1.3 Pebble Flows
13
1.3.6 Review of State-of-the-Art Work
Much research work has been done since the 1950s, in the field of experimental
research on the motion law of pebble flow in high-temperature gas-cooled reactors.
The core research results are not in the public domain. Germany, the USA, South
Africa, China, and other countries have carried out experimental studies on their
respective development plans of the pebble-bed reactors [48, 107, 108]. For example,
the AVR-related pebble flow experiments in Germany [109], MPBR-related pebble
flow experiments in the USA [110], PBMR-related pebble flow experiments in South
Africa [111], and HTR-10-related experiments in China. Many investigations, e.g.,
experimental [11, 61, 110, 112] and numerical [113–118], studies have been carried
out to reveal its features. The practical design of HTGRs is indebted to detail studies
performed on different complications in pebble bed, like two-region arrangement
[119], pebble dispersion [120], stagnant region and optimization of bed configuration
[63]. But the research is still insufficient and the mechanism of this special pebble
flow is poorly understood at present.
Besides, many numerical studies have been done recently. The numerical algorithms for the pebble flow study can be divided into two categories. One is based on
the Lagrangian framework to describe the motion of every discrete pebble, and the
other is based on the Eulerian framework to simulate the flow features in analogies
to the continuum theory. One of the most prevalent Lagrangian approaches is the
discrete element methods and its various extensions [63, 101, 104, 121–125].For
example, numerical simulation of large-scale pebble flow by DEM code have been
performed [102, 126–128]. Particular attention has been paid to the streamlines,
velocity distributions, and pebble diffusion characteristics.
The representative approach in the Eulerian framework is the kinematic model
[129]. As it was difficult to apply the kinematic model to the analysis of the pebble
velocity for reactors having complex geometries, e.g., the PBMR with an annular core
and three defueling chutes, [130] modified it to improve the reconstruction ability
of the pebble velocity profile by using cylindrical core experiments to determine the
specific coefficients used in this modified kinematic model and verifying the model
by the pebble flow experiment of PBMR core. But, these coefficients are still not
widely applicable to other reactor designs.
Although experimental and numerical studies [131], have been done on the flow
characteristics in silo beds with different geometries, those cases mostly belong to
the free outflow and focus on the prediction of outflow rate [40–42, 132]. Other
relevant studies have also been carried out, contributing to various related aspects
such as velocity profiles [113, 128, 130], phenomenological analysis [112], stagnant
zones [133], diffusion and mixing [114], as well as numerical simulations [134, 135],
etc. However, few of the above studies have carried out experimental investigations
on the effect of the bed configuration under the recirculation operation, especially
with the contraction configuration at the bottom of the pebble bed, as well as the
flow uniformity and the very slow region (the funnel flow regime) in such kinds of
configurations.
13
1.3.6 Review of State-of-the-Art Work
Much research work has been done since the 1950s, in the field of experimental
research on the motion law of pebble flow in high-temperature gas-cooled reactors.
The core research results are not in the public domain. Germany, the USA, South
Africa, China, and other countries have carried out experimental studies on their
respective development plans of the pebble-bed reactors [48, 107, 108]. For example,
the AVR-related pebble flow experiments in Germany [109], MPBR-related pebble
flow experiments in the USA [110], PBMR-related pebble flow experiments in South
Africa [111], and HTR-10-related experiments in China. Many investigations, e.g.,
experimental [11, 61, 110, 112] and numerical [113–118], studies have been carried
out to reveal its features. The practical design of HTGRs is indebted to detail studies
performed on different complications in pebble bed, like two-region arrangement
[119], pebble dispersion [120], stagnant region and optimization of bed configuration
[63]. But the research is still insufficient and the mechanism of this special pebble
flow is poorly understood at present.
Besides, many numerical studies have been done recently. The numerical algorithms for the pebble flow study can be divided into two categories. One is based on
the Lagrangian framework to describe the motion of every discrete pebble, and the
other is based on the Eulerian framework to simulate the flow features in analogies
to the continuum theory. One of the most prevalent Lagrangian approaches is the
discrete element methods and its various extensions [63, 101, 104, 121–125].For
example, numerical simulation of large-scale pebble flow by DEM code have been
performed [102, 126–128]. Particular attention has been paid to the streamlines,
velocity distributions, and pebble diffusion characteristics.
The representative approach in the Eulerian framework is the kinematic model
[129]. As it was difficult to apply the kinematic model to the analysis of the pebble
velocity for reactors having complex geometries, e.g., the PBMR with an annular core
and three defueling chutes, [130] modified it to improve the reconstruction ability
of the pebble velocity profile by using cylindrical core experiments to determine the
specific coefficients used in this modified kinematic model and verifying the model
by the pebble flow experiment of PBMR core. But, these coefficients are still not
widely applicable to other reactor designs.
Although experimental and numerical studies [131], have been done on the flow
characteristics in silo beds with different geometries, those cases mostly belong to
the free outflow and focus on the prediction of outflow rate [40–42, 132]. Other
relevant studies have also been carried out, contributing to various related aspects
such as velocity profiles [113, 128, 130], phenomenological analysis [112], stagnant
zones [133], diffusion and mixing [114], as well as numerical simulations [134, 135],
etc. However, few of the above studies have carried out experimental investigations
on the effect of the bed configuration under the recirculation operation, especially
with the contraction configuration at the bottom of the pebble bed, as well as the
flow uniformity and the very slow region (the funnel flow regime) in such kinds of
configurations.