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Evaluation of the CHF, with special regard to the CHFR in the hot channel of the RBMK Smolensk 3 plant
No full textThe EC- (European Commission) EURATOM TACIS project 30303 aims at safety evaluation of VVER-1000 (Russian Design, Gidropress, pressurized water reactor) and of RBMK (Russian design, NIKIET, boiling water cooled, graphite moderated nuclear reactor). Safety improvements for those reactors are also expected from the project. About 50 scientists of various nationalities (primarily Russian and Italian) contributed to the project for around 5 years. A variety of experimental and computational activities was performed. Several hundred reports were issued within a suited Quality Assurance (QA) framework and many scientists contributed to each report (only responsible authors identified here).
The present document deals with the evaluation of the Critical Heat Flux (CHF) in the boiling channels of RBMK characterized by a 8m active length and (nearly) triangular pitch of rods
Experimental study on bubble detachment under variable heat load and the action of electric field
No full textThis paper is aimed at studying single bubble growth and detachment in controlled conditions and under the
action of an electric field. An experimental set-up was built, consisting mainly of a stainless steel lamina with a
nucleation site etched in its center, facing the testing fluid. The required heating power was provided by an SMD
microresistor, stuck by means of epoxy resin on the back side of the plate. A signal generator, connected to the
resistor, allowed for heating power modulation and thus for variation of the bubble size. Two different fluids, FC-72
and HFE-7100 were tested. An external electrostatic field was applied by imposing up to 15 kV dc to a ring of
4mm inner diameter suspended 3mm above the plate: in this way, a very clear axisymmetric configuration was
created, suitable for two-dimensional analyses. High speed images were taken by means of a high-speed camera,
operated at 1000 frames per second with a resolution of about 100 pixel/mm. Images were treated with a dedicated
image processing software to obtain bubble significant geometrical parameters. The preliminary results reported
here show elongation of the detached bubble under the action of the electric field, confirming former results obtained
with nitrogen bubbles originating from an orifice in the same liquids. The detachment volume under the action of
the electric field either increases or decreases, for low and high electric permittivity fluid, respectively, evidencing
the complex nature and role of the electric forces. First results under variable heat load evidence a reduction in
detachment diameter. The dynamical forces outside the microlayer region seem to have a minor role, compared to
the static ones, in vapor bubble growth
On the reciprocal influence of gas injection method and liquid contamination on bubble rising velocity in a liquid column
No full textWater two-phase flow through pressure safety valve with variable backpressure: check of calculation methods to estimate mass flow-rate and critical flow conditions
No full textA careful design of pressure safety valves (PSV) is an essential requirement for safeguarding of industrial plants; reliable correlations are available for PSV design in the cases of liquid or gas discharge. Instead, if two-phase flow is possible, especially for low vapour quality (less than 10%), PSV design becomes very difficult owing to complex thermal hydraulic phenomena that happen between the two phases; moreover, in this situation, the prediction of the critical flow onset (which occurs when the fluid velocity becomes equal to the sound velocity in the fluid) is very important for a correct design because it involves a choked flow situation.
Currently there are some calculation methods, based on different simplifying hypotheses, that try to predict the two-phase flow-rate through a PSV knowing the inlet fluid conditions (pressure, quality or temperature) and the outlet pressure; however, none of them is acknowledged as being reliable for every situation and, therefore, there are not standards for PSV design under two-phase conditions.
This paper shows the results of an experimental research carried out through a PSV with steam-water two-phase flow. The experimental data are compared with the results of a calculation method based on the homogeneous model with non equilibrium hypotheses. The performance of the model is evaluated as a function of inlet and outlet parameters; an analysis of model critical flow prediction capability is presented too, with some uncertainties caused by the test procedures (here the mass flow-rate is imposed
Efflusso bifase attraverso valvole di sicurezza. Confronto tra i dati sperimentali e le previsioni di alcuni modelli omogenei
No full textLa sicurezza è un’esigenza particolarmente sentita negli impianti in cui situazioni accidentali possono comportare un aumento incontrollato della pressione con conseguenti pericoli per l’incolumità del personale, le popolazioni e l’ambiente. Le valvole di sicurezza sono tra i componenti più utilizzati per limitare entro valori accettabili l’incremento di pressione.
Il loro dimensionamento segue delle norme ben precise ed affidabili per gli efflussi monofase mentre manca una standardizzazione nel caso di efflussi bifase. Nel corso degli ultimi cinquanta anni per la rappresentazione dell’efflusso bifase sono stati sviluppati diversi modelli caratterizzati da diverse ipotesi semplificative; al momento, però, nessun metodo di calcolo è riconosciuto a livello di standard per il dimensionamento delle valvole di sicurezza.
In questo articolo sono presentati alcuni risultati di una ricerca sperimentale in corso sull’impianto VASIB, situato presso l’Istituto di Termofluidodinamica Energetica nel centro ricerche dell’ENEA Casaccia. Le esperienze sono condotte, utilizzando acqua come fluido evolvente, su una valvola di sicurezza reale, for=10mm, per differenti condizioni di ingresso: portata (300÷1500 kg/h), titolo (0÷10%), pressione (0.5÷1.75 MPa). I dati sperimentali sono stati comparati, in funzione dei diversi parametri di ingresso, con le previsioni di un metodo di dimensionamento ultimamente suggerito dall’API (American Petroleum Institute) e un metodo semplificato sviluppato per confronto; entrambe le metodologie utilizzano le ipotesi del modello omogeneo di equilibrio (metodo w
Ebollizione in convezione forzata in condizioni di microgravità
No full textRendiconti Accademia Nazionale delle Scienze detta dei XL
Memorie di Scienze Fisiche e Natural
Experimental analysis of heat transfer in a high diameter ratio annular gap filled with a rarefied gas
No full textA first experimental attempt has been realized in order to measure the heat transfer
between a heated platinum wire having a diameter (d) of 0.15 mm, disposed along the axis of a
cylindrical shell in stainless steel having an inner diameter (D) of 100 mm, and air from
atmospheric conditions down to 10-3
mbar. Temperature differences between the wire and the
external stainless steel cylindrical shell in the range of 50-125 K were imposed and the heat
power transferred from the wire to the surround was measured as a function of the gas
pressure. The experimental results demonstrates that for an accurate measurement of the heat
conduction when the pressure goes down to 0.05 mbar is very important to be able to quantify
accurately the radiative contribution which becomes predominant at low pressure. The main
limitations of the test rig described in this paper have been analysed in order to highlight the
modifications which can be suggested to obtain experimental results comparable with
theoretical models
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