13,076 research outputs found
Numerical analysis of reaction-diffusion effects on species mixing rates in turbulent premixed methane-air combustion
Full text linkThe scalar mixing time scale, a key quantity in many turbulent combustion models, is investigated for reactive scalars in premixed combustion. Direct numerical simulations (DNS) of three-dimensional, turbulent Bunsen flames with reduced methane-air chemistry have been analyzed in the thin reaction zones regime. Previous conclusions from single step chemistry DNS studies are confirmed regarding the role of dilatation and turbulence-chemistry interactions on the progress variable dissipation rate. Compared to the progress variable, the mixing rates of intermediate species is found to be several times greater. The variation of species mixing rates are explained with reference to the structure of one-dimensional premixed laminar flames. According to this analysis, mixing rates are governed by the strong gradients which are imposed by flamelet structures at high Damk¨ohler numbers. This suggests a modeling approach to estimate the mixing rate of individual species which can be applied, for example, in transported probability density function simulations. Flame turbulence interactions which modify the flamelet based representation are analyzed
Probability density function modelling of molecular mixing in flames with differential diffusion
No full textMotivated by the fact that differential diffusion can affect pollutant emission and flame stability in practical combustion systems, this paper presents a modified Euclidean Minimum Spanning Tree mixing model which accounts for differential diffusion. The new Probability Density Function modelling satisfies requirements of realizability and conservation of mass, and validation is conducted by comparison with Direct Numerical Simulation dat
Analysis of turbulent flame propagation in equivalence ratio-stratified flow
No full textEffects of equivalence ratio stratification on turbulent combustion processes are investigated using Direct Numerical Simulation. The simulation results are analysed in terms of flame surface area and the burning intensity. The local effects of stratification are then investigated further by examining statistics of the displacement speed conditioned on the flame-normal equivalence ratio gradient. The local burning intensity is found to depend on the orientation of the stratification with respect to the flame front, so that burning intensity is enhanced when the flame speed in the products is faster than in the reactants. The flame surface area is also influenced by equivalence ratio stratification and this may be explained by differences in the surface averaged consumption speed and differential propagation effects due to flame speed variations associated with equivalence ratio fluctuation
Application of PDF mixing models to premixed flames with differential diffusion
Full text linkDifferential diffusion alters the balance of reaction and diffusion in turbulent premixed combustion, affecting the performance and emissions of combustion devices. Modelling combustion devices with Probability or Filtered Density Function (PDF or FDF) methods provides an exact treatment for the change in composition due to chemical reaction, while molecular mixing has to be modelled. Previous PDF molecular mixing models do not account for differential diffusion in a manner which satisfies realizability requirements. A new approach for treating differential diffusion, which ensures realizability, is proposed for pairwise-exchange mixing models in general, and applied in the Interaction by Exchange with the Mean (IEM) model of Dopazo (1975), and in the Euclidean Minimum Spanning Tree (EMST) model of Subramaniam and Pope (1998). The new differential diffusion models are referred to as IEM-DD and EMST-DD respectively.Results from two and three-dimensional DNS of turbulent premixed methane-air combustion show that mixing rates and conditional statistics of species mass fractions depend on species diffusivities and the combustion regime. Zero-dimensional PDF model results obtained for the two-dimensional DNS case show that the EMST-DD model best reproduces the features that characterize differential diffusion in the DNS. The essential feature of the EMSTDDmodel, which accounts for its success in turbulent premixed combustion, is that differential mixing rates are imposed within a model which mixes locally in composition spac
Analysis of turbulent flame propagation in equivalence ratio-stratified flow
Full text linkEquivalence ratio-stratified combustion is an important technology for achieving stable low-emission operation in internal combustion engines and gas turbines. This study examines how equivalence ratio stratification affects the physics of turbulent flame propagation using Direct Numerical Simulation. Three-dimensional simulations of a turbulent slot-Bunsen flame configuration are performed with accurate multi-step kinetic modelling for methane-air combustion. We compare one perfectly-premixed and three equivalence ratio-stratified cases with the mean equivalence ratio gradient aligned with, tangential to or opposed to the mean flame brush. The simulation results are analysed in terms of flame surface area and the burning intensity. The local effects of stratification are then investigated further by examining statistics of the displacement speed conditioned on the flame-normal equivalence ratio gradient. The local burning intensity is found to depend on the orientation of the stratification with respect to the flame front, so that burning intensity is enhanced when the flame speed in the products is faster than in the reactants. This effect of alignment between equivalence ratio gradients and flame fronts has been observed previously in laminar flames and it is found here that it also affects the global behaviour of turbulent flames. The flame surface area is also influenced by equivalence ratio stratification and this may be explained by differences in the surface-averaged consumption speed and differential propagation effects due to flame speed variations associated with equivalence ratio fluctuations
Filtered density functions from direct numerical simulation of a reactive jet in cross-flow
No full textDirect numerical simulation (DNS) with multi-step hydrogen-air chemical kinetics is used to investigate the nearfield of a flame stabilized above a reactive jet in cross-flow (JICF). JICF configurations are typically used where rapid mixing is desirable; classical applications are fuel injection nozzles and dilution holes in gas turbine combustors. Due to the computational cost of DNS, approximate solution methods such as large-eddy simulation (LES) are essential to parametrically study the effect of changing fuel jet configurations on the far field, but these methods require submodels capable of accurately capturing the near-field flame stabilization for success. By incorporating a wealth of turbulence chemistry interactions (between the flame and vorticity generated by the jet shear layer instability as well as product recirculation by a large counter-rotating vortex pair), this DNS is exceptionally well suited to exploration of unclosed terms in LES formulations such as the chemical source-term. One quantity of direct relevance to several models for stratified combustion, such as the Bray-Moss (BM) model and doubly-conditional source-term estimation (DCSE), is the filtered density function for the mixture fraction ! and partially premixed progress variable c. Empirical extraction of the filtered density functions of progress variable and mixture fraction at two representative locations demonstrates the complexity of approximating these two functions from a one- or two-parameter functional form
Analysis of second-order conditional moment closure applied to an autoignitive lifted hydrogen jet flame
No full textThe timing and location of autoignition can be highly sensitive to turbulent fluctuations of composition. Second-order Conditional Moment Closure (CMC) provides transport equations for conditional (co)variances in turbulent reacting flows. CMC equations accounting for compressibility and differential diffusion are analyzed using data from direct numerical simulation of an autoignitive lifted turbulent hydrogen jet flame [1]. At the flame base, second-order moments were required to accurately model the conditional reaction rates. However, over 80% of the second-order reaction rate component was obtainable with a small subset (16%) of the species-temperature covariances. The balance of the second-order CMC equation showed that turbulent transport across spatial composition gradients initiates generation of conditional variances
Terascale direct numerical simulations of turbulent combustion – fundamental understanding towards predictive models
No full textAdvances in high-performance computational capabilities enable scientific simulations with increasingly realistic physical representations. This situation is especially true of turbulent combustion involving multiscale interactions between turbulent flow, complex chemical reaction, and scalar transport. A fundamental understanding of combustion processes is crucial to the development and optimization of next-generation combustion technologies operating with alternative fuels, at higher pressures, and under less stable operating conditions, such as highly dilute, stratified mixtures. Direct numerical simulations (DNS) of turbulent combustion resolving all flow and chemical features in canonical configurations are used to improve fundamental understanding of complex flow processes and to provide a database for the development and validation of combustion models. A description of the DNS solver and its optimization for use in massively parallel simulations is presented. Recent DNS results from a series of three combustion configurations are presented: soot formation and transport in a nonpremixed ethylene jet flame, the effect of fuel stratification in methane Bunsen flames, and extinction and reignition processes in nonpremixed ethylene jet flames
Towards an intermediate water wave model of vocal fold vibration: Evidence from vocal-fold dynamic sonography
No full text
- …
