2
2a) Air at 25°C flows with a velocity of 10m/s over the flat top
surface of a computer chip, the chip surface is a square
0.03m by 0.03m. The air velocity is parallel to one edge of
the square, and the power dissipated through the top
surface is 5W.
The air kinematic viscosity $v = \frac{\mu}{\rho} = 1.6 \times 10^{-5} m^2/s$, thermal
conductivity of air is 0.026W/mK and Pr = 0.7.
The correlation for the average Nusselt number is
$Nu_{avg} = C Re^n Pr^{1/3}$
Where C and n are shown in Table Q2a
Table Q2a
Geometry Re C n
Horizontal plate $< 5 \times 10^5$ 0.68 0.5
Horizontal plate $> 5 \times 10^5$ 0.03 0.8
Determine the top surface temperature of the computer
chip.
2b) The wall is 2m high and 3m wide, the outside wall surface
is maintained at 10°C and outside air at -10°C.
Correlations for the average Nusselt numbers in free
convection are listed in Table 2b.1 and the fluid property
values are listed in Table 2b.2.
Table 2b.1 Average Nusselt numbers
Free convection
from a vertical
surface when $Gr \ Pr < 10^9$
$Nu = 0.59 (Gr Pr)^{0.25}$
when $Gr Pr > 10^9$ $Nu = 0.13 (Gr Pr)^{1/3}$
Table 2b.2 Fluid properties
T $\rho$ $v$ k $\beta$ $C_p$
°C kg/m³ m²/s W/mK K?¹ kJ/kgK
Air 0 1.3 $1.32 \times 10^{-5}$ 0.024 1/T 1.006
Determine the heat transfer rate from the wall.
[10 marks]
[10 marks]
