At our place these are done by CFD analysis. (We have gas, Air as well as mixed flows through different areas, at each of these, the relevant analysis is done)
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Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches
My answer was on this only.
The vanes in our equipments are on different angles, set such that
a) the two edge flows are tangential to the duct (we have rectangular duct)
b) the air flow is stream lined. ie the air paths do not intersect.
The multiple vanes are progressively angled to achieve this. (Fixed vanes for us)
However the details are not unfortunately with me (too complex for me to even understand the mathematics, I only know the broad concept)
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Fantastic ideas for a Fantastic World, I make the illogical logical.They put me in cars,they put me in yer tv.They put me in stereos and those little radios you stick in your ears.They even put me in watches, they have teeny gremlins for your watches