Passive Control of Combustion Instability in Lean Premixed Combustors

2000 ◽  
Vol 122 (3) ◽  
pp. 412-419 ◽  
Author(s):  
Robert C. Steele ◽  
Luke H. Cowell ◽  
Steven M. Cannon ◽  
Clifford E. Smith
Keyword(s):  
Passive Control ◽  
Fuel Injection ◽  
Swirling Flow ◽  
Time Lag ◽  
Stable Configuration ◽  
Cfd Analysis ◽  
Fuel Injector ◽  
Pressure Oscillations ◽  
Excessive Pressure ◽  
Lean Premixed

A Solar fuel injector that provides lean premixed combustion conditions has been studied in a combined experimental and numerical investigation. Lean premixed conditions can be accompanied by excessive combustion driven pressure oscillations which must be eliminated before the release of a final combustor design. In order to eliminate the pressure oscillations the location of fuel injection was parametrically evaluated to determine a stable configuration. It was observed that small axial changes in the position of the fuel spokes within the premix duct of the fuel injector had a significant positive effect on decoupling the excitation of the natural acoustic modes of the combustion system. In order to further understand the phenomenon, a time-accurate 2D CFD analysis was performed. 2D analysis was first calibrated using 3D steady-state CFD computations of the premixer in order to model the radial distribution of velocities in the premixer caused by non-uniform inlet conditions and swirling flow. 2D time-accurate calculations were then performed on the baseline configuration. The calculations captured the coupling of heat release with the combustor acoustics, which resulted in excessive pressure oscillations. When the axial location of the fuel injection was moved, the CFD analysis accurately captured the fuel time lag to the flame-front, and qualitatively matched the experimental findings. [S0742-4795(00)01103-0]

Passive Control of Combustion Instability in Lean Premixed Combustors

1999 ◽  
Author(s):  
Robert C. Steele ◽  
Luke H. Cowell ◽  
Steven M. Cannon ◽  
Clifford E. Smith
Keyword(s):  
Passive Control ◽  
Fuel Injection ◽  
Swirling Flow ◽  
Time Lag ◽  
Stable Configuration ◽  
Cfd Analysis ◽  
Fuel Injector ◽  
Pressure Oscillations ◽  
Excessive Pressure ◽  
Lean Premixed

A Solar fuel injector that provides lean premixed combustion conditions has been studied in a combined experimental and numerical investigation. Lean premixed conditions can be accompanied by excessive combustion driven pressure oscillations which must be eliminated before the release of a final combustor design. In order to eliminate the pressure oscillations the location of fuel injection was parametrically evaluated to determine a stable configuration. It was observed that small axial changes in the position of the fuel spokes within the premix duct of the fuel injector had a significant positive effect on decoupling the excitation of the natural acoustic modes of the combustion system. In order to further understand the phenomenon, a time-accurate 2D CFD analysis was performed. 2D analysis was first calibrated using 3D steady-state CFD computations of the premixer in order to model the radial distribution of velocities in the pre mixer caused by non-uniform inlet conditions and swirling flow. 2D time-accurate calculations were then performed on the baseline configuration. The calculations captured the coupling of heat release with the combustor acoustics, which resulted in excessive pressure oscillations. When the axial location of the fuel injection was moved, the CFD analysis accurately captured the fuel time lag to the flame-front, and qualitatively matched the experimental findings.

Effects of Swirler Configurations on Flow Structures and Combustion Characteristics

2004 ◽  
Author(s):  
Guoqiang Li ◽  
Ephraim J. Gutmark
Keyword(s):  
Fuel Injection ◽  
Vortex Breakdown ◽  
Swirling Flow ◽  
Combustion Characteristics ◽  
Pollutant Emissions ◽  
Flow Structures ◽  
Inlet Temperature ◽  
Nox Emissions ◽  
Atmospheric Conditions ◽  
Fuel Injector

Modern gas turbine combustion technologies are driven by stringent regulations on pollutant emissions such as CO and NOx. A combustion system of multiple swirlers coupled with distributed fuel injection was studied as a new concept for reducing NOx emissions by application of Lean Direct Injection (LDI) combustion. The present paper investigates the effects of swirler configurations on the flow structures in isothermal flow and combustion cases using a multiple-swirlers fuel injector at atmospheric conditions. The swirling flow field within the combustor was characterized by a central recirculation zone formed after vortex breakdown. The differences between the tangential and axial velocity profiles, the shape of the recirculation zones and the turbulence intensity distribution for the different fuel injector configurations impacted the flame structure, the temperature distribution and the emission characteristics both for gaseous and liquid fuels. Co-swirling configuration was shown to have the lowest NOx emission level compared with the counter-swirling ones for both types of fuels with lower inlet temperature. In contrast to this, the swirl configuration had less effect on the combustion characteristics in the case of gaseous fuel with high air inlet temperature. The differences in NOx emissions were shown to be closely related to the Damkohler number or the degree to which the flame resembled well-mixed combustion, which is the foundation for LDI combustion.

Fuel Nozzle Aerodynamic Design Using CFD Analysis

1997 ◽  
Vol 119 (3) ◽  
pp. 527-534 ◽  
Author(s):  
D. S. Crocker ◽  
E. J. Fuller ◽  
C. E. Smith
Keyword(s):  
Fuel Injection ◽  
Swirling Flow ◽  
Turbulence Models ◽  
Cfd Analysis ◽  
Aerodynamic Design ◽  
Flow Area ◽  
Measured Velocity ◽  
Fuel Nozzle ◽  
Cfd Analyses ◽  
Good Agreement

The aerodynamic design of airflow passages in fuel injection systems can be significantly enhanced by the use of CFD analysis. Attempts to improve the efficiency of the fuel nozzle design process by using CFD analyses have generally been unsuccessful in the past due to the difficulties of modeling swirling flow in complex geometries. Some of the issues that have been obstacles to successful and timely analysis of fuel nozzle aerodynamics include grid generation, turbulence models, and definition of boundary conditions. This study attempts to address these obstacles and demonstrate a CFD methodology capable of modeling swirling flow within the internal air passages of fuel nozzles. The CFD code CFD-ACE was used for the analyses. Results of nonreacting analyses and comparison with experimental data are presented for three different fuel nozzles. The three nozzles have distinctly different designs (including axial and radial inflow swirlers) and thus demonstrate the flexibility of the design methodology. Particular emphasis is given to techniques involved in predicting the effective flow area (ACd) of the nozzles. Good agreement between CFD predictions of the ACd (made prior to experiments) and the measured ACd was obtained. Comparisons between predicted and measured velocity profiles also showed good agreement.

3D LES Modeling of Combustion Dynamics in Lean Premixed Combustors

2001 ◽  
Author(s):  
Steven M. Cannon ◽  
Virgil Adumitroaie ◽  
Clifford E. Smith
Keyword(s):  
Turbulence Model ◽  
Time Lag ◽  
Pressure Oscillation ◽  
Operating Conditions ◽  
Pressure Amplitude ◽  
Fuel Injector ◽  
Turbulent Structures ◽  
Combustion Dynamics ◽  
Lean Premixed ◽  
Unsteady Rans

A lean premixed fuel injector/combustor typical of industrial gas turbine combustors has been analyzed using 3D Large Eddy Simulation (LES) methods. The objective of the study was to evaluate the 3D LES modeling approach for predicting combustion dynamics and compare it with simpler unsteady Reynolds Averaged Navier Stokes (RANS) methods using 2D and 3D analyses. Large amplitude pressure oscillations were observed experimentally at the modeled operating conditions, and previous 2D axisymmetric unsteady RANS analysis has shown reasonable, but not perfect, engineering agreement with pressure measurements. Although the pressure amplitude was accurately predicted, the frequency was substantially in error. This study sought to see if 3D modeling would improve the agreement. 2D axisymmetric and full 3D calculations were performed with a state-of-the-art, unstructured-grid, parallel (domain decomposition) CFD code. For the unsteady RANS calculations, the RNG k-ε turbulence model was employed, while for the LES calculation the Smagorinsky subgrid turbulence model was employed. Surprisingly, the 2D unsteady RANS, 3D unsteady RANS, and 3D LES calculations gave nearly identical pressure oscillation predictions, and all calculations had the oscillation frequency around 280 Hertz. This work has shown that smaller turbulent structures captured with 3D LES have very little effect on capturing combustion instability driven primarily by a fuel time-lag.

A Mechanism of Combustion Instability in Lean Premixed Gas Turbine Combustors

1999 ◽  
Author(s):  
Tim Lieuwen ◽  
Hector Torres ◽  
Clifford Johnson ◽  
Ben T. Zinn
Keyword(s):  
Gas Turbines ◽  
Passive Control ◽  
Control Strategies ◽  
Combustion Process ◽  
Fuel Injector ◽  
Mean Flow ◽  
Design Changes ◽  
Lean Premixed ◽  
Flow Oscillations ◽  
Dominant Characteristic

There has been increased demand in recent years for gas turbines that operate in a lean, premixed (LP) mode of combustion in an effort to meet stringent emissions goals. Unfortunately, detrimental combustion instabilities are often excited within the combustor when it operates under lean conditions, degrading performance and reducing combustor life. To eliminate the onset of these instabilities and develop effective approaches for their control, the mechanisms responsible for their occurrence must be understood. This paper describes the results of an investigation of the mechanisms responsible for these instabilities and approaches for their control. These studies found that combustors operating in a LP mode of combustion are highly sensitive to variations in the equivalence ratio (ϕ) of the mixture that enters the combustor. Furthermore, it was found that such ϕ variations can be induced by interactions of the pressure and flow oscillations with the reactant supply rates. The ϕ perturbations formed in the inlet duct (near the fuel injector) are convected by the mean flow to the combustor where they produce large amplitude heat release oscillations that drive combustor pressure oscillations. It is shown that the dominant characteristic time associated with this mechanism is the convective time from the point of formation of the reactive mixture at the fuel injector to the point where it is consumed at the flame. Instabilities occur when the ratio of this convective time and the period of the oscillations equals a specific constant, whose magnitude depends upon the combustor design. Significantly, these predictions are in good agreement with available experimental data, strongly suggesting that the proposed mechanism properly accounts for the essential physics of the problem. The predictions of this study also indicate, however, that simple design changes (i.e., passive control approaches) may not, in general, provide a viable means for controlling these instabilities, due to the multiple number of modes that may be excited by the combustion process. This conclusion indicates that active control strategies may be necessary for controlling these instabilities.

Measurements of Equivalence Ratio Fluctuations in a Lean Premixed Prevaporized (LPP) Combustor and Its Correlation to Combustion Instability

2002 ◽  
Author(s):  
Quang-Viet Nguyen
Keyword(s):  
Real Time ◽  
Equivalence Ratio ◽  
Dynamic Pressure ◽  
Combustion Instability ◽  
Time Lag ◽  
Combustion Stability ◽  
Fuel Injector ◽  
Validation Data ◽  
Frequency Space ◽  
Lean Premixed

Experimental evidence correlating equivalence ratio fluctuations with combustion instabilities and NOX emissions in a jet-A fueled lean premixed prevaporized (LPP) combustor utilizing a non-proprietary ‘generic’ fuel injector is presented. Real-time laser absorption measurements of equivalence ratio, together with dynamic combustor pressure, flame luminosity and fuel pressure were obtained at inlet air conditions up to 16.7 atm and 817 K. From this data, an extensive database of real-time variables was obtained for the purposes of providing validation data for future studies of LPP combustion modeling. In addition, time and frequency space analysis of the data revealed measurable levels of acoustic coupling between all variables. Equivalence ratio and dynamic pressure cross-correlations were found to predict the level of combustion instability. Furthermore, NOX production was found to follow the root-mean-square (RMS) flame luminosity and RMS combustor dynamic pressure. However, the unmixedness of the fuel-air mixture was not found to predict NOX production in this combustor. The generic LPP injector, although not optimized for low-emissions or combustion stability, provides some of the essential features of real injectors for the purposes of studying the relationship between fluctuations in equivalence ratios and combustion instability. In particular, the fuel premixer advection time was found to have a significant and direct impact on the level of combustion instability. The results of this work support the time-lag concept for avoiding combustion instability when designing injector/premixers in LPP combustors.

Measurements of Equivalence Ratio Fluctuations in a Lean Premixed Gas Turbine Combustor Using an IR Absorption Technique

2010 ◽  
Author(s):  
Hyung Ju Lee ◽  
Kyu Tae Kim ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Gas Turbine ◽  
Equivalence Ratio ◽  
Fuel Injection ◽  
Time Lag ◽  
Ir Absorption ◽  
Time Lags ◽  
Gas Turbine Combustor ◽  
Fuel Injectors ◽  
Lean Premixed ◽  
Absorption Technique

An experimental study was conducted to estimate and confirm equivalence ratio fluctuations at the inlet of a lean premixed gas turbine combustor. Fuel injectors were placed at several locations in the mixing section of the combustor, in order to produce different instability characteristics due to the equivalence ratio fluctuations. An IR absorption technique was used to measure the equivalence ratio fluctuations at the inlet of the dump combustor. The measured IR signals were processed in two different ways and the results were compared to confirm the two calibrated equivalence ratio signals. The processed data showed that the two processing methods gave very similar results, and the phase of the measured equivalence ratio fluctuations at the combustor inlet by the IR absorption technique agreed well with that of equivalence ratio fluctuations predicted by time lags in the mixing section. It was, however, not possible to accurately predict the magnitude of the equivalence ratio fluctuations at the combustor inlet by the time lag analysis because the equivalence ratio fluctuations generataed at the fuel injection location is changed by mixing and diffusion as the fuel is convected through the combustor.

scholarly journals Fuel Nozzle Aerodynamic Design Using CFD Analysis

1996 ◽  
Author(s):  
D. Scott Crocker ◽  
Eric J. Fuller ◽  
Clifford E. Smith
Keyword(s):  
Fuel Injection ◽  
Swirling Flow ◽  
Turbulence Models ◽  
Cfd Analysis ◽  
Aerodynamic Design ◽  
Flow Area ◽  
Measured Velocity ◽  
Fuel Nozzle ◽  
Cfd Analyses ◽  
Good Agreement

The aerodynamic design of airflow passages in fuel injection systems can be significantly enhanced by the use of CFD analysis. Attempts to improve the efficiency of the fuel nozzle design process by using CFD analyses have generally been unsuccessful in the past due to the difficulties of modeling swirling flow in complex geometries. Some of the issues that have been obstacles to successful and timely analysis of fuel nozzle aerodynamics include grid generation, turbulence models, and definition of boundary conditions. This study attempts to address these obstacles and demonstrate a CFD methodology capable of modeling swirling flow within the internal air passages of fuel nozzles. The CFD code CFD-ACE was used for the analyses. Results of non-reacting analyses and comparison with experimental data are presented for three different fuel nozzles. The three nozzles have distinctly different designs (including axial and radial inflow swirlers) and thus demonstrate the flexibility of the design methodology. Particular emphasis is given to techniques involved in predicting the effective flow area (ACd) of the nozzles. Good agreement between CFD predictions of the ACd (made prior to experiments) and the measured ACd was obtained. Comparisons between predicted and measured velocity profiles also showed good agreement.

An Experimental Study on the Coupling of Combustion Instability Mechanisms in a Lean Premixed Gas Turbine Combustor

2009 ◽  
Author(s):  
Hyung Ju Lee ◽  
Kyu Tae Kim ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Experimental Study ◽  
Gas Turbine ◽  
Equivalence Ratio ◽  
Acoustic Pressure ◽  
Fuel Injection ◽  
Operating Conditions ◽  
Fuel Injector ◽  
Gas Turbine Combustor ◽  
Lean Premixed ◽  
Injection Location

An experimental study was conducted to characterize the combined effects of flame-vortex interactions and equivalence ratio fluctuations on self-excited combustion instabilities in a swirl-stabilized lean premixed gas turbine combustor. The combustor was designed so that the fuel injector location and the combustion chamber length could be independently varied. In addition, the fuel and air could be mixed upstream of the choked inlet to the combustor, thereby eliminating the possibility of equivalence ratio fluctuations. Experiments were performed over a broad range of operating conditions and at each condition both the combustor length and the fuel injection location were varied. Dynamic pressure in the combustor, acoustic pressure and velocity in the mixing section, and the overall rate of heat release were simultaneously measured at all operating conditions. Two distinct instability regimes were observed; one near 220 Hz and the other near 345 Hz. It was also found that the strength of the instability changed significantly as the fuel injection location was varied, while the phase of the acoustic pressure and velocity fluctuations in the mixing section did not change. A time series of pressure and CH* chemiluminescence signals confirmed constructive or destructive coupling of the two instability mechanisms; the flame-vortex interaction and the equivalence ratio fluctuation interact each other and determine the instability characteristics in partially premixed conditions.

A Mechanism of Combustion Instability in Lean Premixed Gas Turbine Combustors

2000 ◽  
Vol 123 (1) ◽  
pp. 182-189 ◽  
Author(s):  
T. Lieuwen ◽  
H. Torres ◽  
C. Johnson ◽  
B. T. Zinn
Keyword(s):  
Gas Turbines ◽  
Passive Control ◽  
Combustion Process ◽  
Fuel Injector ◽  
Mean Flow ◽  
Design Changes ◽  
Lean Premixed ◽  
Flow Oscillations ◽  
Dominant Characteristic ◽  
Flame Instabilities

There has been increased demand in recent years for gas turbines that operate in a lean, premixed (LP) mode of combustion in an effort to meet stringent emissions goals. Unfortunately, detrimental combustion instabilities are often excited within the combustor when it operates under lean conditions, degrading performance and reducing combustor life. To eliminate the onset of these instabilities and develop effective approaches for their control, the mechanisms responsible for their occurrence must be understood. This paper describes the results of an investigation of the mechanisms responsible for these instabilities. These studies found that combustors operating in a LP mode of combustion are highly sensitive to variations in the equivalence ratio (ϕ) of the mixture that enters the combustor. Furthermore, it was found that such ϕ variations can be induced by interactions of the pressure and flow oscillations with the reactant supply rates. The ϕ perturbations formed in the inlet duct (near the fuel injector) are convected by the mean flow to the combustor where they produce large amplitude heat release oscillations that drive combustor pressure oscillations. It is shown that the dominant characteristic time associated with this mechanism is the convective time from the point of formation of the reactive mixture at the fuel injector to the point where it is consumed at the flame. Instabilities occur when the ratio of this convective time and the period of the oscillations equals a specific constant, whose magnitude depends upon the combustor design. Significantly, these predictions are in good agreement with available experimental data, strongly suggesting that the proposed mechanism properly accounts for the essential physics of the problem. The predictions of this study also indicate, however, that simple design changes (i.e., passive control approaches) may not, in general, provide a viable means for controlling these instabilities, due to the multiple number of modes that may be excited by the combustion process.

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