Dissipation and dilatation rates in premixed turbulent flames (original) (raw)
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Energies
Three-dimensional Direct Numerical Simulations (DNS) data has been utilised to analyse statistical behaviours of the scalar dissipation rate (SDR) and its transport for homogeneous methane-air mixture turbulent Moderate or Intense Low oxygen Dilution (MILD) combustion for different O2 dilution levels and turbulence intensities for different reaction progress variable definitions. Additional DNS has been conducted for turbulent premixed flames and passive scalar mixing for the purpose of comparison with the SDR statistics of the homogeneous mixture MILD combustion with that in conventional premixed combustion and passive scalar mixing. It has been found that the peak mean value of the scalar dissipation rate decreases with decreasing O2 concentration for MILD combustion cases. Moreover, SDR magnitudes increase with increasing turbulence intensity for both MILD and conventional premixed combustion cases. The profiles and mean values of the scalar dissipation rate conditioned upon the ...
A Model for Calculating Heat Release in Premixed Turbulent Flames
Combustion and Flame, 1998
A unified reaction model, being valid in all turbulent combustion regimes, has been developed and tested. Based on a thoroughly validated model for the turbulent burning velocity the Kolmogorov, Petruvski and Piskunuv (KPP) theorem was applied, thus leading to the formulation of the mean reaction rate as a function of local turbulence and kinetic parameters in the flow. Numerical calculations, comprising all flame structures (0.2 Ͻ Da t Ͻ 2.8) of premixed flames were performed and compared with experimental data. At first, an unsteady spherical flame, propagating in a defined turbulence field was simulated, thus emphasizing the correct representation of the turbulent burning velocity. Thereafter, the mean flame lengths of two stationary, highly turbulent Bunsen-type flames were compared with experiments, varying the affecting turbulence parameters. The last premixed example was a strongly swirling flame in a combustor, related to kinetically controlled flame structures. Although those very different flame configurations cover a broad range in the Borghi diagram, the agreement with measured temperature profiles was found satisfactory. Finally, the model was extended to partially premixed systems by accounting for the influence of the mean mixture fraction on local kinetic parameters. The predictions of stability limits of unconfined, strongly swirling flames show satisfying agreement with experiments, supporting the earlier formulated opinion [25] that the blow-off limits of strongly swirling flames are determined by chemical kinetic limitation of the mean reaction rate. The results demonstrate the high performance of the reaction model and recommend its application also in complex 3-D flows, because of its simplicity and numerical robustness even in conjunction with higher order turbulence models.
Experimentally measured burning rates of premixed turbulent flames
Symposium (international) on Combustion, 1996
Integrated burning rates of premixed flames are measured in turbulent opposed axisymmetric flows. The measurement technique is based on the conservation of reactant mass flowing in and out of a control volume. The flow velocities are measured by laser Doppler velocimetry (LDV) using an oil seed to track only reactants. Simultaneous Mie scattering from the LDV probe volume is used to measure the mean progress variable. The mean mass flux of reactants is calculated by combining the velocity and progress variable data. Sixteen experiments were conducted to study the effects of varying turbulent kinetic energy, bulk strain rate, and equivalence ratio of methane-air flames. Experiments were compared in terms of integrated mean rates of creation of products across the flame brush, which have been expressed as turbulent burning speeds.The turbulent burning speed was observed to increase with increased levels of turbulent kinetic energy but also decrease because of the effects of turbulent and bulk straining on the flame. Mixtures near stoichiometric were sufficiently robust that their turbulent burning speed increased monotonically within the range of turbulence tested. Leaner mixtures showed an initial increase in burning speed that became impaired at higher turbulence and bulk strain rates. The integrated burning rates were shown to be two to six times less than that estimated from measured propagation speeds. These results emphasize the differences that exist between propagation speed and burning rate in diverging flows. A refinement to this experiment is proposed to allow the local mean rate of creation of products to be measured.
Scalar dissipation and mean reaction rates in premixed turbulent combustion
Combustion and Flame, 2011
A general equation for a variance parameter, appearing as a crucial quantity in a simple algebraic expression for the mean chemical rate, is derived. This derivation is based on a flamelet approach to model a turbulent premixed flame, for high but finite values of the Damköhler number. Application of this equation to the case of a planar turbulent flame normal to the oncoming flow of reactants gives good agreement with DNS data corresponding to three different values of the Damköhler number and two values of the heat release parameter.
Flow, Turbulence and Combustion, 2005
Direct numerical simulation is a very powerful tool to evaluate the validity of new models and theories for turbulent combustion. In this paper, direct numerical simulations of spherically expanding premixed turbulent flames in the thin reaction zone regime and in the broken reaction zone regime are performed. The flamelet-generated manifold method is used in order to deal with detailed reaction kinetics. The computational results are analyzed by using an extended flame stretch theory. It is investigated whether this theory is able to describe the influence of flame stretch and curvature on the local burning velocity of the flame. It is found that if the full profiles of flame stretch and curvature through the flame front are included in the theory, the local mass burning rate is well predicted. The influence of using a reduced chemistry model is investigated by comparing flamelet simulations with reduced and detailed chemistry. Adding a second dimension to the flamelet-generated manifold increases the accuracy of the reduced model with an order of magnitude.
Journal of Fluid Mechanics, 1982
To study effects of flow inhomogeneities on the dynamics of laminar flamelets in turbulent flames, with account taken of influences of the gas expansion produced by heat release, a previously developed theory of premixed flames in turbulent flows, that was based on a diffusive-thermal model in which thermal expansion was neglected, and that applied to turbulence having scales large compared with the laminar flame-thickness, is extended by eliminating the hypothesis of negligible expansion and by adding the postulate of weak-intensity turbulence. The consideration of thermal expansion motivates the formal introduction of multiple-scale methods, which should be useful in subsequent investigations. Although the hydrodynamic-instability mechanism of Landau is not considered, no restriction is imposed on the density change across the flame front, and the additional transverse convection correspondingly induced by the tilted front is described. By allowing the heat-to-reactant diffusivity...
A flamelet approach is adopted in a study of the factors affecting the volumetric heat release source term in turbulent combustion. This term is expressed as the product of an instability enhanced burning rate factor, P bi , and the mean volumetric heat release rate in an unstretched laminar flamelet of the mixture. Included in the expression for P bi are a pdf of the flame stretch rate and a flame stretch factor. Fractal considerations link the turbulent burning velocity normalised by the effective rms turbulent velocity to P bi . Evaluation of this last parameter focuses on problems of (i) the pdfs of the flame stretch rate, (ii) the effects of flame stretch rate on the burning rate, (iii) the effects of any flamelet instability on the burning rate, (iv) flamelet extinctions under positive and negative flame stretch rates, and (v) the effects of the unsteadiness of flame stretch rates. The Markstein number influences both the rate of burning and the possibility of flamelet instabilities developing which, through their ensuing wrinkling, increase the burning rate. The flame stretch factor is extended to embrace potential Darrieus-Landau thermodiffusive flamelet instabilities. A major limitation is the insufficient understanding of the effects of negative stretch rates that might cause flame extinction. The influences of positive and negative Markstein numbers are considered separately. For the former, a computed theoretical relationship for turbulent burning velocity, normalised by the effective rms velocity, is developed which, although close to that measured experimentally, tends to be somewhat lower at the higher values of the Karlovitz stretch factor. This might be attributed to reduced flame extinction and reduced effective Markstein numbers when the increasingly nonsteady conditions reduce the ability of the flame to respond to changes in flame stretch rates. As the pressure increases, Markstein numbers decrease. For negative Markstein numbers the predicted values of P bi and turbulent burning velocity are significantly increased above the values for positive Markstein numbers. This is confirmed experimentally and these values are close to those predicted theoretically. The increased values are due to the greater stretch rate required for flame extinction, the increased burning rate at positive values of flame stretch rate, and, in some instances, the development of flame instabilities. At lower values of turbulence than those covered by these computations, burning velocities can be enhanced by flame instabilities, as they are with laminar flames, particularly at negative Markstein numbers.
Combustion Science and Technology
The statistical behavior and modeling of scalar dissipation rate (SDR) transport for head-on quenching of turbulent premixed flames by an inert isothermal wall have been analyzed in the context of Reynolds averaged Navier-Stokes simulations based on three-dimensional simple chemistry direct numerical simulation (DNS) data. It has been found that the density variation, scalar-turbulence interaction, reaction rate gradient, molecular diffusivity gradient, and molecular dissipation terms, i.e., T 2 ; T 3 ; T 4 ; f D ð Þ, and ÀD 2 ð Þ, respectively, act as leading order contributors to the SDRε c transport away from the wall and the turbulent transport and molecular diffusion terms remain negligible in comparison to the other terms. The leading order contributors to the SDR transport have been found to be in a rough equilibrium away from the wall before the quenching is initiated but this equilibrium is not maintained during flame quenching. The predictions of the existing models for the unclosed terms of the SDR transport equation have been assessed with respect to the corresponding quantities extracted from DNS data. No existing models have been found to predict the near-wall behavior of the unclosed terms of the SDR transport equation. The models, which exhibit the most satisfactory performance away from the wall, have been modified to account for near-wall behavior in such a manner that the modified models asymptotically approach the existing model expressions away from the wall.
Scalar dissipation rate and scales in swirling turbulent premixed flames
Proceedings of the Combustion Institute, 2017
Simultaneous Rayleigh scattering and OH-PLIF imaging measurements of temperature and OH were used to investigate the properties of turbulent premixed flames, including the nature of the 2D thermal structures and scalar dissipation rate in the Cambridge/Sandia swirling bluff body stabilized flames, with and without the effect of swirl. Swirl creates enhanced turbulence as well as outer flow entrainment, and disrupts the pre-flame zone significantly, whilst the high temperature reaction zone as marked by OH remains relatively intact. In particular, the temperature at the location of maximum OH gradient shows very low variance across the flame region. The 2D image analysis of OH and temperature shows that the corresponding 2D gradients are aligned up to a distance of half the laminar flame thickness away from the flame front, deviating significantly in the case of swirling flames beyond that region. As in previous investigations in diffusion flames, the mean width of the observed thermal structures increases from 300 to 600 microns near the flame, with a main mode around the laminar flame thermal width in the unswirled case. The correlation between 2D thermal fluctuation gradients and variance extracted from the images shows a direct proportionality, with a slope which agrees well with theory in the region of high turbulence away from the base. At the