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I'm trying to find out the temperature drop of flue gas in an uninsulated quartz pipe in ambient air. I am fairly confident with my approach but receive erroneous answers.

I have approached the problem as follows:

1) Calculate the prandtl number

2) Calculate the Re number

3) Calculate the Nu number (for laminar flow in a pipe)

4) Calculate hi (hi = Nu*k/D)

5) Calculate the thermal resistance Rquartz = ln(ro/ri)/(2*pi*k)

6) Calculate the thermal resistance inside convection = 1/(hi*2*pi*ri)

7) Solve for the inside and outside wall temperatures using the following non-linear eqns:

(Tflue-Tinside)/Rinside = (Tinside-Toutside)/Rquartz = 2*pi*ro*(1.32/d^0.25)*(Toutside-Tair)^5/4

The last term comes from the equation for laminar flow to get ho.

8) Calculate ho using ho = 1.32*(Toutside-Tair/D)^0.25

9) Calculate Ro = 1/(ho*2*pi*ro)

10) Calculate U = 1/(Ao*Rtotal)

The problem I have is that the wall temperatures I calculate are odd (inside wall more than 700C different to flue temperature), I would expect them to be very similar. Also, the outside heat transfer coefficient is greater than the inside.

If you could find any faults in my procedure please let me know. Also, if you could suggest the quickest way to get the temperature of the flue gas at the outlet of the pipe from knowing U that would be great.

Thanks,

David

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# Temperature drop along pipe

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