I have created this:
How can i calculate the residence time?
What do you mean by "the residence time"
If you define volume by flowrate you're there?
maybe the simplest way is to use the concept of RTD (Residence Time Distribution) well known in chemical engineering. If you are not familiar with this concept, wikipedia offers a very quick first look:
The cited reference to the Levenspiel book is perfect to go deeper into this.
Practically speaking, you just have to reproduce, in OpenFOAM, the simple experimental determination of this function:
1) firstly determine with a multispecies solver the steady state solution of the flowfield, taking the concentration of one species equal to zero.
2) change the inlet concentration of this species to a value such that it does not change the density of the mixture (f.i. assume the same molecular weight of the average mixture and replace part of the mixture with the new one that represents a tracer, like inked water into blank water).
3) measure in time the concentration at the outlet of the new species, up to the time that the outlet concentration equals the inlet concentration.
As described in wikipedia, this function is your residence time distribution.
Hoping these hints are useful.
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