Showing papers by "Bart Merci published in 2010"

Generalised langevin model in correspondence with a chosen standard scalar-flux second-moment closure

Abstract: In the context of transported joint velocity-scalar probability density function methods, the correspondence between Generalised Langevin Models (GLM) for Lagrangian particle velocity evolution and Eulerian Reynolds-stress turbulence models has been established in the 1990’s by S.B. Pope. It was shown that the GLM representation of a given Reynolds stress model is not unique. It was also shown that a given GLM together with a given mixing model for particle composition evolution implies a differential scalar-flux model. In this paper, we study how extra constraints can be applied on the choice of the GLM coefficients in order to imply a chosen scalar-flux model. This correspondence between GLM-implied and standard scalar-flux models is based on the linear relaxation term and on the mean velocity gradient contributions in the rapid term. In general, GLM-implied models possibly involve more terms (including anisotropy effects in the scalar-flux decay rate and some high-order terms in the rapid-pressure-scrambling term). The proposed form of the GLM supposes a non-constant value for the diffusion coefficient C 0, originally identified as a Kolmogorov constant. Here, the value of C 0 is determined in order to yield the Monin model for linear relaxation of the scalar-flux, and the constant in the rapid-pressure contribution is related to the choice of the parameter β ∗ in the GLM. We finally show how GLM-implied scalar-flux models are in general dependent on the choice of the mixing model and how the proposed GLM can reduce this dependency. These developments are illustrated by results obtained from calculations of the Sydney bluff-body stabilised flame HM1.

Modeling of Turbulent Combustion

Abstract: In turbulent combustion, spatial and temporal fluctuations always play a predominant role. Along with the strong nonlinearity in combustion physics and chemistry, for example, in expressions for reaction rates in terms of species concentrations and temperature (and pressure), this results in a nontrivial closure problem. In this chapter, turbulence modeling and turbulent scales are first briefly discussed, after which details of the two extreme combustion configurations – premixed combustion and nonpremixed combustion – are outlined. Finally, some issues are noted on partially premixed combustion. The text is restricted to single-phase combustion and, for turbulent premixed combustion, the relevant time and length scales are discussed in typical turbulent flame structures. Based on this, the combustion regime diagram is introduced, turbulent flame speed is discussed, and progress variable formalism described. Fast chemistry and finite rate chemistry effects, with associated closure issues, are also considered. For nonpremixed combustion, the eddy break-up model and the eddy dissipation concept are introduced, after which the mixture fraction approach is explained. The preassumed probability density function (PDF) method is introduced, the laminar flamelet concept discussed, and the importance of the scalar dissipation rate highlighted. Finite rate chemistry effects are also discussed. Finally, the transported PDF and conditional moment closure (CMC) frameworks are introduced. Keywords: Turbulent combustion modeling; premixed flame structure; nonpremixed flame structure; combustion regimes; turbulent combustion scales; probability density function

Simulations of upward flame spread by coupling a pyrolysis model with a cfd calculation

Abstract: In simulations of developing fires in enclosures, a correct prediction of flame spread is crucial in the prediction of the fire growth rate. The simulation of this phenomenon requires two different kinds of solvers: one that deals with the solid phase, to determine the thermal degradation and volatile release (pyrolysis); a second one that calculates the combustion process in the flame formed by mixing of the volatiles with the ambient air. The processes in the solid and the gas phase require proper modeling. Moreover, an adequate coupling strategy at the interface between the solid and the gas phase is an important point of interest. The paper focuses on a method to successfully combine these two types of calculations. Two types of flame spread (upward and downward) demonstrate the possibilities of this approach. We conclude that the coupling between the two phases works fine, providing a mechanism of simulating flame spread with two different codes. It is found, however, that the prediction of the gas phase requires an appropriate choice of the CFD sub-models, particularly in case of upward flame spread. The impact of different turbulence models illustrates that totally different flame dynamics can be predicted.