Luc Vervisch

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.

Effects of heat release on triple flames

Heat release effects on laminar flame propagation in partially premixed flows are studied. Data for analysis are obtained from direct numerical simulations of a laminar mixing layer with a uniformly approaching velocity field. The structure that evolves under such conditions is a triple flame, which consists of two premixed wings and a trailing diffusion flame. Heat release increases the flame speed over that of the corresponding planar premixed flame. In agreement with previous analytical work, reductions in the mixture fraction gradient also increase the flame speed. The effects of heat release and mixture fraction gradients on flame speed are not independent, however; heat release modifies the effective mixture fraction gradient in front of the flame. For very small mixture fraction gradients, scaling laws that determine the flame speed in terms of the density change are presented.

Three-dimensional boundary conditions for direct and large-eddy simulation of compressible viscous flows

Navier-Stokes characteristic boundary conditions (NSCBC) usually assume the flow to be normal to the boundary plane. In this paper, NSCBC is extended to account for convection and pressure gradients in boundary planes, resulting in a 3D-NSCBC approach. The introduction of these additional transverse terms requires a specific treatment for the computational domains edges and corners, as well as a suited set of compatibility conditions for boundaries joining regions associated to different flow properties, as inlet, outlet or wall. A systematic strategy for dealing with edges and corners is derived and compatibility conditions for inlet/outlet and wall/outlet boundaries are proposed. Direct numerical simulation (DNS) tests are carried out on simplified flow configurations at first. 3D-NSCBC brings a drastic reduction of flow distortion and numerical reflection, even in regions of strong transverse convection; the accuracy and convergence rate toward target values of flow quantities is also improved. Then, 3D-NSCBC is used for large-eddy simulation (LES) of a free jet and an impinging round-jet. Edge and corner boundary treatment, combining multidirectional characteristics and compatibility conditions, yields stable and accurate solutions even with mixed boundaries characterized by bad posedness issues (e.g. inlet/outlet). LES confirms the effectiveness of the proposed boundary treatment in reproducing mean flow velocity and turbulent fluctuations up to the computational domain limits.

Analysis of weakly turbulent dilute-spray flames and spray combustion regimes

Spray combustion is analysed using a full simulation of the continuous gaseous carrier phase, while dilute-spray modelling is adopted for the discrete phase. The direct numerical simulation of the flow is performed in an Eulerian context and a Lagrangian description is used for the spray. The numerous physical parameters controlling spray flames are first studied to construct two synthetic model problems of spray combustion: a laminar spray flame that propagates freely over a train of droplets and a weakly turbulent spray-jet with coflowing preheated air. It is observed that the flame structures can be classified with respect to three dimensionless quantities, which characterize the fuel/air equivalence ratio within the core of the spray-jet, the ratio between the mean distance between the droplets and the flame thickness, and the ratio between an evaporation time and a flame time. A large variety of reaction zone topologies is found when varying those parameters, and they are scrutinized by distinguishing between premixed and diffusion combustion regimes. Partially premixed combustion is observed in most of the spray-jet flames and the spray parameters that make the flame transition from non-premixed to premixed combustion are determined. A combustion diagram for dilute-spray combustion is then proposed from the identification of those various regimes.

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

Abstract 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.

Partially premixed flamelets in LES of nonpremixed turbulent combustion

Partially premixed flames are observed in nonpremixed turbulent combustion when fuel and oxidizer have mixed before burning. This combustion regime combines the properties of both premixed and diffusion flames. A procedure based on the resolved fields is proposed to associate premixed and diffusion flame descriptions in large eddy simulation. Using basic and well known subgrid modelling of premixed and diffusion flames, the proposed methodology is tested for flames lifted in a two-dimensional turbulent wake. Very recent experimental observations concerning the dynamics of the flow field at the turbulent flame base are reproduced.

Surface density function in premixed turbulent combustion modeling, similarities between probability density function and flame surface approaches

To characterize the dynamics and the physical properties of isoconcentration surfaces of a random reactive scalar field, an instantaneous isosurface quantity and its transport equation are introduced. For turbulent flows, the mean area of isoconcentration surface per unit volume is studied through its transport equation derived by using the probability density function (pdf) formalism. This approach allows the value of the reactive scalar used to define the level surface to be treated as an independent variable. It also leads to the definition of a surface density function. The developed formalism is applied to premixed turbulent combustion and a bridge is built between modeling approaches based on pdf, and others based on the flame surface concept.

Three facets of turbulent combustion modelling: DNS of premixed V-flame, LES of lifted nonpremixed flame and RANS of jet-flame

Three aspects of turbulent combustion modelling are discussed to provide an overview of numerical simulation of turbulent flames. The three examples reported concern direct numerical simulation (DNS), large eddy simulation (LES) and Reynolds average Navier–Stokes (RANS) calculations. Recent developments in DNS deal with the possibility of performing a full simulation of a premixed turbulent V-flame evolving in grid turbulence. The DNS data are useful to improve modelling of turbulent micromixing, in terms of the scalar dissipation rate of a reaction progress variable. Many combustion systems operate with reactants that have been partially premixed by unsteady large-scale motions. In this context, LES of partially premixed turbulent-lifted flame bases are reported, with a subgrid procedure that accounts for the combination of premixed and nonpremixed combustion regimes observed in such flames. Then, some developments are proposed to improve the prediction capabilities of RANS methods applied to complex com...

Edge flames and partially premixed combustion in diffusion flame quenching

The aim is to focus on the development of partially premixed combustion after diffusion flame quenching. To this end, the quenching of a planar two-dimensional diffusion flame is studied by using numerical simulation. A flame hole is obtained by submitting the reaction zone to a high strain and scalar dissipation rate resulting from the interaction between vorticity and upstream triple flame which stabilizes the diffusion flame. The set of control parameters is chosen so that effects of unsteadiness are not expected during quenching, thus, extinction appears for a scalar dissipation rate that is well predicted by one-dimensional flamelet theory. Backward propagating edge flames develop at both extremities of the quenched zone, whereas the combustion regime evolves from diffusion to partially premixed. From the results and the transport equation for a partially premixed fraction, a cross-scalar dissipation rate is introduced as a direct measure of the extent of partial premixing in non-premixed systems. For unity Lewis number, it is shown that the maximum burning rate measured in a one-dimensional planar stoichiometric premixed flame may be used as a reference for a diffusion flame close to extinction and also later when edge flames and triple flames are formed. Finally, the simulations suggest that the scalar dissipation rate controlling the growth of the flame hole is lower than the one that should be applied to first quench the flame.

Effects of heat release in laminar diffusion flames lifted on round jets

Laminar diffusion flames lifted on round jets are simulated using high order accurate numerical schemes. The results are examined in the light of analytical approximations of lift-off heights. A large variety of flame base topologies are observed when the fuel jet velocity is varied. Edge-flames, or triple-flames, progressively evolve into weakly varying partially premixed fronts, before blow-out occurs. The flame base is located on the stoichiometric surface at the point where the flow velocity is of the order of the stoichiometric and planar premixed flame burning velocity. In the simulations, this stabilization point is positioned further upstream than predicted by a frozen flow mixing description of the jet, even when effects of heat release and strain rate are included in the approximation of the triple-flame speed that is used to predict the lift-off height. The numerical results therefore suggest that the well known flow deflection, induced by heat release, brings the flame much closer to the burner than expected. Heat release is found to have a much stronger effect in the round jet than in the previously studied planar mixing layer. In the axisymmetric problem, this is attributed to the intricate coupling between the flow deflection and the position of iso-mixture fraction surfaces relatively to iso-velocity surfaces. Heat release also makes the flame base more robust than predicted by cold flow theory and helps to sustain large velocities before reaching the blow-out condition. Results suggest that the prediction of lift-off height cannot be reached without carefully accounting for the effect of heat release on the flow upstream of the flame base.