Condensation-induced hydraulic shock (CIHS) is a safety issue that requires the accurate prediction of pipe rupture risk due to the formation of a hydraulic shock. The prediction of hydraulic shock formation depends on accurate modeling of slug formation for given conditions of vapor flow rate, liquid height, liquid subcooling, and piping geometry. This paper presents results from the detailed three-dimensional (3-D) computational fluid dynamics (CFD) modeling of this problem applied to twelve different cases from the experimental study of Martin et al. (2007) with different liquid levels, vapor injection rates, and saturation conditions. The results show a remarkable match to experimental results in terms of slug formation, slug propagation, and also in terms of shock formation times and shock amplitudes. The results yield rich information about the mechanisms associated with the formation of interfacial waves as a function of vapor flow rates and liquid level. It also reveals the necessity of condensation to trigger the formation of a slug and the possible modes of condensation (such as interfacial/wall condensation) that result in the propulsion of the slug eventually resulting in the hydraulic shock. This is the first time such a complex problem has been successfully simulated using a 3-D compressible multiphase CFD model, demonstrating the utility of such a tool to increase the operational safety of industrial refrigeration systems.

Accelerator Driven Systems (ADS) are extensively investigated for the transmutation of high-level nuclear waste within many worldwide research programs. The first advanced design of an ADS system is currently developed in SCK.CEN, Mol, Belgium: the Multi-purpose hYbrid Research Reactor for High-tech Applications (MYRRHA). Many European research programs support the design of MYRRHA. In the framework of the Euratom project 'Thermal Hydraulics of Innovative nuclear Systems (THINS)' a liquid-metal free-surface experiment is performed at the Karlsruhe Liquid Metal Laboratory (KALLA) of Karlsruhe Institute of Technology (KIT). The experiment investigates a full-scale model of the concentric free-surface spallation target of MYRRHA using Lead Bismuth Eutectic (LBE) as coolant. In parallel, numerical free surface models are developed and tested which are reviewed in the article. A volume-of-fluid method, a moving mesh model, a free surface model combining the Level-Set method with Large-Eddy Simulation model and a smoothed-particle hydrodynamics approach are investigated. Verification of the tested models is based on the experimental results obtained within the THINS project and on previous water experiments performed at the University Catholic de Louvain (UCL) within the Euratom project 'EUROpean Research Programme for the TRANSmutation of High Level Nuclear Waste in Accelerator Driven System (EUROTRANS)'. The design of the target enables a high fluid velocity and a stable surface at the beam entry. The purpose of this paper is to present an overview of both experimental and numerical results obtained for free surface target characterization. Without entering in technical details, the status, the major achievements and lessons for the future with respect to model development are described as well as some applications, which were carried out within the work package 'multi-phase flow' of THINS. (C) 2014 Elsevier B.V. All rights reserved.

In Pressurized Water Reactors (PWR), convective boiling occurs at the heated walls, which are superheated while liquid bulk is subcooled at a given operating pressure. The physical modeling of the phenomenon is complex, as it requires consideration of phase change and turbulence. The combinations of Reynolds-Averaged Navier Stokes (RANS) and wall boiling models have found moderate success in the past. Succesful modeling depends on the nature of the problem and its operating conditions, the manner of the model implementation and the quality of the computational platform. In the present work recent developments executed in the TransAT code for subcooled boiling are demonstrated. To better capture the phase change, the modified mixture approach is employed where the temperature of the N-phases are resolved separately. This method has been more suitable for phase-change problems, when compared to a single mixture temperature. To check the validity of the modeling, results are evaluated against the DEBORA experiments carried out at CEA Grenoble.

This paper presents results of numerical simulations of various processes that demonstrate phase change heat transfer at high heat fluxes using the level-set method. The model used for the purpose has been first validated for the growth of an evaporating bubble in infinite medium, and fim boiling in 2D and 3D. It has then been applied to simulate the nucleation and departure of a single bubble from a solid body subject to conductive heat transfer. Unlike our previous investigations where phase change induced evaporation rate was incorporated like a sub-grid scale heat transfer model applied to the triple contact line, the present work reports simulations with direct phase change modelling by integrating energy fluxes at the interface. The effect of the conductive heat transfer in the solid from which the bubble departs is also taken into account. Comparison with visual images suggests that accounting for conjugate heat transfer is important to capturing micro-hydrodynamics in nucleate boiling, at least qualitatively.

Direct Numerical Simulations (DNS) of multiphase flows, where all continuum length and time scales are fully resolved have progressed enormously in the last few years. Increases in computer power and new algorithms now make it possible to follow the unsteady motion of several hundred particles (drops, bubbles and solids) for long enough times so that meaningful averages for the fluid mixture can be calculated. However, most progress has so far been made for disperse flow of two-fluid systems. See Prosperetti and Tryggvason (2007) for a review.