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Two-Phase Flow in Fluid Mechanics

Two-phase flow occurs when a mixture of gas and liquid flow together and can take different forms such as separated or dispersed flows. It is categorized based on the superficial velocity of each phase, which is the hypothetical velocity if that phase flowed alone. Two-phase flow has complex characteristics due to factors like surface tension, density differences between phases, varying sound speeds during phase changes, and flow-induced pressure changes causing more phase changes.

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36 views4 pages

Two-Phase Flow in Fluid Mechanics

Two-phase flow occurs when a mixture of gas and liquid flow together and can take different forms such as separated or dispersed flows. It is categorized based on the superficial velocity of each phase, which is the hypothetical velocity if that phase flowed alone. Two-phase flow has complex characteristics due to factors like surface tension, density differences between phases, varying sound speeds during phase changes, and flow-induced pressure changes causing more phase changes.

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engrhussainali3
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Two Phase Flow

In fluid mechanics, two-


phase flow is a flow of
gas and liquid — a
particular example of
multiphase flow. Two-
phase flow can occur in
various forms, such as
flows transitioning from
pure liquid to vapour as
a result of external
heating, separated flows,
and dispersed two-
phase flows where one
phase is present in the
form of particles,
droplets, or bubbles in a
continuous carrier phase
(i.e. gas or liquid).
Categorization
Different modes of two-phase flows.

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The widely accepted method to categorize two-phase flows is to consider
the velocity of each phase as if there is not other phases available. The
parameter is a hypothetical concept called Superficial velocity.

Superficial velocity (or superficial flow velocity), in the engineering multiphase


flows and flows in porous media, is a hypothetical (artificial) flow velocity
calculated as if the given phase or fluid were the only one flowing or present
in a given cross-sectional area. Other phases, particles, the skeleton of the
porous medium, etc. present in the channel are disregarded.

Superficial velocity is used in many engineering equations because it is the


value which is usually readily known and unambiguous, whereas real velocity
is often variable from place to place.

Superficial velocity can be expressed as:

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Using the concept of porosity, the dependence between the advection
velocity and the superficial velocity can be expressed as (for one-
dimensional flow):

The local physical velocity can still be different than the average fluid
velocity because the vector of the local fluid flow does not have to be
parallel to that of average flow. Also, there may be local constriction in the
flow channel.

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Characterisitc of Two Phase Flow
Several features make two-phase flow an interesting and challenging
branch of fluid mechanics:

Surface tension makes all dynamical problems nonlinear.


In the case of air and water at standard temperature and pressure, the
density of the two phases differs by a factor of about 1000. Similar
differences are typical of water liquid/water vapor densities
The sound speed changes dramatically for materials undergoing phase
change, and can be orders of magnitude different.
The phase changes are not instantaneous, and the liquid-vapour
system will not necessarily be in phase equilibrium
The change of phase means flow-induced pressure drops can cause
further phase-change (e.g. water can evaporate through a valve)
increasing the relative volume of the gaseous, compressible medium
and increasing exit velocities, unlike single-phase incompressible flow
where closing a valve would decrease exit velocities
Can give rise to other counter-intuitive, negative resistance-type
instabilities, like Ledinegg instability, geysering, chugging, relaxation
instability, and flow maldistribution instabilities as examples of static
instabilities, and other dynamic instabilities.

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