1,720,990 research outputs found
A Critical Overview of Heat Transfer Phenomena through Different Solutions of In-Pool Condensers for Passive Safety Systems
No full textExperimental Characterization of a Two-Phase Natural Circulation Loop Simulating a Passive Emergency Heat Removal System
No full textPreliminary CFD Study of Flow Oscillations in Parallel Channels Using the Volume Of Fluid (VOF) Method
No full textModelling of Heat Transfer Phenomena for Vertical and Horizontal Configurations of In-Pool Condensers and Comparison with Experimental Findings
No full textDecay Heat Removal (DHR) is a fundamental safety function which is often accomplished in the advanced LWRs relying on natural phenomena. A typical passive DHR system is the two-phase flow, natural circulation, closed loop system, where heat is removed by means of a steam generator or heat exchanger, a condenser, and a pool.
Different condenser tube arrangements have been developed for applications to the next generation NPPs. The two most used configurations, namely, horizontal and vertical tube condensers, are thoroughly investigated in this paper. Several thermal-hydraulic features were explored, being the analysis mainly devoted to the description of the best-estimate correlations and models for heat transfer coefficient prediction. In spite of a more critical behaviour concerning thermal expansion issues, vertical tube condensers offer remarkably better thermal-hydraulic performances.
An experimental validation of the vertical tube correlations is provided by PERSEO facility (SIET labs, Piacenza), showing a fairly good agreement
Analysis of Different Containment Models for IRIS Small Break LOCA, using GOTHIC and RELAP5 Codes
Full text linkAdvanced nuclear water reactors rely on containment behaviour in realization of some of their passive safety functions. Steam condensation on containment walls, where non-condensable gas effects are significant, is an important feature of the new passive containment concepts, like the AP600/1000 ones. In this work IRIS reactor was taken as reference, and the relevant condensation phenomena involved within its containment were investigated with different computational tools. In particular, IRIS containment response to a Small Break LOCA (SBLOCA) was calculated with GOTHIC and RELAP5 codes. IRIS containment drywell was modelled with RELAP according to a sliced approach, based on the two-pipe-with-junction concept, while it was simulated with GOTHIC testing several modelling options, regarding both heat transfer correlations and volume and thermal structure nodalization. The influence on containment behaviour prediction was investigated in terms of drywell temperature and pressure response, Heat Transfer Coefficient (HTC) and steam volume fraction distribution, and internal recirculating mass flowrate. The objective of the paper is to compare the capability of the two codes in modelling of the same postulated accident, thus to check the results obtained with RELAP5, when applied in a situation not covered by its validation matrix. The option to include or not droplets in fluid mass flow discharged to the containment was the most influencing parameter for GOTHIC simulations. Despite some drawbacks, due e.g. to a marked overestimation of internal natural recirculation, RELAP confirmed its capability to satisfactorily model the IRIS containment
Experimental and theoretical studies on density wave instabilities in helically coiled tubes
Full text linkThis paper reports on the advancement in the study of thermal-hydraulic dynamic instabilities with reference to the helical-coiled tube geometry.
A full-scale open-loop experimental facility simulating a helically coiled steam generator was built and operated at SIET labs in Piacenza (Italy). The facility comprises two helical tubes (1 m coil diameter, 32 m length, 8 m height), connected via lower and upper headers. Nearly 100 flow instability threshold conditions were identified, in a test matrix of pressures (80 bar, 40 bar, 20 bar), mass fluxes (600 kg/m2 s, 400 kg/m2 s, 200 kg/m2 s), inlet subcooling (from −30% up to ∼0), and inlet throttling (four different entrance resistance conditions). The long test section feature and the helical-coiled tube geometry render the present facility a quite unique test case in the outline of two-phase flow instability experimental studies. Parametric effects of the operating pressure, flow rate, inlet subcooling and inlet throttling on the threshold power are discussed. The period of oscillations is also discussed. Superimposition of Density Wave Oscillations (DWOs) with Ledinegg flow excursions is finally described.
Theoretical modelling of DWO occurrence in helical pipes was addressed by means of a lumped parameter analytical model, which was exploited to highlight some peculiarities of DWO phenomena and respective stability boundary with respect to classical straight geometry. In the end, numerical simulation results with RELAP5/MOD3.3 code were compared
Time-domain Linear and Non-linear Studies on Density Wave Oscillations
No full textDensity Wave Oscillations (DWOs) are investigated in this paper as the most representative instabilities encountered in the boiling systems. This dynamic type instability mode constitutes an issue of paramount interest for the design of industrial systems and equipments, such as steam generators and boiling water nuclear reactor cores. Suited analytical and numerical modelling tools are useful for grasping the fundamental features of this oscillation mode and for predicting the instability threshold dependence on the main system parameters. A theoretical lumped parameter model – moving boundary type – was developed, based on the integration of mass, energy and momentum 1D equations. Homogeneous two-phase flow model has been assumed within the boiling region. Theoretical predictions on DWOs have permitted to investigate the delay propagation phenomena that, in respect of the constant-pressure-drop boundary condition across the channel, trigger the development of self-sustained flow rate oscillations. Instability threshold calculation has permitted to draw stability maps (Npch-Nsub stability plane), representing e.g. the parametric effect of the inlet subcooling on the instability inception. Several sensitivity studies have permitted finally to identify in the proper simulation of two-phase frictional pressure drops the most critical issue for a correct prediction of the phenomenon. Theoretical calculations from analytical model were then successfully compared with numerical results obtained with the RELAP5 thermal-hydraulic code and the COMSOL multi-physics code. In addition to homogeneous flow model, a drift-flux model for the two-phase flow was also implemented with the latter approach. Topological characterization of DWO instability phenomena was completed by means of a linear stability analysis leading to the definition of the system eigenvalues, both dealing with the analytical model equations and with the thermal-hydraulic model developed in COMSOL. Linear analysis showed to be a quick and powerful tool generally for instability studies and in particular when addressing the influence of the two-phase friction models
Numerical Investigation on Boiling Channel Instabilities by Imposing Constant Pressure Drop Boundary Condition via a Large Bypass
No full textExperimental Characterization of a Passive Emergency Heat Removal System for a GenIII+ Reactor
No full textAmong the several types of passive safety systems adopted in new generation reactor designs, the experimental investigation of a closed loop, two-phase flow, natural circulation system is depicted. Emergency Heat Removal Systems (EHRSs) based on this solution are envisaged as safety-engineered features for advanced nuclear reactors, as in the IRIS reactor. An experimental facility simulating one EHRS-like loop has been built and operated at SIET labs in Piacenza (Italy). The facility is a natural circulation, sliding pressure, and electrically heated loop, with a helical coil steam generator as a heat source and a horizontal tube pool condenser as a heat sink. A steady-state analysis is provided to characterize the system behaviour and its key parameters. Because of the loop limited volume, oscillations of the main parameters (temperatures, flowrate, pressure) may be expected. The oscillating phenomena detected during the experimental campaign are discussed; a reasonable explanation is at last proposed
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