Re# of 8.8 million, What Nu equation should I use?

[JRW261]

I apologize but I am fairly limited with my resources. Basically what I have is Exhaust at 80psi and nearly 7lbm/s at a max temperature of 2000F in a 11" pipe. I know how todo most of the equations but I cannot find a Nu Equation for that type of flow and that high of a reynolds number. I am looking to calculate the heat transfer coef. I used Nud=.023*Red^(4/5)*Pr^n; n=.4 and got h=2.38e3 W/m^2K. That seemed pretty high, so I am going to recheck my calcs. Any insight would be greatly appreciated. I dont have much experience with this so bear with me.

 

[hacksaw]

How are you calculating your reynolds no?

 

[JRW261]

Re=pUD/v

 

[JRW261]

Dens. exh 0.087822147 lbm/Ft^3
Visc. Exh 1.07E-06 lbm/Ft*s
U exh 123.0844865 Ft/s

Here are the values I was using.. If your noticing small discrepencies.. its because I used 80psia, 6.5lbm/s, and D=10.5"... I havn't had a chance to check everything yet, I plan to do it shortly...

Thanks for the help

 

[hacksaw]

I don't get 8 million, check the viscosity units.


At 2000 F, convection is a minor detail more or less. what are you doing?

 

[25362]

For steam at 2000^oF and 80 psia, it seems the viscosity taken is too low by a factor of about 30. Could you please recheck this value ?

 

[JRW261]

Its NG combustion exhaust (assumed dry air).

hacksaw
I am looking at installing a cooled probe in the exhaust, its a custom device and I am doing some preliminary work with it. The surrounding pipes are uninsulated if you were wondering. If you were leading to radiation, that is something I have to look at next

 

[25362]

Assuming Pr^0.4~0.95, Re~250,000, the thermal conductivity at ~0.15 W/m.K, the dia, 0.267 m, and using your formula

h = (0.023)(0.15)(250,000^0.8)(0.95)/0.267 ~ 255 W/m².K​

Is this estimated value a more reasonable one ?

 

[JRW261]

Alright, I see where I went wrong, the source of my viscosity didn't have its english units labeled very well and when I converted it I carried that inaccuracy.. Thanks for the help.