Compressible Flow Sudden Contraction

[RGCook]

I have saturated air (with water) flowing in a large duct at a nominal 1 ft/sec velocity. There is a sudden contraction such that the velocity becomes 45 ft/sec in the duct and the flow continues on inside the smaller duct for a long distance (not a nozzle). I am wondering how I calculate what the change in pressure is in the smaller duct. I am assuming that this is an adiabatic contraction, so it would also be very helpful to know how to calculate the change in temperature.

I assume that the pressure drops on entering the smaller duct and this leads to cooling of the air. Whether the reduction in pressure is enough to offset the cooling such that the moisture does not condense is what I am really after here. Any leads on how to solve this problem would be appreciated.

 

[Latexman]

Can you give us some more specific data? Like duct or pipe sizes, temperatures, pressures, % RH, etc.

Good luck,
Latexman

 

[robsalv]

Being a mech, and having not done thermo since uni... I'm not clear on the adiabatic expansion side of things, but I'd say Bernoulli has at least part of your answer.

Total pressure and mass must stay the same, so therefore the velocity must increases and therefore the static pressure decreases. In simple terms, if the temperature doesn't change, you'll get condensation IF the pressure drops below the saturation pressure.

Hope this helps.

Cheers

Rob