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Multi-species turbulent mixing under supercritical-pressure conditions: modelling, direct numerical simulation and analysis revealing species spinodal decomposition

Masi, Enrica and Bellan, Josette and Harstad, Kenneth G. and Okong'o, Nora A. (2013) Multi-species turbulent mixing under supercritical-pressure conditions: modelling, direct numerical simulation and analysis revealing species spinodal decomposition. Journal of Fluid Mechanics, 721 . pp. 578-626. ISSN 0022-1120.

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A model is developed for describing mixing of several species under high-pressure conditions. The model includes the Peng–Robinson equation of state, a full massdiffusion matrix, a full thermal-diffusion-factor matrix necessary to incorporate the Soret and Dufour effects and both thermal conductivity and viscosity computed for the species mixture using mixing rules. Direct numerical simulations (DNSs) are conducted in a temporal mixing layer configuration. The initial mean flow is perturbed using an analytical perturbation which is consistent with the definition of vorticity and is divergence free. Simulations are performed for a set of five species relevant to hydrocarbon combustion and an ensemble of realizations is created to explore the effect of the initial Reynolds number and of the initial pressure. Each simulation reaches a transitional state having turbulent characteristics and most of the data analysis is performed on that state. A mathematical reformulation of the flux terms in the conservation equations allows the definition of effective species-specific Schmidt numbers (Sc) and of an effective Prandtl number (Pr) based on effective speciesspecific diffusivities and an effective thermal conductivity, respectively. Because these effective species-specific diffusivities and the effective thermal conductivity are not directly computable from the DNS solution, we develop models for both of these quantities that prove very accurate when compared with the DNS database. For two of the five species, values of the effective species-specific diffusivities are negative at some locations indicating that these species experience spinodal decomposition; we determine the necessary and sufficient condition for spinodal decomposition to occur. We also show that flows displaying spinodal decomposition have enhanced vortical characteristics and trace this aspect to the specific features of high-density-gradient magnitude regions formed in the flows. The largest values of the effective speciesspecific Sc numbers can be well in excess of those known for gases but almost two orders of magnitude smaller than those of liquids at atmospheric pressure. The effective thermal conductivity also exhibits negative values at some locations and the effective Pr displays values that can be as high as those of a liquid refrigerant. Examination of the equivalence ratio indicates that the stoichiometric region is thin and coincides with regions where the mixture effective species-specific Lewis number values are well in excess of unity. Very lean and very rich regions coexist in the vicinity of the stoichiometric region. Analysis of the dissipation indicates that it is dominated by mass diffusion, with viscous dissipation being the smallest among the three dissipation modes. The sum of the heat and species (i.e. scalar) dissipation is functionally modelled using the effective species-specific diffusivities and the effective thermal conductivity. Computations of the modelled sum employing the modelled effective species-specific diffusivities and the modelled effective thermal conductivity shows that it accurately replicates the exact equivalent dissipation.

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Bellan, Josette0000-0001-9218-7017
Additional Information:© 2013 Cambridge University Press. Received 5 December 2012; revised 23 January 2013; accepted 28 January 2013; first published online 19 March 2013. This work was conducted at the Jet Propulsion Laboratory (JPL) of the California Institute of Technology (Caltech) and sponsored by the Department of Energy (DOE), Basic Energy Sciences (BES) under the direction of Dr W. Sisk and Dr M. Pederson. Sponsorship from the JPL Research and Technology Development under the Spontaneous Concepts program permitted the development of some ideas which were fully explored under the DOE-BES sponsorship. The computational resources were provided by the JPL Supercomputing Center, by NASA Advanced Supercomputing at Ames Research Center and by National Energy Research Supercomputing Center of the Department of Energy.
Funding AgencyGrant Number
Department of Energy (DOE) Basic Energy Sciences (BES)UNSPECIFIED
JPL Research and Technology Development FundUNSPECIFIED
Subject Keywords:mixing, turbulent mixing
Record Number:CaltechAUTHORS:20130627-094307167
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Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39119
Deposited By: Tony Diaz
Deposited On:28 Jun 2013 15:18
Last Modified:09 Mar 2020 13:18

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