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.

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.

Lowering Equivalence Ratio Limit for Stable Combustion in Gas Turbines by Injection of Free Radicals

2011 ◽  
Author(s):  
Yeshayahou Levy ◽  
Vladimir Erenburg ◽  
Valery Sherbaum ◽  
Vitali Ovcharenko ◽  
Leonid Rosentsvit ◽  
...  
Keyword(s):  
Free Radicals ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Nox Reduction ◽  
Combustion Process ◽  
Combustion Regime ◽  
Combustion Stability ◽  
Nox Formation ◽  
Stable Combustion ◽  
Lean Premixed

Lean premixed combustion is one of the widely used methods for NOx reduction in gas turbines (GT). When this method is used combustion takes place under low Equivalence Ratio (ER) and at relatively low combustion temperature. While reducing temperature decreases NOx formation, lowering temperature reduces the reaction rate of the hydrocarbon–oxygen reactions and deteriorates combustion stability. The objective of the present work was to study the possibility to decrease the lower limit of the stable combustion regime by the injection of free radicals into the combustion zone. A lean premixed gaseous combustor was designed to include a circumferential concentric pilot flame. The pilot combustor operates under rich fuel to air ratio, therefore it generates a significant amount of reactive radicals. The experiments as well as CFD and CHEMKIN simulations showed that despite of the high temperatures obtained in the vicinity of the pilot ring, the radicals’ injection from the pilot combustor has the potential to lower the limit of the global ER (and temperatures) while maintaining stable combustion. Spectrometric measurements along the combustor showed that the fuel-rich pilot flame generates free radicals that augment combustion stability. In order to study the relevant mechanisms responsible for combustion stabilization, CHEMKIN simulations were performed. The developed chemical network model took into account some of the basic parameters of the combustion process: ER, residence time, and the distribution of the reactances along the combustor. The CHEMKIN simulations showed satisfactory agreement with experimental results.

scholarly journals Variable Geometry Fuel Injector for Low Emissions Gas Turbines

1999 ◽  
Author(s):  
K. Smith ◽  
R. Steele ◽  
J. Rogers
Keyword(s):  
Gas Turbines ◽  
Field Tests ◽  
Flow Distribution ◽  
Operating Conditions ◽  
Variable Geometry ◽  
Fuel Injector ◽  
System Variable ◽  
Lean Premixed Combustion ◽  
Stable Operating ◽  
Lean Premixed

To extend the stable operating range of a lean premixed combustion system, variable geometry can be used to adjust the combustor air flow distribution as gas turbine operating conditions vary. This paper describes the design and preliminary testing of a lean premixed fuel injector that provides the variable geometry function. Test results from both rig and engine evaluations using natural gas are presented. The variable geometry injector has proven successful in the short-term testing conducted to date. Longer term field tests are planned to demonstrate durability.

Passive Control of Combustion Instability in a Low Emissions Aeroderivative Gas Turbine

2004 ◽  
Author(s):  
Tomas Scarinci ◽  
Christopher Freeman ◽  
Ivor Day
Keyword(s):  
Gas Turbine ◽  
Gas Turbines ◽  
Laboratory Testing ◽  
Passive Control ◽  
Pressure Ratio ◽  
Combustion Process ◽  
Field Trials ◽  
Low Noise ◽  
Wide Range ◽  
Air Mixing

This paper describes the conceptual ideas, the theoretical validation, the laboratory testing and the field trials of a recently patented fuel-air mixing device for use in high-pressure ratio, low emissions, gaseous-fueled gas turbines. By making the fuel-air mixing process insensitive to pressure fluctuations in the combustion chamber, it is possible to avoid the common problem of positive feedback between mixture strength and the unsteady combustion process. More specifically, a mixing duct has been designed such that fuel-air ratio fluctuations over a wide range of frequencies can be damped out by passive design means. By scaling the design in such a way that the range of damped frequencies covers the frequency spectrum of the acoustic modes in the combustor, the instability mechanism can be removed. After systematic development, this design philosophy was successfully applied to a 35:1 pressure ratio aeroderivative gas turbine yielding very low noise levels and very competitive NOx and CO measurements. The development of the new premixer is described from conceptual origins through analytic and CFD evaluation to laboratory testing and final field trials. Also included in this paper are comments about the practical issues of mixing, flashback resistance and autoignition.

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.

Stagnation-Point Reverse-Flow Combustor Performance With Liquid Fuel Injection

2006 ◽  
Author(s):  
John Crane ◽  
Yedidia Neumeier ◽  
Jeff Jagoda ◽  
Jerry Seitzman ◽  
Ben T. Zinn
Keyword(s):  
Stagnation Point ◽  
Power Density ◽  
Gas Turbines ◽  
Fuel Injection ◽  
Reverse Flow ◽  
Combustion Process ◽  
Liquid Fuels ◽  
Fuel Injector ◽  
Low Velocity ◽  
Pressure Losses

This paper describes an investigation of the performance of the recently developed ultra low emissions, Stagnation-Point Reverse-Flow (SPRF) Combustor when burning liquid fuels (Jet-A and heptane). This study has been undertaken because of the need to burn liquid fuels with low emissions in gas turbines that are used, for example, in aircraft engines, land-based power generation, and marine applications. In contrast with state of the art combustors, in which the reactants and products enter and leave the combustor through opposite ends of the combustor, the reactants and products enter and leave the SPRF combustor through the same plane opposite a closed end. The design of the SPRF combustor allows mixing of reactants with hot combustion products and radicals within the combustor, prior to combustion. Thus, no external premixing of fuel and air is required. Additionally, since the air and fuel enter opposite the closed end of the combustor, they must stagnate near the closed end, thus establishing a region of low velocity just upstream of the closed end that helps stabilize the combustion process. This apparently produces a low temperature, stable, distributed reaction zone. Previous studies with the SPRF combustor investigated its performance while burning natural gas. This paper presents the results of SPRF combustor studies using liquid fuels, both heptane and Jet-A. The performance of the combustor was investigated using an airblast fuel injector, which is suitable for the low fuel flow rates used in laboratory experiments. To reduce pressure losses across the injector, a diffuser was incorporated into the airblast injector. It was found that stable combustor operation was achieved burning Jet-A with emissions of less than 1 ppm NOx and 5 ppm CO, pressure losses less than 5 percent, and a power density on the order of 10 MW/m3 in atmospheric pressure. This power density would linearly scale to 300 MW/m3 in a combustor at a pressure of 30 atmospheres.

CFD Analysis of a Complete Industrial Lean Premixed Gas Turbine Combustor

2000 ◽  
Author(s):  
P. Birkby ◽  
R. S. Cant ◽  
W. N. Dawes ◽  
A. A. J. Demargne ◽  
P. C. Dhanasekaran ◽  
...  
Keyword(s):  
Gas Turbines ◽  
Combustion Process ◽  
Operating Conditions ◽  
Combustion Model ◽  
Reacting Flow ◽  
Lean Premixed Combustion ◽  
Fuel Injectors ◽  
Lean Premixed ◽  
Industrial Gas Turbines ◽  
Combustion Technology

The introduction of lean premixed combustion technology in industrial gas turbines has led to a number of interesting technical issues. Lean premixed combustors are especially prone to acoustically-coupled combustion oscillations as well as to other problems of flame stability such as flashback. Clearly it is very important to understand the physics that lies behind such behaviour in order to produce robust and comprehensive remedies, and also to underpin the future development of new combustor designs. Simulation of the flow and combustion using Computational Fluid Dynamics (CFD) offers an attractive way forward, provided that the modelling of turbulence and combustion is adequate and that the technique is applicable to real industrial combustor geometries. The paper presents a series of CFD simulations of the Rolls-Royce Trent industrial combustor carried out using the McNEWT unstructured code. The entire combustion chamber geometry is represented including the premixing ducts, the fuel injectors and the discharge nozzle. A modified k-ε model has been used together with an advanced laminar flamelet combustion model that is sensitive to variations in fuel-air mixture stoichiometry. Detailed results have been obtained for the non-reacting flow field, for the mixing of fuel and air and for the combustion process itself at a number of different operating conditions. The study has provided a great deal of useful information on the operation of the combustor and has demonstrated the value of CFD-based combustion analysis in an industrial context.

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]

scholarly journals Comprehensive Thermodynamic Analysis of the Humphrey Cycle for Gas Turbines with Pressure Gain Combustion

Energies ◽  
2018 ◽  
Vol 11 (12) ◽  
pp. 3521 ◽  
Author(s):  
Panagiotis Stathopoulos
Keyword(s):  
Gas Turbine ◽  
Gas Turbines ◽  
Combustion Process ◽  
Ideal Gas ◽  
Point Of View ◽  
Pressure Gain Combustion ◽  
Combustion Technology ◽  
Secondary Air ◽  
And Performance ◽  
The Impact

Conventional gas turbines are approaching their efficiency limits and performance gains are becoming increasingly difficult to achieve. Pressure Gain Combustion (PGC) has emerged as a very promising technology in this respect, due to the higher thermal efficiency of the respective ideal gas turbine thermodynamic cycles. Up to date, only very simplified models of open cycle gas turbines with pressure gain combustion have been considered. However, the integration of a fundamentally different combustion technology will be inherently connected with additional losses. Entropy generation in the combustion process, combustor inlet pressure loss (a central issue for pressure gain combustors), and the impact of PGC on the secondary air system (especially blade cooling) are all very important parameters that have been neglected. The current work uses the Humphrey cycle in an attempt to address all these issues in order to provide gas turbine component designers with benchmark efficiency values for individual components of gas turbines with PGC. The analysis concludes with some recommendations for the best strategy to integrate turbine expanders with PGC combustors. This is done from a purely thermodynamic point of view, again with the goal to deliver design benchmark values for a more realistic interpretation of the cycle.

Experimental Investigation of the Acoustic Reflection Coefficient of a Modeled Gas Turbine Impingement Cooling Section

2012 ◽  
Author(s):  
Christoph Jörg ◽  
Michael Wagner ◽  
Thomas Sattelmayer
Keyword(s):  
Reflection Coefficient ◽  
Flow Velocity ◽  
Gas Turbines ◽  
Acoustic Energy ◽  
Quality Data ◽  
Mean Flow ◽  
Impingement Cooling ◽  
Acoustic Reflection ◽  
The Mean ◽  
Mean Flow Velocity

The thermoacoustic stability of gas turbines depends on a balance of acoustic energy inside the engine. While the flames produce acoustic energy, other areas like the impingement cooling system contribute to damping. In this paper, we investigate the damping potential of an annular impingement sleeve geometry embedded into a realistic environment. A cold flow test rig was designed to represent real engine conditions in terms of geometry, and flow situation. High quality data was delivered by six piezoelectric dynamic pressure sensors. Experiments were carried out for different mean flow velocities through the cooling holes. The acoustic reflection coefficient of the impingement sleeve was evaluated at a downstream reference location. Further parameters investigated were the number of cooling holes, and the geometry of the chamber surrounding the impingement sleeve. Experimental results show that the determining parameter for the reflection coefficient is the mean flow velocity through the impingement holes. An increase of the mean flow velocity leads to significantly increased damping, and to low values of the reflection coefficient.

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