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Jeff September 14, 2004 13:55

Particle mass flow rate

There is a command in CFX 4.4, PARTICLE MASS FLOW RATES which tells the software how many particles the tracked particle is representing. What I gather is that we have to only specify mass not the mass flow rate. I would really appreciate if anybody can explain the mathematical significance of the mass flow rate and the equation where it is used.



Jeff September 15, 2004 18:40

Re: Particle mass flow rate

Imagine that each particle track is a string of beads. Particles travel along these tracks at a calculated velocity. What's not known, however, is how close these particles are spaced along the tracks. A slower PARTICLE MASS FLOW RATE means that the particles are "injected" along the tracks at sparse intervals. A very high PARTICLE MASS FLOW RATE means that the particles are packed in a tight stream.

Sources to the fluid are dependent on how fast particles are passing through a given cell. If a track carries one particle/min, it contributes a lower source than if there are 100 particles/min. Note, this particle flow rate is independent of the velocity calculated for the track.

Hope this helps, Jeff

Jeff September 16, 2004 01:45

Re: Particle mass flow rate
But what does mass flow rate signify in steady state flow ? Won't it be sufficient if I only specify mass ? May be answer to this is in the explanation you gave but this remains a confusion.

Also, if i put mass flow rate = 0, what would be it's effect on particle tracking ?


Jeff September 17, 2004 10:46

Re: Particle mass flow rate
Remember, particles are injected and travel through the domain independent of flow, other than the drag between the particles and the fluid. They may be moving faster or slower than the fluid. BECAUSE the flow is steady state, the particles have to be injected at a steady mass flow rate (the rate that particles cross the inlet plane). If I simply specify the total mass of particles to be injected, let's say all at once in a big slug, then the problem becomes transient because we're tracking this cloud of particles down stream only once.

From a process perspective, consider I have a pipeline with fluid moving though it at some flow rate. Also consider that at one location, I am injecting particles from above through a big hopper. If I inject the particles slowly (a low particle mass flow rate) then the fluid downstream will have a light particle loading and the sources on the fluid will be low. If I open the damper and drop the particles in faster (at a high particle mass flow rate) then the particle loading will be much higher and the sources on the fluid will be higher. In this light, a particle mass flow rate of zero should reduce the source term on the fluid to zero (no particle loading at all).

The drag on the particle, for trajectory purposes is independent of the particle mass flow rate. Particles go where particles go. If you are unconcerned about the coupling of forces back onto the fluid, then you can freeze the flow field by setting NUMBER OF HYDRODYNAMIC ITERATIONS to 0, and inject particles on a single iteration using "any arbitrary particle mass flow rate". This will give particle trajectories independent of what the particles are doing to the fluid.

Regards, Jeff

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