The Experts below are selected from a list of 321 Experts worldwide ranked by ideXlab platform
Nilanjan Chakraborty - One of the best experts on this subject based on the ideXlab platform.
-
effects of lewis Number on conditional fluid velocity statistics in low Damkohler Number turbulent premixed combustion a direct numerical simulation analysis
Physics of Fluids, 2013Co-Authors: Nilanjan Chakraborty, Andrei LipatnikovAbstract:The effects of global Lewis Number Le on the statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes simulations have been analysed using a Direct Numerical Simulations (DNS) database of statistically planar turbulent premixed flames with a low Damkohler Number and Lewis Number ranging from 0.34 to 1.2. The conditional velocity statistics extracted from DNS data have been analysed with respect to the well-known Bray-Moss-Libby (BML) expressions which were derived based on bi-modal probability density function of reaction progress variable for high Damkohler Number flames. It has been shown that the Lewis Number substantially affects the mean velocity and the velocity fluctuation correlation conditional in products, with the effect being particularly pronounced for low Le. As far as the mean velocity and the velocity fluctuation correlation conditional in reactants are concerned, the BML expressions agree reasonably well with the DNS data reported in the present work. Based on a priori analysis of present and previously reported DNS data, the BML expressions have been empirically modified here in order to account for Lewis Number effects, and the non-bimodal distribution of reaction progress variable. Moreover, it has been demonstrated for the first time that surface averaged velocity components and Reynolds stresses conditional in unburned reactants can be modelled without invoking expressions involving the Lewis Number, as these surface averaged conditional quantities remain approximately equal to their conditionally averaged counterparts in the unburned mixture.
-
conditional velocity statistics for high and low Damkohler Number turbulent premixed combustion in the context of reynolds averaged navier stokes simulations
Proceedings of the Combustion Institute, 2013Co-Authors: Nilanjan Chakraborty, Andrei LipatnikovAbstract:The statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes (RANS) simulations have been analysed using a Direct Numerical Simulation (DNS) database of statistically planar turbulent premixed flames for both high and low values of Damkohler Number for different values of heat release parameter. It has been found that the contributions arising from chemical reaction to the conditional mean velocities and the conditional Reynolds stresses remain strong under high values of Damkohler Number. The expressions for conditional mean velocity components and conditional Reynolds stresses, which are derived based on bi-modal probability density function of reaction progress variable for unity Lewis Number flames, are modified in this study in such a manner that the new expressions can be used for low Damkohler Number flames where bi-modal distribution is not realised. Suitable models for conditional surface-averaged velocity components and the Reynolds stresses have been identified, which are shown to work satisfactorily for all values of Damkohler Number and heat release parameter considered in this analysed.
-
a priori dns modeling of the turbulent scalar fluxes for low Damkohler Number stratified flames
Combustion Science and Technology, 2012Co-Authors: Sean P Malkeson, Nilanjan ChakrabortyAbstract:Statistically planar turbulent stratified flames for different initial intensities of decaying turbulence have been simulated for global equivalence ratios = 0.7 and = 1.0 using three-dimensional simplified chemistry–based direct numerical simulations (DNS). The simulation parameters are chosen in such a manner that all the cases considered here represent low Damkohler Number thin reaction zones regime combustion. The DNS data have been used to analyze the statistical behaviors of the turbulent fluxes of fuel mass fraction Y F and mixture fraction ξ, and their variances and covariances. It has been found that turbulent scalar flux of fuel mass fraction predominantly exhibits gradient-type transport in all cases considered in the current study, but countergradient-type transport has also been observed in the globally stoichiometric flame for small values of turbulence intensity where the flame normal acceleration due to chemical heat release overcomes the effects of turbulent velocity fluctuation. ...
-
modelling of the tangential strain rate term of the flame surface density transport equation in the context of reynolds averaged navier stokes simulation
Proceedings of the Combustion Institute, 2011Co-Authors: Mohit Katragadda, Sean P Malkeson, Nilanjan ChakrabortyAbstract:Abstract The modelling of the tangential strain rate term in the Flame Surface Density (FSD) transport equation in the context of Reynolds Averaged Navier–Stokes (RANS) simulations of turbulent premixed combustion has been addressed by a priori analysis of a Direct Numerical Simulation (DNS) database of statistically planar freely propagating flames with wide variations of Damkohler Number Da , heat release parameter τ and global Lewis Number Le . It has been found that the dilatation rate contribution to the FSD transport strengthens with increasing value of τ and decreasing value of Le . The behaviour of the normal strain rate term shows significant differences in response to Da and Le . It has been found that the normal strain rate contribution to the FSD transport remains a sink term for the flames with high and small values of Da and Le , respectively, where the effects of strain rate induced by heat release a chem overcome the effects of turbulent straining a turb . By contrast, the effects of a turb overcomes the effects of a chem for low Da flames with Le ⩾ 1 , which leads to a positive value of the normal strain rate term towards the unburned gas side, but this term becomes negative towards the burned gas side due to strong a chem overcoming a turb in the regions of intense heat release. The strengthening of the dilatation rate and a chem at small and large values of Le and Da , respectively, is explicitly taken into account to propose new models for the strain rate contributions to the FSD transport. The new model is shown to satisfactorily capture the effects of Damkohler Number Da , heat release parameter τ , and global Lewis Number Le , on the tangential strain rate term of the FSD transport equation for all the cases considered in this study.
-
a priori direct numerical simulation assessment of algebraic models of variances and dissipation rates in the context of reynolds averaged navier stokes simulations for low Damkohler Number partially premixed combustion
Combustion Science and Technology, 2010Co-Authors: Sean P Malkeson, Nilanjan ChakrabortyAbstract:Statistically planar turbulent premixed and partially premixed flames for different initial turbulence intensity are simulated for global equivalence ratio ⟨φ⟩ = 0.7 and 1.0 using three-dimensional simplified chemistry based Direct Numerical Simulations (DNS). For the simulations of partially premixed flames a bimodal distribution of equivalence ratio variation about the prescribed value of ⟨φ⟩ is introduced in the fresh reactants. The simulation parameters are chosen in such a manner that the combustion situation in all the cases represents the thin reaction zones regime with global Damkohler Number smaller than unity. The DNS data has been used to analyze the statistics of the variances , covariances (where Y, ξ and c are the fuel mass fraction, mixture fraction, and reaction progress variable, respectively, and tilde and double prime represent the Favre mean and Favre fluctuation of the relevant quantities, respectively), scalar dissipation rates (i.e., and ) of active scalar variances, and the cross-s...
N Swaminathan - One of the best experts on this subject based on the ideXlab platform.
-
a priori assessment of closures for scalar dissipation rate transport in turbulent premixed flames using direct numerical simulation
Physics of Fluids, 2008Co-Authors: Nilanjan Chakraborty, J W Rogerson, N SwaminathanAbstract: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 Damkohler Number on turbulence scalar interaction in premixed flames ii model development
Physics of Fluids, 2007Co-Authors: Nilanjan Chakraborty, N SwaminathanAbstract: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...
-
influence of the Damkohler Number on turbulence scalar interaction in premixed flames i physical insight
Physics of Fluids, 2007Co-Authors: Nilanjan Chakraborty, N SwaminathanAbstract: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....
-
study of the conditional covariance and variance equations for second order conditional moment closure
Physics of Fluids, 1999Co-Authors: N Swaminathan, R W BilgerAbstract:Presence of transient events like extinction/ignition in turbulent reacting flows increase conditional fluctuations. Thus, one needs to include the conditional fluctuations of reactive scalars, second order quantities, for conditional moment closure calculation when there are transient events and for predictions of sensitive species like NOx. Transport equations for conditional variances Gii and covariances Gij are derived and studied using a direct numerical simulation (DNS) data base. In high Damkohler Number situations, chemically reactive and turbulent diffusive processes balance the effect of scalar dissipation–scalar fluctuations correlation on Gij evolution. Additionally, the dissipation of scalar fluctuation becomes important for low Damkohler Number situations. Simple models for the different physical processes are proposed and evaluated. Chemical contributions are modeled using a presumed probability density function (PDF) approach which includes second order contributions. The conditional joint...
-
conditional variance equation and its analysis
Symposium (International) on Combustion, 1998Co-Authors: N Swaminathan, R W BilgerAbstract:The conditional moment closure method, found to perform well for reacting flow predictions, needs a second-order closure when there are extinction and ignition processes occurring in the flow. The secondorder contributions include the effect of the conditional variance of reactive species fluctuations. An evolution equation for the conditional variance is derived and is studied using a direct numerical simulation (DNS) database involving no global extinction or ignition. It is found that turbulent dissipative, diffusive, and chemically reactive processes balance the conditional variance production by scalar dissipation-scalar fluctuations correlation. The Damkohler Number is found to play a role in the behavior of the abovementioned physical processes. The evolution of the conditional variance is observed to be a quasi-steady process although the bulk of the flow is decaying.
Yutaka Tada - One of the best experts on this subject based on the ideXlab platform.
-
Miscible viscous fingering involving viscosity increase by a chemical reaction with moderate Damköhler Number
Physics of Fluids, 2011Co-Authors: Yuichiro Nagatsu, Yoshihito Kato, Yusuke Kondo, Yutaka TadaAbstract:In our previous study, we experimentally studied the effects of increased or decreased viscosity of the more-viscous liquid caused by chemical reactions at a very high Damkohler Number, Da (defined as the ratio between a characteristic time of fluid motion and that of a chemical reaction), on miscible viscous fingering [Y. Nagatsu et al., “Experimental study on miscible viscous fingering involving viscosity changes induced by variations in chemical species concentrations due to chemical reactions,” J. Fluid Mech. 571, 475 (2007)]. In another study, we experimentally studied the effects of decreased viscosity caused by chemical reaction at a moderate Da on the fingering [Y. Nagatsu et al., “Effects of moderate Damkohler Number on miscible viscous fingering involving viscosity decrease due to a chemical reaction,” J. Fluid Mech. 625, 97 (2009)]. In the present study, we investigated the effects of increased viscosity caused by chemical reaction at a moderate Da on the fingering in a radial Hele-Shaw cell. W...
-
effects of moderate Damkohler Number on miscible viscous fingering involving viscosity decrease due to a chemical reaction
Journal of Fluid Mechanics, 2009Co-Authors: Yuichiro Nagatsu, Yoshihito Kato, Yusuke Kondo, Yutaka TadaAbstract:We have succeeded in experimentally investigating the effects of a moderate Damkohler Number, Da (defined as the ratio between a characteristic time of fluid motion and that of a chemical reaction), for various Peclet Numbers, Pe, on miscible viscous fingering involving a decrease in the viscosity of the displaced liquid due to a chemical reaction in Hele-Shaw cells. We achieved this by using a chemical reaction between a polymer solution and metal ions. Main analysis has been done for the radial fingering. In the range of Pe employed here, the fingering patterns without the reaction (Da = 0) were independent of Pe. The fingering patterns with the reaction depended on the single parameter, Da, and the area occupied by the fingering pattern near the injection hole increased with Da in the range of Da employed here. The ratio of the area occupied by the fingering pattern within the circle radius of which is the length of longest finger to the area of the circle increased with Da in the range of Da employed here. This result is opposite to that of Nagatsu et al. (J. Fluid Mech., vol. 571, 2007, p. 475), in which the area was decreased by the reaction decreasing the viscosity involving significantly high Da. Experiments in the linear geometry show that the shape of a single finger also depended on the single parameter, Da, and the finger width increased near the base with Da. This result is also opposite to that in the previous case in which the width of a single finger was considered to be decreased by the reaction. These results, interestingly, show that the effects of the decrease in the displaced liquid's viscosity due to chemical reaction on the fingering pattern for moderate Da are opposite to those for significantly high Da. A mechanism for the opposite effects on the fingering pattern depending on Da is discussed.
Andrei Lipatnikov - One of the best experts on this subject based on the ideXlab platform.
-
turbulent diffusion of chemically reacting flows theory and numerical simulations
Physical Review E, 2017Co-Authors: Tov Elperin, Andrei Lipatnikov, Michael A. Liberman, Nathan Kleeorin, Igor Rogachevskii, Rixin YuAbstract:The theory of turbulent diffusion of chemically reacting gaseous admixtures developed previously [T. Elperin, Phys. Rev. E 90, 053001 (2014)PLEEE81539-375510.1103/PhysRevE.90.053001] is generalized for large yet finite Reynolds Numbers and the dependence of turbulent diffusion coefficient on two parameters, the Reynolds Number and Damkohler Number (which characterizes a ratio of turbulent and reaction time scales), is obtained. Three-dimensional direct numerical simulations (DNSs) of a finite-thickness reaction wave for the first-order chemical reactions propagating in forced, homogeneous, isotropic, and incompressible turbulence are performed to validate the theoretically predicted effect of chemical reactions on turbulent diffusion. It is shown that the obtained DNS results are in good agreement with the developed theory. (Less)
-
effects of lewis Number on conditional fluid velocity statistics in low Damkohler Number turbulent premixed combustion a direct numerical simulation analysis
Physics of Fluids, 2013Co-Authors: Nilanjan Chakraborty, Andrei LipatnikovAbstract:The effects of global Lewis Number Le on the statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes simulations have been analysed using a Direct Numerical Simulations (DNS) database of statistically planar turbulent premixed flames with a low Damkohler Number and Lewis Number ranging from 0.34 to 1.2. The conditional velocity statistics extracted from DNS data have been analysed with respect to the well-known Bray-Moss-Libby (BML) expressions which were derived based on bi-modal probability density function of reaction progress variable for high Damkohler Number flames. It has been shown that the Lewis Number substantially affects the mean velocity and the velocity fluctuation correlation conditional in products, with the effect being particularly pronounced for low Le. As far as the mean velocity and the velocity fluctuation correlation conditional in reactants are concerned, the BML expressions agree reasonably well with the DNS data reported in the present work. Based on a priori analysis of present and previously reported DNS data, the BML expressions have been empirically modified here in order to account for Lewis Number effects, and the non-bimodal distribution of reaction progress variable. Moreover, it has been demonstrated for the first time that surface averaged velocity components and Reynolds stresses conditional in unburned reactants can be modelled without invoking expressions involving the Lewis Number, as these surface averaged conditional quantities remain approximately equal to their conditionally averaged counterparts in the unburned mixture.
-
conditional velocity statistics for high and low Damkohler Number turbulent premixed combustion in the context of reynolds averaged navier stokes simulations
Proceedings of the Combustion Institute, 2013Co-Authors: Nilanjan Chakraborty, Andrei LipatnikovAbstract:The statistics of fluid velocity components conditional in unburned reactants and fully burned products in the context of Reynolds Averaged Navier Stokes (RANS) simulations have been analysed using a Direct Numerical Simulation (DNS) database of statistically planar turbulent premixed flames for both high and low values of Damkohler Number for different values of heat release parameter. It has been found that the contributions arising from chemical reaction to the conditional mean velocities and the conditional Reynolds stresses remain strong under high values of Damkohler Number. The expressions for conditional mean velocity components and conditional Reynolds stresses, which are derived based on bi-modal probability density function of reaction progress variable for unity Lewis Number flames, are modified in this study in such a manner that the new expressions can be used for low Damkohler Number flames where bi-modal distribution is not realised. Suitable models for conditional surface-averaged velocity components and the Reynolds stresses have been identified, which are shown to work satisfactorily for all values of Damkohler Number and heat release parameter considered in this analysed.
Yuichiro Nagatsu - One of the best experts on this subject based on the ideXlab platform.
-
Miscible viscous fingering involving viscosity increase by a chemical reaction with moderate Damköhler Number
Physics of Fluids, 2011Co-Authors: Yuichiro Nagatsu, Yoshihito Kato, Yusuke Kondo, Yutaka TadaAbstract:In our previous study, we experimentally studied the effects of increased or decreased viscosity of the more-viscous liquid caused by chemical reactions at a very high Damkohler Number, Da (defined as the ratio between a characteristic time of fluid motion and that of a chemical reaction), on miscible viscous fingering [Y. Nagatsu et al., “Experimental study on miscible viscous fingering involving viscosity changes induced by variations in chemical species concentrations due to chemical reactions,” J. Fluid Mech. 571, 475 (2007)]. In another study, we experimentally studied the effects of decreased viscosity caused by chemical reaction at a moderate Da on the fingering [Y. Nagatsu et al., “Effects of moderate Damkohler Number on miscible viscous fingering involving viscosity decrease due to a chemical reaction,” J. Fluid Mech. 625, 97 (2009)]. In the present study, we investigated the effects of increased viscosity caused by chemical reaction at a moderate Da on the fingering in a radial Hele-Shaw cell. W...
-
effects of moderate Damkohler Number on miscible viscous fingering involving viscosity decrease due to a chemical reaction
Journal of Fluid Mechanics, 2009Co-Authors: Yuichiro Nagatsu, Yoshihito Kato, Yusuke Kondo, Yutaka TadaAbstract:We have succeeded in experimentally investigating the effects of a moderate Damkohler Number, Da (defined as the ratio between a characteristic time of fluid motion and that of a chemical reaction), for various Peclet Numbers, Pe, on miscible viscous fingering involving a decrease in the viscosity of the displaced liquid due to a chemical reaction in Hele-Shaw cells. We achieved this by using a chemical reaction between a polymer solution and metal ions. Main analysis has been done for the radial fingering. In the range of Pe employed here, the fingering patterns without the reaction (Da = 0) were independent of Pe. The fingering patterns with the reaction depended on the single parameter, Da, and the area occupied by the fingering pattern near the injection hole increased with Da in the range of Da employed here. The ratio of the area occupied by the fingering pattern within the circle radius of which is the length of longest finger to the area of the circle increased with Da in the range of Da employed here. This result is opposite to that of Nagatsu et al. (J. Fluid Mech., vol. 571, 2007, p. 475), in which the area was decreased by the reaction decreasing the viscosity involving significantly high Da. Experiments in the linear geometry show that the shape of a single finger also depended on the single parameter, Da, and the finger width increased near the base with Da. This result is also opposite to that in the previous case in which the width of a single finger was considered to be decreased by the reaction. These results, interestingly, show that the effects of the decrease in the displaced liquid's viscosity due to chemical reaction on the fingering pattern for moderate Da are opposite to those for significantly high Da. A mechanism for the opposite effects on the fingering pattern depending on Da is discussed.
