What is a granular two-phase flow?
Particle-fluid two-phase flows are encountered in many industries, including energy, agriculture, mining, food, pharmaceuticals, and more.
It is characterized by particles (discrete phase) entrained and transported by gases or liquids (continuous phase). There is mass, momentum, and energy transfer between particles and fluid media, and collisions occur between particles, particles and solid walls.
Numerical methods for simulating particle flow
Particle and fluid phases can be simulated at different length scales and are generally divided into 2 categories: macroscopic methods (continuum) and microscopic methods (discrete media)
Fluid - Resolved Euler （Marco） Solid - lagrange（micro）
DNS - DEM
Computationally expensive to perform
Fluid - Unresolved Euler（Macro） Solid - lagrange（Micro）
CFD-DEM
Relatively accurate, the calculation cost is moderate
Fluid - Euler（Macro） Solid - Euler（Macro）
TFM
The accuracy is not high
What is the DEM method?
DEM discrete element method means that each unit is discrete and has independent characteristics, which is our common granular material.
The core idea of the discrete element method is to retrieve each particle under the Lagrange coordinate system, calculate the force generated by contact, and then use Newton's second law to calculate the change of acceleration/velocity and displacement of particles, and then obtain the state of the entire system.
Why do we need a CFD+DEM coupling approach?
CFD-DEM
- Collisions between particles and between particles and between containers can be simulated
- Particle build-up and separation in fluid media can be simulated
- The interaction of particles and flow fields can be simulated
- The trajectory of individual particles or groups of particles in the flow field can be tracked
- The fluid force to which particles are subjected can be calculated
Control equation for CFD+DEM coupling method
Fluid phase
Continuity equation:
Momentum equation:
Energy equation:
Granular phase
Newton’s second law of motion (translation):
Euler’s second law of motion (rotation):
Energy equation (particle):
Features of Altair® AcuSolve® and EDEM coupling
- The size of the particles can be much larger than the grid size of the CFD.
- Supports spherical/non-spherical particles, such as the settlement process of irregularly shaped stones in water.
- Energy coupling can be simulated, e.g. hot particles are cooled by air
- CFD dynamic meshes simulate rigid body motion, such as particles passing through a vibrating sieve or rotating fan blades.
- Mass transfer phenomena such as coating on the surface of particles or drying wet particles can be simulated.
- Both CFD and DEM models support GPU-accelerated computing, which has a significant acceleration effect on large models.
Three methods of AcuSolve and EDEM coupling
It can generally be considered that particles of the sparse phase do not affect the flow field, and unidirectional coupling is sufficient, and the calculation cost is small.
If the particles are dense phases and the total volume of particles/fluid volume is much greater than one thousandth, bidirectional coupling is considered. Of course, this is not absolute, mainly depending on whether the presence of particles significantly affects the flow field.
The first method: unidirectional steady-state coupling
- The presence of particles hardly affects the flow field. After AcuSolve completes the flow field calculation, it exports the steady-state results to EDEM.
- EDEM then calculates the trajectory of the particles under a steady flow field.
CFD steady-state flow field
Animation of particle motion
Second method: unidirectional transients
- The presence of particles hardly affects the flow field. However, CFD model parameters can vary over time, such as flow, pressure, physical parameters, and so on.
- AcuSolve and EDEM need to be solved at the same time, AcuSolve is only responsible for data transmission, EDEM is only responsible for data receiving.
The inlet air flow changes accordingly
CFD flow field animation
Animation of particle motion
The third method: bidirectional transient coupling
- The presence of particles significantly affects the flow field. AcuSolve and EDEM need to solve at the same time, exchanging information about the velocity, pressure, temperature and position of particles of the fluid at the end of each iteration of the CFD.
- Model time steps for EDEM are typically on the order of 1e-5 seconds. Therefore, in each time step of acuSolve (usually on the order of 0.001~0.01 seconds), EDEM has undergone multiple rounds of iteration.
Animation of the CFD transient flow field
Animation of particle motion
Let's look at a few examples of AcuSolve and EDEM coupling simulations.
Case: Agricultural machinery
The combine harvester uses a cleaning method in which air flow and sieve work together. The grain mixture vibrates in the sieve, the light impurities are blown away by the air flow, the heavy impurities are removed by the tail sieve, and the grains enter the granary through the sieve hole. The airflow distribution in the sorting chamber has an important influence on the overall screening efficiency.
Schematic diagram of combine harvester (picture from the Internet)
Combine harvester product picture (picture from the Internet)
CFD model: wind speed in the sorting room
EDEM: Grain movement in the sorting room
Case: Cyclone
A cyclone is a kind of equipment used for particle separation. The working principle is the rotational motion caused by the tangential introduction of air flow, so that the solid particles or droplets with large inertial centrifugal force are thrown to the outer wall and separated.
Inlet wind speed 13.4 m/s, 3000 particles, bidirectional coupling
Animation of the CFD transient flow field
Animation of particle motion
Case: Chemical equipment
Fluidized bed is a kind of reactor that uses fluid to pass through the granular solid layer to make solid particles in a suspended motion, and carry out gas-solid phase reaction process or liquid-solid phase reaction process.
Bottom wind speed 2.5m/s, 24750 particles, bidirectional transient coupling
Granulator, the rotation speed of the mixing paddle ensures that the movement produces vortex and the material is fully mixed. At the same time, the grain-knife junction area fully breaks the churning material mass into particles. The rotation speed of the mixing paddle and the granulator can make the material produce three-dimensional movement, and the particles collide, grind and shear, so that the friction is uniform and fine, and finally the spherical particles are stabilized.
Inlet speed 2m/s, rotation speed 600rpm, 5900 particles, bidirectional transient coupling
Case: Food, pharmaceutical industry
Pneumatic conveying is the use of energy from airflow to transport granular materials in the direction of airflow in a closed pipeline, which is a specific application of fluidization technology. The main characteristics are large conveying capacity, long conveying distance and high conveying speed; It can be loaded in one place and then unloaded in multiple places.
Inlet velocity 15 m/s, one million particles, bidirectionally coupled
Pneumatic transport of particles
Medical dry powder inhalers that can be used to simulate the delivery of dry powder drugs to the respiratory tract and identify areas of particle accumulation. Simulate particle build-up and inhaler efficiency.
Air volume flow (50 L/min), 5000 particles
Unidirectional steady-state coupling
Unidirectional transient coupling
Bidirectional transient coupling
Case: Industrial equipment
Vane pump model, pumping liquids with particles, coupled in both directions.
The production line spraying and drying process sprays cryogenic droplets onto a solid surface in a closed cavity in a high-temperature air environment
The VolumeAdded parameter looks at how much moisture there is on the solid surface. When spraying begins, the surface moisture (VolumeAdded) of the particles increases. Once the spraying stops, the coating begins to dry and the moisture gradually decreases.
Case: Home appliances
Vacuum cleaner simulates the process of sucking in and cleaning debris in the shape of a long strip under negative pressure.
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