2 phase flow regimes
DESCRIPTION
hiTRANSCRIPT
INDO – GERMAN WINTER ACADEMY 2010
ANIMESH AGRAWAL
INDIAN INSTITUTE OF TECHNOLOGY, DELHI
TUTOR:Prof. RAVI KUMAR
Two-phase flow is a term covering the interacting flow of two phases (gas, liquid, or solid) where the interface between the phases is influenced by their motion.
2 Phase flow patterns IntroductionFig:The spectrum of two-phase flow
Single-phase flow can be classified according to the external geometry of the flow channel as well as the 'character' of the flow; i.e., laminar( following streamlines) or turbulent (exhibiting fluctuations and chaotic motions).
In contrast multiphase flow is classified according to the internal phase distributions or "flow patterns" or "regimes".
2 Phase flow patterns Introduction
For a two-phase mixture of a gas or vapor and a liquid flowing together in a channel, different internal flow geometries or structures can occur depending on the size or orientation of the flow channel, the magnitudes of the gas and liquid flow parameters, the relative magnitudes of these flow parameters, and on the fluid properties of the two phases
2 Phase flow patterns Introduction
VOID FRACTION,
SLIP RATIO,
For homogeneous flows, slip ratio=1
MASS FLUX, G
2 Phase flow patterns Nomenclature
PHASE VELOCITY,
It is the volume flux divided by the cross-sectional area covered by the phase.
SUPERFICIAL VELOCITY,
It is the volume flux divided by the total cross-sectionalarea of the tube.
2 Phase flow patterns Nomenclature
In gas-liquid two phase flow, a plane normal to the axis of the channel will probably cut many interfaces.
Fortunately, however, the distribution falls into a number of characteristic patterns which can be predicted from the independent variables of the system such as the flow rates of each phase and their physical properties.
It is necessary to define the regimes independently for vertical and horizontal flow.
2 Phase flow patterns Regimes
The different regimes are-
1. Bubble Flow- Here the gas phase is distributed in discrete bubbles withina liquid continuum.
2. Slug Flow- When the concentration ofbubbles in bubble flow becomes high,bubble coalescence occurs and, progressively, the bubble diameter approaches that of the tube. Once thisapproaches, the slug-flow (or plug-flow)regime is entered with the characteristicsbullet shaped bubbles
2 Phase flow patterns Vertical Flow
3. Churn Flow- As the gas flow is increased the velocity of these bubbles increases andultimately, a breakdown of these bubblesoccurs leading to an unstable regime in which there is, a wide bore tubes, an
oscillatory motion of the liquid upwardsand downwards in the tube, thus the name of churned flow is applied. For narrow-bore tubes the oscillations may
not occur and a smoother transition between the slug flow and annular flow may be observed.
2 Phase flow patterns Vertical Flow
4. Annular Flow- The liquid flows on thewall of the tube as a film and the gas phase flows in the centre. Usually, some
liquid phase is entrained as small dropletsin the gas core.
5. Wispy annular flow- As the liquid flow rateis increased the droplet concentration in the
gas core of annular flow increases and, ultimately, droplet coalescence occurs leadingto large lumps or streaks as wispy liquid occurring in the gas core. This regime is characteristics of high mass velocity flows.
2 Phase flow patterns Vertical Flow
Fig: Flow Regimes In Vertical Flow
For horizontal flow, the main complicating feature is the gravitational forces act on the liquid phase causing it to be displaced towards the bottom of the channel.
The various regimes are- Bubble Flow- The bubbles tend to flow at the
top of the tube.
2 Phase flow patterns Horizontal Flow
Plug flow- Bullet shape bubbles occur, but they tend to move along in a position closer to the top of the tube.
Stratified flow- Gravitational spread is complete. Liquid flows along the bottom of the tube and gas along the top part.
Wavy flow- As the gas velocity is increased in stratified flow, large surface waves are formed on the gas liquid interface giving the wavy flow regime.
2 Phase flow patterns Horizontal Flow
Slug flow- As the gas velocity is further increased in the wavy flow region, the waves eventually become high enough to reach the top of the channel.
Annular flow- As the gas velocity increases still further the slugs become pierced with a gas core and the flow becomes annular with a thicker film at the bottom of the channel owing to gravitational effects.
2 Phase flow patterns Horizontal Flow
The rate of exchange of mass, momentum and energy between gas and liquid phases as well as between any multiphase mixture and the external boundaries depends on these internal flow geometries and interfacial area; hence is dependent on flow-pattern.
For instance, the relationships for pressure drop and heat transfer are likely to be different for a dispersed flow consisting of bubbles in a liquid (bubbly flow) than for a separated flow consisting of a liquid film on a channel wall with a central gas core (annular flow).
2 Phase flow patterns Need to Study
This leads to the use of flow-pattern dependent models for mass, momentum and energy transfer, together with appropriate flow-pattern transition criteria.
However, the central task is to predict which flow-pattern will exist under any set of operating conditions as well as to predict the value of characteristic fluid and flow parameters (e.g. bubble or droplet size) at which the transition from one flow-pattern to another will take place.
2 Phase flow patterns Need to Study
What are FLOW REGIME MAPS?
The usual way of presenting results of observations of flow patterns is to plot them on a graph whose axes represent the flow rates of the two phases; an alternative is to plot total mass flux on one axis and the mass fraction of the flow which is vapor or gas on the other axis. When all the observations have been recorded, lines are drawn on the graph to represent the boundaries between the various regimes of flow.
The resultant diagram is called a “flow regime map”.
2 Phase flow patterns Flow Regime Maps
Bennett et al. did studies on steam-water flows.
The resultant flow diagram at 1000 p.s.i.a. is shown-
2 Phase flow patterns Flow Regime Maps
BAKER’S CHART plots against BAKER’S CHART plots against
and are the mass fluxes (lb/h ft2) of liquid and gas phases.
ρ is the density, σ is surface tension, µ is the viscosity
A and W refer to the corresponding values for air and water at atmospheric pressure.
and are the mass fluxes (lb/h ft
2 Phase flow patterns Flow Regime Maps
VERTICAL FLOW
2 Phase flow patterns Flow Pattern Transitions
•Discussion applies to the case of vertical upwards adiabatic flow
BUBBLE FLOW-SLUG FLOW TRANSITION
The random motion of the bubbles will lead to bubble collisions resulting in coalescence of the two bubbles.
Eventually the bubble size will become such that the bubbles begin to have a cross-sectional area which approaches that of the channel.
Radovcich and Moissis considered a cubic lattice of bubbles and showed that bubble collision frequency is proportional to
Where is the mean fluctuating velocity, is the bubble diameter and is the void fraction.
Where is the mean fluctuating velocity, is the bubble diameter and is the void fraction.
Where is the mean fluctuating velocity, is the bubble diameter and Where is the mean fluctuating velocity, is the bubble diameter and
2 Phase flow patterns Flow Pattern Transitions
BUBBLE FLOW-SLUG FLOW TRANSITION
BUBBLE FLOW-SLUG FLOW TRANSITION
The following observations can be made:
At very low void fractions the collision frequency may tend to be zero.
Surface contaminants prevent coalescence of bubbles even if they approach one another, for example, froth produced by addition of a surfactant. In froths bubble flow can exist up to a very high void fractions (0.9 and above).
At high velocity the large bubbles may be broken down by turbulence and also, the residence time in the channel may be insufficient for enough bubble coalescence to occur to give the onset of slug flow.
2 Phase flow patterns Flow Pattern Transitions
If the upwards gas velocity is gradually increased in falling film region, we enter a region in which both climbing and falling film flow are occurring simultaneously. The transition to this region is called “Flooding”.
If the gas velocity is now reduced, a point will be reached at which the liquid phase, in addition to flowing upwards, begins to creep down the tube wall from the injection point. This transition point is called “Flow Reversal” point.
2 Phase flow patterns Flow Pattern Transitions
SLUG FLOW-CHURN FLOW TRANSITION
When the gas velocity in the bubble and the liquid flow ratein the film around the bubble are such that the conditions forflooding are satisfied, then the bubble will break down and thetransition to churn flow will takeplace.
2 Phase flow patterns Flow Pattern Transitions
SLUG FLOW-CHURN FLOW TRANSITION
The following empirical relations are for moderate tube diameter and liquids with low viscosity, specifically when dimensionless * is less than 10-4 where * is
Rise velocity of a vapour bubble for slug flow in a stagnant liquid is given by
2 Phase flow patterns Flow Pattern Transitions
SLUG FLOW-CHURN FLOW TRANSITION
The volume flux and bubble rise velocity at transition is expressed empirically
From above two Equations
2 Phase flow patterns Flow Pattern Transitions
The upper limit for the transition is given by
Golam & Stenning gave the alternative criterion
where
Slug flow is generally associated with the void fraction less than 0.8.
gave the alternative criterion
2 Phase flow patterns Flow Pattern Transitions
CHURN FLOW-ANNULAR FLOW TRANSITION
The transition of flow reversal point can be associated with the lower limit of annular flow.
Wallis suggested the representation of the transitions in terms of the dimensionless groups:
2 Phase flow patterns Flow Pattern Transitions
CHURN FLOW-ANNULAR FLOW TRANSITION
Wallis has found that the critical gas velocity at the flow reversal point can be characterized by the criterion
This criterion may be used to give an approximate prediction of Churn flow to annular transition.
He found out that the transition was not appreciably affected by the liquid flow rate.
2 Phase flow patterns Flow Pattern Transitions
CHURN FLOW-ANNULAR FLOW TRANSITION
Haberstroh and Griffith found out that the transition depends on the liquid flow rate and gave the following correlation:
2 Phase flow patterns Flow Pattern Transitions
ANNULAR FLOW-WISPY ANNULAR FLOW TRANSITION
The wispy annular region usually corresponds to the region where and
This transition is difficult to distinguish visually.
For steam-water flow at 34.5 bar and 69 bar, transition takes place at Vf = 1.05 m/s and 1.35 m/s respectively.
where and
2 Phase flow patterns Flow Pattern Transitions
2 Phase flow patterns Conclusion
The regime that will exist in a two phase flow is guided by several factors like flow rates of each phase, their physical properties, orientation of the tube as well as the manner in which the phases are introduced in the system.
We need to estimate the flow pattern as well as the flow-pattern transition for using the appropriate mass, momentum and energy transfer models.
2 Phase flow patterns References
Boiling Heat Transfer and Two Phase Flow, L.S.Tong and Y.S.Tang Two Phase Flow and Heat Transfer, Butterwoth and Hewitt Annular Two-Phase Flow, G.F.Hewitt and N.S. Hall-Taylor Two Phase Flow and Heat Transfer, P.B.Whalley