2 replies to this topic

[madmanfrommars]

Okay for fluid mechanics, I have a question that involves calculating the reynolds number of air at 180F and at 2atm, but the reference tables in my textbook only give me the viscosity at 1atm and 350F.

I have absolutely no idea what to do at this point. I would appreciate any help that could be provided.



On a side note, how high can reynolds numbers run? For a problem I got a calculation of a reynolds number for water flowing through a pipe at 150,000, but I checked every number and unit 100 times. Is that even possible?

[JEBradley]

To find the viscosity of air at a given temperature just google 'air viscosity calculators' and you should get loads of hits. e.g. http://www.lmnoeng.c...asViscosity.htm

You're unlikely to find anything that calculates viscosity as a function of pressure because it varies little (Crane says no more than 10% up to a pressure of 500 bar - if this can be extrapolated back to 2 bar then the pressure-effect is negligible).

I hope someone else can confirm this but I think it's all down to the ideal gas nature of the gas. I have a vague recollection that the relevant equation has viscosity directly proportional to pV^n - so for an ideal gas n=1 and pV is constant. If its a coursework question just state this as an assumption at the beginning of your answer - 'Assumed air at 2 bar behaves as an ideal gas and therefore pressure effect on viscosity is negligible'.

With regards to Reynolds number - I dont think there's any theoretical limit on the magnitude. Check each value you're putting in and consider if its a realistic value and you should be ok. E.g. there's nothing theoretically wrong with a pipe being 10 miles in diameter (which would give a high Reynold's number) but its pretty unlikely.

However, I made a couple of assumptions - v=10m/s d=3" and Re=150,000 sounds about right.
(If it puts your mind at rest the standard Moody chart plots Re upto 10,000,000.

[JEBradley]

Sorry - I just re-read your post and your Re=150,000 was for water not Air :s

Nvm its still ok. density/viscosity is roughly 1000000. A typical velocity might be 2 m/s. Sooooo that would give a line size of 3" which sounds good.

PS sorry for SI units.