Page 218 - Fluid mechanics, heat transfer, and mass transfer

196 TWO-PHASE FLOW SYSTEMS

FIGURE 7.1 Flow patterns in gas–liquid flow in vertical pipes.

& Using valved auxiliary pipe runs to regulate alterna- entrained as small roughly spherical droplets in the
tive flow rates, avoiding slug flow. vapor or gas stream. In droplet flow, only entrained
& Using a low point effluent drain or bypass. droplets flow with the absence of liquid film. If the
droplet sizes are very small, they can be treated as
& Arranging pipe configurations to protect against slug
mists. These flows are possible at high-velocity vapor
flow; for example, in a pocketed linewhere liquid can
collect, slug flow may develop. or gas flows.
& Spray/Dispersed/Mist Flow: Gas and liquid flow
. What is churn flow?
in dispersed form. All the liquid is carried as fine
& At moderate to high flow velocities and roughly equal
droplets in the gas phase. In such flows, gas velocities
proportions of vapor and liquid, the flow pattern is
involved are high, of the order of 20–30 m/s.
often very irregular and chaotic. If the flow contains
& Figure 7.2 illustrates annular droplet flow.
no distinct entities with spherical or, in a pipe,
cylindrical symmetry, it is called churn flow. . “In two-phase flow, the flow is dispersed when more
than 30 wt% of flow rate is vapor.” True/False?
. Under what circumstances churn or froth flow can occur
in two-phase flow systems? & True.
& Churn flows, sometimes called froth flows, are char- . Illustrate by means of a diagram different two-phase
acterized by strong intermittency and intense mixing, flow patterns in a vertical tube heated from outside.
with neither phase easily described as continuous or & Figure 7.3 illustrates patterns of two-phase flow in a
dispersed. heated vertical pipe.
. What is annular flow?
& When liquid forms a film or ring around the inside
wall of the pipe and gas flows at higher velocity as a
central core, the flow is called annular flow.
. Under what circumstances annular flow develops?
& At high vapor content, the liquid may form a thin film
or ring around the inner wall of the pipe, wetting the
pipe wall, and gas or vapor flows at higher velocity as
a central core. This type offlow is called annular flow.
. What are annular droplet and droplet flows?
& In annular droplet flow, liquid, besides forming
annular film along the walls of the conduit, may get FIGURE 7.2 Annular droplet or mist flows.