Page 220 - Fluid mechanics, heat transfer, and mass transfer

198 TWO-PHASE FLOW SYSTEMS

& Annular Flow: Liquid flows in continuous annular between the types. Figure 7.7 illustrates the typical
ring on the pipe wall and gas/vapor flows through the flow regimes recognized in two-phase flow.
central part of the pipe cross section. & To determine most probable type of two-phase flow
& Misty Flow: Fine liquid droplets are dispersed in the using Figure 7.7,
flowing gas/vapor. 1. Calculate lFG L /G G .
. “When vapor content in a line is more than about 30% 2. Calculate G G .
of total weight flow rate, bubble flow is possible.” & To identify probable type of flow pattern, read inter-
True/False? section of ordinate and abscissa on the figure.
& False.
& Pressure drops for gas/vapor and liquid flows are
. “When vapor content in a line is less than 30% of total separately estimated using appropriate pressure drop
weight flow rate, dispersed flow can occur.” True/False? correlations for single-phase flows.
& False. & For two-phase flow inside vertical pipes, stratified
. Under what conditions stratified flow occurs? and wavy flow regimes do not exist, and bubble, slug,
& Stratified flow occurs when the ratio of liquid to gas annular, and annular with mist are the flow regimes
flow is low. that exist.
. What is the difference between stratified flow and wavy . “If a liquid line has vapor content, frictional loss is
flow? greater than a line having only liquid.” True/False?
& Stratified Flow: Occurs in horizontal lines. Liquid & True.
and gas flow in stratified layers. Liquid flows along . What is Stokes flow?
the bottom of the pipe and gas flows over a smooth & Stokes flow is a type of flow where inertial forces are
gas–liquid interface. small compared to viscous forces: N Re 1.
& Wavy Flow: Gas flows in the top part of the pipe and & This is a typical situation in flows where the fluid
liquid as waves in the lower section over a smooth velocities are very low, viscosities are very high, or
gas–liquid interface. The interface has waves moving the length scales of the flow are very small as found in
in the flow direction. the flow of viscous polymers.
& Figure 7.6 illustrates these flow patterns. . What is slip velocity in two-phase flow?
& As gas flow relative to liquid flow increases, stratified
Slip velocity ¼ V gas V liquid : ð7:1Þ
flow transforms into wavy stratified flow that pro-
. What is the effect of liquid viscosity on slip?
gressively produces high waves and transforms into
& As viscosity increases, slip decreases.
slug flow.
. Does the volume fraction or holdup of each phase in a
. What are the different flow patterns that are normally
two-phase system same as the volumetric flow rate of
considered in two-phase flow systems?
that fraction?
& Six or seven types of flow patterns are usually con-
& No. Because of velocity differences or slip between
sidered in evaluating two-phase flow. Only one type
the phases.
can exist in a line at a time, but as conditions change
. “If a liquid line has vapor content, frictional losses are
(velocity, roughness, elevation, etc.) the type may also
greater than a line having only liquid.” True/False?
change. The unit pressure drop varies significantly

FIGURE 7.6 Stratified and wavy flows.