6
1 Introduction
between solid fuel elements, thermal radiation between the surfaces of adjacent
spheres, and the gas heat convection. According to existing heat transfer researches
for a porous media, the radiation effect will account for more than 50% of total
heat transfer at moderate temperatures in some porous media [26–28], and radiation
heat transfer will be apparently affected by different packing structure and density.
Moreover, in the loss-of-coolant and depressurized accident, the gas heat convection
isn’t essential. Two previous experiments on the effective conductivity of a graphite
pebble bed were conducted by the SANA facility in Germany, in 1996 [29] and the
HTTU in South Africa [30, 31] in 2012. Since it was difficult to maintain the steady
temperature distribution in the test facilities at very high-temperatures, the effective
thermal conductivities obtained by the steady-state method could achieve as high as
1000
◦ C in the SANA and 1200
◦ C in the HTTU, which were inappropriate to present
the drastic growth of effective thermal conductivity (above 1200
◦ C) caused by the
radiation heat transfer at higher temperature.
The distinct knowledge of effective thermal diffusivity and conductivity of pebble
bed contributes to realizing the heat transfer at higher thermal power and outlet
helium temperature, which means a better balance between safety and economic
competitiveness. A full-radius-scaled heat test facility has been developed by the
INET of Tsinghua University in China to measure these two parameters up to 1600
◦ C
under vacuum condition (20 Pa) and atmospheric pressure (10
5 Pa). It’s consistent
with the test facilities of the SANA in Germany, and the HTTU in South Africa,
that the geometry of the bed core is an annulus with a cylindrical graphite heater in
the center [32]. The primary purpose of this test is to obtain the effective thermal
diffusivity from environmental temperature to 1600
◦ C via an inverse method using
the transient temperature of a whole heating process. The effective thermal diffusivity
and conductivity can be calculated through experimental transient temperatures in
the pebble bed, and the relevant data processing algorithm of the inverse method has
been studied theoretically [33]. The secondary purpose is to provide an overview of
the heating process and temperature distribution inside randomly accumulated fuel
balls in the pebble bed. The overview can afford a validation of numerical simulation
of Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) for
the thermal-fluid systems.
1.3 Pebble Flows
1.3.1 Discharging/recirculating Granular Flow
The detailed understanding of slow flow in dense granular systems has remained
one of the central challenges within the field of granular materials. Granular flow is
an attractively simple and yet surprisingly complex subject. Slow and dense flows
pose a considerable challenge to theorists due to many-body interactions and nonthermal fluctuations [34]. Beyond their fundamental scientific interest, such flows
1 Introduction
between solid fuel elements, thermal radiation between the surfaces of adjacent
spheres, and the gas heat convection. According to existing heat transfer researches
for a porous media, the radiation effect will account for more than 50% of total
heat transfer at moderate temperatures in some porous media [26–28], and radiation
heat transfer will be apparently affected by different packing structure and density.
Moreover, in the loss-of-coolant and depressurized accident, the gas heat convection
isn’t essential. Two previous experiments on the effective conductivity of a graphite
pebble bed were conducted by the SANA facility in Germany, in 1996 [29] and the
HTTU in South Africa [30, 31] in 2012. Since it was difficult to maintain the steady
temperature distribution in the test facilities at very high-temperatures, the effective
thermal conductivities obtained by the steady-state method could achieve as high as
1000
◦ C in the SANA and 1200
◦ C in the HTTU, which were inappropriate to present
the drastic growth of effective thermal conductivity (above 1200
◦ C) caused by the
radiation heat transfer at higher temperature.
The distinct knowledge of effective thermal diffusivity and conductivity of pebble
bed contributes to realizing the heat transfer at higher thermal power and outlet
helium temperature, which means a better balance between safety and economic
competitiveness. A full-radius-scaled heat test facility has been developed by the
INET of Tsinghua University in China to measure these two parameters up to 1600
◦ C
under vacuum condition (20 Pa) and atmospheric pressure (10
5 Pa). It’s consistent
with the test facilities of the SANA in Germany, and the HTTU in South Africa,
that the geometry of the bed core is an annulus with a cylindrical graphite heater in
the center [32]. The primary purpose of this test is to obtain the effective thermal
diffusivity from environmental temperature to 1600
◦ C via an inverse method using
the transient temperature of a whole heating process. The effective thermal diffusivity
and conductivity can be calculated through experimental transient temperatures in
the pebble bed, and the relevant data processing algorithm of the inverse method has
been studied theoretically [33]. The secondary purpose is to provide an overview of
the heating process and temperature distribution inside randomly accumulated fuel
balls in the pebble bed. The overview can afford a validation of numerical simulation
of Discrete Element Method (DEM) and Computational Fluid Dynamics (CFD) for
the thermal-fluid systems.
1.3 Pebble Flows
1.3.1 Discharging/recirculating Granular Flow
The detailed understanding of slow flow in dense granular systems has remained
one of the central challenges within the field of granular materials. Granular flow is
an attractively simple and yet surprisingly complex subject. Slow and dense flows
pose a considerable challenge to theorists due to many-body interactions and nonthermal fluctuations [34]. Beyond their fundamental scientific interest, such flows