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

Reactor Engineering Division, Bhabha Atomic Research Centre, Trombay, Mumbai, INDIA - 400 0854th 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

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

Effect of Non-condensable gases on condensationThe 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 condensationCondensation 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

Effect of Non-condensable gases on condensationSchematic representation of the effect of non-condensable gas on condensation

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.

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.

ScenarioIn 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.

HardwarePCCS with isolation Condenser The system is adopted in ESBWR and SBWR

HardwarePCCS with steel containment vessel The Westinghouse AP-600, SPWR, EP-1000, JPSR and AC-600 are the reactors utilizing this concept.

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

HardwareGeneral Arrangement of AHWR with PCCS Passive external condenser

Literature reviewStagnant environment

Literature reviewStagnant environment

Literature reviewFlowing vapor-noncondensable gas mixture

Literature reviewFlowing vapor- noncondensable gas mixture

Heat and mass transfer

Heat and mass transfer coefficientA mass balance at the interface is done to yield the following equation hcond Condensation heat transfer coefficient , hf Film heat transfer coefficienthg - Convective heat transfer coefficientThe 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

Condensate film model

The film heat transfer coefficient on vertical surface is calculated by Nusselt equation for Ref < 30 For condensation on horizontal tube the 0.943 is replaced by 0.725 in Nusselt equationCondensate film heat transfer

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 asBy heat and mass transfer analogyGas/vapor heat transfer- free convectionhg can be obtained from above expression(12)(13)m//cond and hcond can be estimated from equations (11) and (4)

Gas/vapor heat transfer- Forced convectionHeat 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 By heat and mass transfer analogyRe 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

Heat transfer enhancementFollowing 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

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 UchidaCorrelationsThe Tagami correlationCondensation in stagnant atmosphere

CorrelationsThe 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 correlationfor 0.3 m < L < 3.5 m; 1.5 atm. < Pt < 4.5 atm.;10 oC < (Tb-Tw) < 50 oCWhere, C=55.635 W/m2 Pa0.252 oC1.307

CorrelationsCondensation 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).

CorrelationsThe 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.

CorrelationsVierow correlation based on UCB dataPark correlation based on KAIST data1715 < Reg < 216700.83 < Prg < 1.040.111 < Wa < 0.8360.01654 < Ja < 0.07351Which is applicable in the following range The degradation factor is defined as

CorrelationsCorrelation based on non-dimensional numbers Siddique Correlation based on MIT dataWhich 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 This equation is valid in the following range

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

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

Condensation in vertical tube

Forced flow condensationVariation of total heat transfer coefficient along the length of the tube

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 literatureCondensation in stagnant environment

Schematic of the steam condensation experimental set upExperiment set up

Variation of heat transfer coefficient with air mass fractionComparison between experimental and theoretical results

Free and forced convective CondensationComparison of free and forced convective heat transfer coefficients

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