4.5 Regime transition mechanisms

Regime transition mechanisms are crucial in multiphase flow systems. They determine how flow patterns change based on factors like superficial velocities, void fraction, and pressure gradients. Understanding these mechanisms helps predict and control flow behavior in various industrial applications.

Flow pattern maps, stability analysis, and modeling approaches are used to predict transitions. Experimental techniques like flow visualization and tomography provide data to validate models. This knowledge is applied in oil pipelines, nuclear reactors, and other systems where multiphase flow is common.

Regime transition criteria

• Regime transition criteria are used to predict and characterize the changes in flow patterns that occur in multiphase flow systems
• These criteria are based on various parameters such as the superficial velocities of the phases, the void fraction, and the pressure gradients
• Understanding regime transition criteria is crucial for designing and optimizing multiphase flow systems in various industrial applications

Flow pattern maps

Top images from around the web for Flow pattern maps

• 

  Detecting Pipeline Anomalies and Variations in Acoustic Velocity in Multiphase Flow Regimes ... View original

  Is this image relevant?

• 

  Probabilistic approach of a flow pattern map for horizontal, vertical, and inclined pipes | Oil ... View original

  Is this image relevant?

• 

  Frontiers | Petrological Geodynamics of Mantle Melting I. AlphaMELTS + Multiphase Flow: Dynamic ... View original

  Is this image relevant?

• 

  Detecting Pipeline Anomalies and Variations in Acoustic Velocity in Multiphase Flow Regimes ... View original

  Is this image relevant?

• 

  Probabilistic approach of a flow pattern map for horizontal, vertical, and inclined pipes | Oil ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Flow pattern maps

• 

  Detecting Pipeline Anomalies and Variations in Acoustic Velocity in Multiphase Flow Regimes ... View original

  Is this image relevant?

• 

  Probabilistic approach of a flow pattern map for horizontal, vertical, and inclined pipes | Oil ... View original

  Is this image relevant?

• 

  Frontiers | Petrological Geodynamics of Mantle Melting I. AlphaMELTS + Multiphase Flow: Dynamic ... View original

  Is this image relevant?

• 

  Detecting Pipeline Anomalies and Variations in Acoustic Velocity in Multiphase Flow Regimes ... View original

  Is this image relevant?

• 

  Probabilistic approach of a flow pattern map for horizontal, vertical, and inclined pipes | Oil ... View original

  Is this image relevant?

1 of 3

• Flow pattern maps are graphical representations of the different flow regimes that can occur in a multiphase flow system
• These maps are typically plotted with the superficial velocities of the phases on the axes and the flow patterns are identified as regions on the map
• Different types of flow pattern maps include the Baker map, the Mandhane map, and the Taitel-Dukler map
• Flow pattern maps are used to predict the expected flow regime based on the operating conditions of the system

Superficial velocities

• Superficial velocities are the hypothetical velocities of each phase in a multiphase flow system assuming that the phase is flowing alone in the pipe
• The superficial velocity of a phase is calculated by dividing the volumetric flow rate of the phase by the cross-sectional area of the pipe
• The ratio of the superficial velocities of the phases is a key parameter in determining the flow regime
• Examples of superficial velocity ratios include the gas-liquid ratio (GLR) and the liquid-liquid ratio (LLR)

Void fraction

• Void fraction is the ratio of the volume occupied by the gas phase to the total volume of the multiphase mixture
• Void fraction is a crucial parameter in characterizing the flow regime and determining the pressure drop in the system
• The void fraction can be measured using various techniques such as quick-closing valves, conductivity probes, and gamma-ray densitometers
• The void fraction is used in the calculation of the mixture density and the slip velocity between the phases

Pressure gradients

• Pressure gradients are the changes in pressure along the length of the pipe in a multiphase flow system
• The pressure gradient is influenced by factors such as the flow regime, the void fraction, and the superficial velocities of the phases
• The pressure gradient can be used to identify the transition between different flow regimes, such as the transition from bubble flow to slug flow
• Examples of pressure gradient correlations include the Lockhart-Martinelli correlation and the Friedel correlation

Transition mechanisms

• Transition mechanisms are the physical processes that lead to the change from one flow regime to another in a multiphase flow system
• These mechanisms are governed by the interplay between the inertial, viscous, and interfacial forces acting on the phases
• Understanding the transition mechanisms is essential for predicting the onset of flow regime transitions and for developing accurate models of multiphase flow

Kelvin-Helmholtz instabilities

• Kelvin-Helmholtz instabilities occur at the interface between two fluids with different velocities, such as at the interface between a gas and a liquid in stratified flow
• These instabilities are caused by the shear forces acting at the interface and lead to the formation of waves and the eventual breakup of the interface
• Kelvin-Helmholtz instabilities play a crucial role in the transition from stratified flow to slug flow or annular flow
• The onset of Kelvin-Helmholtz instabilities can be predicted using the Kelvin-Helmholtz stability criterion, which compares the destabilizing effect of the shear forces to the stabilizing effect of the surface tension and gravity forces

Rayleigh-Taylor instabilities

• Rayleigh-Taylor instabilities occur when a heavier fluid is accelerated into a lighter fluid, such as in the case of a falling film of liquid in a gas stream
• These instabilities lead to the formation of fingers or droplets of the heavier fluid penetrating into the lighter fluid
• Rayleigh-Taylor instabilities are important in the transition from annular flow to dispersed droplet flow
• The onset of Rayleigh-Taylor instabilities can be predicted using the Rayleigh-Taylor stability criterion, which compares the destabilizing effect of the acceleration to the stabilizing effect of the surface tension and viscous forces

Turbulence vs laminar flow

• The transition from laminar to turbulent flow in multiphase systems is influenced by the interactions between the phases and the flow regime
• In bubbly flow, the presence of bubbles can trigger the transition to turbulence at lower Reynolds numbers compared to single-phase flow
• In slug flow, the turbulence in the liquid slugs can lead to the breakup of the gas bubbles and the transition to churn flow
• The transition to turbulence in multiphase flow can be predicted using modified Reynolds number criteria that account for the presence of the dispersed phase

Coalescence and breakup

• Coalescence and breakup of the dispersed phase (bubbles or droplets) play a significant role in the transition between different flow regimes
• In bubbly flow, coalescence of bubbles can lead to the formation of larger bubbles and the transition to slug flow
• In droplet flow, coalescence of droplets can lead to the formation of a continuous liquid phase and the transition to annular flow
• Breakup of the dispersed phase can be caused by turbulence, shear forces, or interfacial instabilities
• The rates of coalescence and breakup can be modeled using population balance equations that account for the size distribution of the dispersed phase

Modeling approaches

• Various modeling approaches have been developed to predict the flow regime transitions and the behavior of multiphase flow systems
• These approaches range from simple empirical correlations to complex mechanistic models that account for the underlying physical processes
• The choice of the modeling approach depends on the level of detail required, the available computational resources, and the specific application

Stability analysis

• Stability analysis is used to predict the onset of flow regime transitions based on the stability of the interface between the phases
• Linear stability analysis involves linearizing the governing equations and determining the growth rate of small perturbations at the interface
• The most unstable wavelength and the corresponding growth rate can be used to predict the transition from stratified flow to slug flow or annular flow
• Examples of stability analysis methods include the Kelvin-Helmholtz analysis, the Rayleigh-Taylor analysis, and the Orr-Sommerfeld analysis

Critical void fraction

• The critical void fraction is the void fraction at which the flow regime transition occurs
• Empirical correlations have been developed to predict the critical void fraction based on the fluid properties and the flow conditions
• The Taitel-Dukler model uses the critical void fraction to predict the transition from intermittent flow (slug or churn) to annular flow
• The Mishima-Ishii model uses the critical void fraction to predict the transition from bubbly flow to slug flow

Drift flux models

• Drift flux models are used to predict the void fraction and the pressure drop in multiphase flow systems
• These models assume that the phases are interpenetrating continua and that the relative motion between the phases can be described by a drift velocity
• The drift velocity is a function of the fluid properties, the flow conditions, and the flow regime
• Examples of drift flux models include the Zuber-Findlay model, the Wallis model, and the Ishii model

Two-fluid models

• Two-fluid models are based on separate conservation equations for each phase, coupled through interfacial transfer terms
• These models can capture the detailed dynamics of the phases, including the relative motion and the interactions at the interface
• Two-fluid models require closure relations for the interfacial transfer terms, such as the drag force, the virtual mass force, and the turbulent dispersion force
• Examples of two-fluid models include the Eulerian-Eulerian model, the Eulerian-Lagrangian model, and the two-fluid Reynolds-averaged Navier-Stokes (RANS) model

Experimental techniques

• Experimental techniques are essential for validating and improving the models used to predict flow regime transitions and multiphase flow behavior
• These techniques provide detailed measurements of the flow parameters, such as the void fraction, the phase velocities, and the interfacial area concentration
• The choice of the experimental technique depends on the flow conditions, the fluid properties, and the desired spatial and temporal resolution

Flow visualization

• Flow visualization techniques are used to observe the flow patterns and the interactions between the phases
• High-speed video cameras can capture the dynamics of the flow, such as the formation and breakup of bubbles or droplets
• Particle image velocimetry (PIV) can provide instantaneous velocity fields of the phases
• Planar laser-induced fluorescence (PLIF) can visualize the concentration fields of the phases or the distribution of a tracer dye

Conductivity probes

• Conductivity probes are used to measure the local void fraction and the bubble size distribution in gas-liquid flows
• These probes consist of two or more electrodes that detect the presence of the gas phase based on the change in conductivity
• Double-sensor probes can also measure the bubble velocity by cross-correlating the signals from two closely-spaced sensors
• Examples of conductivity probes include the resistive probe, the capacitive probe, and the optical probe

Optical methods

• Optical methods are used to measure the phase distributions and the interfacial area concentration in transparent multiphase flows
• Laser Doppler anemometry (LDA) can measure the velocity of the phases by detecting the Doppler shift of laser light scattered by tracer particles
• Phase Doppler anemometry (PDA) can measure the size and velocity of droplets or bubbles by analyzing the phase shift of the scattered light
• Interferometric particle imaging (IPI) can measure the size and shape of bubbles or droplets by analyzing the interference fringes formed by the scattered light

X-ray and gamma-ray tomography

• X-ray and gamma-ray tomography are used to measure the phase distributions in opaque multiphase flows
• These techniques involve measuring the attenuation of X-rays or gamma-rays as they pass through the flow and reconstructing the phase distributions using tomographic algorithms
• X-ray computed tomography (CT) can provide high-resolution 3D images of the phase distributions
• Gamma-ray densitometry can measure the line-averaged void fraction along a beam path
• Examples of X-ray and gamma-ray tomography systems include the medical CT scanner, the industrial CT scanner, and the gamma-ray densitometer

Industrial applications

• Multiphase flow is encountered in a wide range of industrial applications, from oil and gas production to chemical processing and power generation
• Understanding flow regime transitions is crucial for the design, operation, and optimization of these systems
• The models and experimental techniques discussed in this course are applied to solve practical problems and improve the performance of industrial processes

Oil and gas pipelines

• Multiphase flow is common in oil and gas pipelines, where oil, gas, and water are transported together over long distances
• Flow regime transitions can affect the pressure drop, the phase separation, and the corrosion in the pipeline
• Models are used to predict the flow patterns, the pressure drop, and the phase distributions in the pipeline
• Experimental techniques, such as gamma-ray densitometers and acoustic sensors, are used to monitor the flow conditions and detect any anomalies

Nuclear reactor safety

• Multiphase flow is important in nuclear reactors, where the coolant (water) can undergo phase change (boiling) during normal operation or accident conditions
• Flow regime transitions can affect the heat transfer, the void fraction, and the reactivity in the reactor core
• Models are used to predict the flow patterns, the void fraction distribution, and the critical heat flux in the reactor
• Experimental techniques, such as conductivity probes and X-ray tomography, are used to measure the void fraction and the phase distributions in the reactor

Boiling and condensation

• Boiling and condensation are phase change processes that involve multiphase flow and heat transfer
• Flow regime transitions can affect the heat transfer coefficient, the critical heat flux, and the pressure drop in boiling and condensation systems
• Models are used to predict the flow patterns, the heat transfer coefficient, and the critical heat flux in these systems
• Experimental techniques, such as high-speed video and infrared thermography, are used to visualize the flow patterns and measure the surface temperature distribution

Fluidized bed reactors

• Fluidized bed reactors are used in chemical processing and combustion applications, where a gas is passed through a bed of solid particles
• Flow regime transitions can affect the mixing, the heat transfer, and the reaction rates in the fluidized bed
• Models are used to predict the flow patterns, the pressure drop, and the particle size distribution in the fluidized bed
• Experimental techniques, such as pressure transducers and optical probes, are used to measure the pressure fluctuations and the particle concentration in the fluidized bed