Intro to Convection

Question 1

If the Nusselt number over a radial surface is given, how do you find the average Nusselt Number?

Answer 1

Nu_avg = 1/A_s * integral_(A_s) [h] dA_S

Question 2

How do you solve system with large pipe that looses heat due to natural convection and radiation, where h is not known?

Answer 2

1. Find Rayleigh Number to determine what natural convection scheme to use
2. For Beta we take the average temperature: (T_s + T_infinity)/2 in Kelvin
3. Find Nusselt Number using Natural Convection Table
4. Find h using the Nusselt Number
5. Calculate Heat loss

Question 3

What is the difference between natural convection and forced convection?

Answer 3

For natural convection, the outside fluid doesn’t have a velocity

Question 4

What is film temperature T_f?

Answer 4

T_f = 0.5(T_s + T_infinity)

Question 5

What is an important condition for reynolds analogy?

Answer 5

No streamwise pressure gradient

Question 6

What is the Formula for Reynolds Analogy?

Answer 6

Nu = (C_f / 2) * Re

where Cf = (tau)/(0.5 rho u_infinity^2) = 0.5F / (0.5 L^2 rho u_infinity ^2)

Also works for average values