5.4 CFD-DEM Coupled Simulation and Development
309
for the particle phase [99, 100] and makes it possible to investigate the complex
processes in a Eulerian–Lagrangian framework [79]. Moreover, it is also feasible to
study the effect of non-spherical particles with different sizes and shapes for fluidized
beds [101, 102].
However, the heat transfer model for packed pebble beds needs to be improved
substantially. Three basic modes of heat transfer need to be considered in the simulations, i.e., conduction between particles in contact, fluid–particle convection, and
particle thermal radiation. In many cases, only the conduction and convection models
are applied, and the particle radiation is neglected for low-temperature conditions
[103, 104]. However, when the operating temperature of a packed pebble bed is
very high (about 1,100 K) at normal conditions and the particle size is large (60 mm
in diameter), particle thermal radiation plays a significant role in the heat transport
processes [5, 105]. From all available literature on the CFD-DEM method, only a
simplified radiation model [8, 106] is available, where the averaged surrounding
temperature [107] is replaced by a local environmental temperature. By comparison
with other modes of heat transfer, particle radiation will increase significantly at
high temperatures [8]. However, the effect of material thermal conductivity on the
particle radiation is not considered in this model. Moreover, the radiation model has
not been verified separately for packed pebble beds. Thus, a new radiation model
with an acceptable level of accuracy in engineering is needed for packed pebble beds
In Sects. 5.3.2 and [5], particle thermal radiation was analyzed in detail (with
radiation only and without thermal conduction) for packed pebble beds, and three
numerical models of different spatial scales were proposed. In the Long-Range radiation Model (LRM, Sect. 5.3.4), all possible heat transfers by radiation between
surrounding particles are taken into account. However, from numerical results, the
predicted heat flux from the long-range radiation model is much higher than experimental data because of the ignored material conductivity, which could be viewed
as an inner thermal resistance for radiation. In the Short-range Radiation Model
(SRM), only thermal radiation between direct Voronoï neighbors is considered, and
it is in better agreement with experimental data than the LRM. Because, in the SRM,
the under estimation of the radiation caused by the ignored long-range radiation
heat exchange cancels out the overestimation of the heat flux caused by the ignored
inner thermal resistance, which results in better predictions of the overall radiation
in packed pebble beds.
Based on the short-range radiation model, a complete CFD-DEM method was
developed for packed pebble beds in the present section. Simulations with different
initial conditions or physical properties were performed. The effect of particle thermal
radiation in packed pebble beds is discussed using dimensional analysis. Then, the
present SRM-based CFD-DEM model is assessed and results in being suitable for
engineering applications, especially for HTGRs.
309
for the particle phase [99, 100] and makes it possible to investigate the complex
processes in a Eulerian–Lagrangian framework [79]. Moreover, it is also feasible to
study the effect of non-spherical particles with different sizes and shapes for fluidized
beds [101, 102].
However, the heat transfer model for packed pebble beds needs to be improved
substantially. Three basic modes of heat transfer need to be considered in the simulations, i.e., conduction between particles in contact, fluid–particle convection, and
particle thermal radiation. In many cases, only the conduction and convection models
are applied, and the particle radiation is neglected for low-temperature conditions
[103, 104]. However, when the operating temperature of a packed pebble bed is
very high (about 1,100 K) at normal conditions and the particle size is large (60 mm
in diameter), particle thermal radiation plays a significant role in the heat transport
processes [5, 105]. From all available literature on the CFD-DEM method, only a
simplified radiation model [8, 106] is available, where the averaged surrounding
temperature [107] is replaced by a local environmental temperature. By comparison
with other modes of heat transfer, particle radiation will increase significantly at
high temperatures [8]. However, the effect of material thermal conductivity on the
particle radiation is not considered in this model. Moreover, the radiation model has
not been verified separately for packed pebble beds. Thus, a new radiation model
with an acceptable level of accuracy in engineering is needed for packed pebble beds
In Sects. 5.3.2 and [5], particle thermal radiation was analyzed in detail (with
radiation only and without thermal conduction) for packed pebble beds, and three
numerical models of different spatial scales were proposed. In the Long-Range radiation Model (LRM, Sect. 5.3.4), all possible heat transfers by radiation between
surrounding particles are taken into account. However, from numerical results, the
predicted heat flux from the long-range radiation model is much higher than experimental data because of the ignored material conductivity, which could be viewed
as an inner thermal resistance for radiation. In the Short-range Radiation Model
(SRM), only thermal radiation between direct Voronoï neighbors is considered, and
it is in better agreement with experimental data than the LRM. Because, in the SRM,
the under estimation of the radiation caused by the ignored long-range radiation
heat exchange cancels out the overestimation of the heat flux caused by the ignored
inner thermal resistance, which results in better predictions of the overall radiation
in packed pebble beds.
Based on the short-range radiation model, a complete CFD-DEM method was
developed for packed pebble beds in the present section. Simulations with different
initial conditions or physical properties were performed. The effect of particle thermal
radiation in packed pebble beds is discussed using dimensional analysis. Then, the
present SRM-based CFD-DEM model is assessed and results in being suitable for
engineering applications, especially for HTGRs.