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Residence time distribution via Heat transfer in fluids
Posted Aug 26, 2015, 4:45 a.m. EDT Fluid & Heat, Heat Transfer & Phase Change Version 5.0, Version 5.1 0 Replies
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I want to calculate the residence time distribution of a static mixer. More specifically, I want to replicate an experiment where dye is injected into the fluid stream.
I suppose the module of choice for this type of simulation is the 'transport of diluted species' interface. However, I have had some problems with this.
I started with a simple rectangular duct and first computed a laminar, stationary flow field and then in a second step the time-dependent transport of diluted species. This worked fine, but the time-dependent simulation took a very long time, and when I later used the real mixer geometry, the time step would get smaller and smaller until the simulation crashed.
I then replaced the 'transport of diluted species' physics with 'heat transfer in fluids' and used a higher inlet temperature as my "tracer", instead of a concentration. (I set the case up so that the temperature does not influence the material properties.)
When I ran the simple test case, the results were similar to those obtained before, but the simulation ran about six times faster. Also the simulation of the real mixer geometry ran without problems and the results look plausible.
My questions now are:
1. Why does the 'transport of diluted species' simulation take so much longer, even though the number of degrees of freedom solved for is the same as for the 'heat transfer in fluids' simulation?
And is 'heat transfer' somehow more stable/robust?
2. I'm not sure how much trust to put in the results obtained via the 'heat transfer' approach, even though they look good. The flow field is fixed, since it is calculated separately, the transport is dominated by convection, the density etc. are fixed, so the temperature should just be a passive scalar that is transported. Still, is there anything to look out for? Is this approach permissible at all?
3. Finally, I was wondering whether the particle tracking module would be suitable for this kind of analysis. It seems like overkill to me, since I'm more interested in the residence time of the bulk fluid and not individual particles, but is it commonly used in this way?
Any input would be highly appreciated!
Best regards,
Moritz
Hello Moritz Begall
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