A STUDY OF PASSIVE SCALAR DISSIPATION RATE IN A VARIABLE DENSITY TURBULENT JET

The passive scalar field of a temporally evolving shear layer is investigated using gradient trajectories as a means to analyse the scalar probability density function and the conditional scalar dissipation rate in the presence of external intermittency. These results are of significance for turbulent combustion, where improved predictions of the statistics of the conditional dissipation rate are needed in several models. First, the variation of the conventional first and second moments of the conditional dissipation rate across the layer is quantitatively documented in detail. A strong dependence of the conditional dissipation rate on the lateral position and on the conditioning value of the scalar is observed. The dependence on the transverse distance to the centre-plane partially explains the double-hump profile usually reported when this dependence is ignored. The variation with the scalar observed in the ratio between the second and first moments would invalidate certain assumptions commonly done in turbulent combustion. It is also seen that conditioning on the scalar does not reduce the fluctuation of the dissipation rate with respect to unconditional values. Next, the role of external intermittency in these results is investigated. For that purpose, the flow is partitioned into different zones based on different types of gradient trajectories passing through each point, thereby introducing non-local information in comparison with the standard turbulent/non-turbulent separation based on the conventional intermittency function. In addition to the homogeneous outer regions, three zones are identified: a turbulent zone, a turbulence interface and quasi-laminar diffusion layers. The relative contribution from each of these zones to the conventional intermittency factor is reported. The statistics are then conditioned on each of these zones, and the spatial variation of the scalar distribution and of the conditional scalar dissipation rate is explained in terms of the observed zonal statistics. For the Reynolds numbers of the present simulation, between 1500 and 3000 based on the vorticity thickness and the velocity difference, and a Schmidt number equal to 1, it results that the major contribution to both statistics is due to the turbulence interfaces. At the same time, the turbulent zone shows a distinct behaviour, being approximately homogeneous but anisotropic.

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Implementation of the Scalar Dissipation Rate in the REDIM Concept and its Validation for the Piloted Non-Premixed Turbulent Jet Flames

Eurasian Chemico-Technological Journal ◽

10.18321/ectj1100 ◽

2021 ◽

Vol 23 (3) ◽

pp. 169

Author(s):

C. Yu ◽

U. Maas

Keyword(s):

Dissipation Rate ◽

Transport Model ◽

Turbulent Flames ◽

Turbulent Jet ◽

Scalar Dissipation ◽

Scalar Dissipation Rate ◽

Jet Flames ◽

Reduced Chemistry ◽

The Impact ◽

Turbulent Jet Flames

In order to address the impact of the concentration gradients on the chemistry – turbulence interaction in turbulent flames, the REDIM reduced chemistry is constructed incorporating the scalar dissipation rate, which is a key quantity describing the turbulent mixing process. This is achieved by providing a variable gradient estimate in the REDIM evolution equation. In such case, the REDIM reduced chemistry is tabulated as a function of the reduced coordinates and the scalar dissipation rate as an additional progress variable. The constructed REDIM is based on a detailed transport model including the differential diffusion, and is validated for a piloted non-premixed turbulent jet flames (Sandia Flame D and E). The results show that the newly generated REDIM can reproduce the thermo-kinetic quantities very well, and the differential molecular diffusion effect can also be well captured.

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Scalar dissipation rate modelling in variable density turbulent axisymmetric jets and diffusion flames

Physics of Fluids ◽

10.1063/1.869616 ◽

1998 ◽

Vol 10 (4) ◽

pp. 938-948 ◽

Cited By ~ 35

Author(s):

J. P. H. Sanders ◽

I. Gökalp

Keyword(s):

Dissipation Rate ◽

Diffusion Flames ◽

Variable Density ◽

Scalar Dissipation ◽

Scalar Dissipation Rate ◽

Axisymmetric Jets ◽

And Diffusion

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Correlation between small-scale velocity and scalar fluctuations in a turbulent channel flow

Journal of Fluid Mechanics ◽

10.1017/s0022112008005569 ◽

2009 ◽

Vol 627 ◽

pp. 1-32 ◽

Cited By ~ 49

Author(s):

HIROYUKI ABE ◽

ROBERT ANTHONY ANTONIA ◽

HIROSHI KAWAMURA

Keyword(s):

Strain Rate ◽

Channel Flow ◽

Dissipation Rate ◽

Compressive Strain ◽

Outer Region ◽

Turbulent Channel Flow ◽

Scalar Fields ◽

Scalar Dissipation ◽

Small Scale ◽

Scalar Dissipation Rate

Direct numerical simulations of a turbulent channel flow with passive scalar transport are used to examine the relationship between small-scale velocity and scalar fields. The Reynolds number based on the friction velocity and the channel half-width is equal to 180, 395 and 640, and the molecular Prandtl number is 0.71. The focus is on the interrelationship between the components of the vorticity vector and those of the scalar derivative vector. Near the wall, there is close similarity between different components of the two vectors due to the almost perfect correspondence between the momentum and thermal streaks. With increasing distance from the wall, the magnitudes of the correlations become smaller but remain non-negligible everywhere in the channel owing to the presence of internal shear and scalar layers in the inner region and the backs of the large-scale motions in the outer region. The topology of the scalar dissipation rate, which is important for small-scale scalar mixing, is shown to be associated with the organized structures. The most preferential orientation of the scalar dissipation rate is the direction of the mean strain rate near the wall and that of the fluctuating compressive strain rate in the outer region. The latter region has many characteristics in common with several turbulent flows; viz. the dominant structures are sheetlike in form and better correlated with the energy dissipation rate than the enstrophy.

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Modelling of Conditional Scalar Dissipation Rate in Turbulent Premixed Combustion

Computation ◽

10.3390/computation9030026 ◽

2021 ◽

Vol 9 (3) ◽

pp. 26 ◽

Cited By ~ 1

Author(s):

Shokri Amzin ◽

Mariusz Domagała

Keyword(s):

Dissipation Rate ◽

Premixed Flames ◽

Moment Closure ◽

Navier Stokes ◽

Scalar Dissipation ◽

Flame Surface ◽

Scalar Dissipation Rate ◽

Turbulent Premixed Flames ◽

Conditional Moment ◽

Turbulent Premixed

In turbulent premixed flames, for the mixing at a molecular level of reactants and products on the flame surface, it is crucial to sustain the combustion. This mixing phenomenon is featured by the scalar dissipation rate, which may be broadly defined as the rate of micro-mixing at small scales. This term, which appears in many turbulent combustion methods, includes the Conditional Moment Closure (CMC) and the Probability Density Function (PDF), requires an accurate model. In this study, a mathematical closure for the conditional mean scalar dissipation rate, <Nc|ζ>, in Reynolds, Averaged Navier–Stokes (RANS) context is proposed and tested against two different Direct Numerical Simulation (DNS) databases having different thermochemical and turbulence conditions. These databases consist of lean turbulent premixed V-flames of the CH4-air mixture and stoichiometric turbulent premixed flames of H2-air. The mathematical model has successfully predicted the peak and the typical profile of <Nc|ζ> with the sample space ζ and its prediction was consistent with an earlier study.

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