CHEMILUMINESCENCE MEASUREMENTS OF LOCAL EQUIVALENCE RATIO IN A PARTIALLY PREMIXED FLAME

2006 ◽  
Vol 178 (10-11) ◽  
pp. 1821-1841 ◽  
Author(s):  
T. S. CHENG ◽  
C.-Y. WU ◽  
Y.-H. LI ◽  
Y.-C. CHAO
Keyword(s):  
Equivalence Ratio ◽  
Premixed Flame ◽  
Partially Premixed ◽  
Partially Premixed Flame ◽  
Local Equivalence

The Structure of Two-Stage Counterflow n-Heptane-Air Flames

2000 ◽  
Author(s):  
S. K. Aggarwal ◽  
H. S. Xue
Keyword(s):  
Strain Rate ◽  
Reaction Zone ◽  
Equivalence Ratio ◽  
Flame Structure ◽  
Premixed Combustion ◽  
Premixed Flame ◽  
Partially Premixed Combustion ◽  
Reaction Zones ◽  
Partially Premixed ◽  
Partially Premixed Flame

Partially premixed flames are formed by mixing air (in less than stoichiometric amounts) into the fuel stream prior to the reaction zone, where additional air is available for complete combustion. Such flames can occur in both laboratory and practical combustion systems. In advanced gas turbine combustor designs, such as a lean direct injection (LDI) combustor, partially premixed combustion represents an impotent mode of burning. Spray combustion often involves partially premixed combustion due to the locally fuel vapor-rich regions. In the present study, the detailed structure of n-heptane/air partially premixed flame in a counterflow configuration is investigated. The flame is computed by employing the Oppdif code and a detailed reaction mechanism consisting of 275 elementary reactions and 41 species. The partially premixed flame structure is characterized by two-stage burning or two distinct but synergistically coupled reaction zones, a rich premixed zone on the fuel side and a ‘nonpremixed zone on the air side. The fuel is completely consumed in the premixed zone with ethylene and acetylene being the major intermediate species. The reactions involving the consumption of these species are found to be the key rate-limiting reactions that characterize interactions between the two reaction zones, and determine the overall fuel consumption rate. The flame response to the variations in equivalence ratio and strain rate is examined. Increasing equivalence ratio and/or strain rate to a critical value leads to merging of the two reaction zones. The equivalence ratio variation affects the rich premixed reaction zone, while the variation in strain rate predominantly affects the nonpremixed reaction zone. The flame structure is also characterized in terms of a modified mixture fraction (conserved scalar), and laminar flamelet profiles are provided.

Relative Effects of Velocity- and Mixture-Coupling in a Thermoacoustically Unstable, Partially-Premixed Flame

2021 ◽  
Author(s):  
Ashwini Karmarker ◽  
Jacqueline O’Connor ◽  
Isaac Boxx
Keyword(s):  
Gas Turbine ◽  
Equivalence Ratio ◽  
Premixed Flame ◽  
Turbine Engines ◽  
Gas Turbine Engines ◽  
Real Gas ◽  
Planar Laser ◽  
Partially Premixed ◽  
Partially Premixed Flame ◽  
The Impact

Abstract Combustion instability, which is the result of a coupling between combustor acoustic modes and unsteady flame heat release rate, is a severely limiting factor in the operability and performance of modern gas turbine engines. This coupling can occur through different coupling pathways, such as flow field fluctuations or equivalence ratio fluctuations. In realistic combustor systems, there are complex hydrodynamic and thermo-chemical processes involved, which can lead to multiple coupling pathways. In order to understand and predict the mechanisms that govern the onset of combustion instability in real gas turbine engines, we consider the influences that each of these coupling pathways can have on the stability and dynamics of a partially-premixed, swirl-stabilized flame. In this study, we use a model gas turbine combustor with two concentric swirling nozzles of air, separated by a ring of fuel injectors, operating at an elevated pressure of 5 bar. The flow split between the two streams is systematically varied to observe the impact on the flow and flame dynamics. High-speed stereoscopic particle image velocimetry, OH planar laser-induced fluorescence, and acetone planar laser-induced fluorescence are used to obtain information about the velocity field, flame, and fuel-flow behavior, respectively. Depending on the flow conditions, a thermoacoustic oscillation mode or a hydrodynamic mode, identified as the precessing vortex core, is present. The focus of this study is to characterize the mixture coupling processes in this partially-premixed flame as well as the impact that the velocity oscillations have on mixture coupling. Our results show that, for this combustor system, changing the flow split between the two concentric nozzles can alter the dominant harmonic oscillation modes in the system, which can significantly impact the dispersion of fuel into air, thereby modulating the local equivalence ratio of the flame. This insight can be used to design instability control mechanisms in real gas turbine engines.

Reconstruction of the Heat Release Response of Partially Premixed Flames

2010 ◽  
Author(s):  
Kyu Tae Kim ◽  
Jong Guen Lee ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Heat Release ◽  
Phase Difference ◽  
Equivalence Ratio ◽  
Forced Response ◽  
Premixed Flame ◽  
Physical Parameters ◽  
Ir Absorption ◽  
Inlet Velocity ◽  
Partially Premixed ◽  
Partially Premixed Flame

The forced response of a swirl-stabilized, partially premixed flame to inlet velocity and equivalence ratio oscillations was experimentally investigated in a model lean-premixed gas turbine combustor. Three different forcing mechanisms were studied: the response of a premixed flame to velocity oscillations, the response of a partially premixed flame to equivalence ratio oscillations, and the response of a partially premixed flame to velocity and equivalence ratio oscillations. The overall heat release response of the flame was determined from measurements of the CH* chemiluminescence emission intensity from the entire flame, while the response of the spatially distributed heat release was determined from phase-synchronized chemiluminescence images. In addition, simultaneous measurements were made of the inlet velocity and equivalence ratio oscillations using the two-microphone method and the IR absorption technique, respectively. The results show that in the linear regime, the response of a partially premixed flame to simultaneous velocity and equivalence ratio oscillations can be reconstructed from independent measurements of the flame’s response to velocity oscillations and to equivalence ratio oscillations using a vector summation method. This is the first experimental demonstration of a two-input one-output model of a swirl-stabilized partially premixed flame. It suggests that the response of a partially premixed flame is governed by four physical parameters, i.e., the oscillation frequency, the amplitude of velocity oscillation, the amplitude of equivalence ratio oscillation, and the phase difference between the two oscillations. As a result, the heat release response of a partially premixed flame can be amplified or damped, depending on the phase difference between the velocity and equivalence ratio oscillations at the combustor inlet.

A Study on Flame Structures in CH4 Counterflow Partially Premixed Flame

2014 ◽  
Vol 694 ◽  
pp. 474-477
Author(s):  
Jing Luo ◽  
Lian Sheng Liu ◽  
Zi Zhong Chen
Keyword(s):  
Reaction Zone ◽  
Equivalence Ratio ◽  
Flame Structure ◽  
Premixed Flame ◽  
Counterflow Flame ◽  
Partially Premixed ◽  
Flame Structures ◽  
The Rich ◽  
Partially Premixed Flame ◽  
Two Temperature

An experimental and simulation work had been conducted to study a one-dimensional partially premixed methane/air counterflow flame in this paper. Flame images are obtained through experiments and computations using GRIMech 3.00 chemistry were performed for the flames studied. The partially premixing effects upon the flame were revealed by comparing the flame structures and emissions with premixed flames at the same equivalence ratio. The results show the premixed flame only has a single flame structure. However, PPF has distinct double flame structures at present equivalence ratio. Temperature is relatively high in the whole combustion zone for premixed flame, while, for PPF, there are two temperature peaks in a rich premixed reaction zone on the fuel side and a nonpremixed reaction zone on the oxidizer side respectively. For PPF, NO concentration in the nonpremixed zone is much higher compared to that in the rich premixed zone because of higher OH concentration in the nonpremixed zone.

Effect of Acoustic Feedback on Lagrangian Coherent Structures in a Backward Facing Step Combustor With a Partially Premixed Flame

2017 ◽  
Author(s):  
Ramgopal Sampath ◽  
S. R. Chakravarthy
Keyword(s):  
Coherent Structures ◽  
Large Scale ◽  
Pressure Oscillation ◽  
Velocity Fields ◽  
Premixed Flame ◽  
Time Resolved ◽  
Acoustic Feedback ◽  
Acoustic Characterization ◽  
Partially Premixed ◽  
Partially Premixed Flame

The thermoacoustic oscillations of a partially premixed flame stabilized in a backward facing step combustor are studied at a constant equivalence ratio in long and short combustor configurations corresponding to with and without acoustic feedback respectively. We perform simultaneous time-resolved particle image velocimetry (TR-PIV) and chemiluminescence for selected flow conditions based on the acoustic characterization in the long combustor. The acoustic characterization shows a transition in the dominant pressure amplitudes from low to high magnitudes with an increase in the inlet flow Reynolds number. This is accompanied by a shift in the dominant frequencies. For the intermittent pressure oscillations in the long combustor, the wavelet analysis indicates a switch between the acoustic and vortex modes with silent zones of relatively low-pressure amplitudes. The short combustor configuration indicates the presence of the vortex shedding frequency and an additional band comprising the Kelvin Helmholtz mode. Next, we apply the method of finite-time Lyapunov exponent (FTLE) to the time-resolved velocity fields to extract features of the Lagrangian coherent structures (LCS) of the flow. In the long combustor post transition with the time instants with dominant acoustic mode, a large-scale modulation of the FTLE boundaries over one cycle of pressure oscillation is evident. Further, the FTLEs and the flame boundaries align each other for all phases of the pressure oscillation. In the short combustor, the FTLEs indicate the presence of small wavelength waviness that overrides the large-scale vortex structure, which corresponds to the vortex shedding mode. This behaviour contrasts with the premixed flame in the short combustor reported earlier in which such large scales were found to be seldom present. The presence of the large-scale structures even in the absence of acoustic feedback in a partially premixed flame signifies its inherent unstable nature leading to large pressure amplitudes during acoustic feedback. Lastly, the FTLE boundaries provide the frequency information of the identified coherent structure and also acts as the surrogate flame boundaries that are estimated from just the velocity fields.

Modeling Unsteady Flow of a Lean Partially Premixed Flame on a Dry Low NOx Combustion System

2013 ◽  
Author(s):  
Salvatore Matarazzo ◽  
Hannes Laget ◽  
Evert Vanderhaegen ◽  
Jim B. W. Kok
Keyword(s):  
Gas Turbine ◽  
Limit Cycle ◽  
Gas Turbines ◽  
Premixed Flame ◽  
Computational Domain ◽  
Pressure Oscillations ◽  
Combustion Dynamics ◽  
Partially Premixed ◽  
Partially Premixed Flame ◽  
Limit Cycle Behavior

The phenomenon of combustion dynamics (CD) is one of the most important operational challenges facing the gas turbine (GT) industry today. The Limousine project, a Marie Curie Initial Training network funded by the European Commission, focuses on the understanding of the limit cycle behavior of unstable pressure oscillations in gas turbines, and on the resulting mechanical vibrations and materials fatigue. In the framework of this project, a full transient CFD analysis for a Dry Low NOx combustor in a heavy duty gas turbine has been performed. The goal is to gain insight on the thermo-acoustic instability development mechanisms and limit cycle oscillations. The possibility to use numerical codes for complex industrial cases involving fuel staging, fluid-structure interaction, fuel quality variation and flexible operations has been also addressed. The unsteady U-RANS approach used to describe the high-swirled lean partially premixed flame is presented and the results on the flow characteristics as vortex core generation, vortex shedding, flame pulsation are commented on with respect to monitored parameters during operations of the GT units at Electrabel/GDF-SUEZ sites. The time domain pressure oscillations show limit cycle behavior. By means of Fourier analysis, the coupling frequencies caused by the thermo-acoustic feedback between the acoustic resonances of the chamber and the flame heat release has been detected. The possibility to reduce the computational domain to speed up computations, as done in other works in literature, has been investigated.

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