This paper mainly reviews the research progress on multiphase flow in nuclear engineering. This paper is composed of three parts. The first part documents a literature statistical analysis based on publications, mainly the SCIE-indexed papers collected in the Web of Science. The analysis data of publications on multiphase flows showcase the prosperous future of the current Experimental and Computational Multiphase Flow journal. The second part summarizes advanced measurement technology and microscale and nanoscale multiphase flow research in terms of modeling and algorithms. The third part reviews the current progress in multiphase flow-related research in nuclear engineering. Various aspects of solid–liquid, gas–liquid, and gas–particle flows, in particular, critical heat flux research and deep learning applications in nuclear engineering, have been extensively reviewed. This paper aims to explore multiphase flow research in the future in the nuclear and its related disciplines through the analysis and review of cutting-edge research and development states.

The pebble-bed high-temperature gas-cooled reactor (HTGR) is an advanced nuclear system with inherent safety. The nuclear reactor is operated in high-temperature ranges and its design temperature limit is 1600 °C. It is important to discuss the conduction–radiation heat transfer in the nuclear pebble bed during the thermal hydraulics analysis. Until now, the investigation mainly focuses on large-scale experimental tests, such as the Selbsttätige Abfuhr der Nachwärme (SANA) test by the International Atomic Energy Agency (IAEA), high temperature test unit (HTTU) under low gas pressure by North-West University of South Africa, and the pebble bed equivalent conductivity measurement (PBEC) under vacuum conditions for the high temperature gas-cooled reactor pebble bed module (HTR-PM) project by the Institute of Nuclear and New Energy Technology (INET) of Tsinghua University. In this work, a thermal resistance model was developed to calculate the conduction–radiation effective thermal conductivity of nuclear pebble beds. With the constitutive continuum modeling for conduction, contact resistance, coordination number, and void fraction are considered for physical expressions, and heat conduction in the packed bed will be enhanced directly by higher packing density and more contact. For thermal radiation dominated by the heat transfer process under high temperatures, a sub-cell model with an equivalent resistance network is developed to calculate the radiative exchange factor, and the void fraction effect is implemented by a modification term with thermal ray tracing results. At low packing density, because the thermal rays from the local pebble will travel a further distance until reaching the surrounding pebbles, the local sphere will be in contact with more particles by thermal radiation, and the heat transfer is enhanced. Compared with the empirical correlations, the present model is proven to be applicable for both dense and dilute cases. In the nuclear pebble bed, the present conduction–thermal radiation model agrees generally with experimental data under different temperatures and can be applied in the particle-scale CFD-DEM simulations. The present work provides a meaningful approach for conduction–thermal radiation heat transfer for engineering and nuclear pebble-bed design.

This paper reviews the recent three-year progress on the investigations of pebble bed flows in nuclear engineering. Both the application of pebble beds in the fission reactors and the fusion reactors are included. The fundamental characteristics of packing, flows, conduction, convection, radiation, and the effective thermal conductivity of pebble beds are reviewed. The important issues on the design of the pebble beds as well as that related to the reactor safety are also introduced. In addition, the advances in measurement techniques and numerical coupled methods for exploring the pebble flow characteristics are categorized and summarized too.

The pool boiling characteristics with different boiling surfaces and working fluids play an important part in multiphase flow research. The key parameters of pool boiling, such as heat transfer coefficient (HTC) and critical heat flux (CHF), can be only acquired by experiment. Thus, a pool boiling experimental device is designed and produces the HTC and CHF data on the pure copper heating surface, which are 72.25 kW/(m²·K) and 1093.28 kW/m², respectively. Besides, a series of visualization experimental results of bubble behavior in the pool boiling are taken by the high-speed camera to provide references for the boiling mechanism research. The pool boiling experiment would be the benchmark data to validate the future experiments and computer simulations.

The oscillation and deformation of the tube affect its safe and efficient operation in the shell-and-tube heat exchanger of the nuclear power plant. To offer in-depth understandings, numerical simulation of the flow around the cylinder (or particle) was carried out here by using COMSOL Multiphysics as a research tool. This paper mainly discusses the influence of physical parameters (elastic modulus and Poisson’s ratio) and lateral oscillation on the flow around the circles (cylinder or particle). The physical property parameters have a greater influence on the deformation, lift coefficient, and drag coefficient of the object, and it basically does not affect the vortex shedding frequency. After analyzing the flow around the oscillating particle, four kinds of vortex separation modes (AI, AII, S, S-S modes) are defined. In addition, the lift coefficient and drag coefficient for different modes are discussed. The phenomenon of "frequency locking" occurs in the flow around the oscillating particle. Simulation results prove that the separation frequency of vortex is related to the oscillation frequency.