Numerical study of pebble recirculation in a two-dimensional pebble bed of stationary atmosphere using LB-IB-DEM coupled method Nan Gui a , Zeguang Li a , Zhen Zhang a , Xingtuan Yang a , Jiyuan Tu a,b , Shengyao Jiang a,⇑ a Institute of Nuclear and New Energy Technology, Collaborative Innovation Center of Advanced Nuclear Energy Technology, Key Laboratory of Advanced Reactor Engineering and Safety of Ministry of Education, Tsinghua University, Beijing 100084, PR China b School of Engineering, RMIT University, Melbourne, VIC 3083, Australia article info Article history: Received 21 May 2018 Received in revised form 20 July 2018 Accepted 9 September 2018 Keywords: Pebble bed reactor Pebble recirculation Lattice-Boltzmann Immersed boundary Coupled simulation Discrete element method abstract The pebble bed is one type of the core of the high temperature gas-cooled reactor (HTGR), which is regarded as the candidate of the generation IV advanced reactor. It is important to explore the gas- pebble flow characteristics and the pebble recirculation under the helium atmosphere. In this work, we presented a lattice Boltzmann (LB) method – immersed boundary (IB) method – discrete element method (DEM) coupled approach to simulate a test facility of pebble bed under the recirculation mode of operation. After model validation by an experiment of sphere sedimentation, the process of pebble recirculated at five constant rates are simulated. The correlations of gas motion and pebble motion in the upper and lower half beds are analyzed to uncover the inter-phase relationships for such intermittent pebble flows. Based on the systematic analyses of the two-phase flows, including the mean field and r.m.s field, the historical variation, inter-correlation, and the spectrum and phase space representations, we found sufficient evidences for the characteristics of intermittency, simultaneity, periodicity, and linear dependence for the inter-phase interaction of gas-pebble flows. Ó 2018 Elsevier Ltd. All rights reserved. 1. Introduction The core of pebble bed type-high temperature gas-cooled reac- tor is graphite moderated and helium cooled and can be operated at high temperatures. Therefore the reactor core is composed of a large amount of graphite and fuel pebbles moving in the high tem- perature helium atmosphere. The pebble is discharged continu- ously, slowly and intermittently by a silo at the bottom of the pebble bed and recirculated or reloaded at the top of bed simulta- neously. The helium gas flows through the void space between the pebble elements takes away the heat generated by the fission of U-235 and then transfers the heat to the steam generator. There- fore, it is a special gas-particle system and many important aspects on the gas-pebble system need to be better explored. For example, for fuel management, Tavron and Shwageraus (2016) studied the procedure of optimization of pebble bed reactor fuel management, e.g. OTTO (Once-Through-Then-Out) and MEDUL (German: ‘MEhrfach DUrchLauf’ = multi-pass) fuel man- agements, under the constrained thermal hydraulic conditions. The fuel management performance at equilibrium cycle conditions was evaluated by the VSOP (Very Superior Old Program) code and the pebble bed reactor was found with low sensitivity to fuel man- agement parameters. For pebble flow, Khane et al. (2016) con- ducted an experimental work on the pebble flow dynamics in a scaled-down pebble bed test reactor using radioactive particle tracking (RPT) technique, including the Lagrangian trajectory, velocity field, residence time distributions. They also assessed the possibility of using pebble bed modular reactor as static packed bed approximation and compared the packing characteristics of static and moving pebble beds (Khane et al., 2017). Our group also performed a numerical study on the bed configuration on the peb- ble flow, and implied that the brachistochrone shaped bed config- uration is the best for flow uniformity (Gui et al., 2014). Moreover, a model study was also conducted to explain the normal distribu- tion of pebble concentrations in the pebble trajectory bundles (Gui et al., 2014). For graphite dust deposition, Jayaraju et al. (2016) dealt with Reynolds averaged Navier-Stokes modeling of fluid flow and graphite dust deposition in pebble bed using the standard k   model for the continuum phase and the continuous random walk model for the dispersed phase. After model validation, it was applied to analyze the complex flow behavior and deposition pattern in a structured and an unstructured pebble-bed arrange- ment. In a similar manner, Barth et al. (2014) presented a positron emission tomography (PET) measurement of dust particle deposi- tion and re-suspension in a fluid dynamically scale HTR pebble https://doi.org/10.1016/j.anucene.2018.09.018 0306-4549/Ó 2018 Elsevier Ltd. All rights reserved. ⇑ Corresponding author. E-mail address: guinan@mail.tsinghua.edu.cn (N. Gui). Annals of Nuclear Energy 124 (2019) 58–68 Contents lists available at ScienceDirect Annals of Nuclear Energy journal homepage: www.elsevier.com/locate/anucene