Duct losses

[gyronaut]

I am trying to estimate the losses in a short duct(theory stage at the moment) from which the air used to generate thrust. There will be two 90 degree corners one whith vanes. So far I have used the pipe flow method with the moody diagram and the loss coefficients for bends and change in section. Will this be reasonably close to the mark for low pressure high volume air flow.

Cheers
gyronaut

 

[fredt]

There are a number of texts covering internal flow systems used by fan and hvac people. The problem is quite complex to solve accurately without CFD or physical modelling but an approximate answer can be obtained from the empirical coefficients given in various publications.

Something to be aware of is that when you combine bends and other flow disturbing features the combined loss is usually greater than the sum of the individual losses.

 

[chasbo]

The loss in static thrust from a 90 degree turn is about 50%. This was obtained by field tests on R/C engines. Throw in two and you'll get about 1/4 what you'd get in a straight through duct.