Theory of premixed-flame propagation in large-scale turbulence

1979 ◽  
Vol 90 (3) ◽  
pp. 589-604 ◽  
Author(s):  
P. Clavin ◽  
F. A. Williams
Keyword(s):  
Diffusion Approximation ◽  
Laminar Flame ◽  
Turbulent Transport ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Longitudinal Displacement ◽  
Turbulent Fluctuations ◽  
First Order ◽  
Flame Thickness ◽  
Scale Ratio

A statistical theory is developed for the structure and propagation velocity of premixed flames in turbulent flows with scales large compared with the laminar flame thickness. The analysis, free of usual closure assumptions, involves a regular perturbation for small values of the ratio of laminar flame thickness to turbulence scale, termed the scale ratio ε, and a singular perturbation for large values of the non-dimensional activation temperature β. Any effects of the flame on the flow are considered to be given. In this initial study, molecular coefficients for diffusion of heat and reactants are set equal. The results identify convective-diffusive and reactive-diffusive zones in the flame and predict thickening of the flame by turbulence through streamwise displacement of the reactive-diffusive zone. Profiles for intensities of temperature fluctuations and for streamwise turbulent transport are obtained. A fundamental quantity occurring in the analysis is the longitudinal displacement of the reactive-diffusive zone in an Eulerian frame by turbulent fluctuations, and to first order in the scale ratio this equals the longitudinal displacement of fluid elements in an Eulerian frame by turbulent fluctuations, herein termed simply the Eulerian displacement. To first order in the scale ratio it is found that, if the Eulerian displacement experiences the same type of statistical non-stationarity as the corresponding Lagrangian displacement, then the diffusion approximation is valid for streamwise turbulent transport but the turbulent flame thickens as time increases, while if the Eulerian displacement is statistically stationary then the diffusion approximation necessitates a negative coefficient of diffusion in part of the flame but the flame thickness remains constant. By carrying the analysis to second order in the scale ratio it is shown that the turbulent-flame speed exceeds the laminar-flame speed by an amount proportional to the mean square of the transverse gradient of the Eulerian displacement. This result can be understood from the mechanistic viewpoint of a wrinkled laminar flame in terms of the increase in flame area produced by turbulence. Thus the theory provides a precise statistical quantification of the model of the wrinkled laminar flame for describing structures of turbulent flames.

High-Speed Spectrally Resolved Imaging Studies of Spherically Expanding Natural Gas Flames Under Gas Turbine Operating Conditions

2019 ◽  
Author(s):  
Pradeep Parajuli ◽  
Tyler Paschal ◽  
Mattias A. Turner ◽  
Eric L. Petersen ◽  
Waruna D. Kulatilaka
Keyword(s):  
Natural Gas ◽  
Flame Front ◽  
Gas Turbines ◽  
High Speed ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Initial Pressure ◽  
Flame Speed ◽  
Operating Conditions ◽  
Important Species

Abstract Natural gas is a major fuel source for many industrial and power-generation applications. The primary constituent of natural gas is methane (CH4), while smaller quantities of higher order hydrocarbons such as ethane (C2H6) and propane (C3H8) can also be present. Detailed understanding of natural gas combustion is important to obtain the highest possible combustion efficiency with minimal environmental impact in devices such as gas turbines and industrial furnaces. For a better understanding the combustion performance of natural gas, several important parameters to study are the flame temperature, heat release zone, flame front evolution, and laminar flame speed as a function of flame equivalence ratio. Spectrally and temporally resolved, high-speed chemiluminescence imaging can provide direct measurements of some of these parameters under controlled laboratory conditions. A series of experiments were performed on premixed methane/ethane-air flames at different equivalence ratios inside a closed flame speed vessel that allows the direct observation of the spherically expanding flame front. The vessel was filled with the mixtures of CH4 and C2H6 along with respective partial pressures of O2 and N2, to obtain the desired equivalence ratios at 1 atm initial pressure. A high-speed camera coupled with an image intensifier system was used to capture the chemiluminescence emitted by the excited hydroxyl (OH*) and methylidyne (CH*) radicals, which are two of the most important species present in the natural gas flames. The calculated laminar flame speeds for an 80/20 methane/ethane blend based on high-speed chemiluminescence images agreed well with the previously conducted Z-type schlieren imaging-based measurements. A high-pressure test, conducted at 5 atm initial pressure, produced wrinkles in the flame and decreased flame propagation rate. In comparison to the spherically expanding laminar flames, subsequent turbulent flame studies showed the sporadic nature of the flame resulting from multiple flame fronts that were evolved discontinuously and independently with the time. This paper documents some of the first results of quantitative spherical flame speed experiments using high-speed chemiluminescence imaging.

Fuel Variation Effects in Propagation and Stabilization of Turbulent Counter-Flow Premixed Flames

2018 ◽  
Author(s):  
Ehsan Abbasi-Atibeh ◽  
Sandeep Jella ◽  
Jeffrey M. Bergthorson
Keyword(s):  
High Speed ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Mass Diffusion ◽  
Flame Stabilization ◽  
Turbulent Flames ◽  
Flame Speed ◽  
Flame Surface ◽  
Differential Diffusion ◽  
Turbulent Flame Speed

Sensitivity to stretch and differential diffusion of chemical species are known to influence premixed flame propagation, even in the turbulent environment where mass diffusion can be greatly enhanced. In this context, it is convenient to characterize flames by their Lewis number (Le), a ratio of thermal-to-mass diffusion. The work reported in this paper describes a study of flame stabilization characteristics when the Le is varied. The test data is comprised of Le ≪ 1 (Hydrogen), Le ≈ 1 (Methane), and Le > 1 (Propane) flames stabilized at various turbulence levels. The experiments were carried out in a Hot exhaust Opposed-flow Turbulent Flame Rig (HOTFR), which consists of two axially-opposed, symmetric turbulent round jets. The stagnation plane between the two jets allows the aerodynamic stabilization of a flame, and clearly identifies fuel influences on turbulent flames. Furthermore, high-speed Particle Image Velocimetry (PIV), using oil droplet seeding, allowed simultaneous recordings of velocity (mean and rms) and flame surface position. These experiments, along with data processing tools developed through this study, illustrated that in the mixtures with Le ≪ 1, turbulent flame speed increases considerably compared to the laminar flame speed due to differential diffusion effects, where higher burning rates compensate for the steepening average velocity gradient, and keeps these flames almost stationary as bulk flow velocity increases. These experiments are suitable for validating the ability of turbulent combustion models to predict lifted, aerodynamically-stabilized flames. In the final part of this paper, we model the three fuels at two turbulence intensities using the FGM model in a RANS context. Computations reveal that the qualitative flame stabilization trends reproduce the effects of turbulence intensity, however, more accurate predictions are required to capture the influences of fuel variations and differential diffusion.

scholarly journals Evaluation of Reduced Kinetics in Simulation of Gasified Biomass Gas Combustion

2013 ◽  
Author(s):  
Xiaoxiang Zhang ◽  
Nur Farizan Munjat ◽  
Jeevan Jayasuriya ◽  
Reza Fakhrai ◽  
Torsten Fransson
Keyword(s):  
Gas Turbine ◽  
Chemical Kinetics ◽  
Cfd Simulation ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Turbulent Flame Speed ◽  
Operation Conditions ◽  
Simulation Results ◽  
Gas Turbine Combustion

It is essentially important to use appropriate chemical kinetic models in the simulation process of gas turbine combustion. To integrate the detailed kinetics into complex combustion simulations has proven to be a computationally expensive task with tens to thousands of elementary reaction steps. It has been suggested that an appropriate simplified kinetics which are computationally efficient could be used instead. Therefore reduced kinetics are often used in CFD simulation of gas turbine combustion. At the same time, simplified kinetics for specific fuels and operation conditions need to be carefully selected to fulfill the accuracy requirements. The applicability of several simplified kinetics for premixed Gasified Biomass Gas (GBG) and air combustion are evaluated in this paper. The current work is motivated by the growing demand of gasified biomass gas (GBG) fueled combustion. Even though simplified kinetic schemes developed for hydrocarbon combustions are published by various researchers, there is little research has been found in literature to evaluate the ability of the simplified chemical kinetics for the GBG combustion. The numerical Simulation tool “CANTERA” is used in the current study for the comparison of both detailed and simplified chemical kinetics. A simulated gas mixture of CO/H2/CH4/CO2/N2 is used for the current evaluation, since the fluctuation of GBG components may have an unpredictable influence on the simulation results. The laminar flame speed has an important influence with flame stability, extinction limits and turbulent flame speed, here it is chosen as an indicator for validation. The simulation results are compared with the experimental data from the previous study [1] which is done by our colleagues. Water vapour which has shown a dilution effect in the experimental study are also put into concern for further validation. As the results indicate, the reduced kinetics which are developed for hydrocarbon or hydrogen combustion need to be highly optimized before using them for GBG combustion. Further optimization of the reduced kinetics is done for GBG and moderate results are achieved using the optimized kinetics compared with the detailed combustion kinetics.

Modeling of Inhomogeneously Premixed Combustion With an Extended TFC Model

2000 ◽  
Vol 124 (1) ◽  
pp. 58-65 ◽  
Author(s):  
W. Polifke ◽  
P. Flohr ◽  
M. Brandt
Keyword(s):  
Laminar Flame ◽  
Mixture Fraction ◽  
Turbulent Flame ◽  
Flame Temperature ◽  
Flame Speed ◽  
Premixed Combustion ◽  
Combustion Stability ◽  
Model Parameters ◽  
Turbulent Flame Speed ◽  
Functional Relationships

In many practical applications, so-called premixed burners do not achieve perfect premixing of fuel and air. Instead, fuel injection pressure is limited, the permissible burner pressure drop is small and mixing lengths are curtailed to reduce the danger of flashback. Furthermore, internal or external piloting is frequently employed to improve combustion stability, while part-load operation often requires burner staging, where neighboring burners operate with unequal fuel/air equivalence ratios. In this report, an extension of the turbulent flame speed closure (TFC) model for highly turbulent premixed combustion is presented, which allows application of the model to the case of inhomogeneously premixed combustion. The extension is quite straightforward, i.e., the dependence of model parameters on mixture fraction is accounted for by providing appropriate lookup tables or functional relationships to the model. The model parameters determined in this way are adiabatic flame temperature, laminar flame speed and critical gradient. The model has been validated against a test case from the open literature and applied to an externally piloted industrial gas turbine burner with good success.

Three-dimensional computational fluid dynamics simulation of hydrogen engines using a turbulent flame speed closure combustion model

2012 ◽  
Vol 13 (5) ◽  
pp. 464-481 ◽  
Author(s):  
Udo Gerke ◽  
Konstantinos Boulouchos
Keyword(s):  
Fluid Dynamics ◽  
Computational Fluid Dynamics ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Three Dimensional ◽  
Flame Speed ◽  
Combustion Model ◽  
Laminar Flame Speed ◽  
Release Rates ◽  
Turbulent Flame Speed

The mixture formation and combustion process of a hydrogen direct-injection internal combustion engine is computed using a modified version of a commercial three-dimensional computational fluid dynamics code. The aim of the work is the evaluation of hydrogen laminar flame speed correlations and turbulent flame speed closures with respect to combustion of premixed and stratified mixtures at various levels of air-to-fuel equivalence ratio. Heat-release rates derived from in-cylinder pressure traces are used for the validation of the combustion simulations. A turbulent combustion model with closures for a turbulent flame speed is investigated. The value of the computed heat-release rates mainly depends on the quality of laminar burning velocities and standard of turbulence quantities provided to the combustion model. Combustion simulations performed with experimentally derived laminar flame speed data give better results than those using laminar flame speeds obtained from a kinetic scheme. However, experimental data of hydrogen laminar flame speeds found in the literature are limited regarding the range of pressures, temperatures and air-to-fuel equivalence ratios, and do not comply with the demand of high-pressure engine-relevant conditions.

Ignition Delay Time and Laminar Flame Speed Calculations for Natural Gas/Hydrogen Blends at Elevated Pressures

2012 ◽  
Author(s):  
Marissa Brower ◽  
Eric Petersen ◽  
Wayne Metcalfe ◽  
Henry J. Curran ◽  
Marc Füri ◽  
...  
Keyword(s):  
Natural Gas ◽  
Gas Turbine ◽  
Chemical Kinetics ◽  
Ignition Delay ◽  
Ignition Delay Time ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Flame Speed ◽  
Laminar Flame Speed ◽  
Hydrogen Addition

Applications of natural gas and hydrogen co-firing have received increased attention in the gas turbine market, which aims at higher flexibility due to concerns over the availability of fuels. While much work has been done in the development of a fuels database and corresponding chemical kinetics mechanism for natural gas mixtures, there are nonetheless few if any data for mixtures with high levels of hydrogen at conditions of interest to gas turbines. The focus of the present paper is on gas turbine engines with primary and secondary reaction zones as represented in the Alstom and Rolls Royce product portfolio. The present effort includes a parametric study, a gas turbine model study, and turbulent flame speed predictions. Using a highly optimized chemical kinetics mechanism, ignition delay times and laminar burning velocities were calculated for fuels from pure methane to pure hydrogen and with natural gas/hydrogen mixtures. A wide range of engine-relevant conditions were studied: pressures from 1 to 30 atm, flame temperatures from 1600 to 2200 K, primary combustor inlet temperature from 300 to 900 K, and secondary combustor inlet temperatures from 900 to 1400 K. Hydrogen addition was found to increase the reactivity of hydrocarbon fuels at all conditions by increasing the laminar flame speed and decreasing the ignition delay time. Predictions of turbulent flame speeds from the laminar flame speeds show that hydrogen addition affects the reactivity more when turbulence is considered. This combined effort of industrial and university partners brings together the know-how of applied, as well as experimental and theoretical disciplines.

scholarly journals Investigation of Hydrogen Content and Dilution Effect on Syngas/Air Premixed Turbulent Flame Using OH Planar Laser-Induced Fluorescence

Processes ◽  
2021 ◽  
Vol 9 (11) ◽  
pp. 1894
Author(s):  
Li Yang ◽  
Wubin Weng ◽  
Yanqun Zhu ◽  
Yong He ◽  
Zhihua Wang ◽  
...  
Keyword(s):  
Hydrogen Content ◽  
Laminar Flame ◽  
Turbulent Flame ◽  
Laser Induced Fluorescence ◽  
Flame Speed ◽  
Laminar Flame Speed ◽  
High Hydrogen ◽  
Turbulent Flame Speed ◽  
Planar Laser Induced Fluorescence ◽  
Planar Laser

Syngas produced by gasification, which contains a high hydrogen content, has significant potential. The variation in the hydrogen content and dilution combustion are effective means to improve the steady combustion of syngas and reduce NOx emissions. OH planar laser-induced fluorescence technology (OH-PLIF) was applied in the present investigation of the turbulence of a premixed flame of syngas with varied compositions of H2/CO. The flame front structure and turbulent flame velocities of syngas with varied compositions and turbulent intensities were analyzed and calculated. Results showed that the trend in the turbulent flame speed with different hydrogen proportions and dilutions was similar to that of the laminar flame speed of the corresponding syngas. A higher hydrogen proportion induced a higher turbulent flame speed, higher OH concentration, and a smaller flame. Dilution had the opposite effect. Increasing the Reynolds number also increased the turbulent flame speed and OH concentration. In addition, the effect of the turbulence on the combustion of syngas was independent of the composition of syngas after the analysis of the ratio between the turbulent flame speed and the corresponding laminar flame speed, for the turbulent flames under low turbulent intensity. These research results provide a theoretical basis for the practical application of syngas with a complex composition in gas turbine power generation.

Prediction of Ultra-Lean SI Engine Performance by QD-Combustion Model With an Improved Laminar Flame Speed

2020 ◽  
Author(s):  
Ratnak Sok ◽  
Jin Kusaka ◽  
Kyohei Yamaguchi
Keyword(s):  
Gasoline Engine ◽  
Laminar Flame ◽  
Burning Velocity ◽  
Turbulent Flame ◽  
Combustion Process ◽  
Combustion Characteristics ◽  
Flame Speed ◽  
Combustion Model ◽  
Lean Mixture ◽  
Turbulent Flame Speed

Abstract A quasi-dimensional (QD) simulation model is a preferred method to predict combustion in the gasoline engines with reliable results and shorter calculation time compared with multi-dimensional simulation. The combustion phenomena in spark ignition (SI) engines are highly turbulent, and at initial stage of the combustion process, turbulent flame speed highly depends on laminar burning velocity SL. A major parameter of the QD combustion model is an accurate prediction of the SL, which is unstable under low engine speed and ultra-lean mixture. This work investigates the applicability of the combustion model for evaluating the combustion characteristics of a high-tumble port gasoline engine operated under ultra-lean mixture (equivalence ratio up to ϕ = 0.5) which is out of the range of currently available SL functions initially developed for a single component fuel. In this study, the SL correlation is improved for a gasoline surrogate fuel (5 components). Predicted SL data from the conventional and improved functions are compared with experimental SL data taken from a constant-volume chamber under micro-gravity condition. The SL measurements are done at reference conditions at temperature of 300K, pressure of 0.1MPaa, and at elevated conditions whose temperature = 360K, pressure = 0.1, 0.3, and 0.5 MPaa. Results show that the conventional SL model over-predicts flame speeds under all conditions. Moreover, the model predicts negative SL at very lean (ϕ ≤ 0.3) and rich (ϕ ≥ 1.9) mixture while the revised SL is well validated with the measured data. The improved SL formula is then incorporated into the QD combustion model by a user-defined function in GT-Power simulation. The engine experimental data are taken at 1000 RPM and 2000 RPM under engine load IMEPn = 0.4–0.8 MPa (with 0.1 increment) and ϕ ranges are up to 0.5. The results shows that the simulated engine performances and combustion characteristics are well validated with the experiments within 6% accuracy by using the QD combustion model coupled with the improved SL. A sensitivity analysis of the model is also in good agreement with the experiments under cyclic variation (averaged cycle, high IMEP or stable cycle, and low IMEP or unstable cycle).

Turbulent Consumption Speed Scaling of H2/CO Blends

2011 ◽  
Author(s):  
Prabhakar Venkateswaran ◽  
Andrew D. Marshall ◽  
David R. Noble ◽  
Jerry M. Seitzman ◽  
Tim C. Lieuwen
Keyword(s):  
Laminar Flame ◽  
Turbulent Flame ◽  
Practical Interest ◽  
Flame Speed ◽  
Fuel Composition ◽  
Laminar Flame Speed ◽  
Volume Data ◽  
Mean Flow ◽  
Turbulent Flame Speed ◽  
Kinetic Calculations

This paper describes measurements and analysis of global turbulent consumption speeds, ST,GC, of hydrogen/carbon monoxide (H2/CO) mixtures. The turbulent flame properties of such mixtures are of fundamental interest because of their strong stretch sensitivity and of practical interest since they are the primary constituents of syngas fuels. Data are analyzed at mean flow velocities and turbulence intensities of 4 < U0 < 50 m/s and 1 < u′rms/SL,0 < 100, respectively, for H2/CO blends ranging from 30–90% H2 by volume. Data from two sets of experiments are reported. In the first, fuel blends ranging from 30–90% H2 and mixture equivalence ratio, Φ, were adjusted at each fuel composition to have nominally the same un-stretched laminar flame speed, SL,0. In the second set, equivalence ratios were varied at constant H2 levels. The data clearly corroborate results from other studies that show significant sensitivity of ST,GC to fuel composition. For example, at a fixed u′rms, ST,GC of a 90% H2 case (at Φ = 0.48) is a factor of three times larger than the baseline Φ = 0.9, CH4/air mixture that has the same SL,0 value. We also describe physics-based correlations of these data, using leading points concepts and detailed kinetic calculations of their stretch sensitivities. These results are used to develop an inequality for negative Markstein length flames that bounds the turbulent flame speed data and show that the data can be collapsed using the maximum stretched laminar flame speed, SL,max, rather than SL,0.

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