BARC Vienna, Austria, September 10-13, 2007 N.K. Maheshwari, P.K. Vijayan and D. Saha Reactor Engineering Division, Bhabha Atomic Research Centre, Trombay,

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Vienna, Austria, September 10-13, 2007

N.K. Maheshwari, P.K. Vijayan and D. Saha

Reactor Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai, INDIA - 400 085

4th RCM on the IAEA CRP on Natural Circulation Phenomena, Modelling and Reliability of Passive Safety Systems

that Utilize Natural Circulation

Effect of non-condensable gases on condensation heat transfer

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The problem is relevant to containment cooling using Passive Containment Cooling System (PCCS).

Containment of a nuclear reactor is a key component of the mitigation part of the defence in depth philosophy, since it is the last barrier designed to prevent large radioactive releases to the environment.

To provide safety-grade heat sink for preventing the containments exceeding its design pressure, passive systems for condensing steam are used in the nuclear reactors.

Effect of Non-condensable gases on condensation

The present talk deals with state of art on the effect of non-condensable gases on condensation heat transfer

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The other important system encountering condensation in presence of noncondensable gas is the power plant condenser.

The presence of noncondensable gas greatly influences the condensation process warranting in-depth study of the phenomena.


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• Condensation occurs when the temperature of vapor is reduced below its saturation temperature.

• Presence of even a small amount of Non-condensable gas (e.g. air, N2, H2, He, etc.) in the condensing vapor leads to a significant reduction in heat transfer during condensation.

• The buildup of non-condensable gases near the condensate film inhibits the diffusion of vapor from the bulk mixture to the liquid film.

Definition

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Schematic representation of the effect of non-condensable gas on condensation

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The geometries of interest are tubes, plates, annulus, etc. and the flow orientation (horizontal, vertical) can be different for various applications.

The condensation heat transfer is affected by parameters such as

Mass fraction of non-condensable gas System pressure Gas/vapor mixture Reynolds number Orientations of surface Interfacial shear Prandtl number of condensate Multi-component non-condensable gases, etc.

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Scenario

During a loss-of-coolant accident (LOCA) or a main-steam-line-break (MSLB) accident, or any other accident that causes a coolant release into the containment.

A large amount of steam is released into the containment which mixes with the noncondensable gases.

There are cooling surfaces provided for condensing the steam from steam/non-condensable gas mixture.

During condensation process, the steam condenses on the surfaces, while the non-condensable gases are accumulated on the film condensate layer creating an additional thermal resistance resulting in a degradation of the heat transfer to the wall.

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Scenario

In the design and operation of a steam turbine the exit temperature of the process fluid is kept as low as possible so that a maximum change in enthalpy occurs during the conversion of heat into work. The presence of small proportion of air in the vapor can reduce heat transfer performance in a marked manner which increases the condenser pressure.

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Hardware

PCCS with isolation Condenser

The system is adopted in ESBWR and SBWR

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Hardware

PCCS with steel containment vessel

The Westinghouse AP-600, SPWR, EP-1000, JPSR

and AC-600 are the reactors utilizing this concept.

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Hardware

PCCS with Building Condenser

SWR-1000: Containment Pressure Reduction and Heat Removal following a LOCA using Steam Condensation on Condenser Tubes.

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Hardware

General Arrangement of AHWR with PCCS

FIG 1. SCHEM ATIC OF PASSIVE CONTAINM ENT COOLER

GDW P

Steam—noncondensable gas

Condensate

FIG 1. SCHEM ATIC OF PASSIVE CONTAINM ENT COOLER

GDW P

Steam—noncondensable gas

Condensate

Passive external condenser

Passive External condensed

Secondary Containment

Primary Containment

Core

Gravity driven water pool

Turbine

Condenser

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Literature review

Test performed

Geometry and size

Working fluid Remarks

Othmer Copper tube

D= 76.2 mm, L=1.22 m

Air/steam Reduction in heat transfer coefficient (HTC) by 50% when 0.5% air is present in steam

Uchida Vertical tube

D=0.2 m, L=0.3 m

Air, Nitrogen and Argon with Steam

The correlation developed is widely used in nuclear reactor containment analysis

Al-Diwani and Rose

Cooled vertical copper plate, 97 x 97 mm

Air, Argon and Helium with

Experimental data show good agreement with the published data

Dehbi et al. Vertical copper tube

D=38 mm, L=3.5 m

Air/Steam

Air-Helium-Steam

Developed correlations for air/steam and air-Helium and steam mixture. Heat transfer coefficient estinated by heat and mass transfer model agree well with exptl. data

Liu et al. Vertical copper tube

D=40 mm, L=2 m

Air, Helium with Steam Developed a correlation and found that HTC is 2.2 times higher than HTC estimated by Uchida correlation

Stagnant environment

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Literature review

Test performed Geometry, orientation and size

Working fluid Remarks

Maheshwari et al. Horizontal tube

D=21.3 mm, L=0.75 m

Air/Steam HTC for horizontal tube is higher than vertical tube

Anderson et al. Vertical and Horizontal Condensing plates

Characteristic length, L= 0.91 m

Air/steam and Air-Helium-Steam

Effect of orientation of condensing surface was found to be small

Stagnant environment

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Literature review

Test performed

Geometry and size Working fluid

Remarks

Nagasaka et al. Vertical SS tube

(Full scale SBWR PCC tube)

Nitrogen/Steam

Helium/Steam

Facility is called GIRAFFE system. The results for average HTC were presented in terms of degradation coefficient (ratio of actual HTC and pure steam HTC by Nusselt theory)

Masoni et al. Vertical tube

(Full scale SBWR PCC tube)

Air/steam PANTHERS exptl. Facility. The results are given in terms of condenser efficiency as a function of inlet pressure and air mass fraction

Ogg Vertical SS tube

ID=49.0 mm, L=2.44 m

Air/Steam and

Helium/ Steam

A correlation for heat transfer coefficient was developed based on the experiment in term of Nusselt’s pure steam heat transfer coefficient and degradation factor consisting the two separate factors which involves mixture Reynolds number and air mass fraction.

Hassanein et al. Vertical SS tube

ID=46 mm, L=2.54 m

Air/Steam and Helium/Steam

The local Nusselt number was correlated as a function of local mixture Reynolds number, Jakob number and gas mass fraction and Schmidt number.

Vierow Vertical coper tube

ID=22.1 mm, L=2.13 m

Air/Steam The authors found that at an air inlet mass fraction of 14% the heat transfer coefficients were reduced to one-seventh the values of pure steam. Instabilities were observed at high air contents. Vierow developed a correlation for local heat transfer coefficient

Flowing vapor-noncondensable gas mixture

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Literature review

Test performed

Geometry and size

Working fluid

Remarks

Siddique Vertical tube

ID= 25.27 mm,

L=1.22 m

Air/Steam and

Helium/ Steam

For same mole fraction, compared to helium air has more inhibiting effect on condensation heat transfer, but for the same mass ratio, helium is found to be more inhibiting. They developed correlations.

Araki Vertical tube

ID=49.5 mm, L=1.21 m

Air/Steam Correlations for condensation HTC for laminar and turbulent range are developed in terms of Reynolds number and air mass fraction

Kuhn Vertical SS tube

ID=50.8 mm, L=2.4 m

Air/Steam and

Helium/ Steam

The local Nusselt number was correlated as a function of local mixture Reynolds number, Jakob number and gas mass fraction and Schmidt number. .

Park et al. Vertical tube Air/Steam Correlation for local HTC in terms of degradation factor is developed. The range of validity for Jakob number in the correlation is smaller than that of the correlation developed by Siddique et al.

Maheshwari et al. Vertical tube

ID=42.76 mm, L=1.6 m

Air/Steam Experiments were performed with natural convection of water outside the tube and with forced flow of water flowing in a cooling jacket surrounding the tube. Correlation is developed. A strong dependency of heat transfer coefficient on Reynolds number of the inlet mixture was also found

Flowing vapor- noncondensable gas mixture

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Heat and mass transfer coefficient

A mass balance at the interface is done to yield the following equation

Heat and mass transfer

bnc,inc,

inc,//cond

WW

W

ρD

LmSh

fgH//

condm )

iT -

b(T

condh

1

gh

condh

1

fh

1 =

toth

hcond – Condensation heat transfer coefficient , hf – Film heat transfer coefficient

hg - Convective heat transfer coefficient

The heat transfer through the condensate film is balanced by the heat transfer through the gas/vapor interface which is sum of latent heat and sensible heat. This yields

Where, hcond is given by eq.,

where, L is the characteristic length which is outer diameter for horizontal tube and length of the tube for vertical tube

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Condensate film model

The film heat transfer coefficient on vertical surface is calculated by Nusselt equation

4

1

wil

3

lfggll

f TTLμ

kHgρρρ0.943h

for Ref < 30

For condensation on horizontal tube the 0.943 is replaced by 0.725 in Nusselt equation

Condensate film heat transfer fg

H//cond

m ) i

T -b

(T cond

h

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Heat transfer at gas/vapor boundary layer

In case of stagnant gas environment, the natural convection boundary layer approach provides the expressions for sensible heat transfer through the gas/vapor boundary layer formed during condensation of vapor.

)10Gr(for Pr13.0

)10Gr(for Pr56.0933.0

925.0

GrNu

GrNu

)10Gr(for 13.0

)10Gr(for 56.0933.0

925.0

GrScSh

GrScSh

)( ,,2

3,

bgigbg gLGr

The Grashof number is defined as

By heat and mass transfer analogy

Gas/vapor heat transfer- free convection

hg can be obtained from above expression

(12)

(13)

m//cond and hcond can be estimated

from equations (11) and (4)

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Heat transfer at gas/vapor boundary layer

In case of vapor/gas mixture flowing inside a vertical tube, the forced convective boundary layer approach provides the expressions for sensible heat transfer through the gas/vapor boundary layer formed during condensation of vapor. The following Gnielinski correlation is used

Gas/vapor heat transfer- Forced convection

1)-2/3(Pr1/2/2)s

12.7(f +1

1000)Pr-/2)(Res

(fNu

By heat and mass transfer analogy

1)-2/3(Sc1/2/2)s

12.7(f +1

1000)Pr-/2)(Res

(fSh

Re is local mixture Reynolds number in the bulk fluid, and fs is the friction factor for smooth tube

When the Reynolds number is less than 2300, a fully developed laminar flow regime is assumed. A value of 3.66 is assigned for Nu and Sh

2300< Re < 5 x 106

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Heat transfer enhancement

Following modifications are carried out to account for the

• Film Waviness/ripple effect on condensate film heat transfer coefficient

• Condensate film roughness effect on condensation and convective heat transfer

• Suction effect

• Developing flow effect on heat and mass transfer

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Some of the correlations available in literature

Number of correlations are available in the literature. Some of the

correlations developed are given below.

7.0

)( 1380

nc

ncUchidatot W

Wh

The correlation developed by Uchida

Correlations

nc

nctot W

Wh

12844.11

The Tagami correlation

Condensation in stagnant atmosphere

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0.3070.252tot

2.344stot dTPCXh

The correlation developed by Liu et al.

2.533 x 105 Pa < Ptot < 4.559 x 105 Pa4 oC < dT < 25 oC; 0.395 < Xs < 0.873

Dehbi correlation

0.25

wb

ncttot0.05

tot(Dehbi)TT

logW458.3P243828.7P3.7Lh

for 0.3 m < L < 3.5 m; 1.5 atm. < Pt < 4.5 atm.;10 oC < (Tb-Tw) < 50 oC

Where, C=55.635 W/m2 Pa0.252 oC1.307

Correlations

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Correlations

Condensation inside the vertical tube

There are two types of correlations for estimating the heat transfer coefficient.

The local heat transfer coefficient is expressed in the form of a degradation factor defined as the ratio of the experimental heat transfer coefficient (when noncondensable gas is present) and pure steam heat transfer coefficient.

The degradation factor is a function of local noncondensable gas mass fraction and mixture Reynolds number (or condensate Reynolds number).

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Correlations

The local heat transfer coefficient is expressed in the form of dimensionless numbers and does not require information of condensation heat transfer coefficient for pure steam.

In these correlations, local Nusselt number is expressed as a function of mixture Reynolds number, Jacob number, noncondensable gas mass fraction and condensate Reynolds number, etc.

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Correlations

Vierow correlation based on UCB data1.1Re0.0050f o.45

filma

W

0.24filmRe0.63Ja1.4

aW0.0012f

Park correlation based on KAIST data

1715 < Reg < 216700.83 < Prg < 1.040.111 < Wa < 0.8360.01654 < Ja < 0.07351

Which is applicable in the following range

The degradation factor is defined as

filmhtoth

f

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Correlations

Correlation based on non-dimensional numbers

Siddique Correlation based on MIT data

0.741Ja1.105aW1.137ReNu(x) 0.404

g

Which applies in the following range of experiments

0.1 < Wa < 0.95 ; 445 < Reg < 22700 ; 0.004 < Ja < 0.07

Maheshwari correlation based on BARC experiments

5.08.085.015.0 ReRe15.0)( gafilm JaWxNu

This equation is valid in the following range

0.1 < Wa < 0.68000 < Reg < 227000.005 < Ja < 0.07

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Condensation inside a vertical tube

Work done in BARC on condensation inside vertical tube

• Experimental studies on condensation in presence of air in vertical tube

• Development of a theoretical model to investigate condensation in presence of noncondensable gas when steam/air mixture is flowing down inside the tube

• Studies on the effects of various parameters on condensation in presence of noncondensable gas

• Comparison of theoretical results with BARC experimental data and data available in literature

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Condensation in vertical tube400

260

100

100

48.3

1000

1600

950

50

Air vent lineCondensate

line

A A

Section -AA

Geometry and Dimensions of the model

Test set-up

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Forced flow condensation

Variation of total heat transfer coefficient along the length of the tube

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Work done in BARC on condensation in stagnant environment • Experimental studies on condensation in presence of air over

horizontal tube

• Development of a theoretical model to investigate condensation in presence of noncondensable gas when steam/air mixture is non-flowing

• Studies on the effects of various parameters on condensation in presence of noncondensable gas

• Comparison of theoretical results with BARC experimental data and data available in literature

Condensation in stagnant environment

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Schematic of the steam condensation experimental set up

Pressure regulator

Compressed air

condensing Section21.3 mm OD tube

Insulated lines

20001000

750

To drain

Water inlet

Heater0-18 kW

Water

LT

P

P

T T

T

P

ThermocouplePressure transmitterLevel transmitterLT

Relief valve and rupture dick

Noz

zles

for

ve

rtic

al

inst

alla

tion

of

mod

elRot

amet

er (

0-8

lpm

)

Experiment set up

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Variation of heat transfer coefficient with air mass fraction

Comparison between

experimental and theoretical results

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Free and forced convective Condensation

Comparison of free and forced convective heat transfer coefficients

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Summary

Work done by various researchers is reviewed

The report deals with the following

- Condensation in stagnant steam/non-condensable environment - Condensation in a flowing steam/non-condensable mixture - Geometry considered -tubes with different orientations, plate, etc.

Recent work performed in BARC is also presented

Documents

BARC Vienna, Austria, September 10-13, 2007 N.K. Maheshwari, P.K. Vijayan and D. Saha Reactor Engineering Division, Bhabha Atomic Research Centre, Trombay,