Abstract: An experimental investigation on the role of noncondensable gases during condensation of steam inside horizontal/slightly inclined tubes is presented. In a condenser of a thermal desalination unit, the noncondensable gases flowing within the gas phase cause reduction of performance and efficiency. Many researchers have investigated in-tube condensation for horizontal heat exchangers, but very few works have been performed to study the condensation in slightly inclined tubes with noncondensable gases. Inclination of tubes may be requested to achieve suitable and reliable draining. Experiments were conducted in the following conditions: tube internal diameter 12.6 mm, 20 mm and 26.8 mm; tube inclination 7°; inlet noncondensable gas mass fraction Ïin = 5%â42%; inlet mixture Reynolds number Rem,in = 5000â20,000; local noncondensable gas mass fraction Ïin = 5%â60%; local mixture Reynolds number Rem = 500â20,000; saturated steam at atmospheric pressure, gravity controlled with stratified flow regime, in which the condensate is collected mainly in the bottom part of the tube due to gravity and it is drawn out by its own momentum. A correlation of mixed gas heat transfer coefficient along a slightly inclined tube, in a gravity controlled flow regime, has been developed, showing a good agreement with experimental results.
Abstract: Considering the importance of safety features in the development of Generation IV nuclear reactors, an innovative and passive decay heat removal system (DHRS) has been proposed for liquid metal cooled reactors. The attention is here focused on the direct heat exchanger (DHX) of the system constituted by a bayonet tube that allows to remove the decay heat from the primary coolant; both primary and secondary fluids flow in natural circulation. Since each bayonet tube is equipped with a vacuum gap, the most important heat transfer mechanism characterizing the DHX is radiation. Furthermore, the presence of the vacuum gap guarantees a physical separation and a complete decoupling between primary and secondary fluids, enhancing the safety features of the whole system. Several CFD analyses have been carried out in order to obtain a charcterizazion of the DHX both for sodium and lead cooled fast reactors, and the results are discussed in the paper
Abstract: An experimental investigation on the role of noncondensable gasses during condensation of steam inside inclined tubes is presented. In a condenser, noncondensable gasses flowing with steam cause reduction of condenser performance and efficiency. Many researchers have investigated in-tube condensation for vertical heat exchangers, but very few works have been performed to study condensation in inclined tubes with noncondensable gasses. In the paper, experiments dedicated to this situation are described, with reference to the following conditions: tube internal diameter: 12.6 mm, 20 mm and 26.8 mm; tube inclination: 7°, 15°, 30° and 45°; inlet noncondensable gas mass fraction Ïin = 2%â42%; inlet mixture Reynolds number Rem,in = 5000â20000; local noncondensable gasmass fraction Ï = 2%â70%; localmixture Reynolds number Rem = 400â21000; local condensate Reynolds number Rel = 10â290; saturated steam at atmospheric pressure; and gravity controlled flow regime. A limited influence of the inclination angle on heat transfer coefficient has been observed. Correlations to evaluate the local heat transfer coefficient along inclined tubes, in a gravity controlled flow regime, have been developed and they are in good agreement with the experimental results.
Abstract: The MARS nuclear power plant is a cheap, simple, extremely safe PWR plant incorporating all main well-proven features of a âtraditionalâ PWR plant. A production plant is made of medium-power modules: each reactor module may produce up to some 1000 MWth; the design here presented refers to a size of 600 MWth. The whole design is strongly simplified, thanks to safety characteristics based on simple, almost completely static and exclusively passive-type emergency systems. The probability of core integrity failure is a vanishing event (according to rigorous PRA evaluations, core damage probability in the range of 1E-7 event/year). The core cooling system includes one loop only, with recirculation-type steam generator; during normal operation, the primary coolant is pumped in the reactor vessel and in the primary loop, while, in emergency conditions, the coolant flow in the core is completely guaranteed through an independent cooling system, transferring heat to the external atmosphere with natural circulation and relying on only static components and one non-static, passive component (check valves, 400% redundant on two independent cooling trains). In the paper the peculiar design features and the experimental activities performed to support the design of Mars Reactor are presented.
Abstract: Experiments on steam condensation inside inclined tubes were carried out with the following aims: a) to investigate the physical phenomena involved in condensation of steam within tubes; b) to study the influence of the geometry (namely, tube inclination) on the heat transfer rate, also in presence of high concentration of non-condensables; c) to develop models and heat transfer correlations for these conditions; d) to produce a database for modeling in-tube condensation with high percentage of non-condensable gases. Steam and steam-air condensation experiments were carried out in gravity controlled stratified flow regime inside a horizontal and inclined tube (22 mm inside diameter) and the average heat transfer coefficient has been evaluated. For pure steam condensation, the experimental data were compared with literature correlations and their agreement has been verified, suggesting some minor modifications. A limited influence of tube inclination on heat transfer has been observed: condensation in the presence of non-condensable gases is not sensibly affected by inclination, especially at high gas concentrations. Two empirical correlation are proposed to be used in the preliminary design of a condenser in a passive containment cooling system, as in thermal-hydraulic simulation, especially in transient conditions, when a high gas concentration is present.
Abstract: The present work is aimed at describing an innovative plant solution able to greatly simplify and to make a fast neutron Sodium cooled nuclear reactor plant cheaper, without impairing safety. An innovative system has been developed, aimed at eliminating the intermediate sodium loop. It is based on a special heat exchanger that âjoinsâ the intermediate heat exchanger, typical SFRs component, to the steam generator. The presence of an intermediate loop in Sodium Fast Reactors (SFR) is due to the high reactivity features of sodium with air or water. Thanks to this circuit a reaction between primary sodium (radioactive) and water/steam, within the energy conversion system, is prevented and, at the same time, the following presence of reaction product in the primary circuit, with the safety implication it would have, is avoided. Moreover, the plant solution proposed allows maintaining a physical double separation between primary sodium and water/steam, but avoiding any moving component within the fluid between primary sodium and water/steam. The research leading to these results has received funding from the European Communityâs Seventh R&TD Framework Programme (FP7/2007-2011) under grant agreement n° 232658
Abstract: The MARS nuclear power plant is a cheap, simple, extremely safe PWR plant incorporating all main well-proven features of a âtraditionalâ PWR plant. A production plant is made of medium-power modules: each reactor module may produce up to some 1000 MWth; the design here presented refers to a size of 600 MWth.  The whole design is strongly simplified, thanks to safety characteristics based on simple, almost completely static and exclusively passive-type emergency systems. The probability of core integrity failure is a vanishing event (according to rigorous PRA evaluations, core damage probability in the range of 1E-7 event/year). The core cooling system includes  one loop only, with recirculation-type steam generator; during normal operation, the primary coolant is pumped in the reactor vessel and in the primary loop, while, in emergency conditions, the coolant flow in the core is completely guaranteed through an independent cooling system, transferring heat to the external atmosphere with natural circulation and relying on only static components and one non-static, passive component (check valves, 400% redundant on two independent cooling trains). In the paper the peculiar design features and the experimental activities performed to support the design of Mars Reactor are presented.
Abstract: For the future development of Generation IV nuclear reactors, both safety and economic targets have to be achieved. In order to increase, at the same time, the power density generation and the safety features, a huge R&D effort is still required. Referring especially to Liquid Metal Cooled Fast Reactors, much attention is placed on Gas Entrainment phenomena, which could cause unlikely positive reactivity insertion accident. The GETS experimental facility (Gas Entrainment Test Section), especially aimed at studying the free surface vortices occurrence, has been built in the thermal-hydraulics laboratory of the DIAEE. The main purpose of this facility is to identify the most important parameters affecting the whirlpools formation and evolution. Experimental tests and preliminary observations have been performed. Different vortex behaviours related to different experimental conditions have been identified and presented in the present paper. 2D occurrence maps as function of different dimensionless groups (Reynolds, Froude and Weber numbers and H* = H/d ratio) have been defined. In the present paper, the results of a first experimental campaign, carried out with cold water, are discussed.
Abstract: In the perspective of Generation IV nuclear reactors development, and before reaching a standardized industrial diffusion, some problems have to be overcome. Concerning the Sodium Fast Reactors (SFRs), one of the main issues is the Sodium-Water Reaction (SWR) within the Steam Generators (SGs). If sodium comes into contact with steam or water within a failed SG, a violent chemical reaction producing explosive (H2), caustic and corrosive (NaOH) reaction products would occur. In order to prevent any SWR, an innovative solution based on a double barrier separation between the water and the primary sodium, the âCandleâ concept, has been developed. The present paper is aimed at evaluating, by means of a simplified model, the mechanical consequences of a SWR occurring within a Candle.
Abstract: In the perspective of a future development of nuclear reactors, many targets should be reached in order to satisfy the GEN IV requirements. Among them, beyond the safety criteria and targets, the economy of energy produced is an important issue. With this aim, mainly for Sodium cooled Fast Reactors, reactor designs with compact vessels and high thermal power are being studied and designed. Nevertheless, owing to the high coolant velocity, the cover gas entrainment into sodium results as one of the most important safety issues in SFRs design. The gas entrainment phenomenon, in a SFR, may cause reactivity increase or fluctuation in the core, heat transfer capability reduction, entrained gas activation and instrumentation noise increase. Several studies, aimed at avoiding the formation of vortices leading gas entering into the pumps, were carried out with reference to several hydraulic systems. More recently, several studies have also been carried out. The gas entrainment, due to free surface vortex, is a very complex and unsteady phenomenon and it has been deeply studied by means of three different approaches: analytical, experimental and numerical. In this paper, several studies and models already proposed will be critically analysed with the aim as identifying advantages, disadvantages and specific limits for each model, in the view at developing an innovative approach.
Abstract: Lâoggetto del presente studio è un impianto di cogenerazione alimentato ad idrometano asservito alla gestione termica di una piscina costruita allâinterno del centro sportivo universitario di Tor di Quinto. Il laboratorio di sperimentazione connesso alla realizzazione di tale impianto ha come obiettivi la promozione di sistemi innovativi per la gestione energetica ed economica dellâAteneo e la sostenibilità ambientale allâinterno di una grande città come Roma. Di fatto, tale laboratorio si configura come uno dei nuovi centri di autoproduzione facente capo alla smart grid della Sapienza costituendo il primo sistema ibrido realizzato in Italia che impiega, in modo sinergico e razionale, le tecnologie della cogenerazione, del fotovoltaico e della produzione di idrogeno a bassa pressione ( 15 bar ) mediante processo elettrolitico. A partire dai risultati ottenuti da uno studio di fattibilità sulla piscina si è determinata la taglia di 60 kW elettrici per il cogeneratore. Per lâ alimentazione del motore a combustione interna è stata prevista la possibilità di operare con gas metano al 100 %, oppure con una miscela arricchita di idrogeno di composizione modulabile ( nel range 0 - 10 % in volume ) mediante interposizione di un buffer di disaccoppiamento (capacità : 60 litri) ed apposita apparecchiatura miscelatrice. Lâidrogeno viene prodotto da un elettrolizzatore alcalino (potenzialità : 1.1 Nm3/h ) che assorbe una potenza elettrica di 5.5 kW in parte dalla rete e in parte da un impianto fotovoltaico (6 kW) grid - connected totalmente integrato in facciata di una palazzina adiacente. Con il completamento dellâinstallazione del sistema di analisi fumi in continuo sarà possibile effettuare i test sul motore e valutare le prestazioni energetiche ed ambientali dellâintero sistema ibrido
Abstract: An experimental apparatus has been built in order to perform sensitivity analysis on the performance of a natural draft-dry cooling tower. This component plays an important role in the passive system for the residual heat decay removal foreseen in the MARS reactor and in the GCFR of the Generation IV reactors. The sensitivity analysis has investigated: 1) the heat exchanger arrangement; two different arrangements have been considered: a horizontal arrangement, in which a system of electrical heaters are placed at the inlet cross section of the tower, and a vertical arrangement, with the heaters distributed vertically around the circumference of the tower. 2) The shape of the cooling tower; by varying the angle of the shell inclination it is possible to obtain a different shape for the tower itself. An upper and a lower angle inclination were modified and by a calculation procedure eleven different configuration were selected. 3) The effect of cross wind on the tower performance. An equation-based procedure to design the dry-cooling tower is presented. In order to evaluate the influence of the shape and the heat exchanger arrangement on the performance of the cooling tower, a geometrical factor (FG) and a thermal factor (FT) are introduced. By analyzing the experimental results, engineering design relations are obtained to model the cooling tower performance. The comparison between the experimental heat transfer coefficient and the heat transfer coefficient obtained by the mathematical procedure shows that there is a good agreement. The influence of wind has been investigated on selected configurations and it has been studied by introducing several coefficients for the comparison: a wind effect and a pressure coefficients were used to evaluate the effect at the throat; a power coefficient, a mean static and mean dynamic pressure coefficients were also defined in order to compare the influence of the wind on different configurations. The obtained results show that it is possible to evaluate the shape and the heat exchanger arrangement to optimize the performance of the cooling tower either in windless condition either in presence of cross wind.
Abstract: Buildings with large enclosure often present unresolved problems related to energy and air flows such as unwanted thermal stratification, local overheating, uncontrolled contaminant spreading. These kind of constructions are often found in unique buildings where innovative designs are developed. Consequently, there exists no previous experience and very careful analysis on the ventilation design is advisable [1]. This is particularly true for the halls of the âCittà dello Sportâ in Rome, designed by Santiago Calatrava, because of their singularity in term of dimension and shape. The CFD simulations here presented are playing a significant role in the on to go design of these unique large enclosure buildings for the optimisation of the HVAC strategies. The simulated heat, cooling and ventilation system defines three different climatic zones in order to minimize the energy consumption: the swimming pool plane and the playground where optimal condition for the athletes have to be assumed; the stands where the people watching the event is placed; the huge volume under the roofing where particular climate conditions arenât required. The simulated system provides a particular ventilation system using a controlled airflow to heat and cool the mass of the stands, which afterwards operates directly in the zone occupied by the spectators. The swimming pool level and the basketball playground are heated, cooled and ventilated by a dedicated HVAC system. Air velocity, temperature and relative humidity fields in the indoor space of the building, in condition of maximum external and internal thermal loads have been obtained from simulations, in order to evaluate suggested value for the designing process of the HVAC system.
Abstract: With the intention to place the University City of Rome in a vanguard position in the use of renewable and low-emission energy sources, it has been planned to subdivide the district in âislandsâ, each characterized from different energy sources. One phase of this new energetic organization involves the installation of a trigeneration system based on a microturbine for the electric power production and an absorption chiller, fed directly from the exhaust, to supply thermal energy for the satisfaction of the winter energy requirements and of the necessities of summer air conditioning. In the paper, a methodology for the preliminary choice of the main equipment, based on the data provided in the technical descriptions of the suppliers, in order to obtain an objective âevaluation parameterâ, is described. The methodology could be useful to synthetize the valence of the project, not only from the economic point of view but also considering the environmental value.
