P1252122140UriXb - PowerPoint PPT Presentation

Title: P1252122140UriXb


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• MULTIPHASE FLOW
• More complicated than single phase flow.
• Flow pattern is not simply laminar or turbulent.
• Types of multiphase flow
• Solid-fluid flows (e.g. particulate flows)
• Liquid-liquid flows
• Gas-liquid flows
• Three-phase flows.
• Due to density differences, horizontal flows are
  different than vertical flows. Cocurrent flows
  are different than countercurrent flows. Phase
  changes should be taken into account when
  present.

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Multiphase Flow (gas-liquid)
Horizontal Vertical
Dispersed Annular Stratified Churn or
froth Wavy Slug Plug Bubble
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Multiphase Flow (gas-liquid) Note Each phase
travels with its own velocity. Flow regime is a
matter of visual interpretation and subjective to
the person who takes the measurements. Transition
from one regime to another is gradual. Cocurrent
Horizontal flows Low liquid velocity Stratified
flow, wavy flow, annular flow Intermediate liquid
velocity Plug flow, slug flow, annular flow
Large liquid velocity Bubbly flow, spray or mist
flow. Gas velocity ?
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• Importance of flow regime predictions
• Better predictions of DP and Holdup (volume
  fraction), if flow regime is known.
• Flow regime prediction is not only important for
  reliable design, but for pipeline operability.
• Phenomena like pipe corrosion and erosion depend
  on flow regimes.
• Distribution of corrosion, hydrate and was
  inhibitors depend on flow regimes.
• Flow regime at pipe outlet affects gas-liquid
  separation efficiency.

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Multiphase Flow (gas-liquid) Typical Velocities
(1in pipe) Regime Liquid Velocity Vapor
Velocity (ft/sec)
(ft/Sec) Dispersed Close to vapor gt
200 Annular lt0.5 gt 20 Stratified lt0.5
0.5-10 Slug Less than vapor vel.
3-50 Plug 2 lt 4 Bubble 5-15
0.5-2
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Multiphase Flow (gas-liquid) Flow regime
maps Good for approximate prediction of flow
characteristics.
Baker plot (1954)
WG , WL gas or liquid mass velocity
(lb/h) viscosity in cp, surface tension in
dyn/cm, density in lb/ft3, area in ft2.
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• Flow regime maps
• ? and ? depend on the fluid property only.
• BX depends on the ratio of flows (Known
  beforehand. Not a design parameter)
• BY depends on the vapor/gas superficial
  velocity. This is the only parameter the designer
  can change (through A)
• Transition boundaries are not at all that
  sharp.
• Trajectories On The Baker Plot.
• How regimes change through a pipe.
• As the pressure drops, the density of the vapor
  becomes lower.
• 1) ? ? ? BX ? BX decreases
• 2) 1/ ? ? BY ? BY increases
• Thus trajectories are always "up" and "to the
  left"

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• Shortcomings of the Taitel - Dukler flow regime
  models
• Poor prediction of stratified flow for inclined
  pipes.
• Stratified flow model used for flow regime
  prediction contradicts pressure drop and liquid
  holdup data.
• Poor prediction of high pressures and low surface
  tension fluids.
• Near vertical flow regime better predicted than
  near horizontal.
• Viscosity effect not properly described.
• Out of 10,000 gas liquid flow pattern
  observations over the last 30 years, only 67 of
  all observations were predicted correctly. (Shell
  Research - Development, 1999)

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Flow regime maps Mandhane Plot (Mandhane et al.,
1974) Claimed that the Baker correlation
overestimates the effect of fluid
properties. Claimed that a plot with superficial
velocities rather than superficial mass
velocities is better. Suggested a slight
correction for fluid properties by using a
corrected superficial gas velocity
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Flow regime maps Weisman Plot (Weisman et al.,
1979) Found that Mandhanes suggestion for
plotting VL versus VG is a good first order
approximation. Presented updated corrections for
fluid properties. This paper provides the most
up-tp-date correlations for predicted flow
regimes (horizontal pipes). Note that all the
experiments were for pipes 1/2in to
2in. Weisman, J., Duncan, D., Gibson, J., and T.
Crawford, Int. J. Multiphase Flow, 5, pp.437-462,
1979. We know fairly well what happens in a 1in
horizontal pipe for air and water flow.
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Pressure Drop - homogeneous model Assume 1)
Zero slip between phases. 2) Uniform flow. 3)
Phase equilibrium. 4) Friction factor given by an
eqn. similar to that for single phase
flow. Define xquality, mass fraction that is
vapor or gas fraction of cross-section that
is gas Mixture density, ?H(Mass Flow)/(Volume
flow) Then
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Pressure Drop - homogeneous model Comments 1)
Drops in air, uG?uL In this case ti? 1/2 f
?G uG2 (shear stress at the interface) and
ti1/2 f ?H uG2 overpredicts 2) Bubbles in
liquids In this case ti? 1/2 f ?L uL2
Now f ? (D uL ?L )/ mL and ti1/2 f ?H uL2
underpredicts It might be better to use f ?
(D uL ?L )/ m2-p since ?H lt ?L and m2-p lt m2-p
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Limitations of the homogeneous model 1)
Assumption of equilibrium between phases often
not correct. Only way to deal with this problem
is to use a two - fluid model. 2) Use of
single phase equations with ?H and m2-p not very
good. 3) There can be appreciable slip between
the phases, so the calculation of from x can be
incorrect. This can affect calculations of
pressure drop due to hydrostatic head. The
separated flow model, is based on calculations
of slip SuG / uL . However, it needs more
equations to calculate x and
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Pressure Drop - horizontal pipes Lockhart-Martine
lli (1949) (exps. with 1 in pipe) Approximated
2-phase flow pressure drop from single phase flow
results (when the other phase is not
present). Lockhart-Martinelli parameter Two
phase pressure drop or The factors FL2 or
FG2 are read from a figure (see fig 6-26 in
Perrys handbook). High predictions for
stratified, wavy, slug flows. Low predictions for
annular flow.
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Pressure Drop - horizontal pipes Lockhart-Martine
lli (1949) Two phase pressure drop where
and X is the L-M parameter as before.
a b Bubble
14.2 0.75 Slug
1190 0.82 Stratified
15400 1 (horizontal) Plug
27.3
0.86 Annular 4.8 -0.3125D
0.343-0.021D D is the ID. If D gt 12in, then use
D12 in
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Pressure Drop - horizontal pipes Dispersed flow
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Pressure Drop - horizontal pipes Wavy flow