scholarly journals Predicting the consumption speed of a premixed flame subjected to unsteady stretch rates

2021 ◽  
Author(s):  
Meysam Sahafzadeh ◽  
Seth B. Dworkin ◽  
Larry W. Kostiuk
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Time Scale ◽  
Transfer Functions ◽  
Laminar Flame ◽  
Premixed Flames ◽  
Turbulent Flames ◽  
Response Analysis ◽  
Phase Lag

The stretched laminar flame model provides a convenient approach to embed realistic chemical kinetics when simulating turbulent premixed flames. When positive-only periodic strain rates are applied to a laminar flame there is a notable phase lag and diminished amplitude in heat release rate. Similar results have being observed with respect to the other component of stretch rate, namely the unsteady motion of a curved flame when the stretch rates are periodic about zero. Both cases showed that the heat release rate or consumption speed of these laminar-premixed flames vary significantly from the quasi-steady flamelet model. Deviation from quasi-steady behaviour increases as the unsteady flow time scale approaches the chemical time scale that is set by the stoichiometry. A challenge remains in how to use such results predictively for local and instantaneous consumption speed for small segments of turbulent flames where their unsteady stretch history is not periodic. This paper uses a frequency response analysis as a characterization tool to simplify the complex non-linear behaviour of premixed methane air flames for equivalence ratios from 1.0 down to 0.7, and frequencies from quasi-steady up to 2000 Hz using flame transfer functions. Various linear and nonlinear models were used to identify appropriate flame transfer functions for low and higher frequency regimes, as well as extend the predictive capabilities of these models. Linear models were only able to accurately predict the flame behaviour below a threshold of when the fluid and chemistry time scales are the same order of magnitude. Other proposed transfer functions were tested against arbitrary multi-frequency stretch inputs and were shown to be effective over the full range of frequencies.

scholarly journals Predicting the consumption speed of a premixed flame subjected to unsteady stretch rates

2021 ◽  
Author(s):  
Meysam Sahafzadeh ◽  
Seth B. Dworkin ◽  
Larry W. Kostiuk
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Time Scale ◽  
Transfer Functions ◽  
Laminar Flame ◽  
Premixed Flames ◽  
Turbulent Flames ◽  
Response Analysis ◽  
Phase Lag

The stretched laminar flame model provides a convenient approach to embed realistic chemical kinetics when simulating turbulent premixed flames. When positive-only periodic strain rates are applied to a laminar flame there is a notable phase lag and diminished amplitude in heat release rate. Similar results have being observed with respect to the other component of stretch rate, namely the unsteady motion of a curved flame when the stretch rates are periodic about zero. Both cases showed that the heat release rate or consumption speed of these laminar-premixed flames vary significantly from the quasi-steady flamelet model. Deviation from quasi-steady behaviour increases as the unsteady flow time scale approaches the chemical time scale that is set by the stoichiometry. A challenge remains in how to use such results predictively for local and instantaneous consumption speed for small segments of turbulent flames where their unsteady stretch history is not periodic. This paper uses a frequency response analysis as a characterization tool to simplify the complex non-linear behaviour of premixed methane air flames for equivalence ratios from 1.0 down to 0.7, and frequencies from quasi-steady up to 2000 Hz using flame transfer functions. Various linear and nonlinear models were used to identify appropriate flame transfer functions for low and higher frequency regimes, as well as extend the predictive capabilities of these models. Linear models were only able to accurately predict the flame behaviour below a threshold of when the fluid and chemistry time scales are the same order of magnitude. Other proposed transfer functions were tested against arbitrary multi-frequency stretch inputs and were shown to be effective over the full range of frequencies.

scholarly journals Heat release rate correlation and combustion noise in premixed flames

2011 ◽  
Vol 681 ◽  
pp. 80-115 ◽  
Author(s):  
N. SWAMINATHAN ◽  
G. XU ◽  
A. P. DOWLING ◽  
R. BALACHANDRAN
Keyword(s):  
Spatial Distribution ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Time Scale ◽  
Reaction Rate ◽  
Laminar Flame ◽  
Premixed Flames ◽  
Length Scale ◽  
Turbulent Premixed Flames

The sound emission from open turbulent flames is dictated by the two-point spatial correlation of the rate of change of the fluctuating heat release rate. This correlation in premixed flames can be represented well using Gaussian-type functions and unstrained laminar flame thermal thickness can be used to scale the correlation length scale, which is about a quarter of the planar laminar flame thermal thickness. This correlation and its length scale are observed to be less influenced by the fuel type or stoichiometry or turbulence Reynolds and Damkohler numbers. The time scale for fluctuating heat release rate is deduced to be about τc/34 on an average, where τc is the planar laminar flame time scale, using direct numerical simulation (DNS) data. These results and the spatial distribution of mean reaction rate obtained from Reynolds-averaged Navier–Stokes (RANS) calculations of open turbulent premixed flames employing the standard model and an algebraic reaction rate closure, involving a recently developed scalar dissipation rate model, are used to obtain the far-field sound pressure level from open flames. The calculated values agree well with measured values for flames of different stoichiometry and fuel types, having a range of turbulence intensities and heat output. Detailed analyses of RANS results clearly suggest that the noise level from turbulent premixed flames having an extensive and uniform spatial distribution of heat release rate is low.

scholarly journals Effects of the Injector Design on the Transfer Function of Premixed Swirling Flames

2017 ◽  
Author(s):  
M. Gatti ◽  
R. Gaudron ◽  
C. Mirat ◽  
T. Schuller
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Bluff Body ◽  
Transfer Functions ◽  
Thermal Power ◽  
Phase Lag ◽  
Swirl Number ◽  
Injector Design

This article reports a series of experiments on the dynamics of lean-premixed swirl-stabilized flames submitted to harmonic flowrate modulations. The flame transfer function is analyzed for different injector designs with a specific focus on conditions leading to the lowest heat release rate response for a given flowrate perturbation. Experiments are carried out at a fixed equivalence ratio and fixed thermal power. Transfer functions are measured for radial swirling vanes by modifying the diameter of the swirler injection holes, the diameter of the injection tube at the top of the swirler and the end piece diameter of a central insert serving as a bluff body. It is found that the lowest response depends on the forcing frequency and is obtained when the injector design features the largest swirl number. The transfer function of the studied flames features a minimum gain value which decreases for increasing swirl levels. This minimum value is found to be independent of the velocity forcing level and is only controlled by the level of swirl. An excessive swirl level however leads to flash-back of the perturbed flames inside the injector. The way the flame behaves at this forcing frequency is analyzed for a set of injectors featuring the same radial swirling vane design and different injection tube diameters or conical end pieces. It is found that at the condition corresponding to the lowest FTF gain, i.e. the injector with the largest swirl number, the upper and lower parts of the flame contribute to out of phase heat release oscillations, but they also both feature a reduced level of fluctuations. When the swirl number decreases, the FTF gain increases due to a reduction of the phase lag between heat release rate oscillations in the lower and the upper parts of the flame and more importantly due to a general increase of the level of heat release oscillations in both parts of the flame.

Optical Measurement of Local and Global Transfer Functions for Equivalence Ratio Fluctuations in a Turbulent Swirl Flame

2013 ◽  
Author(s):  
Bernhard C. Bobusch ◽  
Bernhard Ćosić ◽  
Jonas P. Moeck ◽  
Christian Oliver Paschereit
Keyword(s):  
Transfer Function ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Equivalence Ratio ◽  
Transfer Functions ◽  
Optical Measurement ◽  
Low Frequencies ◽  
Fuel Flow ◽  
Calibration Measurement

Equivalence ratio fluctuations are known to be one of the key factors controlling thermoacoustic stability in lean premixed gas turbine combustors. The mixing and thus the spatio-temporal evolution of these perturbations in the combustor flow is, however, difficult to account for in present low-order modeling approaches. To investigate this mechanism, experiments in an atmospheric combustion test rig are conducted. To assess the importance of equivalence ratio fluctuations in the present case, flame transfer functions for different injection positions are measured. By adding known perturbations in the fuel flow using a solenoid valve, the influence of equivalence ratio oscillations on the heat release rate is investigated. The spatially and temporally resolved equivalence ratio fluctuations in the reaction zone are measured using two optical chemiluminescence signals, captured with an intensified camera. A steady calibration measurement allows for the quantitative assessment of the equivalence ratio fluctuations in the flame. This information is used to obtain a mixing transfer function, which relates fluctuations in the fuel flow to corresponding fluctuations in the equivalence ratio of the flame. The current study focuses on the measurement of the global, spatially integrated, transfer function for equivalence ratio fluctuations and the corresponding modeling. In addition, the spatially resolved mixing transfer function is shown and discussed. The global mixing transfer function reveals that despite the good spatial mixing quality of the investigated generic burner, the ability to damp temporal fluctuations at low frequencies is rather poor. It is shown that the equivalence ratio fluctuations are the governing heat release rate oscillation response mechanism for this burner in the low-frequency regime. The global transfer function for equivalence ratio fluctuations derived from the measurements is characterized by a pronounced low-pass characteristic, which is in good agreement with the presented convection–diffusion mixing model.

scholarly journals A Comparison of the Transfer Functions and Flow Fields of Flames With Increasing Swirl Number

2018 ◽  
Author(s):  
M. Gatti ◽  
R. Gaudron ◽  
C. Mirat ◽  
L. Zimmer ◽  
T. Schuller
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Flow Fields ◽  
Phase Lag ◽  
Swirl Number ◽  
Gain Curve ◽  
Cold Flow ◽  
Flow Response ◽  
Swirled Flames

The frequency response of premixed swirled flames is investigated by comparing their Transfer Function (FTF) between velocity and heat release rate fluctuations. The equivalence ratio and flow velocity are kept constant and four different swirling injectors are tested with increasing swirl numbers. The first injector features a vanishing low swirl number S = 0.20 and produces a flame anchored by the recirculating flow in the wake of a central bluff body. The three other swirling injectors produce highly swirled flows (S > 0.6) leading to a much larger internal recirculation region, which size increases with the swirl level. When operating the burner at S = 0.20, the FTF gain curve smoothly increases to reach a maximum and then smoothly decreases towards zero. For the highly swirled flames (S > 0.6), the FTF gain curve shows a succession of valleys and peaks attributed to interferences between axial and azimuthal velocity fluctuations at the injector outlet. The FTF phase-lag curves from the vanishing low and highly swirled flames are the same at low frequencies despite their large differences in flame length and flame aspect ratio. Deviations between the FTF phase lag curves of the different swirled flames start above the frequency corresponding to the first valley in the FTF gain of the highly swirled flames. Phase averaged images of the axial flow fields and of the flame chemiluminescence are used to interpret these features. At forcing frequencies corresponding to peak FTF gain values, the cold flow response of all flames investigated is dominated by large coherent vortical structures shed from the injector lip. At forcing frequencies corresponding to a valley in the FTF gain curve of the highly swirled flames, the formation of large coherent structures is strongly hindered in the cold flow response. These observations contrast with previous interpretations of the mechanisms associated to the low FTF response of swirled flames. It is finally found that for flames stabilized with a large swirl number, heat release rate fluctuations result both from large flame luminosity oscillations and large flame volume oscillations. For conditions leading to a small FTF gain value, both the flame luminosity and flame volume fluctuations are suppressed confirming the absence of strong perturbations within the flow at these frequencies. The experiments made in this work reveal a purely hydrodynamic mechanism at the origin of the low response of swirling flames at certain specific frequencies.

Determination of the Heat Release Distribution in Turbulent Flames by a Model Based Correction of OH* Chemiluminescence

2011 ◽  
Vol 133 (12) ◽  
Author(s):  
Martin Lauer ◽  
Mathieu Zellhuber ◽  
Thomas Sattelmayer ◽  
Christopher J. Aul
Keyword(s):  
Strain Rate ◽  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Correction Method ◽  
Turbulent Flames ◽  
Lookup Table ◽  
Rate Model ◽  
Model Based

Imaging of OH* or CH* chemiluminescence with intensified cameras is often employed for the determination of heat release in premixed flames. Proportionality is commonly assumed, but in the turbulent case this assumption is not justified. Substantial deviations from proportionality are observed, which are due to turbulence-chemistry interactions. In this study a model based correction method is presented to obtain a better approximation of the spatially resolved heat release rate of lean turbulent flames from OH* measurements. The correction method uses a statistical strain rate model to account for the turbulence influence. The strain rate model is evaluated with time-resolved velocity measurements of the turbulent flow. Additionally, one-dimensional simulations of strained counterflow flames are performed to consider the nonlinear effect of turbulence on chemiluminescence intensities. A detailed reaction mechanism, which includes all relevant chemiluminescence reactions and deactivation processes, is used. The result of the simulations is a lookup table of the ratio between heat release rate and OH* intensity with strain rate as parameter. This lookup table is linked with the statistical strain rate model to obtain a correction factor which accounts for the nonlinear relationships between OH* intensity, heat release rate, and strain rate. The factor is then used to correct measured OH* intensities to obtain the local heat release rate. The corrected intensities are compared to heat release distributions which are measured with an alternative method. For all investigated flames in the lean, partially premixed regime the corrected OH* intensities are in very good agreement with the heat release rate distributions of the flames.

On the correlation of heat release rate in turbulent premixed flames

2011 ◽  
Vol 33 (1) ◽  
pp. 1533-1541 ◽  
Author(s):  
N. Swaminathan ◽  
R. Balachandran ◽  
G. Xu ◽  
A.P. Dowling
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Premixed Flames ◽  
Turbulent Premixed Flames ◽  
Turbulent Premixed

Influence of Lewis Number on Heat Release Rate in Premixed Syngas Flames

2019 ◽  
Author(s):  
Kedar G. Bhide ◽  
Sheshadri Sreedhara
Keyword(s):  
Heat Release ◽  
Fuel Consumption ◽  
Heat Release Rate ◽  
Release Rate ◽  
Consumption Rate ◽  
Lewis Number ◽  
Turbulent Flames ◽  
Attractive Alternative ◽  
Differential Diffusion ◽  
Fuel Consumption Rate

Abstract Syngas is an attractive alternative to currently popular hydrocarbon fuels due to its ability to be synthesized from multiple sources and lower carbon content. Direct Numerical Simulation (DNS) studies on premixed and non-premixed syngas flames have recently received attention. In this light, DNS of turbulent premixed syngas has been performed. Influence of turbulence and differential diffusion effects on chemical pathways of fuels like Hydrogen and methane has been studied in the past. Similar study on syngas flame has not been reported in the literature. Two cases with variation in the intensity of turbulence have been reported in this study. Effect of differential diffusion and turbulence on heat release rate and fuel consumption rate has been discussed. The behavior of heat release rate and fuel consumption rate was largely similar between laminar and turbulent flames considered in this study. Influence of species Lewis number was found to be more pronounced than that of turbulence.

Response of Heat Release Rate to Flame Straining in Swirling Hydrogen-Air Premixed Flames

2020 ◽  
Vol 104 (2-3) ◽  
pp. 451-478
Author(s):  
Kozo Aoki ◽  
Masayasu Shimura ◽  
JoonHwi Park ◽  
Yuki Minamoto ◽  
Mamoru Tanahashi
Keyword(s):  
Heat Release ◽  
Heat Release Rate ◽  
Release Rate ◽  
Premixed Flames
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