Matteo Bucci

Critical heat flux maxima resulting from the controlled morphology of nanoporous hydrophilic surface layers

Porous hydrophilic surfaces have been shown to enhance the critical heat flux (CHF) in boiling heat transfer. In this work, the separate effects of pore size and porous layer thickness on the CHF of saturated water at atmospheric pressure were experimentally investigated using carefully engineered surfaces. It was shown that, for a fixed pore diameter (∼20 nm), there is an optimum layer thickness (∼2 μm), for which the CHF value is maximum, corresponding to ∼115% enhancement over the value for uncoated surfaces. Similarly, a maximum CHF value (∼100% above the uncoated surface CHF) was observed while changing the pore size at a constant layer thickness (∼1 μm). To explain these CHF maxima, we propose a mechanistic model that can capture the effect of pore size and pore thickness on CHF. The good agreement found between the model and experimental data supports the hypothesis that CHF is governed by the competition between capillary wicking, viscous pressure drop and evaporation, as well as conduction heat t...

Experiments and Modelling Techniques for Heat and Mass Transfer in Light Water Reactors

The paper summarizes the lesson learned from theoretical and experimental activities performed at the University of Pisa, Pisa, Italy, in past decades in order to develop a general methodology of analysis of heat and mass transfer phenomena of interest for nuclear reactor applications. An overview of previously published results is proposed, highlighting the rationale at the basis of the performed work and its relevant conclusions. Experimental data from different sources provided information for model development and assessment. They include condensation experiments performed at SIET (Piacenza, Italy) on the PANTHERS prototypical PCCS module, falling film evaporation tests for simulating AP600-like outer shell spraying conditions, performed at the University of Pisa, experimental data concerning condensation on finned tubes, collected by CISE (Piacenza, Italy) in the frame of the INCON EU Project, and experimental tests performed in the CONAN experimental facility installed at the University of Pisa. The experience gained in these activities is critically reviewed and discussed to highlight the relevant obtained conclusions and the perspectives for future work.

Analysis of the NUPEC PSBT Tests with FLICA-OVAP

This paper discusses the results of a computational activity devoted to the prediction of two-phase flows in subchannels and in rod bundles. The capabilities of the FLICA-OVAP code have been tested against an extensive experimental database made available by the Japanese Nuclear Power Energy Corporation (NUPEC) in the frame of the PWR subchannel and bundle tests (PSBT) international benchmark promoted by OECD and NRC. The experimental tests herein addressed involve void fraction distributions and boiling crisis phenomena in rod bundles with uniform and nonuniform heat flux conditions. Both steady-state and transient scenarios have been addressed, including power increase, flow reduction, temperature increase, and depressurization, representative of PWR thermal-hydraulics conditions. After a brief description of the main features of FLICA-OVAP, the relevant physical models available within the code are detailed. Results obtained in the different tests included in the PSBT void distribution and DNB benchmarks are therefore reported. The relevant role of selected physical models is discussed.

Validation of a CFD Condensation Model Based on Heat and Mass Transfer Analogy by TOSQAN Facility ISP47 Test

The content of this paper is focused on a computational fluid dynamics analysis of the test performed within the facility TOSQAN as a part of the International Standard Problem 47 (ISP 47). The aim of the study is to contribute to the understanding of the heat and mass transfer mechanisms and to check the possibility to use a commercial CFD code for simulating the mass transfer phenomena of interest in nuclear reactor containment design and safety analysis. In this aim, the FLUENT 6.2 code has been used. The effect of the condensation rate onto the vessel walls was simulated by appropriate source terms introduced by user-defined subroutines into the mass, energy and momentum balance equations. In the paper the time trends of the average temperature and pressure of the atmosphere inside the TOSQAN vessel have been compared with the available experimental data, obtaining a good agreement. Spatial profiles have been also analysed and compared with the experimental ones for the main physical variables in the first, second and fourth steady-state phases which the test consists of. (authors)