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With the implementation of efficient algorithms for the accurate calculation of reaction source terms, computational fluid dynamics (CFD) is now a powerful tool for the simulation and design of chemical reactors with complex kinetic schemes. The example studied in this work is the methane chlorination reaction for which the detailed chemistry scheme has 152 reactions and 38 species. The adiabatic, jet-stirred chlorination reactor used for the CFD simulations is an insulated right cylinder with a coaxial premixed feed stream at one end. In order for this reactor to remain lit, recirculation of hot products is crucial, and hence, reactor stability is sensitive to both macroscale and microscale mixing. By neglecting density variations, a Lagrangian composition probability density function (PDF) code with a novel chemistry tabulation algorithm (in-situ adaptive tabulation or ISAT) for handling complex reactions is used to simulate the species concentrations and temperature field inside of the reactor. In addition, a reduced mechanism with 21 reactions and 15 species is tested for accuracy against the detailed chemistry scheme, a simplified CSTR model is used to illustrate the shortcomings of zero-dimensional models, and a pair-wise mixing stirred reactor (PMSR) model is used to show the stabilizing effect of micromixing on reactor stability. The CFD simulations are generally in good agreement with results from pilot-scale reactors for the outlet temperature and major species. Albert D. Harvey and David H. WestThe Dow Chemical Company, Freeport, With the implementation of efficient algorithms for the accurate calculation of reaction source terms, computational fluid dynamics (CFD) is now a powerful tool for the simulation and design of chemical reactors with complex kinetic schemes. The example studied in this work is the methane chlorination reaction for which the detailed chemistry scheme has 152 reactions and 38 species. The adiabatic, jet-stirred chlorination reactor used for the CFD simulations is an insulated right cylinder with a coaxial premixed feed stream at one end. In order for this reactor to remain lit, recirculation of hot products is crucial, and hence, reactor stability is sensitive to both macroscale and microscale mixing. By neglecting density variations, a Lagrangian composition probability density function (PDF) code with a novel chemistry tabulation algorithm (in-situ adaptive tabulation or ISAT) for handling complex reactions is used to simulate the species concentrations and temperature field inside of the reactor. In addition, a reduced mechanism with 21 reactions and 15 species is tested for accuracy against the detailed chemistry scheme, a simplified CSTR model is used to illustrate the shortcomings of zero-dimensional models, and a pair-wise mixing stirred reactor (PMSR) model is used to show the stabilizing effect of micromixing on reactor stability. The CFD simulations are generally in good agreement with results from pilot-scale reactors for the outlet temperature and major spe...

Chlorination of hydrocarbons is an industrially important process used for the production of commercially viable environmentally friendly chemicals. The highly exothermic nature of these reactions necessitates a thorough study of reactor stability and product feasibility. Here, computational fluid dynamics (CFD) is used to analyze the performance of a coaxial rightcylindrical insulated reactor for different inlet flow configurations. Chlorination reactions involve a large number of radicals and other intermediates, and hence, direct simulations using traditional CFD techniques are difficult because of the stiff nature of the reaction scheme involved. A novel algorithm for reaction computation, in situ adaptive tabulation (ISAT), is used to obtain considerable computational gains. The joint probability density function ( JPDF) transport equation for the scalars with closed terms for reaction is solved using a Monte Carlo particle algorithm in tandem with a finitevolume (FV) Reynolds-averaged Navier-Stokes (RANS) method. The particle method handles transport of 15 scalars along with enthalpy and feeds back mean field values of temperature and molecular weight that are used by the FV code to correct the flow for reaction. The scalar scatter plots conditioned on the mixture fraction are used to study the details of the kinetics in different reactor zones. Comparison of premixed and segregated inlets is done to determine reactor stability and product yield. Conclusions are then drawn about fundamental properties of the reactor and broad considerations for reactor design. Chlorination of hydrocarbons is an industrially important process used for the production of commercially viable environmentally friendly chemicals. The highly exothermic nature of these reactions necessitates a thorough study of reactor stability and product feasibility. Here, computational fluid dynamics (CFD) is used to analyze the performance of a coaxial rightcylindrical insulated reactor for different inlet flow configurations. Chlorination reactions involve a large number of radicals and other intermediates, and hence, direct simulations using traditional CFD techniques are difficult because of the stiff nature of the reaction scheme involved. A novel algorithm for reaction computation, in situ adaptive tabulation (ISAT), is used to obtain considerable computational gains. The joint probability density function (JPDF) transport equation for the scalars with closed terms for reaction is solved using a Monte Carlo particle algorithm in tandem with a finite-volume (FV) Reynolds-averaged Navier-Stokes (RANS) method. The particle method handles transport of 15 scalars along with enthalpy and feeds back mean field values of temperature and molecular weight that are used by the FV code to correct the flow for reaction. The scalar scatter plots conditioned on the mixture fraction are used to study the details of the kinetics in different reactor zones. Comparison of premixed and segregated inlets is done to determine reactor stability and prod...

No abstract

We study the nucleation and growth of a nanobubble on rough surfaces using molecular dynamics simulations. A nanobubble nucleates and grows by virtue of a heterogeneous surface reaction which results in the production of gas molecules near the surface. We study the role of surface roughness in the nucleation and growth behavior of a nanobubble. We perform simulations at various reaction rates and surface morphology and quantified the growth dynamics of a nanobubble. Our simulations show that after the onset of nucleation, the nanobubble grows rapidly with radius following t 1/3 behavior followed by a diffusive growth regime which is marked by t 1/2 growth behavior. This growth behavior remains independent of surface roughness and reaction rates over the range considered in this study. We also quantified the oversaturation of gas required for nucleation of a nanobubble and demonstrated its dependence on the surface morphology.

A framework is demonstrated to explore the performance limits and investigate the trade-offs between key performance metrics for membraneless electrolyzers.

An approximate steady-state method is devised for computing the flow field in a baffled, impeller-stirred tank reactor. The flow field in a cylindrical tank with a 45" pitched-blade impeller rotating at 100 rpm and four stationary rectangular side-wall bafles is simulated using a new approximate steady-state approach. The method provides an alternative to a full unsteady Navier -Stokes simulation. The new steady-state analysis involves accurately defining the geometly of the mixing tank using a multiblock grid technique. The flow is solved from a rotating frame of reference jfbr a single position of the impeller relative to the side-wall baffles. The steady-state numerical results are then spatially averaged and compared with time-averaged data obtained experimentally using laser Doppler velocimetry (LDV). Spatially averaged numerical predictions obtained using this approximate steady-state method for the radial and axial velocity components agree well with the L D V data. The predicted magnitude (of the tangential velocity component, however, is higher than the experimentally measured values. Closer agreement of the tangential velocities with experimental values is obtained using a finer grid and it is found that a relatively fine grid is needed for accurately predicting the tangential velocity magnitude. Use of this approximate steady-state method allows designers of mixing vessels to obtain flow-field results for baffled vessels much more efficiently than using full unsteady Navier -Stokes simulations.introduction Correspondence concerning this arlicle should be addressed to A. D. Harvey Present address for C. K. Lee: Midland, MI 48674.ble. Also, this scale-up is sufficiently complex so as to warrant a procedure whereby scale-up takes place in several successive stages, each stage becoming larger than its successor, incurring not only the cost of the final process, but also the cost of each intermediate process.Computational techniques can significantly reduce the cost associated with this scale-up process by verifying the performance of large-scale units before their actual manufacture, and provide an aid in making critical reactor design decisions. Before any computational results can be deemed reliable enough to influence design decisions, the computational model must be validated using experimental data. The present work is part of a research effort aimed at developing and validating a set of computational tools to numerically study the flow field inside an impeller-stirred mixing tank. These computational tools include a multiblock grid-generation program, and an incompressible Navier-Stokes code. a where the inviscid fluxes in the transformed curvilinear coordinate system are written as 2 J SC,, = -Vol. 41, No. 10 AICbE Journal

In this study, the predictive performance of six different two-equation turbulence models on the flow in an unbaffled stirred tank has been investigated. These models include the low Reynolds number k-ε model of Rodi, W., and Mansour, N. N., “Low Reynolds Number k-ε Modeling With the Aid of Direct Simulation Data,” J. Fluid Mech., Vol. 250, pp. 509–529, the high and low Reynolds number k-ω models of Wilson, D. C., 1993, Turbulence Modeling for CFD, DCW Industries, La Canada, CA., the RNG k-ε model, and modified k-ω and k-ε models which incorporate a correction for streamline curvature and swirl. Model results are compared with experimental laser Doppler velocimetry (LDV) data for the turbulent velocity field in an unbaffled tank with a single paddle impeller. An overall qualitative agreement has been found between the experimental and numerical results with poor predictions observed in some parts of the tank. Discrepancies in model predictions are observed in the anisotropic regions of the flow such as near the impeller shaft and in the impeller discharge region where the model overpredicts the radial velocity component. These results are discussed and a strategy for improving two-equation models for application to impeller stirred tanks is proposed.