2019 NCEES T&F Problem #10

[BuffaloWings]

Could someone please explain why the solution just added the radiation and convection heat transfer coefficients instead of treating them like resistors in parallel?

 

[BuffaloWings]

Sorry this is from the 2011 Exam (not 2001)

 

[Dr. Barber]

Could someone please explain why the solution just added the radiation and convection heat transfer coefficients instead of treating them like resistors in parallel?

I have the 2011 NCEES Practice Exam, Thermal and Fluid Systems. ISBN 978-1-932613-61-2

There does not seem to be a problem #10. There's a #110 in the AM section and a #510 in the PM section, but none of those seem to match what you're describing.

Can you provide a screenshot of the question and answer?

 

[BuffaloWings]

Sorry it’s actually 2019

 

[Dr. Barber]

They did treat it as resistances in parallel.

The convection resistance is 1/(h_conv*A) and the radiation resistance is 1/(h_rad*A).

For parallel you find the total resistance as 1/R_total = 1/R_conv + 1/R_rad hence 1/R_total = (h_conv + h_rad)*A

 

[BuffaloWings]

Thank you. I worked it out again and realized my mistake.