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Unformatted text preview: ber. 11 Convec'on Heat Transfer • Isoﬂux Surfaces: – Solu'on is similar, but changes if surface is isoﬂux rather than isothermal '
q'w x
Nux =
k f (Tw ( x ) − T∞ ) – If 0.1 < Pr < 100 we have € – Nu is used to obtain the local surface temperature 12 Convec'on Heat Transfer • A general correla'on for all Prandtl numbers, i.e. Pr > 0 is: It has the following special limits: 13 Convec'on Heat Transfer • There is no mean Nusselt number for Isoﬂux condi'ons. If we desire a mean surface temperature then we can integrate: • or 2 ''
qw L
3
Tw − T∞ =
0.453k f Re1 / 2 Pr1 / 3
L • which gives: € € Nu0 − L '
q'w L
=
= 0.679 Re1 / 2 Pr1 / 3
L
k f (Tw − T∞ ) 14 Convec'on Heat Transfer • Delayed Hea9ng – In many problems there may be an unheated star'ng length. 15 Convec'on Heat Transfer • The solu'on for Nu must be modiﬁed as follows if hea'ng is delayed downstream. For an isothermal surface with an unheated length xo we have: 16 Example 3 • An electrical classmate is designing a circuit board that contains four heat producing components that must be kept cool. Since he is an electrical and knows nothing about heat transfer, he asks (pleads) for your help. The circuit board is 40 [cm] long in the ﬂow direc'on and 30 [cm] wide. The chips are to have a layout as shown below. Your electrical buddy also informs you that the circuit board is to be part of a larger system where an oversized fan (chosen by your buddy who also lacks knowledge in fan selec'on/sizing principles) supplies air with a ﬂow of U = 12 m/s and free stream temperature T = 25 [C]. Using the proposed layout and environment condi'ons you choose to evaluate the best and wo...
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 Fall '12
 DrMuzychka
 Heat Transfer

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