Professionally I work at BPT and focus on creating value adding App for Process Simulation. You can find more info on those on the BPT website. Below are posts that should help HYSYS, PetroSIM and UNISIM users alike in their day to day challenges to produce accurate yet fast models efficiently.

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Wednesday, 14 October 2009

What does "Compressible Flow" mean? Isn't any gas compressible?

It is a question I have asked myself a couple of times, so when I got a good answer to this one, I thought it might be useful to share it. Explanation courtesy of Biswas Souvik, Aspentech.

Compressible flow is definened as any flow where the density is NOT constant along the flow field. If in a flow field where density is completely constant that flow is called incompresible flow. This should not be confused with compressible fluid though. For example, air is a compressible fluid but if there is one flow field where air is flowing but its density is not changing then that flow will be considered incompressible. In reality, few flows are exactly incompressible because density always changes even if by a minute amount. So there is basically a rule of thumb to differentiate between compressible & incompressible flow: if density changes by more than 5% then the flow is compressible, if not the flow is incompressible. For ideal gases with constant Cp/Cv ratio of 1.4, under adiabatic flow conditions, it can be theoritcally shown that for an 5% change of density the Mach No. has to be >= 0.3. So there is another generally accepted rule of thumb,saying if Mach No.>= 0.3 the flow is compressible, otherwise incompressible.

1 comment:

Elsherif said...

Dear Esteemed Mr Wim Van Wassenhove
I Would like to ask simple Question here regarding the compressible and incompressible flow

for the case of Heavy Virgin feed to Reforming unit is it considered as incompressible since its density change due to various reactions in the unit?

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