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Class 9

Class 9. 2012-04-03 Introduction to mixture models Overview of 3D approaches. ‘Fine’ models: Resolving interfaces Modelling approaches: Phase-by-phase + jump conditions on moving interfaces One-fluid description. ‘Rough’ or ‘mixture’ models: Statistical description as mixtures of phases

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Class 9

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  1. Class 9 2012-04-03 Introduction to mixture models Overview of 3D approaches

  2. ‘Fine’ models: Resolving interfaces Modelling approaches: Phase-by-phase + jump conditions on moving interfaces One-fluid description ‘Rough’ or ‘mixture’ models: Statistical description as mixtures of phases All sub-scale inter-phase processes are parameterised Systems: 3D or 1D (pipe flow) Modelling approaches in 3D: Multi-fluid mixture One-fluid mixture Modelling discrete phases Strategies and approaches

  3. Two phase systems: Gas–liquid air–water, e.g. airlift pump steam–water Liquid–liquid oil–water Liquid–solid slurries Gas–solid pneumatic transport Three phase systems: Gas–liquid–liquid natural gas–petroleum–water Gas–liquid–solid Liquid–liquid–solid Classification of mixtures

  4. Classical steam technology Nuclear industry Refrigerator technology Petroleum industry Chemical processes Food technology Major industrial developments

  5. Interfacial area density Volume fraction also known as void fraction in gas–liquid systems porosity in gas–solid systems Two key quantities

  6. Approaches in 3D modelling

  7. Multi-fluid mixture models • Fluid elements (computational cells) are large, typically contain both/several phases • This is described by volume fraction fields, α(p) • Each phase is described by its own phasic transport equations • A common pressure field is shared • There is no thermal equilibrium • The interfaces are not resolved, but inter-phase processes must be parameterised and included in constitutional relations

  8. Balance Equations Inter-phase processes

  9. Primary field varaibles: Constitutive equations Intrinsic: Heat sources: Phase transition fluxes Inter-phase processes: Work Heat transfer It is possible to generalise to multi-component phases Constitutive Relations

  10. Pros and Cons TOO MANY EQUATIONS • Intrinsic constitutive equations are the same as the single-phase ones • One needs a lot of external constitutive equations • Some of these require empirical correlations • Sometimes there is not enough experimental data to establish such correlations • Risk of unsubstantiated assumptions • High computational demand (w.r.t. the one-fluid models) • Low computational demand (w.r.t. fine models) • Flexibility If it makes sense, some simplification can be achieved by assuming thermal equilibrium among the phases

  11. One-fluid mixture model • Derived by averaging and using simplifying assumptions • The mixture is considered as a single fluid • Common T and p • The interfaces are ignored • All interface processes are transferred to constitutive laws

  12. Pros and Cons TOO FEW EQUATIONS • Do not describe small-scale phenomena at all • The external constitutive equations must be based on empirical correlations • Not even the intrinsic constitutive equations are general, they are problem-dependent • Too much constrained to describe adequately the flow phenomena • Lowest computational demand (w.r.t. fine models)

  13. Remedies It is possible to extend the model by adding new primary fields to the model in addition to Example: volume fraction field, α(p) • Homogenous model • Generalised homogenous model • Slip model • Non-equilibrium model • Diffusion model • These include more constitutive equations and thus need more correlations • The consistency of the system cannot be assured

  14. Modelling Discrete Phases The previous models were based on the ‘Eulerian’ approach (time- and position-dependent fields) • Ambient fluid: single-phase ‘Eulerian’ model • Disperse phase — ‘Lagrangian’ model: • Establish equation of motion of particles subject to fluid forces • Solve this for each particle, and follow their path in the fluid • Draw conclusions from statistics upon particles Mixed ‘Eulerian–Lagrangian’ approach

  15. Degrees of Disperse Phase Modelling • Flow→particle: Track individual particles subject to ambient flow • Particle↔particle coupling: include interactions • Flow↔particles coupling: include effect of particles on the ambient flow • Consider particle–particle contacts Increasing particle loading

  16. Features of Disperse Phase Modelling • Effects of various fluid dynamical actions • Particle-wall interactions, depositions • Sedimentation • Bubbles and drops: • Growth and collapse • Coalescence and breakup • Studying varying particle size distribution

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