A Detailed Analysis of Thermoacoustic Interaction Mechanisms in a Turbulent Premixed Flame

2004 ◽  
Author(s):  
Bruno Schuermans ◽  
Valter Bellucci ◽  
Felix Guethe ◽  
Franc¸ois Meili ◽  
Peter Flohr ◽  
...  
Keyword(s):  
Heat Release ◽  
Acoustic Field ◽  
Turbulence Intensity ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Turbulent Flame ◽  
Interaction Mechanism ◽  
Flame Speed ◽  
Fuel Concentration ◽  
Turbulent Flame Speed

A combined theoretical and experimental analysis of thermoacoustic interaction mechanisms of a lean pre-mixed swirl-stabilized gas turbine burner is presented. A full-scale gas turbine burner has been tested in an atmospheric test rig. The test facility was equipped with loudspeakers to excite the acoustic field and with arrays of microphones to measure the response of the acoustic field to the forcing signal. With this set-up transfer matrices relating the acoustic pressure and velocity on both sides of the flame front have been measured. A laser absorption measurement technique allowed for measurement of the fluctuations of fuel concentration in the mixture. Heat release fluctuations were monitored using a photo-multiplier. The measurement of the acoustic field, heat release and equivalence ratio fluctuations have been measured simultaneously. Special attention has been given to the role of fuel concentration fluctuations in the thermoacoustic interaction mechanism. In order to be able to clearly separate this mechanism from other possible mechanisms, all the experiments have been performed in pre-premixing mode as well. In pre-premixing mode the fuel is injected far upstream of the burner in order to avoid fuel concentration fluctuations at the burner location. This is in contrast with premixing mode where fuel and air are mixed in the burner. An acoustic flame model has been derived. The model includes the well-known interaction mechanism of equivalence ratio fluctuations but also includes a novel mechanism that is caused by fluctuations of vorticity. This latter mechanism relates the turbulent flame speed via turbulence intensity fluctuations to the acoustic field. The idea is that periodic acoustic fluctuations cause periodic changes of the turbulence intensity. The turbulence intensity strongly affects the turbulence flame speed. The fluctuations of the turbulent flame speed result in fluctuations of the heat release. This turbulence intensity fluctuation model has been compared with the measured pre-premix transfer functions and shows an excellent agreement. The measured transfer functions in premix mode have been compared with the model that includes fluctuations of fuel concentration and turbulence intensity. Also in this case a very good agreement is found. Moreover, it has been demonstrated that the phase relation between measured equivalence ratio fluctuation and heat release corresponds to the model.

Flame Characteristics and Turbulent Flame Speeds of Turbulent, High-Pressure, Lean Premixed Methane/Air Flames

2005 ◽  
Author(s):  
P. Griebel ◽  
R. Bombach ◽  
A. Inauen ◽  
R. Scha¨ren ◽  
S. Schenker ◽  
...  
Keyword(s):  
Flame Front ◽  
Gas Turbines ◽  
Equivalence Ratio ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Operating Conditions ◽  
Turbulent Flame Speed ◽  
Front Position ◽  
Preheating Temperature ◽  
Lean Premixed

The present experimental study focuses on flame characteristics and turbulent flame speeds of lean premixed flames typical for stationary gas turbines. Measurements were performed in a generic combustor at a preheating temperature of 673 K, pressures up to 14.4 bars (absolute), a bulk velocity of 40 m/s, and an equivalence ratio in the range of 0.43–0.56. Turbulence intensities and integral length scales were measured in an isothermal flow field with Particle Image Velocimetry (PIV). The turbulence intensity (u′) and the integral length scale (LT) at the combustor inlet were varied using turbulence grids with different blockage ratios and different hole diameters. The position, shape, and fluctuation of the flame front were characterized by a statistical analysis of Planar Laser Induced Fluorescence images of the OH radical (OH-PLIF). Turbulent flame speeds were calculated and their dependence on operating conditions (p, φ) and turbulence quantities (u′, LT) are discussed and compared to correlations from literature. No influence of pressure on the most probable flame front position or on the turbulent flame speed was observed. As expected, the equivalence ratio had a strong influence on the most probable flame front position, the spatial flame front fluctuation, and the turbulent flame speed. Decreasing the equivalence ratio results in a shift of the flame front position farther downstream due to the lower fuel concentration and the lower adiabatic flame temperature and subsequently lower turbulent flame speed. Flames operated at leaner equivalence ratios show a broader spatial fluctuation as the lean blow-out limit is approached and therefore are more susceptible to flow disturbances. In addition, because of a lower turbulent flame speed these flames stabilize farther downstream in a region with higher velocity fluctuations. This increases the fluctuation of the flame front. Flames with higher turbulence quantities (u′, LT) in the vicinity of the combustor inlet exhibited a shorter length and a higher calculated flame speed. An enhanced turbulent heat and mass transport from the recirculation zone to the flame root location due to an intensified mixing which might increase the preheating temperature or the radical concentration is believed to be the reason for that.

Aerodynamic Quenching and Burning Velocity of Turbulent Premixed Methane-Air Flames

2015 ◽  
Author(s):  
Girish V. Nivarti ◽  
R. Stewart Cant
Keyword(s):  
Turbulence Intensity ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Premixed Flames ◽  
Experimental Studies ◽  
Flame Speed ◽  
Flame Surface ◽  
Turbulent Flame Speed ◽  
Combustion Models ◽  
Turbulent Premixed

Industry-relevant turbulent premixed combustion models continue to rely on empirical expressions for turbulent flame speed in closure modelling for the mean turbulent reaction rate. To date, an accurate sub-model for turbulent flame speed has not been proposed for flows with high turbulence intensities. Experimental studies in the pertinent combustion regime, known as the Thin Reaction Zones (TRZ) regime, are limited by the existing techniques of turbulence generation whereas, until recently, the high computational expense involved in solving such problems has restricted theoretical studies. We investigate the behaviour of premixed flames in the TRZ regime by conducting a parametric 3D Direct Numerical Simulation (DNS) study of stoichiometric methane-air mixtures using single-step chemistry in an inflow-outflow configuration. Inflow turbulence intensity is varied while keeping the integral length scale constant across six separate simulations which span altogether a significant portion of the TRZ regime. The resulting variation of turbulent flame speed with turbulence intensity demonstrates the well-known bending phenomenon and conforms with recent experimental observations of freely-propagating premixed flames in this regime. As turbulence intensity is increased, the calculated flame surface exhibits an increasing degree of wrinkling and pocket-formation. In addition, the internal thermo-chemical structure of the flame is greatly affected when the turbulence intensity is more than an order of magnitude higher than the laminar flame speed. These qualitative observations establish the present DNS framework as a powerful tool for capturing turbulence-chemistry interactions that influence the bending phenomenon. Hence, this work forms the basis for further analysis using a detailed chemical description to investigate these interactions and, thereby, improve combustion models of industrial relevance.

Investigation of the Turbulent Flame Speed for Natural Gas and Natural Gas/Hydrogen Mixtures at High Turbulence Levels and Volumetric Heat Release Rates

2013 ◽  
Author(s):  
Martin Zajadatz ◽  
Nikolaos Zarzalis ◽  
Wolfgang Leuckel
Keyword(s):  
Natural Gas ◽  
Heat Release ◽  
Gas Turbines ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Release Rates ◽  
Fuel Gas ◽  
Lean Premixed Combustion ◽  
Number Range ◽  
Turbulent Flame Speed

In gas turbine combustion application, there is a strong tendency towards high volumetric heat release rates without compromising ignition stability and the requirement of low emission concentrations of NOx, CO and unburned hydrocarbons. In order to meet these demands for industrial gas turbines the lean premixed combustion concept has been developed. In the scope of this paper fundamental experimental work, which has been carried out in order to analyze the important topic of turbulence/chemistry interaction on a semi-technical scale, will be reported. The turbulent intensity and length scales have been varied by a generic burner system, which consists of four geometrically scaled burners. At atmospheric pressure conditions more than 700 Bunsen type flames in a Reynolds number range from 21000 to 128000 have been investigated. Gas/air mixture preheating has been included in the tests as a typical boundary condition for combustion in gas turbines. The natural gas was blended with 25 vol. % and 50 vol. % hydrogen in order to alter the kinetics of the fuel gas. The influence of the aforementioned parameters on the turbulent flame speed were assessed and compared with existing correlations for the turbulent flame speed. Special emphasis has been taken on the influence of gas/air mixture preheating and kinetics.

Investigation of Turbulence Models Applied to Premixed Combustion Using a Level-Set Flamelet Library Approach

2004 ◽  
Vol 126 (4) ◽  
pp. 701-707 ◽  
Author(s):  
Ulf Engdar ◽  
Per Nilsson ◽  
Jens Klingmann
Keyword(s):  
Level Set ◽  
Turbulence Intensity ◽  
Turbulent Flame ◽  
Turbulence Models ◽  
Flame Speed ◽  
Premixed Combustion ◽  
Chemical Mechanism ◽  
Turbulent Flame Speed ◽  
The Mean ◽  
The Impact

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by an external code, which employs a detailed chemical mechanism. However, a model for the turbulent flame speed is required, which, among other considerations, depends on the turbulence intensity, i.e., these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-Cd, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-ε model, a cubic nonlinear k-ε model, the standard k-ω model and the shear stress transport (SST) k-ω model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the nonlinear k-ε model offers no advantage over the standard model, regardless of flame speed model. The k-ω based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Mu¨ller flame speed model shows the least sensitivity to the choice of turbulence model.  

Investigation of Turbulence Models Applied to Premixed Combustion Using a Level-Set Flamelet Library Approach

2003 ◽  
Author(s):  
Ulf Engdar ◽  
Per Nilsson ◽  
Jens Klingmann
Keyword(s):  
Level Set ◽  
Turbulence Intensity ◽  
Turbulent Flame ◽  
Turbulence Models ◽  
Flame Speed ◽  
Premixed Combustion ◽  
Chemical Mechanism ◽  
Turbulent Flame Speed ◽  
Non Linear ◽  
The Mean

Most of the common modeling approaches to premixed combustion in engineering applications are either based on the assumption of infinitely fast chemistry or the flamelet assumption with simple chemistry. The level-set flamelet library approach (FLA) has shown great potential in predicting major species and heat release, as well as intermediate and minor species, where more simple models often fail. In this approach, the mean flame surface is tracked by a level-set equation. The flamelet libraries are generated by an external code, which employs a detailed chemical mechanism. However, a model for the turbulent flame speed is required, which, amongst other considerations, depends on the turbulence intensity, i.e. these models may show sensitivity to turbulence modeling. In this paper, the FLA model was implemented in the commercial CFD program Star-CD, and applied to a lean premixed flame stabilized by a triangular prism (bluff body). The objective of this paper has been to investigate the impact on the mean flame position, and hence on the temperature and species distribution, using three different turbulent flame speed models in combination with four different turbulence models. The turbulence models investigated are: the standard k-ε model, a cubic non-linear k-ε model, the standard k-ω model and the Shear Stress Transport (SST) k-ω model. In general, the computed results agree well with experimental data for all computed cases, although the turbulence intensity is strongly underestimated at the downstream position. The use of the non-linear k-ε model offers no advantage over the standard model, regardless of flame speed model. The k-ω based turbulence models predict the highest turbulence intensity with the shortest flame lengths as a consequence. The Mu¨ller flame speed model shows the least sensitivity to the choice of turbulence model.

Effects of Flame Development and Structure on Thermo-Acoustic Oscillations of Premixed Turbulent Flames

Volume 4 ◽  
2004 ◽  
Author(s):  
Pratap Sathiah ◽  
Andrei N. Lipatnikov ◽  
Jerzy Chomiak
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Turbulent Flame ◽  
Kinematic Model ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Oncoming Flow ◽  
Turbulent Flame Speed ◽  
Premixed Turbulent Flames

Non-stationary confined premixed turbulent flames stabilized behind a bluff body are studied. A simple kinematic model of such flames was developed by Dowling [9] who reduced the combustion process to the propagation of an infinitely thin flame at a constant speed. The goal of this work is to extend the model by taking into account the structure of premixed turbulent flames and the development of turbulent flame speed and thickness. For these purposes, the so-called Flame Speed Closure model for multi-dimensional simulations of premixed turbulent flames is adapted and combined with the aforementioned Dowling model. Simulations of the heat release rate dynamics for ducted flames due to oncoming flow oscillations have been performed. Typical results show that the oscillations of the integrated heat release rate follow the oncoming flow velocity oscillations with certain time delay. The delays computed using the Dowling and the above approach are different, thus indicating the importance of resolving flame structure when modeling ducted flame oscillations.

The structure and propagation of turbulent flames

1975 ◽  
Vol 344 (1637) ◽  
pp. 217-234 ◽  
Keyword(s):  
Turbulence Intensity ◽  
Combustion Zone ◽  
Laminar Flame ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Flame Structure ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Turbulence Scale ◽  
Turbulent Flame Speed

The influence of turbulence intensity, scale and vorticity on burning velocity and flame structure is examined by using premixed propane-air mixtures supplied at atmospheric pressure to a combustion chamber 31cm long and lOcmx 10 cm cross-section. The chamber is fitted with transparent side walls to permit flame observations and schlieren photography. Control over the turbulence level is achieved by means of grids located upstream of the combustion zone. By suitable modifications to grid geometry and flow velocity, it is possible to vary turbulence intensity and scale independently within the combustion zone in such a manner that their separate effects on burning velocity and flame structure are readily distinguished. From analysis of the results obtained three distinct regions may be identified, each having different characteristics in regard to the effect of scale on turbulent burning velocity. For each region a mechanism of turbulent flame propagation is proposed which describes the separate influences on burning velocity of turbulence intensity, turbulence scale, laminar flame speed and flame thickness. The arguments presented in support of this 3-region model are substantiated by the experimental data and by the pictorial evidence on flame structure provided by the schlieren photographs. This model also sheds light on some of the characteristics which turbulent flames have in common with laminar flames when the latter are subjected to pressure and velocity fluctuations. Finally the important role of vorticity is examined and it is found that turbulent flame speed is highest when the rate of production of vorticity is equal to about half the rate of viscous dissipation.

scholarly journals Numerical Investigation of Pressure Influence on the Confined Turbulent Boundary Layer Flashback Process

Fluids ◽  
2019 ◽  
Vol 4 (3) ◽  
pp. 146 ◽  
Author(s):  
Aaron Endres ◽  
Thomas Sattelmayer
Keyword(s):  
Boundary Layer ◽  
Gas Turbines ◽  
Separation Zone ◽  
Turbulent Flame ◽  
Pressure Rise ◽  
Flame Speed ◽  
Zone Size ◽  
Detailed Chemical Kinetics ◽  
Turbulent Flame Speed ◽  
Quenching Distance

Boundary layer flashback from the combustion chamber into the premixing section is a threat associated with the premixed combustion of hydrogen-containing fuels in gas turbines. In this study, the effect of pressure on the confined flashback behaviour of hydrogen-air flames was investigated numerically. This was done by means of large eddy simulations with finite rate chemistry as well as detailed chemical kinetics and diffusion models at pressures between 0 . 5 and 3 . It was found that the flashback propensity increases with increasing pressure. The separation zone size and the turbulent flame speed at flashback conditions decrease with increasing pressure, which decreases flashback propensity. At the same time the quenching distance decreases with increasing pressure, which increases flashback propensity. It is not possible to predict the occurrence of boundary layer flashback based on the turbulent flame speed or the ratio of separation zone size to quenching distance alone. Instead the interaction of all effects has to be accounted for when modelling boundary layer flashback. It was further found that the pressure rise ahead of the flame cannot be approximated by one-dimensional analyses and that the assumptions of the boundary layer theory are not satisfied during confined boundary layer flashback.

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