All of the fluid flow relationships discussed
previously are for the flow of a single phase of fluid
whether liquid or vapor. At
certain important locations in fluid flow systems the
simultaneous flow of
liquid water and steam occurs, known as two-phase flow.
These simple relationships used
for analyzing single-phase flow are insufficient for
analyzing two-phase flow.
There are several techniques used to predict
the head loss due to fluid friction for two-phase flow.
Two-phase flow friction is greater than single-phase friction
for the same conduit dimensions
and mass flow rate. The difference appears to be a function
of the type of flow and results
from increased flow speeds. Two-phase friction losses are
experimentally determined by measuring
pressure drops across different piping elements. The
two-phase losses are generally related
to single-phase losses through the same elements.
One accepted technique for determining the
two-phase friction loss based on the single-phase loss
involves the two-phase friction
multiplier (R), which is defined as the ratio of the
two-phase head loss
divided by the head loss evaluated using saturated liquid
properties.
The friction multiplier (R) has been found to
be much higher at lower pressures than at higher pressures.
The two-phase head loss can be many times greater than the
single-phase head loss.
Although a wide range of names has been used
for two-phase flow patterns, we shall define only three
types of flow. The flow patterns to be used are defined as
follows:
1. Bubbly flow: there is dispersion of vapor
bubbles in a continuum of liquid.
2. Slug flow: in bubbly flow, the bubbles
grow by coalescence and ultimately become
of the same order of diameter as the tube. This generates the
typical bullet-shaped
bubbles that are characteristic of the slug-flow regime.
3. Annular flow: the liquid is now
distributed between a liquid film flowing up the wall
and a dispersion of droplets flowing in the vapor core of the
flow.
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