Denis Veynante

Turbulent combustion modeling

Numerical simulation of flames is a growing field bringing important improvements to our understanding of combustion. The main issues and related closures of turbulent combustion modeling are reviewed. Combustion problems involve strong coupling between chemistry, transport and fluid dynamics. The basic properties of laminar flames are first presented along with the major tools developed for modeling turbulent combustion. The links between the available closures are illuminated from a generic description of modeling tools. Then, examples of numerical models for mean burning rates are discussed for premixed turbulent combustion. The use of direct numerical simulation (DNS) as a research instrument is illustrated for turbulent transport occurring in premixed combustion, gradient and counter-gradient modeling of turbulent fluxes is addressed. Finally, a review of the models for non-premixed turbulent flames is given.

A thickened flame model for large eddy simulations of turbulent premixed combustion

A subgrid scale model for large eddy simulations of turbulent premixed combustion is developed and validated. The approach is based on the concept of artificially thickened flames, keeping constant the laminar flame speed sl0. This thickening is simply achieved by decreasing the pre-exponential factor of the chemical Arrhenius law whereas the molecular diffusion is enhanced. When the flame is thickened, the combustion–turbulence interaction is affected and must be modeled. This point is investigated here using direct numerical simulations of flame–vortex interactions and an efficiency function E is introduced to incorporate thickening effects in the subgrid scale model. The input parameters in E are related to the subgrid scale turbulence (velocity and length scales). An efficient approach, based on similarity assumptions, is developed to extract these quantities from the resolved velocity field. A specific operator is developed to exclude the dilatational part of the velocity field from the estimation of...

Vortex-driven acoustically coupled combustion instabilities

Combustion instability is investigated in the case of a multiple inlet combustor with dump. It is shown that low-frequency instabilities are acoustically coupled and occur at the eigenfrequencies of the system. Using spark-schlieren and a special phase-average imaging of the C 2 -radical emission, the fluid-mechanical processes involved in a vortex-driven mode of instability are investigated. The phase-average images provide maps of the local non-steady heat release. From the data collected on the combustor the processes of vortex shedding, growth, interactions and burning are described. The phases between the pressure, velocity and heat-release fluctuations are determined. The implications of the global Rayleigh criterion are verified and a mechanism for low-frequency vortex-driven instabilities is proposed.

Quenching processes and premixed turbulent combustion diagrams

The structure of premixed turbulent flames is a problem of fundamental interest in combustion theory. Possible flame geometries have been imagined and diagrams indicating the corresponding regimes of combustion have been constructed on the basis of essentially intuitive and dimensional considerations. A new approach to this problem is described in the present paper. An extended definition of flamelet regimes based on the existence of a continuous active (not quenched) flame front separating fresh gases and burnt products is first introduced. Direct numerical simulations of flame/vortex interactions using the full Navier–Stokes equations and a simplified chemistry model are then performed to predict flame quenching by isolated vortices. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effect of stretch, curvature, transient dynamics and viscous dissipation can be accounted for. As a result, flame quenching by vortices (which is one of the key processes in premixed turbulent combustion) may be computed accurately. The effects of curvature and viscous dissipation on flame/vortex interactions may also be characterized by the same simulations. The influence of non-unity Lewis number and of thermo-diffusive processes in turbulent premixed combustion is discussed by comparing flame responses for two values of the Lewis number ( Le = 0.8 and 1.2). An elementary (‘spectral’) diagram giving the response of one flame to a vortex pair is constructed. This spectral diagram is then used, along with certain assumptions, to establish a turbulent combustion diagram similar to those proposed by Borghi (1985) or Williams (1985). Results show that flame fronts are much more resistant to quenching by vortices than expected from the classical theories. A cut-off scale and a quenching scale are also obtained and compared with the characteristic scales proposed by Peters (1986). Results show that strain is not the only important parameters determining flame/vortex interaction. Heat losses, curvature, viscous dissipation and transient dynamics have significant effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. It is found, for example, that the Klimov–Williams criterion which is generally advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.

A power-law flame wrinkling model for LES of premixed turbulent combustion Part I: non-dynamic formulation and initial tests

Abstract A model of turbulent sub-grid scale flame speed for premixed combustion is proposed and tested in Large Eddy Simulation (LES). The model is based on writing the unresolved flame surface density in terms of a general power-law expression that involves an inner cutoff scale. This scale is derived from an equilibrium assumption of flame-surface production and destruction. The flame-surface production term is modeled using a parameterization of the effective flame stretch, obtained from a spectral superposition of earlier DNS results of single vortex-flame interactions [8] . The model is implemented in a LES combustion code in the context of Thickened Flame LES (TF-LES) using an empirically chosen (non-dynamic) value for the power-law exponent. Three-dimensional simulations of premixed flame embedded in a time decaying isotropic turbulent flow are performed in several different parameter ranges. Comparisons between direct numerical simulation (DNS) and LES using different resolutions and thickness factors show that the LES reproduces the total reaction rate of the DNS quite well and independently on the thickness factor, resolution, and sub-grid scale model used for the turbulent eddy viscosity. Comparisons between the predicted overall turbulent flame speed sT as function of the r.m.s. velocity and experimental data show good agreement over a significant range of parameters.

Transformation thermodynamics: cloaking and concentrating heat flux

We adapt tools of transformation optics, governed by a (elliptic) wave equation, to thermodynamics, governed by the (parabolic) heat equation. We apply this new concept to an invibility cloak in order to thermally protect a region (a dead core) and to a concentrator to focus heat flux in a small region. We finally propose a multilayered cloak consisting of 20 homogeneous concentric layers with a piecewise constant isotropic diffusivity working over a finite time interval (homogenization approach).

A comparison of flamelet models for premixed turbulent combustion

Abstract Five flamelet models for premixed turbulent combustion are described and compared in the case of a one-dimensional turbulent flame propagating in frozen turbulence. This simple configuration allows ananalytical solution (KPP) to be obtained as devised by Kolmogorov, Petrovski, and Piskunov and performed by Hakberg and Gosman [1]. The explicit solution obtained by this analysis provides the turbulent flame speed as a function of the model parameters and of the turbulence characteristics. These results are compared with experimental data of Abdel-Gayed et al. [2] and with results obtained with the classical eddy break-up model. The realizability of the models is also studied. Recent models based on direct numerical simulation results correctly predict the “bending” of the turbulent burning velocity U T as a function of the RMS turbulent velocity u ′ as well as the total quenching of the flame in sufficiently intense turbulence. It is shown that this bending effect may be obtained without taking into account strain effects on the laminar burning velocity. However, no model is able to reproduce all data of Abdel-Gayed and Bradley in a satisfactory way. The reasons for these discrepancies are discussed.

Stabilization of a Turbulent Premixed Flame Using a Nanosecond Repetitively Pulsed Plasma

A nanosecond repetitively pulsed plasma (NRPP) produced by electric pulses of 10 kV during 10 ns at a frequency of up to 30 kHz has been used to stabilize and improve the efficiency of a 25-kW lean turbulent premixed propane/air flame (ReD=30000) at atmospheric pressure. We show that, when placed in the recirculation zone of the flow, the plasma significantly increases the heat release and the combustion efficiency, thus allowing to stabilize the flame under lean conditions where it would not exist without plasma. Stabilization is obtained with a very low level of plasma power of about 75 W, or 0.3% of the maximum power of the flame. In addition, they find that at high flow rates, where the flame should normally blow out, the NRPP allows the existence of an intermittent V-shaped flame with significant heat release, and at even higher flow rates the existence of a small dome-shaped flame confined near the electrodes that can serve as a pilot flame to reignite the combustor. Optical emission spectroscopy measurements are presented to determine the temperature of the plasma-enhanced flame, the electron number density, and to identify the active species produced by the plasma, namely O, H, and OH

A power-law flame wrinkling model for LES of premixed turbulent combustion Part II: dynamic formulation

Abstract A new power-law model of flame wrinkling for LES of premixed turbulent combustion has been proposed (Part I, [1] ). In the present paper, a dynamic formulation is developed to obtain the required power-law exponent from test-filtering the resolved scales during the simulation. First, it is shown that when the dynamic procedure is used to determine unknown multiplicative model coefficients that occur in classical models for premixed combustion, the approach is ill-posed asymptotically. Instead, it is well posed in formulations that aim to find unknown scaling exponents, this observation is used presently to formulate a new power-law dynamic model. The model is implemented in a LES combustion code in the context of Thickened Flame LES (TF-LES). Three-dimensional simulations of premixed flame in decaying isotropic turbulent flow are performed in several different parameters ranges. Comparisons between direct numerical simulation (DNS) and LES using different resolutions and thickness factors show that the proposed dynamic procedure allows the LES to reproduce the total reaction rate of the DNS quite well and irrespective of thickness factor and test-filter size. Comparisons between predicted overall turbulent flame speed s T as function of the r.m.s. velocity and experimental data show good agreement over a significant range of parameters. The simulations show that the exponent is not constant, but depends on time, turbulence level, and Reynolds number.

Diagrams of premixed turbulent combustion based on direct simulation

The structure and morphology of premixed turbulent flames is a problem of fundamental interest in combustion theory. Diagrams indicating the typical structure of a flame submitted to a given turbulent flow have been constructed on the basis of essentially intuitive and dimensional considerations. Knowing the turbulence integral scale and the turbulent kinetic energy, these diagrams indicate if the flow will feature flamelets, pockets or distributed reaction zones. A new approach to this problem is described in the present paper. The method is based on direct numerical simulations of flame/vortex interactions. The interaction of a laminar flame front with a vortex pair is computed using the full Navier-Stokes equations. The formulation includes non-unity Lewis number, non-constant viscosity and heat losses so that the effects of stretch, curvature, transient dynamics and viscous dissipation can be accounted for. As a result, flame quenching by vortices (which is one of the key-processes in premixed turbulent combustion) may be computed accurately. An elementary (‘spectral’) diagram giving the response of one flame to a vortex pair is constructed. This spectral diagram is then used along with certain assumptions to establish a turbulent combustion diagram similar to those proposed by Borghi 2 or Williams 5 . Results show that flame fronts are more resistant to quenching by vortices than expected from the classical theories. A cut-off scale and a quenching scale are also obtained and compared to the characteristic scales proposed by Peters. 3 Stretch is not the only important parameter determining flame/vortex interaction. Curvature, viscous dissipation and transient dynamics have large effects, especially for small scales and they strongly influence the boundaries of the combustion regimes. For example, the Klimov-Williams criterion which has been advocated to limit the flamelet region, underestimates the size of this region by more than an order of magnitude.