Effect of dilatation on scalar dissipation in turbulent premixed flames

Flow Turbulence Combust

Swaminathan

2015

Dynamic algebraic closure of scalar dissipation rate (SDR) of reaction progress variable in the context of Large Eddy Simulations (LES) of turbulent premixed combustion has been addressed here using a power-law based expression and a model, which was originally proposed for Reynolds Averaged Navier Stokes (RANS) simulations, but has recently been extended for LES. The performances of these models have been assessed based on a-priori analysis of a Direct Numerical Simulations (DNS) database of statistically planar turbulent premixed flames with a range of different values of heat release parameter τ , turbulent Reynolds number Re t and global Lewis number Le. It has been found that the power-law model with a single constant exponent α D does not adequately capture the volume-averaged behaviour of density-weighted SDR and this problem is particularly severe especially for Le << 1 flames. The deficiency of the power-law model with a single power-law exponent arises due to multi-fractal nature of SDR. The dynamic evaluation of the model parameter for the algebraic model, which was originally proposed in the context of RANS and has been extended here for LES, has been shown to capture the local behaviour of SDR better than the power-law model. It has been demonstrated that the empirical parameterisation of a model parameter for the static version of the RANS-extended SDR model can be avoided using a dynamic formulation which captures the local behaviour of SDR either comparably or better than the static formulation for a range of different values of τ, Le and Re t , without sacrificing the prediction of the volume-averaged SDR. Keywords

Section: Mathematical Backgroundmentioning

confidence: 92%

Section: Introductionmentioning

confidence: 99%

Dynamic Closure of Scalar Dissipation Rate for Large Eddy Simulations of Turbulent Premixed Combustion: A Direct Numerical Simulations Analysis

Gao

Flow Turbulence Combust

Swaminathan

2015

“…cases D and E, respectively) are representative of hydrogen-blended methane-air mixtures and the Lewis number 1.2 [30]). Case A is taken from a well-known DNS database [31] which was used extensively in the past for the analysis of turbulent premixed combustion in the corrugated flamelets regime [10,27,32,33]. Cases B-L have been simulated using a well-known compressible DNS code called SENGA [34] which has been used successfully in a number of previous analyses [10][11][12][13][17][18][19][20]27], where the standard conservation equations of mass, momentum, species and energy have been solved in non-dimensional form.…”

Section: For Cases a B C D E F G And H-l Respectivelymentioning

confidence: 99%

Modelling of the Curvature Term in the Flame Surface Density Transport Equation: A Direct Numerical Simulations Based Analysis

Katragadda

Malkeson

International Journal of Spray and Combustion Dynamics

2014

A simple chemistry based three-dimensional Direct Numerical Simulations (DNS) database of freely propagating statistically planar turbulent premixed flames with a range of different values of Karlovitz number Ka, turbulent Reynolds number Re t , heat release parameter τ and global Lewis number Le has been used for the modelling of the curvature term of the generalised Flame Surface Density (FSD) transport equation in the context of Reynolds Averaged Navier Stokes (RANS) simulations. The curvature term has been split into the contributions arising due to the reaction and normal diffusion components of displacement speed (i.e. T 1 ) and the term arising due to the tangential diffusion component of displacement speed (i.e. T 2 ). Subsequently, the subterms (i.e. T 1 and T 2 ) of the curvature contribution to the FSD transport have been split into the closed (i.e. T 1r and T 2r ) and unclosed (i.e. T 1ur and T 2ur ) components. It has been found that T 2 remains deterministically negative throughout the flame brush. However, the qualitative behaviour of T 1 changes significantly depending upon the values of Ka, Re t and Le. Detailed physical explanations have been provided for the observed behaviours of the components of the curvature term. Moreover, it has been observed that the closed contributions of T 1 and T 2 (i.e. T 1r and T 2r ) remains negligible in comparison to the unclosed contributions (i.e. T 1ur and T 2ur ). Suitable model expressions have been identified for T 1ur and T 2ur in the context of RANS simulations, which are shown to perform satisfactorily in all cases considered in the current analysis, accounting for the variations in Ka, Re t , τ and Le.Keywords: Direct Numerical Simulation, Flame Surface Density, Curvature, Lewis Number, Turbulent Reynolds Number, Reynolds Averaged Navier Stokes Modelling [14] demonstrated that algebraic models may not be adequate for simulating incylinder processes in Spark Ignition (SI) piston engines. Therefore, it may be necessary to consider a modelled FSD transport equation for simulating turbulent premixed flames under some conditions. Moreover, despite ever increasing computational power and capacity, Large Eddy Simulations (LES) are still not routinely used in industrial calculations due to large computational costs associated with it, and, consequently, RANS is still the most commonly used simulation method for industrial engineering problems. The accurate modelling of the chemical reaction rate is a necessity for highfidelity Computational Fluid Dynamics (CFD) simulations, which play important roles in the development of new generation energy-efficient and low-pollution SI engines and Lean Premixed Prevaporaised (LPP) gas turbines. Thus, it is important to have highfidelity models for FSD transport in the context of RANS simulations. The FSD transport equation has a number of unclosed terms, and previous analyses [1][2][3][4][5][6][7][8][9][10][11][12] indicated that the terms due to fluid-dynamic straining and flame curvature act as leading order c...

“…It is worth noting that Soret and Dufor effects are ignored here following several previous analyses on entropy generation in turbulent reacting flows [4][5][6][7][8][10][11][12][13][14][15]17]. Moreover, there have been several previous DNS based computational analyses [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34], which ignored Dufor and Sorret effects without much loss of generality. These effects are not expected to play important roles in most hydrocarbon-air and hydrogen-air flames [35] unless extremely lean hydrogen-air flames are considered.…”

Section: Mathematical Backgroundmentioning

confidence: 99%

“…Single step chemistry has been used successfully to obtain fundamental physical insight and to develop high-fidelity models in several analyses in the past [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34] and the same methodology has been followed here. In the context of simplified chemistry the species field is characterized by a reaction progress variable c , which can be defined in terms of a suitable reactant mass fraction R Y as follows:…”

Section: Products Reactantsmentioning

confidence: 99%

A Direct Numerical Simulation-Based Analysis of Entropy Generation in Turbulent Premixed Flames

Farran

2013

Entropy

A compressible single step chemistry Direct Numerical Simulation (DNS) database of freely propagating premixed flames has been used to analyze different entropy generation mechanisms. The entropy generation due to viscous dissipation within the flames remains negligible in comparison to the other mechanisms of entropy generation. It has been found that the entropy generation increases significantly due to turbulence and the relative magnitudes of the augmentation of entropy generation and burning rates under turbulent conditions ultimately determine the value of turbulent second law efficiency in comparison to the corresponding laminar values. It has been found that the entropy generation mechanisms due to chemical reaction, thermal conduction and mass diffusion in turbulent flames strengthen with decreasing global Lewis number in comparison to the corresponding values in laminar flames. The ratio of second law efficiency under turbulent conditions to its corresponding laminar value has been found to decrease with increasing global Lewis number. An increase in heat release parameter significantly augments the entropy generation due to thermal conduction, whereas other mechanisms of entropy generation are marginally affected. However, the effects of augmented entropy generation due to thermal conduction at high values of heat release parameter are eclipsed by the increased change in availability due to chemical reaction, which leads to an increase in the second law efficiency with increasing heat release parameter for identical flow conditions. The combustion regime does not have any major influence on the augmentation of entropy generation due to chemical reaction, thermal conduction and mass diffusion in turbulent flames in comparison to corresponding laminar flames, whereas the extent of augmentation of entropy generation due to viscous dissipation in turbulent conditions in comparison to OPEN ACCESSEntropy 2013, 15 1541 corresponding laminar flames, is more significant in the thin reaction zones regime than in the corrugated flamelets regime. However, the ratio of second law efficiency under turbulent conditions to its corresponding laminar value does not get significantly affected by the regime of combustion, as viscous dissipation plays a marginal role in the overall entropy generation in premixed flames.