scholarly journals G-equation modelling of thermoacoustic oscillations of partially premixed flames

2017 ◽  
Vol 9 (4) ◽  
pp. 260-276 ◽  
Author(s):  
Bernhard Semlitsch ◽  
Alessandro Orchini ◽  
Ann P Dowling ◽  
Matthew P Juniper
Keyword(s):  
Heat Release ◽  
Limit Cycle ◽  
Heat Release Rate ◽  
Release Rate ◽  
Large Scale ◽  
Describing Function ◽  
Describing Functions ◽  
Wide Range ◽  
Flame Describing Function ◽  
Thermoacoustic Oscillations

Numerical simulations aid combustor design to avoid and reduce thermoacoustic oscillations. Non-linear heat release rate estimation and its modelling are essential for the prediction of saturation amplitudes of limit cycles. The heat release dynamics of flames can be approximated by a flame describing function. To calculate a flame describing function, a wide range of forcing amplitudes and frequencies needs to be considered. For this reason, we present a computationally inexpensive level-set approach, which accounts for equivalence ratio perturbations on flames with arbitrarily complex shapes. The influence of flame parameters and modelling approaches on flame describing functions and time delay coefficient distributions are discussed in detail. The numerically obtained flame describing functions are compared with experimental data and used in an acoustic network model for limit cycle prediction. A reasonable agreement of the heat release gain and limit cycle frequency is achieved even with a simplistic, analytical velocity fluctuation model. However, the phase decay is over-predicted. For sophisticated flame shapes, only the realistic modelling of large-scale flow structures allows the correct phase decay predictions of the heat release rate response.

A Weakly Nonlinear Approach Based on a Distributed Flame Describing Function to Study the Combustion Dynamics of a Full-Scale Lean-Premixed Swirled Burner

2017 ◽  
Vol 139 (9) ◽  
Author(s):  
Davide Laera ◽  
Sergio M. Camporeale
Keyword(s):  
Heat Release ◽  
Limit Cycle ◽  
Heat Release Rate ◽  
Release Rate ◽  
Gas Turbines ◽  
Full Scale ◽  
Describing Function ◽  
Weakly Nonlinear ◽  
Lean Premixed ◽  
Flame Describing Function

Modern combustion chambers of gas turbines for power generation and aero-engines suffer of thermo-acoustic combustion instabilities generated by the coupling of heat release rate fluctuations with pressure oscillations. The present article reports a numerical analysis of limit cycles arising in a longitudinal combustor. This corresponds to experiments carried out on the longitudinal rig for instability analysis (LRIA) test facility equipped with a full-scale lean-premixed burner. Heat release rate fluctuations are modeled considering a distributed flame describing function (DFDF), since the flame under analysis is not compact with respect to the wavelengths of the unstable modes recorded experimentally. For each point of the flame, a saturation model is assumed for the gain and the phase of the DFDF with increasing amplitude of velocity fluctuations. A weakly nonlinear stability analysis is performed by combining the DFDF with a Helmholtz solver to determine the limit cycle condition. The numerical approach is used to study two configurations of the rig characterized by different lengths of the combustion chamber. In each configuration, a good match has been found between numerical predictions and experiments in terms of frequency and wave shape of the unstable mode. Time-resolved pressure fluctuations in the system plenum and chamber are reconstructed and compared with measurements. A suitable estimate of the limit cycle oscillation is found.

scholarly journals Flame Dynamics Intermittency in the Bi-Stable Region Near a Subcritical Hopf Bifurcation

2017 ◽  
Author(s):  
D. Ebi ◽  
A. Denisov ◽  
G. Bonciolini ◽  
E. Boujo ◽  
N. Noiray
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Acoustic Pressure ◽  
Oscillation Amplitude ◽  
Stable Region ◽  
Describing Function ◽  
Acoustic Damping ◽  
Intermittent Behavior ◽  
Flame Describing Function

We report experimental evidence of thermoacoustic bi-stability in a lab-scale turbulent combustor over a well-defined range of fuel-air equivalence ratios. Pressure oscillations are characterized by an intermittent behavior with “bursts”, i.e. sudden jumps between low and high amplitudes occurring at random time instants. The corresponding probability density functions of the acoustic pressure signal show clearly separated maxima when the burner is operated in the bi-stable region. A flame describing function, which links acoustic pressure to heat release rate fluctuations, is estimated at the modal frequency from simultaneously recorded flame chemiluminescence and acoustic pressure. The representation of its statistics is new and particularly informative. It shows that this describing function is characterized, in average, by a nearly constant gain and by a significant drift of the phase as function of the oscillation amplitude. This finding suggests that the bi-stability does not result from an amplitude-dependent balance between flame gain and acoustic damping, but rather from the non-constant phase difference between the acoustic pressure and the coherent fluctuations of heat release rate.

Nonlinear combustion instability analysis of a bluff body burner based on the flame describing function

2021 ◽  
pp. 095441002110440
Author(s):  
Yipin Lu ◽  
Yinli Xiao ◽  
Juan Wu ◽  
Liang Chen
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Premixed Combustion ◽  
Single Frequency ◽  
Describing Function ◽  
Lean Premixed Combustion ◽  
Lean Premixed ◽  
Flame Describing Function ◽  
Frequency Harmonic

Lean premixed combustion is a common form of combustion organization in power equipment and propulsion systems. In order to understand the dynamic characteristics of lean premixed flame and predict and control its combustion instability, it is necessary to obtain its flame describing function (FDF). Based on the open source CFD toolbox, OpenFOAM, the dynamic K-equation model, and the finite rate Partially Stirred Reactor (PaSR) model were used to perform large eddy simulations (LES) of lean premixed combustion, and the response of the unsteady heat release rate to single-frequency harmonic disturbances was studied. The response of the unsteady heat release rate was characterized by the FDF, and the response of the unsteady heat release rate to the two-frequency harmonic disturbance was studied. The results show that the quantitative heat release rate response and flame dynamics have very proper accuracy. In the single-frequency harmonic disturbance, as the forcing frequency increases, the curling behavior of the flame surface and the instantaneous vortex structure change; the nonlinear kinematics effect is manifested by the entrainment of the vortex. At lower forcing frequencies, the heat release response changes linearly with the increase of forcing amplitude; at intermediate frequencies, the heat release response exhibits obvious nonlinear behavior; at high frequencies, the heat release response to amplitude changes decreases. The introduction of the second harmonic disturbance will significantly reduce the response range of the total heat release rate and make the combustion more stable.

Synchronization During Multi-Mode Thermoacoustic Oscillations in a Liquid-Fueled Gas Turbine Combustor at Elevated Pressure

2019 ◽  
Author(s):  
Mitchell L. Passarelli ◽  
J. D. Maxim Cirtwill ◽  
Timothy Wabel ◽  
Adam M. Steinberg ◽  
A. J. Wickersham
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Heat Release Rate ◽  
Release Rate ◽  
High Speed ◽  
Elevated Pressure ◽  
Fuel Injector ◽  
Frequency Pulling ◽  
Industrial Gas Turbine ◽  
Thermoacoustic Oscillations

Abstract This paper analyzes intermittent self-excited thermoacoustic oscillations in which the pressure (P′) and heat release rate (q̇′) fluctuations are harmonically coupled. That is to say, P′ and q̇′ do not oscillate at the same frequencies, but rather at frequencies in integer ratios. Thus, this system represents a case dominated by nonlinear cross-mode coupling. The measurements were obtained in an optically-accessible combustor equipped with an industrial gas turbine fuel injector operating with liquid fuel under partially-premixed conditions at elevated pressure. High-speed chemiluminescence (CL) imaging of OH* was used as an indicator of the heat release rate. The data was processed using spectral proper orthogonal decomposition (SPOD) to isolate the dominant heat release and pressure modes. Synchronization theory was used to determine when the modes are coupled and how their interaction manifests in the measurements, particularly how it relates to the observed intermittency. The results show three distinct intervals of synchronized oscillation shared by all the mode pairs analyzed. The first interval exhibits the same characteristics as a pair of noisy, phase-locked self-oscillators, with phase-slipping and frequency-pulling. While the behaviour of the second interval differs among mode pairs, strong frequency-pulling is observed during the third interval for all pairs.

Flame Area Fluctuation Measurements in Velocity-Forced Premixed Gas Turbine Flames

2015 ◽  
Author(s):  
Alexander J. De Rosa ◽  
Janith Samarasinghe ◽  
Stephen J. Peluso ◽  
Bryan D. Quay ◽  
Domenic A. Santavicca
Keyword(s):  
Heat Release ◽  
Gas Turbine ◽  
Heat Release Rate ◽  
Release Rate ◽  
Large Scale ◽  
Large Scale Structure ◽  
Scale Structure ◽  
Small Scale ◽  
Flame Velocity ◽  
Unstable Combustion

Fluctuations in the heat release rate that occur during unstable combustion in lean premixed gas turbine combustors can be attributed to velocity and equivalence ratio fluctuations. For a fully premixed flame, velocity fluctuations affect the heat release rate primarily by inducing changes in the flame area. In this paper, a technique to analyze changes in flame area using chemiluminescence-based flame images is presented. The technique decomposes the flame area into separate components which characterize the relative contributions of area fluctuations in the large scale structure and the small scale wrinkling of the flame. The fluctuation in the wrinkled area of the flame which forms the flame brush is seen to dominate its response in the majority of cases tested. Analysis of the flame area associated with the large scale structure of the flame resolves convective perturbations that move along the mean flame position. Results are presented that demonstrate the application of this technique to both single-nozzle and multi-nozzle flames.

Fire Growth in Standard Tests and Railcars

2018 ◽  
Author(s):  
Charles Luo ◽  
Soroush Yazdani ◽  
Brian Y. Lattimer
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Large Scale ◽  
Growth Conditions ◽  
Standard Test ◽  
Fire Growth ◽  
Gas Temperature ◽  
Fire Dynamics ◽  
Room Corner Test

Large scale flammability performance of interior finish used on railcars has been evaluated in previous studies using the NFPA 286 room corner fire test, which has a cross-section similar to a railcar. In some studies, the wall containing the door was removed to account for the shorter length of the room compared to the railcar length. The focus of this study is to assess whether the NFPA 286 standard room-corner test with a door represents conditions that developed inside a railcar during a fire. Fire Dynamics Simulator (FDS) was used to model the fire growth in a NFPA 286 standard room-corner test with a door, NFPA 286 room without the wall containing the door, and railcar geometry with a single door open. All three cases had the same exposure fire in a corner and the same lining material. In predictions of the NFPA 286 room-corner test with a door, gas temperature, heat release rate, and time to flashover agreed well with available NFPA 286 standard test data. The simulation results of fire growth inside a railcar with one side door open produced similar conditions and fire growth compared with the standard NFPA 286 room with a door. For simulations on the NFPA 286 room with the wall containing the door removed, it was found that removal of the wall with the door resulted in non-conservative fire growth conditions with the gas temperature and heat release rate under-estimated compared to the standard NFPA 286 room with a door. These simulations indicate that the standard NFPA 286 room-corner test with a door is representative of conditions that would develop inside of a railcar.

scholarly journals Experimental Study on Occurrence Limit Heat Release Rate of Flashover in a Building Fire

2021 ◽  
Vol 21 (2) ◽  
pp. 65-71
Author(s):  
Seunggoo Kang ◽  
Yi Chul Shin
Keyword(s):  
Experimental Study ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Large Scale ◽  
Fire Behavior ◽  
Design Calculation ◽  
Building Fire ◽  
Fire Source ◽  
Combustion Tests

In this study, to allow the flashover to occur, combustion tests were conducted by setting the conditions of a fire source using a large-scale compartment and changing the opening condition. As a result, the inside temperature of the compartment was measured under the fire source conditions. Moreover, according to the “Handbook on Design Calculation &#x0004d;ethods of Fire Behavior” by the Architectural Institute of Japan, the validity of the heat release rate required for the flashover to occur was verified through the correlation between <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>Q</mi><mrow><mi>F</mi><mi>O</mi></mrow></msub><mo>/</mo><msub><mi>Q</mi><mrow><mi>v</mi><mi>m</mi><mi>a</mi><mi>x</mi></mrow></msub></math> and <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>A</mi><mi>T</mi></msub><msup><mrow><mo>(</mo><mi>k</mi><mi>p</mi><mi>c</mi><mo>)</mo></mrow><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup><mo>/</mo><msub><mi>c</mi><mrow><mi>P</mi></mrow></msub><mn>0</mn><mo>.</mo><mn>5</mn><mi>A</mi><msup><mi>H</mi><mrow><mn>1</mn><mo>/</mo><mn>2</mn></mrow></msup></math>.

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