Spray coating of non-spherical particles

In Aspherix there are several ways for simulating the process of spray coating. In this tutorial we demonstrate a method where liquid is added to particles that are on the surface of a particle bed and in a specific region. The region mimics the area that can be reached by a nozzle.

This tutorial consists of two simulations. In the first one a packing of non-spherical particles, represented by superquardics is generated inside a drum geometry. In the second phase a rotational motion is applied to the geometry, making it a rotating drum and the spray is activated. The image below shows the particles colored by surface liquid content.

Name of the case: Project_Liquid_Transport_Spray

Simulation 1: Inserting nonspherical particles in the drum

General settings

We start with selecting “superquadric” as particle shape:

We use two materials (one for the wall, one for the particles):

and create a simulation domain that reaches from -0.05 to 0.05 in all three dimensions. Furthermore we select a periodic boundary condition in x-dimension.

Meshes and regions

For the drum geometry we import a cylindric mesh command and assign materia1 to it. The mesh must be translated by 0.05 meters in z direction and rotated by and angle of 90 degrees around the y-axis:

In the next step we generate a cylindric insertion region. The bottom center is set to (-0.0495, 0, 0) and the top center is set to (0.0495, 0, 0). The radius is set to 0.0495 meters.

Physics models

We use the standard contact model normal hertz and tangential history. As rolling friction model epsd2 is chosen. For simulating a liquid film at the particle surface we activate the cohesion model washino/capillary/viscous. Since we are using superquadric particles the computeDistance switch must be set to yes. Furthermore we set the limitLiquidContent setting to on.

For the particle-wall contacts the wall contact model normal hertz, tangential history and rolling friction epsd2 is selected. There is no liquid transfer between particles and walls.

Rotating drums are typically inclined a bit in order to induce the motion of the material in axial direction. We mimic that by using a gravity-vector that slightly deviates from negative z-direction. The magnitude is, per default, set to 9.807.

We also activate the liquid transfer model:

Particles

We set the material_properties command for the particle material:

	material1	material2
Density	3000	3000
Young’s modulus	2.5e7	2.5e7
Poisson ratio	0.25	0.25
Coefficient of restitution	1.0	0.3
Coefficient of friction	0.7	0.7
Contace angle	70	70
Max. liquid content	0.1	0.1
Coefficient of rolling friction	0.01	0.01
Latent heat	0	0
Evaporation temperature	0	0
Liquid heat capacity	0	0

Please note that the latent heat, the evaporation temperature and the liquid heat capacity are not used in this case and that their values will be ignored.

Since two different materials are used,we must also material interaction properties are defined:

	material1 & material2
Coefficient of restitution	0.3
Coefficient of friction	0.7
Coefficient of rolling friction	0.01

The selected physical models also require the definition of several global scalars:

	value
Fluid viscosity	0.131
Maximal relative separation distance	0.2
Minimal relative separation distance	0.08
Surface liquid content initial	1e-6
Surface tension	23e-3

A superquadric particle template with semiaxis of 0.002 is generated and blockiness 3 is generated and used in a particle distribution:

Please note that the Superquadric Builder (Tools / Non-spherical particles / Superquadric builder) can be used to generate, visualize and add supequadric particle templates.

An insertion command command of type pack is created, which adds 2000 particles with a velocity of 1 m/s in negative z-direction. The orientation of the particles during insertion is set to random for avoiding undesired and unphysical alignment.

Simulation control

In the next section we define the simulation time step and the total simulation time. Since the filling simulation should terminate as soon as a settled particle bed is obtained, the simulate command is used with mode “until_settled”.

For an optimal usage of the resources the load balancing is switched on:

Output settings

In this tab we first define the timestep width for the file and the terminal output:

The default output_settings command command is used:

Simulation and results

This was the last setting in for this calculation. Please save the project and change to the Simulation tab. After running the simulation the following curve for the kinetic energy shows:

The result of this simulation is a settled particle bed that can be visualized with Paraview:

Simulation 2: Spray coating

We use the result of Simulation 1 as starting point for the spray coating simulation by deriving a simulation. The main changes in this case are the following: we add a rotational motion to the drum, the surface of the particle bed is detected and the spray is activated.

Meshes and regions

First, a mesh module of type motion is generated. The drum will rotate around the x-axis with a period of 1:

The mesh module must be added to the mesh in order to activate it:

Instead of an insertion region we are now using a wetting region:

Particles

Since no further particles will be inserted, the insertion command, particle distribution and particle template can be removed (in that order).

Physics models

In the physics models tab we first add a command that detects particles that are at the surface of the particle bed:

Then we use the update_particle command command to increase the surface liquid content of surface particles inside the specified wetting region:

Simulation control

In the simulation control tab we change the run mode from “until settled” to a run time of 1 second.

Simulation and results

With this last setting the case setup is completed and the simulation can be launched. The graph below shows the kinetic energy of the particle bed:

The results can also be visualized in Paraview, the image below shows the particles colored according to the sorface liquid content after a second of simulation time.