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BARC 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 th 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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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 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. Effect of Non-condensable gases on condensation

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BARC Vienna, Austria, September 10-13, 2007 Effect of Non-condensable gases on condensation 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, N 2, H 2, 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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BARC Vienna, Austria, September 10-13, 2007 Effect of Non-condensable gases on condensation Schematic representation of the effect of non-condensable gas on condensation

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BARC Vienna, Austria, September 10-13, 2007 Effect of Non-condensable gases on condensation 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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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 Hardware PCCS with isolation Condenser The system is adopted in ESBWR and SBWR

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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 Hardware General Arrangement of AHWR with PCCS Passive external condenser Passive External condensed Secondary Containment Primary Containment Core Gravity driven water pool Turbin e Condenser

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BARC Vienna, Austria, September 10-13, 2007 Literature review Test performed Geometry and size Working fluidRemarks OthmerCopper tube D= 76.2 mm, L=1.22 m Air/steamReduction in heat transfer coefficient (HTC) by 50% when 0.5% air is present in steam UchidaVertical 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 SteamDeveloped a correlation and found that HTC is 2.2 times higher than HTC estimated by Uchida correlation Stagnant environment

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BARC Vienna, Austria, September 10-13, 2007 Literature review Test performedGeometry, orientation and size Working fluidRemarks Maheshwari et al.Horizontal tube D=21.3 mm, L=0.75 m Air/SteamHTC 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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BARC Vienna, Austria, September 10-13, 2007 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/steamPANTHERS exptl. Facility. The results are given in terms of condenser efficiency as a function of inlet pressure and air mass fraction OggVertical 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 Nusselts 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. VierowVertical coper tube ID=22.1 mm, L=2.13 m Air/SteamThe 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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BARC Vienna, Austria, September 10-13, 2007 Literature review Test performed Geometry and size Working fluid Remarks SiddiqueVertical tube ID= 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. ArakiVertical tube ID=49.5 mm, L=1.21 m Air/SteamCorrelations for condensation HTC for laminar and turbulent range are developed in terms of Reynolds number and air mass fraction KuhnVertical 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 tubeAir/SteamCorrelation 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/SteamExperiments 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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BARC Vienna, Austria, September 10-13, 2007 Heat and mass transfer coefficient A mass balance at the interface is done to yield the following equation Heat and mass transfer h cond – Condensation heat transfer coefficient, h f – Film heat transfer coefficient h g - 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, h cond 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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BARC Vienna, Austria, September 10-13, 2007 Condensate film model The film heat transfer coefficient on vertical surface is calculated by Nusselt equation for Re f < 30 For condensation on horizontal tube the is replaced by in Nusselt equation Condensate film heat transfer

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BARC Vienna, Austria, September 10-13, 2007 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. The Grashof number is defined as By heat and mass transfer analogy Gas/vapor heat transfer- free convection h g can be obtained from above expression (12) (13) m // cond and h cond can be estimated from equations (11) and (4)

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BARC Vienna, Austria, September 10-13, 2007 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 By heat and mass transfer analogy Re is local mixture Reynolds number in the bulk fluid, and f s 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 10 6

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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 Some of the correlations available in literature Number of correlations are available in the literature. Some of the correlations developed are given below. The correlation developed by Uchida Correlations The Tagami correlation Condensation in stagnant atmosphere

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BARC Vienna, Austria, September 10-13, 2007 The correlation developed by Liu et al x 10 5 Pa < P tot < x 10 5 Pa 4 o C < dT < 25 o C; < X s < Dehbi correlation for 0.3 m < L < 3.5 m; 1.5 atm. < P t < 4.5 atm.;10 o C < (T b -T w ) < 50 o C Where, C= W/m 2 Pa o C Correlations

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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 Correlations Vierow correlation based on UCB data Park correlation based on KAIST data 1715 < Re g < < Pr g < < W a < < Ja < Which is applicable in the following range The degradation factor is defined as

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BARC Vienna, Austria, September 10-13, 2007 Correlations Correlation based on non-dimensional numbers Siddique Correlation based on MIT data Which applies in the following range of experiments 0.1 < W a < 0.95 ; 445 < Re g < ; < Ja < 0.07 Maheshwari correlation based on BARC experiments This equation is valid in the following range 0.1 < W a < < Re g < < Ja < 0.07

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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 Condensation in vertical tube Geometry and Dimensions of the model Test set-up

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BARC Vienna, Austria, September 10-13, 2007 Forced flow condensation Variation of total heat transfer coefficient along the length of the tube

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BARC Vienna, Austria, September 10-13, 2007 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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BARC Vienna, Austria, September 10-13, 2007 Schematic of the steam condensation experimental set up Pressure regulator Compressed air condensing Section 21.3 mm OD tube Insulated lines To drain Water inlet Heater 0-18 kW Water LT P P T T T P Thermocouple Pressure transmitter Level transmitter LT Relief valve and rupture dick Nozzles for vertical installation of model Rotameter (0-8 lpm) Experiment set up

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BARC Vienna, Austria, September 10-13, 2007 Variation of heat transfer coefficient with air mass fraction Comparison between experimental and theoretical results

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BARC Vienna, Austria, September 10-13, 2007 Free and forced convective Condensation Comparison of free and forced convective heat transfer coefficients

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BARC Vienna, Austria, September 10-13, 2007 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

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