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-   -   DPM limitation for continuous suspensions (https://www.cfd-online.com/Forums/fluent/28932-dpm-limitation-continuous-suspensions.html)

 Matthew Brannock September 18, 2001 21:52

DPM limitation for continuous suspensions

Hi,

My question is with regards to this comment in the fluent manual (chpt 14.1): "Limitation on Modeling Continuous Suspensions of Particles: The Lagrangian discrete phase model described in this chapter is suited for flows in which particle streams are injected into a continuous phase flow with a well-defined entrance and exit condition. The Lagrangian model does not effectively model flows in which particles are suspended indefinitely in the continuum, as occurs in solid suspensions within closed systems such as stirred tanks, mixing vessels, or fluidized beds."

The system I want to model involves concentric cylinders, with liquid (water) and a low concentration of small particles in between. One cylinder is moving so a flow-field is developed. Does the above statement from the manual imply that I cannot model this system using the DPM provided by fluent? And why is this so (ie why are defined inlet and exit conditions required for the particle phase)? Are there any other methods of modelling the movement of particles induced by the forces from a velocity profile? Perhaps a scalar transport UDF?

 Lanre September 24, 2001 14:19

Re: DPM limitation for continuous suspensions

The particle trajectory "enters" the domain from any position and "exits" through a outlet, wall (sticking or trapping) or mass transfer to the continuous phase (evaporation).

There are a few reasons the DPM model is not well suited to modelling suspensions, a couple are:

1. Particle-particle interaction is ignored and the additional drag is missing from the analysis. This is important if the suspension is dense. 2. The DPM model treats the secondary phase in a Lagrangian frame that is inhenrently time-dependent. This means that the onus is on the analyst to specify the time duration of the particle motion for which it is expected that all particles would have completed their trajectories. For settling suspensions, the particles can be removed from the calculation when they settle on the bottom wall/boundary of the domain. If the suspension is dilute, the DPM model is valid. However, the settling time can be impractically long. For non-settling suspensions, the DPM model is not valid.

Your options to model the suspension are:

1. separate, homogenous phases, eg. liquid and solid

2. a single, homogenous liquid phase with specific treatment of the suspension viscosity (suspensions/slurries typically exhibit non-Newtonian rheology). This ignores the individual particles.

3. multi, homogeneous scalars (species) for the liquid and solids. Also ignores individual particles.

If you chose #2 or #3, you will not be able to predict slip (settling) of the solids and the subsequent settling (and displacement of fluid) on the bottom of the container.

 robjac August 18, 2010 02:38

Quote:
 Originally Posted by Lanre ;98468 The particle trajectory "enters" the domain from any position and "exits" through a outlet, wall (sticking or trapping) or mass transfer to the continuous phase (evaporation). There are a few reasons the DPM model is not well suited to modelling suspensions, a couple are: 1. Particle-particle interaction is ignored and the additional drag is missing from the analysis. This is important if the suspension is dense. 2. The DPM model treats the secondary phase in a Lagrangian frame that is inhenrently time-dependent. This means that the onus is on the analyst to specify the time duration of the particle motion for which it is expected that all particles would have completed their trajectories. For settling suspensions, the particles can be removed from the calculation when they settle on the bottom wall/boundary of the domain. If the suspension is dilute, the DPM model is valid. However, the settling time can be impractically long. For non-settling suspensions, the DPM model is not valid. Your options to model the suspension are: 1. separate, homogenous phases, eg. liquid and solid 2. a single, homogenous liquid phase with specific treatment of the suspension viscosity (suspensions/slurries typically exhibit non-Newtonian rheology). This ignores the individual particles. 3. multi, homogeneous scalars (species) for the liquid and solids. Also ignores individual particles. If you chose #2 or #3, you will not be able to predict slip (settling) of the solids and the subsequent settling (and displacement of fluid) on the bottom of the container.
Hi ,
I am facing problem in Discrete phase modelling with suspended particles in a closed vessel of fluid.The suspended particles in the fluid is to be heated in a closed vessel at a fixed volume concentration.But how to make the particle suspended before the heating is done at a fixed concentration, without any inlet and outlet flow?

Suggestion from ur side would be of great help..
Regards,
Rob

 robjac August 18, 2010 02:38

Hi ,
I am facing problem in Discrete phase modelling with suspended particles in a closed vessel of fluid.The suspended particles in the fluid is to be heated in a closed vessel at a fixed volume concentration.But how to make the particle suspended before the heating is done at a fixed concentration, without any inlet and outlet flow?

Suggestion from ur side would be of great help..
Regards,
Rob

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