Td Dunstan

Erratum: Geometrical properties and turbulent flame speed measurements in stationary premixed v-flames using direct numerical simulation (Flow Turbulence Combust (2011) 87 (237-259) DOI 10.1007/s10494-010-9284-1)

Three dimensional, fully compressible direct numerical simulations (DNS) of premixed turbulent flames are carried out in a V-flame configuration. The governing equations and the numerical implementation are described in detail, including modifications made to the Navier–Stokes Characteristic Boundary Conditions (NSCBC) to accommodate the steep transverse velocity and composition gradients generated when the flame crosses the boundary. Three cases, at turbulence intensities, u′/sL, of 1, 2, and 6 are considered. The influence of the flame holder on downstream flame properties is assessed through the distributions of the surface-conditioned displacement speed, curvature and tangential strain rates, and compared to data from similarly processed planar flames. The distributions are found to be indistinguishable from planar flames for distances greater than about 17δth downstream of the flame holder, where δth is the laminar flame thermal thickness. Favre mean fields are constructed, and the growth of the mean flame brush is found to be well described by simple Taylor type diffusion. The turbulent flame speed, sT is evaluated from an expression describing the propagation speed of an isosurface of the mean reaction progress variable

Effect of flow-geometry on turbulence-scalar interaction in premixed flames

\tilde{c}

Flame Interactions in Turbulent Premixed Twin V-Flames

in terms of the imbalance between the mean reactive, diffusive, and turbulent fluxes within the flame brush. The results are compared to the consumption speed, sC, calculated from the integral of the mean reaction rate, and to the predictions of a recently developed flame speed model (Kolla et al., Combust Sci Technol 181(3):518–535, 2009). The model predictions are improved in all cases by including the effects of mean molecular diffusion, and the overall agreement is good for the higher turbulence intensity cases once the tangential convective flux of

Prediction of Noise Source for an Aeroengine Combustor

\tilde{c}

The Effects of Non-Unity Lewis Numbers on Turbulent Premixed Flame Interactions in a Twin V-Flame Configuration

is taken into account.

Assessment of Reynolds Averaged Navier–Stokes Modeling of Scalar Dissipation Rate Transport in Turbulent Oblique Premixed Flames

Turbulent combustion of stoichiometric hydrogen-air mixture is simulated using direct numerical simulation methodology, employing complex chemical kinetics. Two flame configurations, freely propagating and V-flames stabilized behind a hot rod, are simulated. The results are analyzed to study the influence of flame configuration on the turbulence-scalar interaction, which is critical for the scalar gradient generation processes. The result suggests that this interaction process is not influenced by the flame configuration and the flame normal is found to align with the most extensive strain in the region of intense heat release. The combustion in the rod stabilized flame is found to be flamelet like in an average sense and the growth of flame-brush thickness with the downstream distance is represented well by Taylor theory of turbulent diffusion, when the flame-brushes are non-interacting. The thickness is observed to saturate when the flame-brushes interact, which is found to occur in the simulated rod st...

Computation of Forced Premixed Flames Dynamics

Multiple flame–flame interactions in premixed combustion are investigated using direct numerical simulations of twin turbulent V-flames for a range of turbulence intensities and length scales. Interactions are identified using a novel automatic feature extraction (AFE) technique, based on data registration using the dual-tree complex wavelet transform. Information on the time, position, and type of interactions, and their influence on the flame area is extracted using AFE. Characteristic length and time scales for the interactions are identified. The effect of interactions on the flame brush is quantified through a global stretch rate, defined as the sum of flamelet stretch and interaction stretch contributions. The effects of each interaction type are discussed. It is found that the magnitude of the fluctuations in flamelet and interaction stretch are comparable, and a qualitative sensitivity to turbulence length scale is found for one interaction type. Implications for modeling are discussed.

Scalar dissipation rate modelling for Large Eddy Simulation of turbulent premixed flames

Combustion noise may become an important noise source if not well understood in the design stage of lean burn gas turbine combustors. This work aims to predict the combustion noise source and broadband spectrum for a real aeroengine combustor, which has rarely been reported in past studies, and to compare with measured noise data on a demonstrator aeroengine. A spatial-temporal correlation model of fluctuating heat release rate is developed by analysing recent results of turbulent DNS V-flames, and the aeroengine combustor flow is calculated using RANS. A low-order linear network model is applied to the demonstrator aeroengine combustor to obtain the transfer function relating the heat release and acoustic fluctuations. The initial results for a low-medium power setting indicate that the prediction model captures the main characteristics of the broadband spectral shape of combustion noise but underestimates the spectral level. Empirical length and time scales in the correlation model are hence employed to obtain good agreement with the measured spectral level. Further work is underway to improve the model in predicting the combustion noise level.

DNS of EGR-type turbulent flame in MILD condition

The influence of Lewis number on turbulent premixed flame interactions is investigated using automatic feature extraction (AFE) applied to high-resolution flame simulation data. Premixed turbulent twin V-flames under identical turbulence conditions are simulated at global Lewis numbers of 0.4, 0.8, 1.0, and 1.2. Information on the position, frequency, and magnitude of the interactions is compared, and the sensitivity of the results to sample interval is discussed. It is found that both the frequency and magnitude of normal type interactions increases with decreasing Lewis number. Counternormal type interactions become more likely as the Lewis number increases. The variation in both the frequency and the magnitude of the interactions is found to be caused by large-scale changes in flame wrinkling resulting from differences in the thermo-diffusive stability of the flames. During flame interactions, thermo-diffusive effects are found to be insignificant due to the separation of time scales.

Influence of flame geometry on turbulent premixed flame propagation: a DNS investigation

The statistical behavior of scalar dissipation rate (SDR) transport of reaction progress variable and its modeling in the context of Reynolds averaged Navier–Stokes (RANS) simulations has been analyzed based on a-priori analysis of direct numerical simulation (DNS) data for a turbulent premixed V-flame configuration. It has been found that the terms in the scalar dissipation rate transport equation originating from density variation due to heat release, chemical reaction rate gradient, molecular dissipation, and diffusivity gradient are the major contributors to the scalar dissipation rate transport for different downstream axial locations from the flame holder. By contrast, the contribution of turbulent transport of scalar dissipation rate assumes negligible values in comparison to the unclosed terms due to density variation, chemical reaction rate gradient, molecular dissipation, and diffusivity gradient. The statistical behaviors of the unclosed terms arising from density variation, chemical reaction rate gradient, and molecular dissipation for V-flame have been found to be qualitatively consistent with previous results for statistically planar flames. The models for the unclosed turbulent transport, density variation, chemical reaction rate gradient, and molecular dissipation terms of the scalar dissipation rate transport equation, which were proposed based on previous a-priori analysis of statistically planar flames, have been found to predict the corresponding terms satisfactorily at different downstream axial locations in the V-flame configuration considered here. The present analysis shows that the unclosed term in the scalar dissipation rate transport equation originating from diffusivity gradient can assume magnitudes comparable to the density variation contribution to the SDR transport equation, and a model has been proposed for the combined contribution of the chemical reaction rate gradient, molecular dissipation, and diffusivity gradient terms, which has been found to satisfactorily predict the corresponding quantity extracted from DNS data at different axial locations.