Direct Contact Condensation (DCC) in Nuclear Field Riccardo Mereu Research Associate - Department of Energy, Politecnico di Milano Visiting Researcher – Tokyo Institute of Technology Takahashi’s class, TITech, 07/15/2014 DCC – Nuclear Eng. Field Nuclear Engineering Field: Thermal-Hydraulics • • Safety systems for pressure suppression in BWRs Safety/Relief Valves (SRVs) actuation Loss-Of-Coolant Accident (LOCA) Rapid depressurization systems in PWRs Steam – steam/gas injection in a condensing pool 2 Nuclear Applications BWRs - pressure suppression safety systems 3 Nuclear Applications BWRs - pressure suppression safety systems • Safety/Relief Valves (SRVs) located on main steam lines • closure of the main steam isolation valves or the sudden closure of the turbine admission or stop valves and failure of the turbine bypass system to relieve the excess pressure • SRVs discharge steam into water pools located inside the containment 4 Nuclear Applications 5 BWRs - pressure suppression safety systems • Loss-of-coolant-Accident (LOCA) is related with a decrease of reactor coolant inventory (e.g. break of primary system pipe) • A large amount of non-condensable and condensable (steam) gas is blown from the upper drywell of the containment to the condensation pool through the blowdown pipes at the Olkiluoto type BWRs. • The wetwell pool serves as the major heat sink for condensation of steam. The blowdown causes both dynamic and structural loads to the condensation pool. Direct Contact Condensation 6 DCC of steam injected into water • DCC common phenomenon occuring when steam is introduced into subcooled water (pool) • Steam contacts and condenses on a sub-cooled liquid interface directly • Behaviour of interface is governed by heat transfer processes in steam and liquid regions around the interface • DCC occurs for steam discharged as a forced jet or buoyant plume through a sparger • Strong dependence on mass flux (kg/s2m), sub-cooling (Tsteam-Twater) and injector size... Direct Contact Condensation 7 Regions Song et al. (2012) Petrovic et al. (2007) Direct Contact Condensation Subcooling [K] Regimes Diameter [m] Steam mass flux [kg/m2-s] 9 Direct Contact Condensation Effect of non-condensable gas • Presence of non-condensable gas into steam flow influences the heat and mass transfer mechanisms • Effects of non-condensable gas on bubble dynamics and pressure loads during chugging mixing is verified • Even small quantities of air (<1%) reduce the condensation rate considerably • Increasing air quantities (2%) the pressure oscillation at the bottom of the pool is also influenced, both amplitude and frequency decrease considerably NKS-148 (2006) Direct Contact Condensation 10 Thermo-hydrodynamic issues of steam jet into water pool • bubble dynamics • thermal stratification • mixing • steam condensation within water pool, within ducts, and at wall surfaces… • heat transfer, rapid condensation and momentum exchange (i.e. hydrodynamic loads to the pool structures) Detailed analysis of these phenomena either by experiments or with numerical simulations/modeling are mandatory Q&A 11 BACKUP 12 Direct Contact Condensation 13 Regions • Steam plume is the first region in the condensation region, present at the steam pipe exit • Interface separates the steam plume from a hot water layer • Condensation occurs in the hot water layer through the interface via a convective heat and mass transfer • Shape of interface depends on the interfacial eddies and local temperature, the size on the rate of condensation and viceversa • Flow at interface mixed with bubbles and presence of turbulence, formed by the momentum substracted from the condensing steam Direct Contact Condensation 14 Regimes • Chugging Curved or flat shape of steam plume, steam plume size close to cross sectional area of injectors Steam inflow rates up to 80 kg/m2s (depending on Twater and Dinjector) Steam inflow rate lower than the condensation rate (suction of surrounding water into steam injector) Chugging inside the injector with pulsanting mode is called interfacial condensation oscillation (usually steam inflow rate < 5 kg/m2s) Petrovic et al. (2007) SAFFIR Final report (2006) Direct Contact Condensation 15 Regimes • Jetting Steam plume holding approx. constant size and shape Steam inflow rates higher than 100 kg/m2s (depending on Twater and Dinjector) Three different types: conical, ellipsoidal and divergent Petrovic et al. (2007) Direct Contact Condensation 16 Regimes • Bubbling Steam generates regular or irregular bubbles at the edge of the injector Steam inflow rates between chugging and jetting Possible collpase of original bubble after detachment Clerx et al. (2009) Direct Contact Condensation 17 3-D regime diagram Petrovic et al. (2007) Direct Contact Condensation 18 3-D regime diagram – low steam inflow rate Petrovic et al. (2007) Direct Contact Condensation 19 2-D regime diagram – more regimes Song et al. (2012)
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