Chapter 1. Introduction

1. Applications of Two-Phase Flow

a) power and propulsion system

b) process system

c) transport and transmission system

d) environment control system

e) information system

f) biological system and devices

g) geometeological phenomena

2. Characteristics of Two-Phase System

. existance of moving and deformable interface

. internal discontinuities of values

. complicated flow fluid due to interfacial structure

. mathmetical difficulties for a local-instant formulation

. difficulties in measurements and scaling problem

3. Methods of Two-Phase Analysis

a) empirical correlations

b) phenomenological models

c) integral analysis

d) differential analysis

4. Classifications of Two-Phase Flow

o # of Phases

; solid, liquid, gas

o composition

; water/steam,

air/water, sodium/vapor

o geometry

; upflow, downflow,

horizontal

o flow regime

. seperated flow regime

. transitional flow regime

. dispersed flow regime

5. Two-Phase Problems in Power Industries

(1) Fossil plants

☞ boiler ; dryout

☞ condenser - heat transfer coefficient, Δp

- flow distribution in inlet and outlet manifold

- direct contact spray condenser

(2) Alternative energy sources

☞ OTEC(Ocean-Thermal Energy Conversion)

- accurate knowledge of evaporating and condensing coefficient

☞ Geothermal energy

- informations for Δp and void fraction

- undesirable flow pattern

☞ Solar power plant - sodium heat transfer

(3) Nuclear power plants

☞ accident analysis like LOCA

- nonequllibrium model to describe two-phase phenomena

- interfacial phenomena between phases

☞ coolant behaviour in core and steam generator

- flow instability in BWR

- DNB problem in PWR

- predictions of two-phase pressure drop and void fraction

- subchannel analysis

☞ evaporation and condensation within horizontal tube

☞ fouling and corrosion

6. Key issues in two-phase flow

① treatment of gas-liquid interface

▶ worse than treatment of turbulence in single-phase flow

▶ interfacial area and interfacial drag or heat trransfer

- flow regime dependent

- unsteady and undeveloped flow condition

- different geometry

② prediction of local and average void fraction

▶ not cover all the flow directions and geometrical variations existing

in a complex system

▶ interpolation procedures to describe transitions from one flow

pattern to another in most system codes

③ two-phase flow turbulence model

▶ interface-induced turbulence

④ three-dimensional and nonthermal equlibrium condition

▶ countercurrent flow in downcomer or small-break critical flow

▶ subcooled boiling, heat transfer beyond CHF or surface dryout,

flashing during critical flow depressurization, direct contact

condensation with and without gas

▶ empirical adjustment because of their inadequate and gross mixing

description among adjacent nodes

⑤ outside issues

a) microchannel two-phase flow

b) global two-fluid problems

- identify the similarities between global climate and nuclear system

- treatment of vertical movement of air with moisture

- aerosols in atmosphere vs nucleation site for cloud droplets

- atmosphere general circulation model

7. Multi-scale approach for nuclear thermal-hydraulic research

macro-scale

multi-field modeling

transport of interfacial area

turbulence modeling

o CFD scale

o component scale

o system scale

(1) CFD scale

- open medium

- turbulence modelling using RANS or LES

- ITM(Interface Tracking Method) and DNS

● In bubbly flow,

o liquid turbulence by wall shear

o random stirring due to bubble motion

o vortex sheding in wake of bubble

o deformation of interface

⇒ velocity fluctuation in liquid field

● Successive steps to investigate a two-phase flow with CFD

o Identification of all important flow processes

o Selecting a basic model : Single fluid, two-fluid or multi-field model

o Filtering turbulent scales and two-phase intermittency scales: RANS, LES

o Identification of Local Interface Structures : statistical approach for such                                             as the void fraction, the interfacial area

o Use of Interface Tracking Method : depending on the flow configuration

and on the expected resolution scale

o Selecting a turbulence modeling:  k-ε, Rij-ε, etc.

o Modeling Interfacial transfers and validation

o Modeling Turbulent transfers and validation

o Modeling Wall transfers and validation

(2) Component scale

- design, safety and operation studies for reactor core, steam generator or

condenser, etc

- minimum spatial resolution ; subchannel size (∼ 1 cm)

- FLICA, COBRA, and THYC

(3) System scale

- RELAP or CATHARE

o two-fluid 6-eq. model

o 0D, 1D, 3D