Fanno flow constant area duct Re number

[jlchard1]

I am trying to calculate the friction factor for a Fanno flow in a constant area duct. I know the friction factor is based on Reynolds number however does the Reynolds number not vary along the pipe due to a change in dynamic viscosity? (caused by a decrease in temperature along the pipe). And if so, does this not mean that the friction factor also changes along the pipe?

 

[boneh3ad]

The Reynolds number would tend to change if your viscosity is changing, though I can't imagine it changes very much. I don't know your specific situation, though. You can factor that in if you'd like by using Sutherland's law for viscosity, which approximately (but accurately) describes the temperature dependence of viscosity. That should give you a temperature dependence of your Reynolds number, and if you know your temperature profile as your flow moves downstream, you should be able to get what you need.

Or you could just approximate the flow as constant temperature if the temperature change isn't too drastic.