Non-premixed Combustion Model Research Articles

A robust and accurate algorithm of the β-pdf integration and its application to turbulent methane–air diffusion combustion in a gas turbine combustor simulator

The β-pdf has been widely assumed for the probability distribution of the mixture fraction in many turbulent mixing and turbulent non-premixed combustion models in the literature. The numerical integration of the β-pdf often encounters the singularity difficulties and only few publications have addressed this issue. An efficient, accurate and robust numerical treatment of the β-pdf integration was proposed. The present treatment of the β-pdf integration was implemented into a flamelet model to calculate turbulent methane–air combustion in a model gas turbine combustor. Numerical results obtained using the present β-pdf integration method and those based on the properties of the beta and gamma functions were compared to illustrate the accuracy of the present method. Effect of assuming the β-pdf to the mass-weighted pdf and unweighted pdf of the mixture fraction on the calculated density field was also investigated.

The effect of turbulence on mixing in prototype reaction‐diffusion systems

The effect of turbulence on mixing in prototype reaction-diffusion systems is analyzed here in the special situation where the turbulence is modeled ideally with two separated scales consisting of a large-scale mean flow plus a small-scale spatiotemporal periodic flow. In the limit of fast reaction and slow diffusion, it is rigorously proved that the turbulence does not contribute to the location of the mixing zone in the limit and that this mixing zone location is determined solely by advection of the large-scale velocity field. This surprising result contrasts strongly with earlier work of the authors that always yields a large-scale propagation speed enhanced by small-scale turbulence for propagating fronts. The mathematical reasons for these differences are pointed out. This main theorem rigorously justifies the limit equilibrium approximations utilized in non-premixed turbulent diffusion flames and condensation-evaporation modeling in cloud physics in the fast reaction limit. The subtle nature of this result is emphasized by explicit examples presented in the fast reaction and zero-diffusion limit with a nontrivial effect of turbulence on mixing in the limit. The situation with slow reaction and slow diffusion is also studied in the present work. Here the strong stirring by turbulence before significant reaction occurs necessarily leads to a homogenized limit with the strong mixing effects of turbulence expressed by a rigorous turbulent diffusivity modifying the reaction-diffusion equations. Physical examples from non-premixed turbulent combustion and cloud microphysics modeling are utilized throughout the paper to motivate and interpret the mathematical results. © 2000 John Wiley & Sons, Inc.

Spray vaporization in nonpremixed turbulent combustion modeling: a single droplet model

The injection of liquid fuel is a common procedure in turbulent combustion devices operating in the nonpremixed regime. Various numerical models may be found in the literature to calculate such turbulent flames, using either Reynolds averaged Navier-Stokes techniques (RANS) or large eddy simulation (LES). The typical inputs of nonpremixed turbulent combustion modeling are the mean and the fluctuations of the mixture fraction. In computational fluid dynamics codes, the mean source of mixture fraction may be provided by Euler-Lagrange spray modeling. However, the sources of fluctuations of mixture fraction due to vaporization require more closures. Direct numerical simulation (DNS) provides a way of estimating these sources and, using DNS of droplets evaporating in a turbulent flow, it is described how they play an important role in the time evolution of fuel/air mixing in a dilute spray. The statistical properties of the spray and of the scalar field are analyzed to propose a single droplet model (SDM) to evaluate these sources. SDM calculates mean values of the Eulerian source of fuel conditioned on the mixture fraction.

Regimes of non-premixed flame-vortex interactions

Detailed studies of flame-vortex interactions are extremely valuable to improve our understanding of turbulent combustion regimes. Combined experimental and numerical studies have already been performed in the premixed case during previous investigations. Therefore, we decided to carry out a detailed experimental investigation on the regimes observed during in teraction of a vortex ring and a non-premixed, diluted, hydrogen/air, laminar counterflow flame. To obtain the needed information, several optical diagnostic techniques have been used, in particular, planar laser-induced fluorescence (PLIF) of acetone to quantify vortex structure and speed, simultaneous OH PLIF and Rayleigh measurements, and simultaneous OH PLIF and particle-imaging velocimetry (PIV) measurements. A post-processing of the results combined with direct simulations using detailed chemistry and transport models to check the quality of the postprocessing procedures has led to the construction of a spectral interaction diagram. Eight interaction types were found, emphasizing the relative importance of competing physical phenomena such as straining, curvature, wrinkling, roll-up, and extinction. In particular, we observe two different types of extinction, one due to the combined action of curvature and straining, and the other purely due to straining effects. It was also observed that many vortices are too small or dissipate too rapidly to influence the flame. In other cases, the vortex ring can lead to the formation of pockets of oxidizer burning in the fuel part of the domain. These regimes and the limits between them have important implications for the modeling of turbulent non-premixed combustion.

The effect of turbulence on mixing in prototype reaction-diffusion systems

The effect of turbulence on mixing in prototype reaction-diffusion systems is analyzed here in the special situation where the turbulence is modeled ideally with two separated scales consisting of a large-scale mean flow plus a small-scale spatiotemporal periodic flow. In the limit of fast reaction and slow diffusion, it is rigorously proved that the turbulence does not contribute to the location of the mixing zone in the limit and that this mixing zone location is determined solely by advection of the large-scale velocity field. This surprising result contrasts strongly with earlier work of the authors that always yields a large-scale propagation speed enhanced by small-scale turbulence for propagating fronts. The mathematical reasons for these differences are pointed out. This main theorem rigorously justifies the limit equilibrium approximations utilized in nonpremixed turbulent diffusion flames and condensation-evaporation modeling in cloud physics in the fast reaction limit. The subtle nature of this result is emphasized by explicit examples presented in the fast reaction and zero-diffusion limit with a nontrivial effect of turbulence on mixing in the limit. The situation with slow reaction and slow diffusion is also studied in the present work. Here the strong stirring by turbulence before significant reaction occurs necessarily leads to a homogenized limit with the strong mixing effects of turbulence expressed by a rigorous turbulent diffusivity modifying the reaction-diffusion equations. Physical examples from non-premixed turbulent combustion and cloud microphysics modeling are utilized throughout the paper to motivate and interpret the mathematical results. c 2000 John Wiley & Sons, Inc.

96/05267 Laboratory study of N2O formation from 15N-isotope-marked NO while burning char particles at FBC conditions

Detailed studies of flame-vortex interactions are extremely valuable to improve our understanding of turbulent combustion regimes. Combined experimental and numerical studies have already been performed in the premixed case during previous investigations. Therefore, we decided to carry out a detailed experimental investigation on the regimes observed during in teraction of a vortex ring and a non-premixed, diluted, hydrogen/air, laminar counterflow flame. To obtain the needed information, several optical diagnostic techniques have been used, in particular, planar laser-induced fluorescence (PLIF) of acetone to quantify vortex structure and speed, simultaneous OH PLIF and Rayleigh measurements, and simultaneous OH PLIF and particle-imaging velocimetry (PIV) measurements. A post-processing of the results combined with direct simulations using detailed chemistry and transport models to check the quality of the postprocessing procedures has led to the construction of a spectral interaction diagram. Eight interaction types were found, emphasizing the relative importance of competing physical phenomena such as straining, curvature, wrinkling, roll-up, and extinction. In particular, we observe two different types of extinction, one due to the combined action of curvature and straining, and the other purely due to straining effects. It was also observed that many vortices are too small or dissipate too rapidly to influence the flame. In other cases, the vortex ring can lead to the formation of pockets of oxidizer burning in the fuel part of the domain. These regimes and the limits between them have important implications for the modeling of turbulent non-premixed combustion.

Lagrangian simulation of a thin non-premixed flame in the field of an asymmetric layer

Development of an analytical, subgrid-scale non-premixed combustion model for simulation of two-dimensional reacting shear flow at conditions of fast chemistry is described. The model is based on generalization of the classical one-dimensional flamelet representation to multi-dimensional flow conditions. and incorporation of resulting combustion model into adaptive, Lagrangian vortex element techniques. Evolution of the external flow field is computed by tracking the motion of vortex elements, while flame front topology is described by a collection of thin flamelets, which are also used to compute the unsteady response of the local flame structure to the prevailing strain. One of the interesting features of the model described herein is that, while maintaining computational efficiency, it does not oversimplify the flame structure and/or flow-field dynamics. In particular, all the essential features of flow-combustion interactions are retained, namely the effect of flow-induced stretch on the local flame structure and burning rates, and the impact of heat release on flow-field divergence and baroclinic vorticity generation. The efficiency of the numerical scheme is guaranteed by adaptively concentrating computational resources along the flame front and regions of finite vorticity. Implementation of the model is illustrated by computing the combustion field of a reacting layer in which the characteristic vorticity length scale is several orders of magnitude larger than the initial flame thickness.