Nedunchezhian Swaminathan

Scalar Dissipation Rate Modeling and its Validation

A simple algebraic model for the Favre averaged scalar dissipation rate, c, in high Damkohler number premixed flames is obtained from its transport equation by balancing the leading order terms. Recently proposed models for the dominant terms in the transport equation are revisited and revised. The algebraic model incorporates essential physics of turbulent premixed flames, namely, dilatation rate, its influence on turbulence-scalar interaction, chemical reactions, and dissipation processes. A realizability analysis is carried out to show that the algebraic model is always unconditionally realizable. The model predictions of dissipation rate are compared with the DNS results, and the agreement is good over a range of flame conditions. Application of the Kolmogorov-Petrovski-Piskunov (KPP) theorem along with the above algebraic model gives an expression for the turbulent flame speed. Its prediction compares well with a range of experimental data with no modifications to the model constants.

Influence of the Damköhler number on turbulence-scalar interaction in premixed flames. I. Physical insight

Scalar dissipation rate is a central quantity in turbulent flame modeling as it is closely related to the reaction rate. It is well known that turbulence-scalar interaction plays a vital role in turbulent flows with scalar mixing and thus on the scalar dissipation rate. This interaction process is characterized by the tensor inner product between the scalar gradient vector and the turbulence strain rate tensor and it is found to depend strongly on the Damkohler number, Da. Two direct numerical simulation data sets are analyzed in detail in order to understand the physics of Da dependence. The well known alignment of scalar gradient with the most compressive principal strain rate resulting in production of the scalar gradient by turbulence is observed for low (Da<1) Damkohler number flame, whereas the turbulence dissipates the scalar gradient in high Da flame. This dissipation of the scalar gradient in the high Da flame is because of its preferential alignment with the most extensive principal strain rate....

Interaction of turbulence and scalar fields in premixed flames

The interaction of turbulence with progress variable, c, field in a premixed flame is studied. This interaction is characterized by the correlation c,ieijc,j¯, where the overbar denotes an appropriate averaging process, c,i is the gradient of c in spatial direction i and eij is the turbulence strain rate. The importance of this term is recognized via a transport equation for the square of the magnitude of the scalar gradient in turbulent premixed flames. It is also shown that the above correlation forms a natural part of the tangential strain rate which appears in the flame surface density, Σ, equation. It is well known that this correlation is negative signifying the scalar gradient production by incompressible turbulence via the preferential alignment of the scalar gradient with the most compressive principal strain rate. This physical picture is commonly adopted for turbulent premixed flames also. Contrary to this, analyses of direct numerical simulation data for a turbulent premixed flame having flame...

Effect of heat release on turbulence and scalar-turbulence interaction in premixed combustion

Stereoscopic particle image velocimetry and planar laser induced fluorescence measurements of hydroxyl radical are simultaneously applied to measure, respectively, local turbulence intensities and flame front position in premixed ethylene-air flames stabilized on a bluff body. Three different equivalence ratios, 0.55, 0.63, and 0.7, and three different Reynolds numbers, 14 000, 17 000, and 21 000, are considered. Laser measurements were made for five different flame configurations within the ranges above and in the corresponding cold flows. By comparing the measurements of the cold and the corresponding hot flows, the effect of heat release on the turbulence and its interaction with the flame front is studied. All the flames are in the thin reaction zone regime. Typical flow features forming behind the bluff body are observed in the cold flows, whereas in the reacting flows the mean velocities and thus the shape, size, and characteristics of the recirculating eddy behind the bluff body are strongly influenced by the heat release. The strong acceleration across the mean flame and the radial outward shift of the stagnation plane of the recirculating eddy yield negative radial velocities which are absent in the corresponding cold flow cases. The spatial intermittency of the flame front leads to an increase in the turbulent kinetic energy. Although a decrease in the mean and rms values of the strain rate tensor eij components is observed for the reacting case as one would expect, the local flow acceleration across the flame front leads to a substantial increase in the skewness and the kurtosis of the probability density functions (PDFs) of eij components. The turbulence-scalar interaction is studied by analyzing the orientation of the flame front normal with the eigenvectors of eij. The PDFs of this orientation clearly show that the normals have an increased tendency to align with the extensive strain rate, which implies that the scalar gradients are destroyed by the turbulence as the scalar isosurfaces are pulled apart. This result questions the validity of passive scalar turbulence physics commonly used for premixed flame modeling. However, the influence of Lewis number on this alignment behavior is not clear at this time.Stereoscopic particle image velocimetry and planar laser induced fluorescence measurements of hydroxyl radical are simultaneously applied to measure, respectively, local turbulence intensities and flame front position in premixed ethylene-air flames stabilized on a bluff body. Three different equivalence ratios, 0.55, 0.63, and 0.7, and three different Reynolds numbers, 14 000, 17 000, and 21 000, are considered. Laser measurements were made for five different flame configurations within the ranges above and in the corresponding cold flows. By comparing the measurements of the cold and the corresponding hot flows, the effect of heat release on the turbulence and its interaction with the flame front is studied. All the flames are in the thin reaction zone regime. Typical flow features forming behind the bluff body are observed in the cold flows, whereas in the reacting flows the mean velocities and thus the shape, size, and characteristics of the recirculating eddy behind the bluff body are strongly influe...

A priori assessment of closures for scalar dissipation rate transport in turbulent premixed flames using direct numerical simulation

The scalar dissipation rate transport in both the corrugated flamelet and thin reaction zone regimes is studied using three-dimensional direct numerical simulation (DNS) databases for freely propagating statistically planar turbulent premixed flames. Both flames have comparable turbulent Reynolds number but the flame representing the corrugated flamelet combustion regime has a global Damkohler number, Da>1, whereas the second flame representing the thin reaction zone regime has Da 1 flame, while it produces the scalar gradients in the Da<1 flame. Simple algebraic models for the con...

Influence of the Damköhler number on turbulence-scalar interaction in premixed flames. II. Model development

The modeling of Damkohler number, Da, effects on the interaction of turbulence and scalar fields in premixed flames is studied using two freely propagating statistically planar flames calculated by direct numerical simulation (DNS). One flame having Da>1 shows attributes of turbulent combustion in the corrugated flamelets regime while the other having Da 1 flame whereas it produces the scalar gradients in the Da<1 flame. It is argued and also shown that the Damkohler number dependence should explicitly appear in the models for the turbulence-scalar interaction in order to represent the above physics correctly. Simple unified models, in the Reynolds averaged Navier-Stokes (RANS) framework, for the interaction processes involving a local Damkohler number depend...

Assessment of combustion submodels for turbulent nonpremixed hydrocarbon flames

Data bases generated by direct numerical simulation (DNS) of nonpremixed combustion are used to evaluate stationary laminar flamelet and conditional moment closure (CMC) models of turbulent combustion. The chemical kinetics used for the simulation and modeling is a systematically reduced two-step mechanism for hydrocarbon combustion. Heat release effects on the chemistry are included but a constant density assumption is used. Three different Reynolds numbers and a range of Damkohler numbers are considered. Two different versions of stationary laminar flamelet models are considered. In one version, the instantaneous turbulent scalar dissipation rate at stoichiometry is used to match the laminar flamelets, whereas in another version the conditional average of scalar dissipation at stoichiometry is used. In the CMC calculations, turbulent mixing is modelled by a presumed beta function pdf with the mixture fraction variance being the only input quantity and this is obtained from the DNS. CMC predictions of major and minor species are excellent and are always within 6% despite the presence of some local extinction. Both versions of flamelet models predict the major species with much less accuracy than this. The minor species predictions, and hence the reaction rate predictions, are even less accurate.

Turbulent premixed flames

1. Fundamentals and challenges K. N. C. Bray and N. Swaminathan 2. Modelling D. Bradley, L. Vervisch, V. Moureau, P. Domingo, D. Veynante, N. Chakraborty, M. Champion, A. Mura, N. Swaminathan and R. P. Lindstedt 3. Combustion instabilities A. M. K. P. Taylor and Y. Urata, A. P. Dowling, A. S. Morgans, L. Gicquel, F. Nicoud and T. Poinsot 4. Practice Y. Urata, A. M. K. P. Taylor, Y. Urata and B. Jones 5. Future directions S. Hayashi, Y. Mizobuchi, J. F. Driscoll, K. N. C. Bray and N. Swaminathan.

Relationship between turbulent scalar flux and conditional dilatation in premixed flames with complex chemistry

Abstract The behavior of the turbulent scalar flux, u″c″ , in hydrogen- and methane-air flames has been studied by using two-dimensional, direct numerical simulation databases involving a multi-step chemical mechanism. The relationship between local dilatation and the scalar flux has been investigated. As observed previously in simulations with a single irreversible chemical reaction, the predominant effect of turbulence is on the conditional diffusion of the scalar, c. The peak conditional dilatation occurs in fluid mixtures with low values of progress variable at all locations inside the flame brush. This yielded a negative correlation between dilatation and the scalar fluctuations. This negative correlation dictates the scalar flux to be of gradient type. The inter-relationship between the correlation and the scalar flux was observed to be true in all the data sets considered here. Calculation of unstrained and strained laminar flame indicate that the peak dilatation occurs in mixtures with low values of the progress variable for hydrogen flames with equivalence ratio of ∼0.5 and above. Thus, these flames in statistically stationary and one-dimensional situation may never exhibit counter-gradient flux for the turbulent scalar flux. However, the above flames in statistically multi-dimensional situation may exhibit counter-gradient flux.

Validation of a Turbulent Flame Speed Model across Combustion Regimes

A flame speed expression proposed recently is studied in detail and extensively validated in this study. This expression has no adjustable parameters, and its constants are closely tied to the physics of scalar mixing at small scales. In the weak turbulence limit, the flame speed expression recovers a linear dependence of the turbulent to laminar flame speed ratio, , on the normalized turbulence rms velocity, , in accordance with Damköhlers classical result. However, in the limit of intense turbulence, Damköhlers result gives a square-root dependence, while the new expression gives a combination of linear and square-root terms. Predictions of the new expression are compared to a wide range of experimental data of S T from various flame configurations and conditions, with the same values of the model constants. The quantitative comparisons are found to be very good with experimental data beyond the usually restricted range of of existing models. Predictions of S T for high-pressure turbulent flames, up to 3 MPa, also compare acceptably well with experimental measurements. The flame speed is found to increase when the mean curvature of the flame brush is positive, and this dependence seems to be linear.