Page 217 - Fluid mechanics, heat transfer, and mass transfer

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TWO-PHASE FLOW SYSTEMS

7.1 Two-Phase Flow 195 & When vapor/gas bubbles, distributed in the liquid,
7.1.1 Gas/Vapor–Liquid Flow 195 move along at about the same velocity as the liquid,
7.1.2 Fluid–Solid Systems 199 the flow is described as bubble flow.
7.1.2.1 Packed Beds 199 & Bubble flow describes the flow of distinct, roughly
7.1.2.2 Fluidization 200
spherical vapor regions surrounded by continuous
7.1.2.3 Gas–Solids Transport 208
liquid. The diameter of the bubbles is generally
7.1.2.3.1 Pneumatic Conveyance 208
considerably smaller than that of the container
7.1.2.4 Solid–Liquid (Slurry) Flow 217
through which they flow.
. Under what conditions bubble flow can be expected?
& Bubble flow usually occurs at low vapor concentra-
tions. When vapor content is less than 30% of total
weight flow rate, bubble flow is possible.
7.1 TWO-PHASE FLOW . “Bubble flow in a horizontal pipe is prevalent at high
ratios of gas to liquid flow rates.” True/False?
7.1.1 Gas/Vapor–Liquid Flow & False.
. Explain what is meant by slug flow?
. Give examples of industrially important applications of
& Slug Flow: Slugs of gas bubbles flow through the
gas/vapor–liquid flows.
liquid. If the vapor and liquid are flowing through a
& Gas bubbles in oil, wet steam, vapor–liquid flow in
pipe, bubbles may coalesce into long vapor regions
refrigeration systems, steam–water flows in boilers
that have almost the same diameter as the pipe. This
and condensers, vapor–liquid flows in heat exchan-
is called slug flow. Slug flow develops when high
gers, evaporators, distillation columns, and reactors,
waves of liquid develop with progressive increase of
and so on.
gas or vapor content in wavy flow.
. What are the various types of flow patterns in gas–liquid
. What are the undesirable effects of slug flow?
flow in vertical pipes? Illustrate.
& Slug flow can cause vibrations in equipment because
& Bubble flow, slug flow, churn flow, annular flow, and
of high-velocity slugs of liquid impinging against
mist flow. The flow regime depends on the interaction
fittings and bends.
of two forces, namely, gravity and vapor shear, acting
& Can cause flooding in gas–liquid separators.
in different directions. At low vapor flow rates,
gravity dominates, giving rise to stratified, slug–plug, & Pressure fluctuations develop in the piping, which
or bubble flow, depending on the relative amounts of can upset process conditions and cause inconsistent
liquid present. At high vapor velocities, vapor shear instrument sensing.
dominates, giving rise to wavy, annular, or annular– . How is slug flow avoided in process piping?
mist flow. & Reducing line sizes to minimum.
& Figure 7.1 illustrates the flow patterns. & Designing parallel pipe runs that will increase flow
. What is bubble flow? capacity without increasing overall friction losses.

Fluid Mechanics, Heat Transfer, and Mass Transfer: Chemical Engineering Practice, By K. S. N. Raju
Copyright Ó 2011 John Wiley & Sons, Inc.
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