JORDANIAN - GERMAN WINTER ACADEMY

2006

Typical Study Of Two-phase Flow Industrial Applications

Pressure Drop and Flow Regimes.

Dr. B. Al-Shannak

Eng. Kh. Al-Qudah

Basic Definition of Two-Phase Flow

•A phase is simply one of the state of matter and can be either a gas , a liquid or a solid.

•Multiphase flow is a simultaneous flow of several phases . Two-phase flow is the simplest case of multiphase flow .

•Gas-liquid mixtures are referred to as two-phase two-component flow where as liquid -vapor mixture referred to as two-phase single- component mixture.

•common examples of two-phase flows, some such as rain, clouds , smoke ,fog ,snow ,dust storms are occur in nature . Others such as boiling water, cooking are frequent occurrences and several every day processes involve a sequence of different two-phase flow configuration or flow patterns .

•Two-phase flow involving a mixture of gas or vapor and liquid is very common in various industrial and scientific applications, and there have been many experimental and theoretical studies

Two-phase Flow ApplicationsThe practical importance in many common engineering and industrial applications are:

Steam generators and condensers, steam turbines ( Power Plants ).

Refrigeration .

Coal fired furnaces .

Fluidized bed reactors .

Liquid sprays .

Separation of contaminants from a carrier fluid

Free surface flows, where sharp interfaces exist .

pumping of slurries .

pumping of flashing liquids .

raining bed driers .

oil industry two phase flow occurs in pipelines carrying oil and natural gas.

energy conversion .

paper manufacturing .

food manufacturing .

medical applications .

•-The laws governing two phase flow are identical to those for single phase flow. However, the equations are more complex and/or more numerous than those of single phase flow.

•The description of the two-phase flow is complicated due to the existence of interface between the phases depending on a large number of variables such as :

quality (x).

phase physical properties .

flow patterns .

pipe geometry .

orientation of flow .

A general classifications divide two-phase flow into four groups depending on the mixtures of phases in the flow. The four groups are the flow of gas-liquid, gas-solid, liquid-solid and immiscible liquid-liquid mixtures. The last case is technically not a two-phase mixture, it is rather a single phase two-component flow, but for all practical purposes it can be considered as a two-phase mixture.

Two-Phase Flow Regimes

The description of two-phase flow in tubes is complicated by the existence of an interface between the two-phases. For gas –liquid two-phase flow the interface exists in a wide variety of forms, depending on the flow rates and physical properties of the phases, and also on the geometry and orientation of the tube.

The different interfacial structures are called flow patterns or flow regimes.

There are various flow patterns common in two-phase flow system, each having different characteristics and associated pressure drops. A number of different methods have been proposed for the recognition of flow patterns, ranging from visual observation to characteristic fluctuation in hold up.

Flow Regimes In Horizontal Flow

1. Bubble flow .

2. Plug flow .

3. Stratified flow (layered, separated) .

4. Wavy flow (ripple flow, cresting) .

5. Slug flow .

6. Semi-annular flow .

7. Annular flow (ringed) .

8. Spray flow (mist, froth, dispersed) .

Vertical flow Regimes

1. Annular flow.

2. Bubble flow.

3. Slug flow.

4. Churn flow.

5. Ripple Flow.

6. Mist Flow .

Slug Bubble Separated Annular

Two Phase Flow Regimes Mapping

Mapping of flow patterns that occur in pipe flow has always been a popular means of describing the behaviors of flow at different conditions.  The superficial velocity of the gas and liquid are usually put on the two different axes, and supply an efficient method of comparing and contrasting the effects of different flow conditions.

Dispersed Bubble

Mandhani et al Map(1974)

Analysis of two-phase flows

How do we analyze two-phase flows?

The main analysis techniques divide into the following categories.

A-Simple Correlations

•based on experiment .

•often quoted in dimensionless form.

•may or may not have scientific/physical basis.

•often restricted in area of application .

B-Simple Analytical Models .

Due to the large number of applications where multiphase flow occurs, it is important to have accurate models.1-Homogeneous model .

Suitable average properties are determined and the mixture is treated as a single fluid in the

•take average of properties for both phases .

•used, e.g., for suspension, foam, mist, dispersed bubble.

•no detail of the flow considered .

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2-Separated flow model .

•assume phases flow side by side .

•use separate equations for each phase .

•consider interaction between the phases .

3-Drift flux model .

•focuses on relative motion between phases .

C-Integral Analysis .

•assume velocity, temperature or concentration profile

•fit to boundary conditions and apply integrated fluid mechanical equations

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D-Differential Analysis.

•use of time-averaged equations of motion .

E-Universal Phenomena .

•certain phenomena apply regardless of the regime, e.g. wave theory

THE NUMERICAL MODELS

•The numerical simulation of industrial flows is an increasingly important means to solve a large variety of fluid flow problems such as internal flows, external aerodynamics, spray cooling, film coating, environmental and biological flows, and power generation. Several general-purpose codes and a large number of specific VKI codes are now available.

•In contrast to the standard one-dimensional lumped parameter simulation and modeling of two-phase flows, the more general methods of computational fluid dynamics (CFD) are based on the conservation equations of mass, momentum and energy in the three spatial dimensions of a flow field. CFD-methods are now established as engineering tools for reactor safety analysis.

•Four basic approaches can be applied in the CFD modelling of multiphase flows. These are: the porous medium, the Lagrangian, the Eulerian and the interface models.

Multiphase models in FLUENT

•Discrete Phase Model (DPM).

•Mixture Model .

•Volume of Fluid Model (VOF) .

•Eulerian Multiphase Flow Model .

Case Study Modeling and Experimental investigation of Two-phase flow contraction coefficient and pressure drop at the branching pipes .

PhD Proposal

Supervised By:

Dr . A . Salaymeh.

Dr . B. Shannak .

 

+1

3

2

+Ac-12

+Ac-13

Stream Lines

   

Problem When the fully developed, two-phase flow passes through an inlet of the branching junction, the flow separates from the wall at the deflecting legs (branches) and reaches narrowest cross section that called contraction region. These separations and contraction regions cause high pressure losses and therefore a high-energy dissipation. The resistance to flow offered by T-junction is larger than that implied by the equations for the single phase flow. Consequently, the laws of friction in T-junction are of great practical importance, and experimental work on them is very necessary.

-Due to the lack of readily available standard design methods to calculate pressure drop and mass flow rate in two-phase flow applications, design methodologies have been routinely based on previous experiences. Depending on the strength of the interaction between phases, different modeling approaches have been proposed for two-phase flows but very little of these conducted the two-phase flow contraction .

So the present study will also expand our knowledge of the state-of-the art of contraction coefficient and pressure drop of two-phase flow in pipe branching.

Objective The overall objective of the present study is to focus some sheds on the modeling and investigations of the contraction coefficient and predict the two-phase flow pressure drop and mass flow rate at the branching junction in order to produce a new reliable computational relation for the contraction coefficient and to develop an empirical model for pressure drop The relevance of two-phase flow problems in industrial applications have motivated to this investigation . New formula may be produced related the pressure drop , mass flow rates and contraction coefficient relations.

Method of Analysis

The contraction coefficient should be Modeled with the help of the following laws:

.Conservation law of mass .

•conservation law of momentum.

•conservation law of energy.

 

+1

3

2

+Ac-12

+Ac-13

Stream Lines

   

1-The general principle of conservation of mass that the mass within the system remains constant with time :

0.0

cscv

VdAdvt

0.0t

0.0)2(

233322222111 ph

phphph dt

dmVAVAVA

phphph VAVAVA 233322222111

for steady state flow

Where , , represent average velocities over the cross sections.

1V 2V 3V

2-Energy Conservation

12H E-EW-Q

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(2ph)

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(2ph)

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3-Conservation of momentum

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.d(mv)

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surfacebody FFdt

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..0lim

cssys

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dt

dF ).(VdmVc

cssys

dAVVdt

dm

dt

dF ).(VdmVc

)2(2

111 )sin(0 phVAFy

)2(2

222 )(0 phVAFx

)2(2

111)2(2

333 )cos()( phph VAVA

defined the contraction coefficient as:

2

2)21( A

AcCc

33

)31( A

AcCc

The contraction coefficient Cc can be represented as

phAVpfCc 2),,,,(

All these primary influencing parameters should be taken into consideration for the development of anew pressure loss formula.

Comparison will be made between model and experimental data .

�Stream lines

θ

P3.A3

P1,A1

P2,A2

Ac12

Ac13