[nirmal_singh]

Hi Members,

I had a small confusion.

I want to simulate a shell and tube heat exchanger with gravity conditions(real case), but when I consulted with some CFD people, they told me that I need to simulate transient state (time-dependent) if I want to simulate gravity conditions accurately. But as far as I think that it can be simulated in steady state only (want to save extra time that will be required for transient case).

The heat exchanger has mixed convection process (but strong forced convection effects).

Am I incorrect in my thought process?

Thanks in advace

[Boris_M]

Well if it is expected that a steady state will exist in your model at some time then you should be able to use steady state as this will be reached.
However if the flow is very turbulent and fluctuations due to flow separation etc. will appear then the results would be more fixed single solution out of a transient result.
You can take simply the karman vortex street cylinder and run such a cylinder transient or steady state. There will be a difference of course as it is a transient case. If you assume it is steady state then you have to take into account some error.

Usually this is something that needs to be tested to see how big the error in order to say if that is acceptable for you or not.

In nature almost every flow case you can simulate with FloEFD is transient but most cases can be calculated steady state, except for those where you want to see the transient behavior or need to consider it as transient.
For me it is really hard to tell from your model description to judge if I would rather consider transient or steady state.

Boris