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A.n. Lipatnikov - One of the best experts on this subject based on the ideXlab platform.
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Evaluation of mean species mass fractions in Premixed Turbulent Flames: A DNS study
Proceedings of the Combustion Institute, 2020Co-Authors: A.n. Lipatnikov, Vladimir SabelnikovAbstract:Abstract Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air Flame associated with the thin-reaction-zone regime of Premixed Turbulent burning are analyzed (by adapting five alternative definitions of combustion progress variable c) in order to examine three different models that (i) are based on the Flamelet paradigm and (ii) aim at evaluating mean concentrations of various species in applied CFD research into Turbulent combustion. Mean mole fractions of all considered species and mean density are predicted if the laminar-Flame profiles of species mole fractions and density, respectively, are directly averaged using a Probability Density Function (PDF) P(c). The best predictions are obtained by extracting P(c) from the DNS data and defining c based on hydrogen mass fraction. These predictions suggest that mean mole fractions of various species in a Premixed Turbulent Flame can be evaluated at a post-processing stage of a CFD study by adopting P(c), obtained at the major stage of the simulations, to average a Flamelet library. When applied in such a way, the Flamelet paradigm is useful even for lean hydrogen-air Flames and even at Karlovitz number as large as 13. If the same PDF is applied to average reaction rates from the same Flamelet library, the mean rates of production/consumption of species n are poorly predicted, e.g. for radicals H, O, OH, HO2, and H2O2 if c is defined using hydrogen mass fraction. A hypothesis that conditioned rates 〈Wn|c〉 can be predicted using conditioned mole fractions 〈Xn|c〉, temperature 〈T|c〉, and density 〈ρ|c〉 is not supported either, e.g. for radicals O and OH. These differences between predictive capabilities of the first approach (directly averaging concentration profiles) and two other approaches (averaging reaction rates) are attributed to weakly (highly) non-linear dependencies of the concentrations (rates, respectively) on c.
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evolution of averaged local Premixed Flame thickness in a Turbulent flow
Combustion and Flame, 2019Co-Authors: Rixin Yu, Thommie Nillson, A.n. LipatnikovAbstract:Abstract In the combustion literature, contradictory results on the influence of turbulence on the local thickness of a Premixed Flame can be found and the present paper aims at contributing to reconcile this issue. First, different measures of local Flame thickness in a Turbulent flow, e.g. area-weighted and unweighted surface-averaged values of (i) |∇c|, i.e., the absolute value of 3D gradient of the combustion progress variable c, or (ii) 1/|∇c|, are studied and analytical relationships/inequalities between them are obtained. Second, the evolution of the different Flame thickness measures is explored by numerically evaluating them, as well as various terms in relevant evolution equations derived analytically. To do so, various measures and terms are extracted from DNS data obtained from (i) a highly Turbulent, constant-density, dynamically passive, single-reaction wave, (ii) moderately and highly Turbulent, single-step-chemistry Flames, and (iii) moderately and highly Turbulent, complex-chemistry lean methane-air Flames. In all those cases, all studied Flame thickness measures are reduced during an early stage of Premixed Turbulent Flame development, followed by local Flame re-broadening at later stages. Analysis of various terms in the aforementioned evolution equations shows that the initial local Flame thinning is controlled by Turbulent strain rates. The subsequent local Flame re-broadening is controlled by (i) curvature contribution to the stretch rate, which counter-balances the strain rate, (ii) spatial non-uniformities of the normal diffusion contribution to the local displacement-speed vector Sdn, and (iii) dilatation, which plays an important role in moderately Turbulent Flames, but a minor role in highly Turbulent Flames. Moreover, the present study shows that differently defined measures of a local Flame thickness can be substantially different. This difference should also be borne in mind when comparing data that indicate local Flame thinning with data that indicate local Flame broadening.
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Assessment of a transport equation for mean reaction rate using DNS data obtained from highly unsteady Premixed Turbulent Flames
International Journal of Heat and Mass Transfer, 2019Co-Authors: A.n. Lipatnikov, Vladimir Sabelnikov, Alexei PoludnenkoAbstract:Recently, a transport equation for the mean rate of product creation in a Premixed Turbulent Flame was derived and a joint closure relation for dominant terms in that equation was developed and validated against Direct Numerical Simulation (DNS) data obtained from weakly Turbulent Flames. Present communication reports results of further DNS tests of this newly introduced approach, performed using recent data computed under substantially different Turbulent conditions. In particular, the ability of the approach to predict significant oscillations of the Turbulent burning velocity with time is shown under conditions associated with not only the Flamelet but also the thin-reaction-zone regimes of Premixed Turbulent burning. While such oscillations are documented in the present and other DNS studies, capabilities of other models of Premixed Turbulent combustion to reproduce this challenging phenomenon have not yet been demonstrated in the literature.
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transport equations for reaction rate in laminar and Turbulent Premixed Flames characterized by non unity lewis number
International Journal of Hydrogen Energy, 2018Co-Authors: A.n. Lipatnikov, Nilanjan Chakraborty, Vladimir SabelnikovAbstract:Abstract Transport equations for (i) the rate W of product creation and (ii) its Favre-averaged value W ˜ are derived from the first principles by assuming that W depends solely on the temperature and mass fraction of a deficient reactant in a Premixed Turbulent Flame characterized by the Lewis number L e different from unity. The right hand side of the transport equation for W ˜ involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum T Σ ¯ of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar Premixed Flames characterized by L e 1 are performed and the linear relation between T Σ integrated along the normal to a laminar Flame and a product of (i) the consumption velocity u c and (ii) the stretch rate s ˙ w evaluated in the Flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of T Σ ¯ ∝ ρ W s ˙ ¯ is hypothesized, where ρ is the density and s ˙ is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, Premixed Turbulent Flames in the case of a single-step chemistry and L e = 0.34 , 0.6, or 0.8. In all three cases, the agreement between T Σ ¯ and ρ W s ˙ ¯ extracted from the DNS is good with exception of large ( c ¯ > 0.4 ) values of the mean combustion progress variable c ¯ in the case of L e = 0.34 . The developed linear relation between T Σ ¯ and ρ W s ˙ ¯ helps to understand why the leading edge of a Premixed Turbulent Flame brush can control its speed.
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A balance equation for modeling conditioned enthalpies in Premixed Turbulent Flames
Combustion and Flame, 2015Co-Authors: A.n. LipatnikovAbstract:In order to model the difference between enthalpies conditioned to unburned and burned gases, caused by compression of the mixture in the combustion chamber of a spark ignition engine, a new balance equation for the correlation between fluctuations in the enthalpy and combustion progress variable is proposed to be used, derived and closed invoking a few assumptions that appear to be plausible at low Mach and unity Lewis numbers. Subsequently, the difference in the conditioned enthalpies is determined within the framework of the well-known BML paradigm of Premixed Turbulent burning in the Flamelet regime. In the paper, this new approach is analytically compared with commonly used balance equations for enthalpy conditioned to unburned gas. Moreover, in order to investigate basic features of the newly proposed balance equation, it is applied to numerical simulations of a statistically stationary, planar, one-dimensional Premixed Turbulent Flame subject to increasing pressure. Instead of modeling the mean Flame structure, it is approximated using complementary error function, with the Flame speed and thickness being input parameters in the present study. Computed results show that the difference in the conditioned enthalpies is increased when the density ratio or the pressure growth rate is increased and when the Turbulent Flame speed or mean Flame brush thickness is decreased. Moreover, computed enthalpy difference is sensitive to a submodel of Turbulent flux of the combustion progress variable, with the use of gradient diffusion approximation appearing to be an improper solution. Furthermore, computed enthalpy difference is sensitive to a closure relation for enthalpy conditioned to Flamelets.
Vladimir Sabelnikov - One of the best experts on this subject based on the ideXlab platform.
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Evaluation of mean species mass fractions in Premixed Turbulent Flames: A DNS study
Proceedings of the Combustion Institute, 2020Co-Authors: A.n. Lipatnikov, Vladimir SabelnikovAbstract:Abstract Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air Flame associated with the thin-reaction-zone regime of Premixed Turbulent burning are analyzed (by adapting five alternative definitions of combustion progress variable c) in order to examine three different models that (i) are based on the Flamelet paradigm and (ii) aim at evaluating mean concentrations of various species in applied CFD research into Turbulent combustion. Mean mole fractions of all considered species and mean density are predicted if the laminar-Flame profiles of species mole fractions and density, respectively, are directly averaged using a Probability Density Function (PDF) P(c). The best predictions are obtained by extracting P(c) from the DNS data and defining c based on hydrogen mass fraction. These predictions suggest that mean mole fractions of various species in a Premixed Turbulent Flame can be evaluated at a post-processing stage of a CFD study by adopting P(c), obtained at the major stage of the simulations, to average a Flamelet library. When applied in such a way, the Flamelet paradigm is useful even for lean hydrogen-air Flames and even at Karlovitz number as large as 13. If the same PDF is applied to average reaction rates from the same Flamelet library, the mean rates of production/consumption of species n are poorly predicted, e.g. for radicals H, O, OH, HO2, and H2O2 if c is defined using hydrogen mass fraction. A hypothesis that conditioned rates 〈Wn|c〉 can be predicted using conditioned mole fractions 〈Xn|c〉, temperature 〈T|c〉, and density 〈ρ|c〉 is not supported either, e.g. for radicals O and OH. These differences between predictive capabilities of the first approach (directly averaging concentration profiles) and two other approaches (averaging reaction rates) are attributed to weakly (highly) non-linear dependencies of the concentrations (rates, respectively) on c.
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Assessment of a transport equation for mean reaction rate using DNS data obtained from highly unsteady Premixed Turbulent Flames
International Journal of Heat and Mass Transfer, 2019Co-Authors: A.n. Lipatnikov, Vladimir Sabelnikov, Alexei PoludnenkoAbstract:Recently, a transport equation for the mean rate of product creation in a Premixed Turbulent Flame was derived and a joint closure relation for dominant terms in that equation was developed and validated against Direct Numerical Simulation (DNS) data obtained from weakly Turbulent Flames. Present communication reports results of further DNS tests of this newly introduced approach, performed using recent data computed under substantially different Turbulent conditions. In particular, the ability of the approach to predict significant oscillations of the Turbulent burning velocity with time is shown under conditions associated with not only the Flamelet but also the thin-reaction-zone regimes of Premixed Turbulent burning. While such oscillations are documented in the present and other DNS studies, capabilities of other models of Premixed Turbulent combustion to reproduce this challenging phenomenon have not yet been demonstrated in the literature.
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transport equations for reaction rate in laminar and Turbulent Premixed Flames characterized by non unity lewis number
International Journal of Hydrogen Energy, 2018Co-Authors: A.n. Lipatnikov, Nilanjan Chakraborty, Vladimir SabelnikovAbstract:Abstract Transport equations for (i) the rate W of product creation and (ii) its Favre-averaged value W ˜ are derived from the first principles by assuming that W depends solely on the temperature and mass fraction of a deficient reactant in a Premixed Turbulent Flame characterized by the Lewis number L e different from unity. The right hand side of the transport equation for W ˜ involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum T Σ ¯ of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar Premixed Flames characterized by L e 1 are performed and the linear relation between T Σ integrated along the normal to a laminar Flame and a product of (i) the consumption velocity u c and (ii) the stretch rate s ˙ w evaluated in the Flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of T Σ ¯ ∝ ρ W s ˙ ¯ is hypothesized, where ρ is the density and s ˙ is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, Premixed Turbulent Flames in the case of a single-step chemistry and L e = 0.34 , 0.6, or 0.8. In all three cases, the agreement between T Σ ¯ and ρ W s ˙ ¯ extracted from the DNS is good with exception of large ( c ¯ > 0.4 ) values of the mean combustion progress variable c ¯ in the case of L e = 0.34 . The developed linear relation between T Σ ¯ and ρ W s ˙ ¯ helps to understand why the leading edge of a Premixed Turbulent Flame brush can control its speed.
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Transport equations for reaction rate in laminar and Turbulent Premixed Flames characterized by non-unity Lewis number
International Journal of Hydrogen Energy, 2018Co-Authors: Andrei N. Lipatnikov, Nilanjan Chakraborty, Vladimir SabelnikovAbstract:Transport equations for (i) the rate of product creation and (ii) its Favre-averaged value are derived from the first principles by assuming that depends solely on the temperature and mass fraction of a deficient reactant in a Premixed Turbulent Flame characterized by the Lewis number different from unity. The right hand side of the transport equation for involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar Premixed Flames characterized by are performed and the linear relation between integrated along the normal to a laminar Flame and a product of (i) the consumption velocity and (ii) the stretch rate evaluated in the Flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of is hypothesized, where is the density and is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, Premixed Turbulent Flames in the case of a single-step chemistry and , 0.6, or 0.8. In all three cases, agreement between and extracted from the DNS is good with exception of large () values of the mean combustion progress variable in the case of. The developed linear relation between and helps to understand why the leading edge of a Premixed Turbulent Flame brush can control its speed.
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SCALAR FLUX AT THE LEADING EDGE OF Premixed Turbulent Flame BRUSH
2013Co-Authors: A.n. Lipatnikov, Vladimir SabelnikovAbstract:It is widely accepted that Turbulent scalar flux can show the countergradient behavior almost everywhere within a Premixed Flame brush with the exception of a narrow zone at the leading edge (LE) of the Flame where the flux always shows the gradient behavior. Moreover, many experts consider the existence of such a zone to be of crucial importance in order for the Flame to be able to propagate into unburned mixture. The goal of the present work is to dispute this widely-recognized belief by studying an asymptotic case of density variations localized to infinitely thin, wrinkled Flamelets that separate unburned and burned mixture and self-propagate at a finite speed into the former mixture. First, simple mathematical and physical examples are discussed in order to argue that a Premixed Flame can propagate into unburned mixture even if averaged scalar flux does not show the gradient behaviour at the LE. This phenomenon is associated with the straightforward influence of large-scale velocity oscillations (i.e. Turbulent diusion as far as a Premixed Turbulent Flame is concerned) on the mean rate of product creation at the LE. Second, by considering a fully-developed, statistically stationary, planar, one-dimensional Turbulent Premixed Flame, the following criterion is obtained. Turbulent scalar flux shows the countergradient behavior at the LE if Turbulent burning velocity is less than the laminar Flame speed multiplied by the density ratio and by a factor that (i) is equal to unity if perturbations of the local burning rate in Flamelets are disregarded, but (ii) can substantially depend on the Lewis number and preferential diusion eects if such perturbations
I.g. Shepherd - One of the best experts on this subject based on the ideXlab platform.
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numerical simulation of lewis number effects on lean Premixed Turbulent Flames
Proceedings of the Combustion Institute, 2007Co-Authors: John B. Bell, R K Cheng, I.g. ShepherdAbstract:Abstract A dominant factor in determining the burning rate of a Premixed Turbulent Flame is the degree to which the Flame front is wrinkled by turbulence. Higher Turbulent intensities lead to greater wrinkling of the Flame front and an increase in the Turbulent burning rate. This picture of Turbulent Flame dynamics must be modified, however, to accommodate the affects of variations in the local propagation speed of the Flame front. Classical Flame analysis characterizes these local variations in propagation speed by the Markstein number which represents the response of the Flame front to curvature and strain. In this paper, we consider lean Premixed Flames for three different fuels having widely varying fuel Lewis numbers corresponding to widely varying Markstein numbers. In particular, we present numerical simulations of Premixed Turbulent Flames for lean hydrogen, propane and methane mixtures in two dimensions. Each simulation is performed at turbulence conditions similar to those found in laboratory-scale experiments and is performed using detailed chemical kinetics and transport properties. We discuss the effect of Lewis number on the overall Flame morphology and explore the dependence of local Flame propagation speed on Flame curvature. We also explore the relationship between local Flame speed and experimentally accessible variables such as OH concentration. Finally, we focus on the low Lewis number case, hydrogen, in which the Flame front is broken indicating local extinction.
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Flame front analysis of high-pressure Turbulent lean Premixed methane–air Flames
Proceedings of the Combustion Institute, 2005Co-Authors: Thierry Lachaux, Fabien Halter, Christian Chauveau, Iskender Gökalp, I.g. ShepherdAbstract:An experimental study on lean Turbulent Premixed methane–air Flames at high pressure is conducted by using a Turbulent Bunsen Flame configuration. A single equivalence ratio Flame at U = 0.6 is explored for pressures ranging from atmospheric pressure to 0.9 MPa. LDA measurements of the cold flow indicate that turbulence intensities and the integral length scale are not sensitive to pressure. Due to the decreased kinematic viscosity with increasing pressure, the Turbulent Reynolds numbers increase, and isotropic turbulence scaling relations indicate a large decrease of the smallest turbulence scales. Available experimental results and PREMIX code computations indicate a decrease in laminar Flame propagation velocities with increasing pressure, essentially between the atmospheric pressure and 0.5 MPa. The u0/SL ratio increases therefore accordingly. Instantaneous Flame images are obtained by Mie scattering tomography. The images and their analysis show that pressure increase generates small scale Flame structures. In an attempt to generalize these results, the variance of the Flamelet curvatures, the standard deviation of the Flamelet orientation angle, and the Flamelet crossing lengths have been plotted against Re1=2 t which is proportional to the ratio between the integral and Taylor length scales, and which increases with pressure. These three parameters vary linearly with the ratio between large and small turbulence scales and clearly indicate the strong effect of this parameter on Premixed Turbulent Flame dynamics and structure. An obvious consequence is the increase in Flame surface density and hence burning rate with pressure, as confirmed by its direct determination from 2D tomographic images.
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Premixed Turbulent Flame structures in moderate and intense isotropic turbulence
Combustion Science and Technology, 2002Co-Authors: Robert K. Cheng, I.g. Shepherd, B. Bédat, L. TalbotAbstract:Several 2-D imaging techniques including planar laser induced fluorescence for OH (OH-PLIF) have been used to investigate Premixed Turbulent Flame structures under moderate to intense isotropic turbulence. Unconditioned velocity statistics were measured by laser Doppler anemometry. The experiments used a low-swirl burner that produces high intensity near-isotropic turbulence. The goal is to gain better insights into the Flame structures at high turbulence and to test and verify the concept of the "distributed reaction zones" regime. Four methane/air Flames (φ=0.7) have been studied with 0.5< u ′
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PLIF Investigation of the Evolution of Premixed Turbulent Flame Structures
1996Co-Authors: B. Bédat, I.g. Shepherd, R K ChengAbstract:Planar Laser Induced Fluorescence (PLIF) technique of OH has been used to investigate Premixed Turbulent Flame structures under moderate and intense isotropic turbulence. The goal is to test and verify theoretical assumptions regarding classification of Premixed Turbulent Flames. The experiments use a weak-swirl burner that supports stable combustion in laminar and Turbulent Flames with incident turbulence intensities exceeding 20%. OH-PLIF results obtained for a Flame with Ka = 0.8 (i.e. corrugated Flamelet regime) show that the Flame forms deep Flame cusps but not Flame pockets. For a Flame with Ka = 3.1 (i.e. distributed reaction zone regime), the Flame front is more fragmented with pockets form both in the products and in the reactants. Sharp rises in the OH fluorescence intensity profiles deduced from both Flames suggest Flamelet behavior even for conditions well within the regime of {open_quotes}distributed reaction zones{close_quotes}.
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The influence of burner geometry on Premixed Turbulent Flame propagation
Combustion and Flame, 1991Co-Authors: Robert K. Cheng, I.g. ShepherdAbstract:Abstract The influence of burner geometry on Premixed Turbulent Flame propagation has been studied experimentally by investigating the Turbulent transport, turbulence production and burning rate in three Flame configurations: they are rod-stabilized v-Flames, tube stabilized conical Flames and Flames stabilized in a stagnation flow. In order to make comparisons of the three Flames with different Flame shapes and flow patterns, it was necessary to measure the velocity and scalar properties along flowlines through the Flame zone. Under similar flow and mixture conditions, the magnitudes of turbulence transport within the three Flames are significantly different. In contrast, the levels of turbulence production are more consistent and the turbulence kinetic energies evolve to the same level downstream of the Flame zone. The burning rates of the v-Flames and the conical Flames are found to be consistent with Turbulent-to-laminar-burning speed ratio of the stagnation flow stabilized Flames. These results show that turbulence transport processes are sensitive to the Flame geometry but the burning rates and turbulence production are not. The merits of these laboratory Flames for the development of current theoretical Turbulent combustion models are also discussed.
Yasuhiko Ohta - One of the best experts on this subject based on the ideXlab platform.
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Thermal-field Structure of a Non-Premixed Turbulent Flame Formed in a Strong Pressure-Gradient Flow
International Journal of Heat and Fluid Flow, 2005Co-Authors: Masato Tagawa, M. Fukatsu, Y. Nabata, Yasuhiko OhtaAbstract:Statistical characteristics of a non-Premixed Turbulent Flame formed in a curved-rectangular duct and spatio-temporal structures of the thermal field were investigated experimentally. The Flame was much affected by a strong pressure gradient in the radial direction of the duct curvature, which caused strong gradient diffusion in Turbulent heat transfer on the inner-wall side of the Flame and, in contrast, counter-gradient heat transfer on the outer-wall side. Two-point correlation measurement of temperature fields revealed that, in the strong gradient diffusion region, a spatial thermal pattern generated by Turbulent mixing of high- and low-temperature fluid parcels was advected downstream with little diffusion. In contrast, the pattern was attenuated and diffused rapidly in the counter-gradient diffusion region. These results accurately correspond to the generation mechanism of the counter-gradient heat transport so far observed in stably stratified Turbulent flows.
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thermal field structure and characteristics of unburned fuel emission of a Turbulent diffusion Flame under a strong pressure gradient
Transactions of the Japan Society of Mechanical Engineers. B, 2005Co-Authors: Masato Tagawa, M. Fukatsu, Takaya Katsuragawa, Yasuhiko OhtaAbstract:Spatio-temporal structures of the thermal field of a non-Premixed Turbulent Flame formed in a curved rectangular duct and the relevant unburned-fuel emission characteristics were investigated experimentally. The combustion field was dominated by a pressure gradient in the radial direction of the duct curvature, which caused strong gradient diffusion in Turbulent heat transfer on the inner-wall side of the Flame and, conversely, counter-gradient heat transfer on the outer-wall side. Two-point correlation measurement of temperature fields revealed that, in the strong gradient-diffusion region, a spatial thermal pattern generated by Turbulent mixing of high-and low-temperature fluid parcels was advected downstream with little diffusion. In contrast, the pattern was attenuated and rapidly diffused in the counter-gradient diffusion region. The behaviors of the unburned-fuel emission were also dominated by the pressure gradient and showed quite different characteristics between the inner- and outer-wall regions of the curved duct. In the downstream combustion field, the volume concentration of the unburned fuel remained much higher on the outer-wall side of the Flame than the inner-wall side. This characteristic conforms closely to the turbulence structures of the thermal field.
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Heat transfer characteristics of a non-Premixed Turbulent Flame formed in a curved rectangular duct
Combustion and Flame, 2002Co-Authors: Masato Tagawa, F. Matsubara, Yasuhiko OhtaAbstract:Abstract Heat transfer characteristics of a non-Premixed Turbulent Flame formed in a curved rectangular duct (180° bend) were investigated experimentally. Key turbulence quantities of velocity and thermal fields such as Reynolds stress components and Turbulent heat fluxes were measured using a combined LDV and fine-wire thermocouple technique. These measurements provided direct evidence of the occurrence of the anomalous phenomenon of counter-gradient heat transfer, which can be ascribed to the presence of a strong pressure-gradient in the radial direction of the curved duct. The experiment also revealed that the onset region of this “counter-gradient” diffusion was adjacent to the strong “gradient” diffusion region. The quantitative appraisal of the production terms for the Turbulent heat flux showed that the pressure gradient promoted gradient diffusion on the inner-wall (low-pressure) side of the curved-duct Flame and caused counter-gradient diffusion on the outer-wall (high-pressure) side. The schlieren photography for visualizing the density field showed a totally different behavior of the burned gas parcels between the high- and low-pressure sides of the Flame. The essential mechanism causing the counter-gradient diffusion can be explained by the unique motion of the high-temperature (low-density) gas parcel on the high-pressure side of the Flame. High-temperature fluid motions tend to be preferentially damped by the pressure gradient imposed on the flow field. The occurrence of the counter-gradient diffusion phenomenon will of course lead to the collapse of the “gradient-diffusion hypothesis,” on which most conventional turbulence models rely. In such a field, the analogy between heat and mass transfer processes, which holds almost always in normal Turbulent passive-scalar transport, can disappear.
Nilanjan Chakraborty - One of the best experts on this subject based on the ideXlab platform.
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transport equations for reaction rate in laminar and Turbulent Premixed Flames characterized by non unity lewis number
International Journal of Hydrogen Energy, 2018Co-Authors: A.n. Lipatnikov, Nilanjan Chakraborty, Vladimir SabelnikovAbstract:Abstract Transport equations for (i) the rate W of product creation and (ii) its Favre-averaged value W ˜ are derived from the first principles by assuming that W depends solely on the temperature and mass fraction of a deficient reactant in a Premixed Turbulent Flame characterized by the Lewis number L e different from unity. The right hand side of the transport equation for W ˜ involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum T Σ ¯ of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar Premixed Flames characterized by L e 1 are performed and the linear relation between T Σ integrated along the normal to a laminar Flame and a product of (i) the consumption velocity u c and (ii) the stretch rate s ˙ w evaluated in the Flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of T Σ ¯ ∝ ρ W s ˙ ¯ is hypothesized, where ρ is the density and s ˙ is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, Premixed Turbulent Flames in the case of a single-step chemistry and L e = 0.34 , 0.6, or 0.8. In all three cases, the agreement between T Σ ¯ and ρ W s ˙ ¯ extracted from the DNS is good with exception of large ( c ¯ > 0.4 ) values of the mean combustion progress variable c ¯ in the case of L e = 0.34 . The developed linear relation between T Σ ¯ and ρ W s ˙ ¯ helps to understand why the leading edge of a Premixed Turbulent Flame brush can control its speed.
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Transport equations for reaction rate in laminar and Turbulent Premixed Flames characterized by non-unity Lewis number
International Journal of Hydrogen Energy, 2018Co-Authors: Andrei N. Lipatnikov, Nilanjan Chakraborty, Vladimir SabelnikovAbstract:Transport equations for (i) the rate of product creation and (ii) its Favre-averaged value are derived from the first principles by assuming that depends solely on the temperature and mass fraction of a deficient reactant in a Premixed Turbulent Flame characterized by the Lewis number different from unity. The right hand side of the transport equation for involves seven unclosed terms, with some of them having opposite signs and approximately equal large magnitudes when compared to the left-hand-side terms. Accordingly, separately closing each term does not seem to be a promising approach, but a joint closure relation for the sum of the seven terms is sought. For this purpose, theoretical and numerical investigations of variously stretched laminar Premixed Flames characterized by are performed and the linear relation between integrated along the normal to a laminar Flame and a product of (i) the consumption velocity and (ii) the stretch rate evaluated in the Flame reaction zone is obtained. Based on this finding and simple physical reasoning, a joint closure relation of is hypothesized, where is the density and is the stretch rate. The joint closure relation is tested against 3D DNS data obtained from three statistically 1D, planar, adiabatic, Premixed Turbulent Flames in the case of a single-step chemistry and , 0.6, or 0.8. In all three cases, agreement between and extracted from the DNS is good with exception of large () values of the mean combustion progress variable in the case of. The developed linear relation between and helps to understand why the leading edge of a Premixed Turbulent Flame brush can control its speed.
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measurement of Flame surface density for Turbulent Premixed Flames using plif and dns
Proceedings of the Combustion Institute, 2007Co-Authors: Johan Hult, Nilanjan Chakraborty, Sara Gashi, M Klein, Karl W Jenkins, Stewart Cant, Clemens F KaminskiAbstract:Abstract Results for Flame surface density (FSD) in Premixed Turbulent Flame kernels have been obtained from OH planar laser induced fluorescence (PLIF) and direct numerical simulations (DNS), and have been compared for similar values of global Lewis number and normalised turbulence intensity. Stoichiometric methane–air and lean hydrogen–air mixtures were studied, and the same post-processing techniques were employed for both experimental and DNS data in order to evaluate FSD statistics from spatial gradients of the reaction progress variable. Full 3D FSD statistics were obtained from the DNS data sets. Also, FSD statistics were obtained from two-dimensional cross-sections extracted from the DNS data sets which were found to be in qualitative agreement with the FSD statistics of PLIF data. The location of maximum FSD within the Flame was found to be close to the middle of the Flame brush for both methane–air and hydrogen–air Flames, and was found to be slightly skewed about the middle of the Flame brush for some methane–air Flames. The PLIF data for both fuels showed a decrease in the maximum FSD with increasing turbulence intensity. This effect was not observed in the three-dimensional DNS analysis for methane–air Flames, but was found to be consistent with both two-dimensional and three-dimensional analysis of the DNS data for hydrogen–air Flames. The findings have been compared with the results of other experimental and DNS work reported in the literature and mechanisms have been suggested to explain the observed behaviour.