  
Combined Cycle ◽  
Nox Emissions ◽  
Scr System

Environmental regulations are very stringent in the U.S., requiring very low emissions of nitrogen oxides (NOx) from combined cycle power plants. Selective Catalytic Reduction (SCR) systems utilizing vanadium pentoxide (V2O5) as the active material in the catalyst are a proven method of reducing NOx emissions in the exhaust stack of gas turbines with heat recovery steam generators (HRSG) to 2–4 ppmvd. These low NOx emissions levels require an increase of SCR removal efficiency to the level of 90+ % with limited ammonia slip. The distribution of flow velocities, temperature, and NOx mass flow at the inlet of the SCR are critical to minimizing NOx and ammonia (NH3) concentrations in HRSG stack. The short distance between the ammonia injection grid and the catalyst in the HRSG complicates the achievement of homogeneous NH3 and NOx mixture. To better understand the influence of the above factors on overall SCR system performance, field testing of combined cycle power plants with an SCR installed in the HRSG has been conducted. Uniformity of exhaust flow, temperature and NOx emissions upstream and downstream of the SCR were examined and the results served as a basis for SCR system tuning in order to increase its efficiency. NOx mass flow profiles upstream and downstream of the SCR were used to assess ammonia distribution enhancement. Ammonia flow adjustments within a cross section of the exhaust gas duct yielded significantly improved NOx mass flow uniformity after the SCR while reducing ammonia consumption. Based on field experience, a procedure for ammonia distribution grid tuning was developed and recommendations for SCR performance improvement were generated.

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