Xu Cheng

2nd Sino-German Symposium on Fundamentals of Advanced Nuclear Safety Technology

The present issue contains x elaborations of outstanding presentations held at the 2 Sino-German Symposium on Fundamentals of Advanced Nuclear Safety Technology (SG-FANS) on September 12–15 in Karlsruhe. The symposium was a follow-up event to the symposium in Shanghai on March 8–12, 2015, about which the KERNTECHNIK reported in detail in its special issue March 2016 (Vol. 81, No. 2). The symposium was co-organized by Gesellschaft für Anlagenund Reaktorsicherheit (GRS) gGmbH, Karlsruhe Institute of Technology (KIT) and Shanghai Jiao Tong University (SJTU), and financed by the Sino-German Center for Research Promotion (Chinesisch-Deutsches Zentrum für Wissenschaftsförderung, CDZ). It was hosted by the Karlsruhe Institut of Technology (KIT). The symposium was organized and structured in such a way that there was sufficient opportunity for the participants to exchange information and to have detailed technical discussions. The great interest and success of the event is impressively demonstrated by the large number of participants and the number of presentations. For the first time, a poster session with 22 contributions was held as part of the symposium. The symposium was made perfect by a visit of important nuclear Institutes and selected unique test facilities of KIT, a welcome reception and a conference dinner. The main motivation of the symposium was the importance of nuclear safety, which is attracting enormous attention of the international nuclear community and is becoming a significant topic for international collaboration, in particular after the Fukushima accident. Cooperation in nuclear safety is a win-win situation for both countries. China has worldwide the largest nuclear power development program, requires high standards of nuclear safety and looks for international collaboration, while Germany is one of the countries with the most advanced nuclear safety technologies looking for international markets. The main purposes of the symposium and this special issue are

Mechanistic prediction of post dryout heat transfer and rewetting

Abstract Post dryout heat transfer and rewetting are important processes determining the level and the time duration of high temperature phase and the integrity of the fuel cladding. In spite of extensive studies in the past decades, reliable prediction methods are still missing due to the complexity of processes involved, which consist mainly of interaction between solid wall, main gas flow and droplets. In the present study, a phenomenological model is proposed considering the three individual heat transfer processes between the three parts. Main new features of the present model compared to the models available in the open literature are the mechanistic modeling of (a) droplet concentration and droplet size, (b) turbulent fluctuation velocity of droplets and its critical value, (c) evaporation rate of droplets arriving the wall. Comparison of the new model with selected experimental data shows at least qualitatively good agreement. The experimental behavior of wall temperature can be well explained. According to the new model the Leidenfrost effect results in the hysteresis behavior of wall temperature.

Comparison of heat transfer models with databank of supercritical fluid

Abstract Because of the large variation of thermal–physical properties near pseudo-critical point, heat transfer of supercritical water shows abnormal behavior. Thus, an accurate prediction of heat transfer between the cladding and fluid plays a very important role. It is necessary to investigate the reliably of heat transfer models in the vicinity of the pseudo-critical point. In the frame of evaluating the present HT models and developing new reliable prediction models, databank of heat transfer of supercritical water flowing in tube were established by Karlsruhe Institute of Technology (KIT) with more than 35,000 experimental data. A thorough analysis and assessment was carried out, to give an insight into the characters of the database. Experimental data from different sources are compared to figure out the deviation between different sources and to present the reliability of the database.

Phase-Field Modeling of Binary Eutectic Alloy Solidification with Convection

Abstract In the framework of a description of melt pool heat transfer under severe accident conditions, we introduce a computational fluid dynamics approach for the phase change based on the phase-field method. The approach is derived using the formalism of irreversible thermodynamics and depends on a phenomenological expression for the free energy of binary eutectic alloys. The free energy is constructed to describe sharp interfaces on sufficiently small length scales and is capable of representing the appearance of mushy layers in a volume-averaged large-scale perspective. In particular, a dynamic calculation procedure for the diffuse interface width is introduced based on free energy minimization. Numerical simulations using this approach are performed and compared with experimental and numerical results from the literature. These comparisons demonstrate that the new model improves numerical simulation results and is able to describe the dynamics of sharp and diffuse interfaces.

Selected contributions from 1th Sino-German Symposium on Fundamentals of Advanced Nuclear Safety Technology

The present special issue contains 20 elaborations of outstanding presentations held at the 1 Sino-German Symposium on Fundamentals of Advanced Nuclear Safety Technology (SG-FANS) on March 8–12, 2015 in Shanghai. The symposium was co-organized by Shanghai Jiao Tong University (SJTU) and Karlsruhe Institute of Technology (KIT) and financed by the Sino-German Center for Research Promotion (Chinesisch-Deutsches Zentrum für Wissenschaftsförderung, CDZ). It was hosted by the School of Nuclear Science and Engineering (SNSE) of SJTU. The symposium was organized and structured in such a way that there was sufficient opportunity for the participants to exchange information and to have detailed technical discussions. The main motivation of the symposium was the importance of nuclear safety, which is attracting enormous attention of the international nuclear community and is becoming a significant topic for international collaboration, in particular after the Fukushima accident. Cooperation in nuclear safety is a win-win situation for both countries. China has worldwide the largest nu-clear power development program, requires high standards of nuclear safety and looks for international collaboration, while Germany is one of the countries with the most advanced nuclear safety technologies looking for international markets. The main purposes of the symposium and this special issue are

Numerical investigation of bubble wake properties in the moving liquid with LES model

ABSTRACT Wake flow caused by the relative motions between bubble and liquid phase influences bubble motions and enhances turbulent properties in the liquid phase. This phenomenon has been stressed for a better understanding of bubbly flow. In this paper, large eddy simulation (LES) is performed to simulate a single bubble rising in the moving liquid, with volume of fluid (VOF) method to capture the interface movements between bubble and liquid phase. The simulation results are firstly compared with the numerical and experimental data from the literature. A good agreement demonstrated the capability of the employed computational fluid dynamics (CFD) approach to predict turbulent properties in the liquid phase and capture interface movement as well as its induced wake flow. Consequently, the dynamic behaviors of a single bubble rising in the moving liquid were investigated. An ensemble averaged has been employed to evaluate the velocity distribution composed by wake velocity and liquid velocity quantitatively as well as the velocity fluctuations enhanced by bubble motion. Their dependency was also evaluated based on a systematic CFD simulation which covers a wide range of liquid velocity. With comparisons of the single phase flow, the influence from the existence of bubble on turbulent properties was determined.

Pseudo-Transient Simulation of Turbulent Mixing in a Rectangular Channel

Fast and robust numerical approach for turbulent heat transfer in case of high gradients of physical properties and unsteady heat transfer in case of strong temperature fluctuations is developed. This pseudo transient Large Eddy Simulation approach is applied to perform transient calculations of turbulent mixing between helium and air in a rectangular mixing channel for Atwood numbers of 0.04 and 0.6. Comparisons of these numerical results with the experimental results by Banerjee, Kraft, Andrews (2010) show a good agreement. The results confirmed applicability of the pseudo transient approach also for higher Atwood numbers while wider time stepping and higher Courant numbers are used. Also reported are simulation results using Reynolds averaged (RANS) method where standard k–e and k–ω SST models are applied.Copyright

Application of a CFD-Based Forced Cross Flow Model to a Sub-Channel Analysis Tool

A detailed CFD analysis was performed to investigate the effect of mixing vanes on single-phase flow distribution in a 5×5 rod bundle simulating PWR operating conditions. The simulation model was 1 m long and contains one split-type mixing vane grid. For the inlet, a fully developed velocity profile from a previous rod-bundle simulation was used. The simulation case was isothermal to avoid a superposition with wall boiling effects. For the simulation of turbulence effects a non-linear k-e model was used. This model is able to reproduce the swirling flow in the wake of the grid spacer as well as secondary flow patterns that typically occur in sub-channel flows and needs less computational time compared to Reynolds Stress models. The results show that two large vortices are formed at the mixing vane tips in the center of an interior sub-channel. These vortices are co-rotating within the sub-channel and counter-rotating compared to the adjacent sub-channels. The presence of these vortices influence the radial pressure distribution and the cross-flows significantly. A new model approach was developed, which accounts for the effects of mixing vanes and the vortex structures. The new model requires additional input parameters that can be derived from the CFD results. The proposed model was implemented into the sub-channel analysis tool COBRA-FLX™. A comparison between the old and the new approaches for modeling the forced cross-flow shows a significant improvement of prediction of the mass flux distribution.Copyright

A Correlation of Heat Transfer Non-Uniformity in Rod Bundle and its Application to Sub-Channel Code

A strong non-uniformity of the circumferential distribution of the wall temperature, heat flux and heat transfer coefficient around the rod surface is observed by previous CFD investigations, especially in the tight lattice. This phenomenon is important for core design of some Generation IV reactors such as supercritical water-cooled reactors (SCWR) and liquid metal fast reactors (LMFR). In this paper important dimensionless numbers that affect the non-uniformity of heat transfer are summarized and CFD calculations are carried out to study the local heat transfer behavior of central, corner and wall sub-channels respectively. An empirical correlation is proposed based on CFD data.Using this correlation, a three-dimensional fuel model is being developed in the sub channel code MATRA to incorporate some of this effect into the thermal-hydraulic analysis of SCWR.Copyright