Hybrid RANS/PDF calculations of sydney swirling flames

In this work, we perform steady 2D axisymmetric RANS and hybrid RANS/PDF calculations to predict the turbulent flow and mixing fields of swirling inert flows and flames. The cases studied, N29S054 and SM1 respectively, are bluff body burner flows, studied experimentally at Sydney University. Turbulence is modeled with a non-linear k-ε type model, taking into account effects of rotation and streamline curvature on the turbulence. Flow field predictions are in reasonable agreement with experimental data. For the reacting flow, agreement for mean mixture fraction and mixture fraction variance with experimental results is less satisfactory. Yet, the mean temperature field is quite well reproduced. We compare presumed and transported scalar PDF simulation results, with the same laminar flamelet model for chemistry. The influence of the micro-mixing model is small in our case. The mixing model constant Cφ, has a stronger influence, through the mixture fraction variance

Joint scalar PDF simulations of a bluff-body stabilised flame with the REDIM approach

Transported joint scalar probability density function (PDF) results are presented for ‘Sydney Flame HM3’, a jet type turbulent flame with strong turbulence – chemistry interaction, stabilized behind a bluff body. We apply the novel Reaction-Diffusion Manifold (REDIM) technique, by which a detailed chemistry mechanism is reduced, including diffusion effects. Only N2 and CO2 mass fractions are used as reduced coordinates. A second-moment closure RANS turbulence model is applied. As micro-mixing model, the modified Curl’s coalescence/dispersion (CD) and the Euclidean Minimum Spanning Tree (EMST) models are used. In physical space, agreement between experimental data and simulation results is good up to the neck zone, for the unconditional mean values of velocity, mixture fraction, major and some minor chemical species. Conditional mean profiles in mixture fraction space are also in reasonable agreement with experiments up to the neck zone, though conditional fluctuations tend to be under-predicted. CD generally yields better predictions for the level of fluctuations in mixture fraction space than EMST, but this is partly due to unrealistic particle evolution in composition space. In general, simulations using the REDIM approach for reduction of detailed C2-chemistry confirm earlier findings for micro-mixing model behaviour, obtained with C1-chemistry

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

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 beta (auaEuro parts per thousand) 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

RANS modelling of a lifted H2/N2 flame using an unsteady flamelet progress variable approach with presumed PDF

An unsteady flamelet/progress variable (UFPV) approach is used to model a lifted H2/N2 flame in a RANS framework together with presumed PDF. We solve the unsteady flamelets both in physical space and in mixture fraction space. We show that in the former case, the scalar dissipation rate profile strongly varies in time (while it is assumed to be fixed in time in the latter). However, this does not result in significant qualitative differences in the corresponding flamelet libraries. The progress variable is carefully defined, including both the main combustion product (H2O) and a key radical species in ignition process (HO2). The presumed-PDF model is proposed in terms of the non-normalised progress variable, without assuming its statistical independence with mixture fraction. We introduce a modelled transport equation for the mean progress variable which is consistent with the basic underlying UFPV assumption, derived from the Lagrangian flamelet model. The influence of different model parameters on the results for the mean temperature and mean species mass fractions and their fluctuations is discussed. Good results are obtained for the conditions of the considered lifted flame where detailed experimental data is available. However, at low coflow temperature the modelled flame lift-off height is shorter than expected.This work is supported by the Comunidad de Madrid through Project HYSYCOMB P2009/ENE-1597 and by the Spanish Ministry of Economy and Competitiveness under Projects ENE2008-06515-004-02 and CSD2010-00011. This research was also partially supported by the Generalitat Valenciana inside the program Ajudes per a la realitzacio de projectes d'I+D per a grups de investigaclel emergent (Reference GV/2013/041), which is gratefully acknowledged.Naud, B.; Novella Rosa, R.; Pastor Enguídanos, JM.; Winklinger, JF. (2015). RANS modelling of a lifted H2/N2 flame using an unsteady flamelet progress variable approach with presumed PDF. Combustion and Flame. 162(4):893-906. https://doi.org/10.1016/j.combustflame.2014.09.014S893906162

Turbulent scalar fluxes from a generalized Langevin model: Implications on mean scalar mixing and tracer particle dispersion

A Generalized Langevin Model (GLM) formulation to be used in transported joint velocity-scalar probability density function methods is recalled in order to imply a turbulent scalar-flux model where the pressure-scrambling term is in correspondence with standard Monin's return-to-isotropy term. The proposed non-constant C0 formulation is extended to seen-velocity models for particle dispersion modeling in dispersed two-phase flows. This allows us to correct the wrong turbulent scalar-flux modeling in the limit of tracer particles. Moreover, this allows us to have a more general formulation in order to consider advanced Reynolds-stress models. The cubic model of Fu, Launder, and Tselepidakis is considered, together with the model of Merci and Dick for turbulent dissipation. Results are presented for different swirling and recirculating single-phase and two-phase flows, showing the capabilities of the proposed non-constant C0 GLM formulations compared to the standard GLM.Accepted Author ManuscriptFluid Mechanic