Thermodynamics HW Solutions 717

Thermodynamics HW Solutions 717 - Chapter 9 Natural...

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
Chapter 9 Natural Convection 9-26 A printed circuit board (PCB) is placed in a room. The average temperature of the hot surface of the board is to be determined for different orientations. Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas with constant properties. 3 The local atmospheric pressure is 1 atm. 3 The heat loss from the back surface of the board is negligible. Properties The properties of air at 1 atm and the anticipated film temperature of ( T s + T )/2 = (45+20)/2 = 32.5 ° C are (Table A-15) 1 - 2 5 K 003273 . 0 K ) 273 5 . 32 ( 1 1 7275 . 0 Pr /s m 10 631 . 1 C W/m. 02607 . 0 = + = = = × = ° = f T k β υ Analysis The solution of this problem requires a trial-and-error approach since the determination of the Rayleigh number and thus the Nusselt number depends on the surface temperature which is unknown L = 0.2 m Insulation Air T = 20 ° C PCB, T s 8 W ( a ) Vertical PCB . We start the solution process by “guessing” the surface temperature to be 45 ° C for the evaluation of the properties and h . We will check the accuracy of this guess later and repeat the calculations
Background image of page 1
This is the end of the preview. Sign up to access the rest of the document.
Ask a homework question - tutors are online