Gas-Solid Spouted Bed

It is strongly recommended to visit DEM parameters and CFD-DEM parameters for more detailed information on the concepts and physical meaning of the parameters ind DEM and CFD-DEM.

Features

• Solvers: lethe-particles and lethe-fluid-particles

• Three-dimensional problem

• Displays the selection of models and physical properties

• Simulates a solid-gas spouted bed

Files Used in This Example

Both files mentioned below are located in the example’s folder (examples/unresolved-cfd-dem/gas-solid-spouted-bed).

• Parameter file for particle generation and packing: packing-particles.prm

• Parameter file for CFD-DEM simulation of the spouted bed: gas-solid-spouted-bed.prm

Description of the Case

This example simulates the spouting of spherical particles in air. First, we use lethe-particles to fill the bed with particles. We enable check-pointing in order to write the DEM checkpoint files which will be used as the starting point of the CFD-DEM simulation. Then, we use the lethe-fluid-particles solver within Lethe to simulate the spouting of the particles by initially reading the checkpoint files from the DEM simulation.

DEM Parameter File

All parameter subsections are described in the parameter section of the documentation.

To set-up the rectangular spouted bed case, we first fill the bed with particles.

We introduce the different sections of the parameter file packing-particles.prm needed to run this simulation.

Mesh

In this example, we are simulating a rectangular spouted bed. In order to ensure that the flow is developed at the inlet of the bed and that the void fraction does not interfere with the Dirichlet condition of the inlet velocity imposed at the inlet of the spout, we introduce the flow through a channel that is connected to the inlet of the bed. The geometry of the bed was created using GMSH. The following portion of the DEM parameter file shows the function called:

subsection mesh
  set type                                = gmsh
  set file name                           = ./mesh/spouted_structured.msh
  set expand particle-wall contact search = false
end

where the file name includes the path to the mesh file.

Simulation Control

Another subsection, which is generally the one we put at the top of the parameter files, is the simulation control . time step, end time, log frequency, and output frequency are defined here. Additionally, users can specify the output folder for the simulation results in this subsection. The log frequency parameter controls the frequency at which the iteration number is printed on the terminal. If log frequency = 1000 the iteration number will be printed out every 1000 iterations. This is an easy way to monitor the progress of the simulation. A simulation time of 1 s was chosen with a time step of 0.000005. It is important to choose a long enough time as to allow all particles to come to rest. We store the output files generated in the folder output_dem:

subsection simulation control
  set time step        = 0.00001
  set time end         = 0.8
  set log frequency    = 1000
  set output frequency = 10000
  set output path      = ./output_dem/
end

Restart

The lethe-fluid-particles solver requires reading several DEM files to start the simulation. For this, we have to write the DEM simulation information. This is done by enabling the check-pointing option in the restart subsection. We give the written files a prefix “dem” set in the “set filename” option. The DEM parameter file is initialized exactly as the cylindrical packed bed example. The difference is in the number of particles, their physical properties, and the insertion box defined based on the new geometry. For more explanation about the individual subsections, refer to the DEM parameters and the CFD-DEM parameters .

subsection restart
  set checkpoint = true
  set frequency  = 10000
  set restart    = false
  set filename   = dem
end

Model Parameters

The section on model parameters is explained in the DEM examples. We show the chosen parameters for this section:

subsection model parameters
  subsection contact detection
    set contact detection method                = dynamic
    set dynamic contact search size coefficient = 0.9
    set neighborhood threshold                  = 1.3
  end
  subsection load balancing
    set load balance method     = dynamic
    set threshold               = 0.5
    set dynamic check frequency = 10000
  end
  set particle particle contact force method = hertz_mindlin_limit_overlap
  set particle wall contact force method     = nonlinear
  set integration method                     = velocity_verlet
end

We enable dynamic load balancing in order to fully take advantage of the parallelization of the code.

Lagrangian Physical Properties

The physical properties section of the particles allows us to specify the different parameters related to the particle such as its density, diameter, and the different coefficients that dictates the collision behavior of the particles. Also, in this section we define the total number of particles for the simulation. The gravitational acceleration as well as the physical properties of particles and walls are specified in the Lagrangian physical properties subsection. These properties include diameter and density of particles, Young’s modulus, Poisson’s ratio, restitution coefficient, friction and rolling friction coefficients. We insert 31,050 particles with a 2.5 mm diameter in the simulation.

subsection lagrangian physical properties
  set g                        = 0.0, -9.81, 0.0
  set number of particle types = 1
  subsection particle type 0
    set size distribution type            = uniform
    set diameter                          = 0.0025
    set number                            = 31050
    set density particles                 = 2526
    set young modulus particles           = 1e6
    set poisson ratio particles           = 0.25
    set restitution coefficient particles = 0.97
    set friction coefficient particles    = 0.4
    set rolling friction particles        = 0.3
  end
  set young modulus wall           = 1e6
  set poisson ratio wall           = 0.25
  set restitution coefficient wall = 0.33
  set friction coefficient wall    = 0.2
  set rolling friction wall        = 0.3
end

Insertion Info

The insertion info subsection manages the insertion of particles. It allows us to control the insertion of particles at each time step. This section is already explained in the DEM examples. However, further information regarding the information box will be given. The volume of the insertion box should be large enough to fit all particles. Also, its bounds should be located within the mesh generated in the Mesh subsection.

subsection insertion info
  set insertion method                               = volume
  set inserted number of particles at each time step = 31050
  set insertion frequency                            = 2000
  set insertion box points coordinates               = -0.075, 0.0, 0 : 0.075, 0.3, 0.015
  set insertion distance threshold                   = 1.05
  set insertion maximum offset                       = 0.3
  set insertion prn seed                             = 19
end

Floating Walls

We need to pack the particles in the bottom of the rectangular bed while preventing them from going down inside the inlet channel. Therefore, we create a stopper (floating wall) at the top of the channel. We chose the point with a y-coordinate of 0 to create the wall. We then define a normal to the wall at this point. Make sure that the end time of the floating wall is bigger than the simulation time to ensure that the particles remain outside the channel during the entire simulation time. This is shown in:

subsection floating walls
  set number of floating walls = 1
  subsection wall 0
    subsection point on wall
      set x = 0
      set y = 0
      set z = 0
    end
    subsection normal vector
      set nx = 0
      set ny = 1
      set nz = 0
    end
    set start time = 0
    set end time   = 50
  end
end

Running the DEM Simulation

Launching the simulation is as simple as specifying the executable name and the parameter file. Assuming that the lethe-particles executable is within your path, the simulation can be launched in parallel as follows:

mpirun -np 8 lethe-particles packing-particles.prm

Note

Running the packing should take approximately 10-15 minutes on 8 cores.

After the particles have been packed inside the square bed, it is now possible to simulate the fluidization of particles.

CFD-DEM Parameter File

The CFD simulation is to be carried out using the packed bed simulated in the previous step. We will discuss the different parameter file sections. The mesh section is identical to that of the DEM so it will not be shown here.

Simulation Control

The simulation is run for 5 s with a time step of 0.0001 s. The time scheme chosen for the simulation is first order backward difference method (BDF1). The simulation control section is shown:

subsection simulation control
  set method               = bdf1
  set output frequency     = 50
  set time end             = 5
  set time step            = 0.0001
  set subdivision          = 1
  set log precision        = 10
  set output path          = ./output/
end

Physical Properties

The physical properties subsection allows us to determine the density and viscosity of the fluid. We choose a density of 1 and a viscosity of 0.0000181 as to simulate the flow of air.

subsection physical properties
  subsection fluid 0
    set kinematic viscosity = 0.0000181
    set density             = 1
  end
end

Initial Conditions

For the initial conditions, we choose zero initial conditions for the velocity.

subsection initial conditions
  subsection uvwp
      set Function expression = 0; 0; 0; 0
  end
end

Boundary Conditions

For the boundary conditions, we choose a slip boundary condition on all the walls of the bed and the channel except the inlet at the bottom of the channel and the bottom of the bed and the outlet on the top of the bed where an outlet boundary conditions was imposed. At the base of the channel and bottom walls of the bed, we impose a Dirichlet boundary condition with an inlet velocity of 0.2 m/s and a background velocity of 1.25 respectively. For more information about the boundary conditions, please refer to the Boundary Conditions Section

subsection boundary conditions
  set time dependent = false
  set number         = 4
  subsection bc 0
    set id   = 0
    set type = slip
  end
  subsection bc 1
    set id   = 2
    set type = outlet
  end
  subsection bc 2
    set id   = 1
    set type = function
    subsection u
      set Function expression = 0
    end
    subsection v
      set Function expression = 20
    end
    subsection w
      set Function expression = 0
    end
  end
  subsection bc 3
    set id   = 3
    set type = function
    subsection u
      set Function expression = 0
    end
    subsection v
      set Function expression = 1.25
    end
    subsection w
      set Function expression = 0
    end
  end
end

The additional sections for the CFD-DEM simulations are the void fraction subsection and the CFD-DEM subsection. These subsections are described in detail in the CFD-DEM parameters .

Void Fraction

Since we are calculating the void fraction using the packed bed of the DEM simulation, we set the mode to dem. For this, we need to read the dem files which we already wrote using check-pointing. We, therefore, set the read dem to true and specify the prefix of the dem files to be dem. We choose to use the quadrature centered method (QCM) to calculate the void fraction. This method does not require smoothing the void fraction as it is space and time continuous. For this simulation, we use a reference sphere having the same volume as the mesh elements as the averaging volume to calculate the void fraction. For this, we specify the mode to be qcm. We want the volume of the volume averaging sphere to be equal to the volume of the element. For this, we set the qcm sphere equal cell volume equals to true. Since we want to keep the mass conservative properties of the \(L^2\) projection, we do not bound the void fraction and as such we set bound void fraction to false.

subsection void fraction
  set mode                         = qcm
  set qcm sphere equal cell volume = true
  set read dem                     = true
  set dem file name                = dem
  set bound void fraction          = false
end

CFD-DEM

We also enable grad-div stabilization in order to improve local mass conservation. The void fraction time derivative is enabled to account for the time variation of the void fraction.

Note

For certain simulations, this parameter should be disabled to improve stability of the solver.

subsection cfd-dem
  set grad div                      = true
  set void fraction time derivative = true
  set drag force                    = true
  set buoyancy force                = true
  set shear force                   = true
  set pressure force                = true
  set saffman lift force            = false
  set drag model                    = rong
  set coupling frequency            = 100
  set implicit stabilization        = false
  set grad-div length scale         = 0.005
  set vans model                    = modelA
end

We determine the drag model to be used for the calculation of particle-fluid forces. We enable buoyancy, drag, shear and pressure forces. For drag, we use the Rong model to determine the momentum transfer exchange coefficient. The VANS model we are solving is model A. Other possible option is model B.

Finally, the linear and non-linear solver controls are defined.

Non-linear Solver

subsection non-linear solver
  subsection fluid dynamics
    set solver           = inexact_newton
    set tolerance        = 1e-8
    set max iterations   = 20
    set verbosity        = verbose
    set matrix tolerance = 0.75
  end
end

We use the inexact_newton solver as to avoid the reconstruction of the system matrix at each Newton iteration. For more information about the non-linear solver, please refere to the Non Linear Solver Section

Linear Solver

subsection linear solver
  subsection fluid dynamics
    set method                                = gmres
    set max iters                             = 1000
    set relative residual                     = 1e-3
    set minimum residual                      = 1e-10
    set preconditioner                        = ilu
    set ilu preconditioner fill               = 1
    set ilu preconditioner absolute tolerance = 1e-12
    set ilu preconditioner relative tolerance = 1
    set verbosity                             = verbose
    set max krylov vectors                    = 200
  end
end

For more information about the linear solver, please refer to the Linear Solver Section

Running the CFD-DEM Simulation

The simulation is run using the lethe-fluid-particles application. Assuming that the lethe-fluid-particles executable is within your path, the simulation can be launched as per the following command:

lethe-fluid-particles spouted-bed.prm

Results

The results are shown in an animation below. We show the spouting of the particles as the gas is introduced from the channel at the base of the bed. Additionally, the void fraction profile is shown. The bubble formation as well as the spouting strength are highly dependent on the drag model used. It would be interesting to try this case for different drag models.