Part I: Basics
Part I covers the common background material and emphasises the latest empirical and mechanistic modelling, computational and instrumentation aspects of multiphase flows. A tutorial text is e-mailed to the participants before the course to introduce the very basic concepts and fill any basic gaps in their background, so that they can participate in the best possible way.
Monday (9:00-12:30 and 14:00-17:30)
1. Introduction to multiphase flows
Definition of multiphase flows; types of flow (two-phase, three-phase, single component, multi-component). Applications: power generation, hydrocarbon recovery, chemical processing, etc. Differences to single phase flows; examples of characteristic multiphase phenomena (Counter Current Flow Limitation, Departure from Nucleate Boiling, Dryout, instability). History of development of the subject; principal sources of information.
2. Two-phase flow instrumentation and visualization
Objective of measurements in two-phase flows, quantities characterizing a gas-liquid flow: liquid and gas holdup (“void fraction”), phase and superficial velocities, phase slip, volume flow and volume flow ratio, bubble size, interfacial area density, statistical functions. Methods to measure gas fractions: cut-off valves, differential pressure method, local (needle) probes, mesh sensors, liquid film sensors. Animations of measured data, flow maps for vertical and horizontal two-phase flows.
3. Basic modeling approaches
Conservation equations and constitutive models. Interfacial and boundary conditions. Nondimensionalization and scaling. Ensemble, volume and cross-sectional area averaging. Averaging for turbulent flows. Large eddy simulation. Multiphase flow applications.
4. Empirical and phenomenological models
Empirical models and correlations for void fraction and pressure drop; the drift flux model. Examples of phenomenological modelling: modelling of annular flow, flooding (CCFL).
Tuesday (9:00-12:30 and 14:00-17:30)
5. Instability of the gas-liquid interface and flow regime maps
Basic theory of the interfacial instability (Rayleigh-Taylor and Kelvin-Helmholtz instability) and numerous applications. Flow regime maps based on phenomenological modelling; stability of stratified flow as basis of flow regime maps.
6. Basics of phase transition, pool boiling
Thermodynamics of phase transition. Bubble nucleation. Bubble growth and departure, including the effect of microlayers and conjugate heat transfer. Macroscopic view on boiling and heat transfer: saturated and subcooled pool boiling. Transition to film boiling. Rewetting. Microscopic view on boiling (novel results obtained with micro-engineered heaters and modern diagnostics such as infrared thermography). Effects of nano- and micro-scale surface features on nucleation, nucleate boiling, CHF and rewetting. Correlations and models.
7. Flow boiling and condensation
Flow boiling models for heat transfer in a boiling channel, including onset of nucleate boiling, subcooled boiling, pool and flow boiling correlations, critical heat flux, introduction to post-CHF heat transfer, quenching. Condensation models for heat transfer, including laminar and turbulent films, influence of non-condensible gases.
8. Phenomenological models for the flow boiling crisis
Definition of the boiling crisis or Critical Heat Flux (CHF) in flow systems. Types of flow boiling crisis: Departure from Nucleate Boiling (DNB) vs Film Dryout. Physical limits of CHF. General trends of CHF with respect to mass flux, equilibrium quality, channel length, pressure and inlet subcooling. Mechanistic models for the DNB-type and Film Dryout-type boiling crisis.
Wednesday (9:00-12:30 and 14:00-17:30)
9. Thermal non-equilibrium flows
Importance of Departure from Mechanical and Thermal Equilibrium. Subcooled Boiling: Net Vapor Generation, Fully-Developed Subcooled Flow Boiling. Post-CHF Heat Transfer: Inverted-Annular and Dispersed-Flow Film Boiling. Quenching.
10. Multifield models
The need for multifield models. Interpenetrating continua and Lagrangian-Eulerian approaches. Closure requirements. One-dimensional form – structure, strengths and weaknesses. Multidimensional aspects – applicability and limitations.
11. Advanced two-phase flow instrumentation
Void fraction measurement by attenuation of ionizing radiation. Phase distribution and flow structure: Gamma, X-ray and neutron tomography, dual energy tomography, impedance tomography. Velocity measurements: hot film probes, laser methods, ultrasonic sensors.
12. Numerical methods
Introduction. Initial and boundary conditions. Method of characteristics. Finite difference methods. Stability. Explicit and implicit methods. Methods used in computer codes.