helpQUESTION $2$

A $15-$cm $20-$cm printed circuit board whose components are not allowed to comet into direct contact

with air for reliability reasons is to be cooled by passing cool air through a $20-cm$ long channel. of

rectangular cross section $0\mathrm{.}2cm14cm$ drilled into the board. The heat generated by the electronic

components is conducted across the thin layer of the board to the channel, where it is removed by air

that enters the channel at $15C.$ The heat flux at the top surface of the channel can be considered to

be uniform, and heat transfer through other surfaces is negligible. If the velocity of the air at the inlet

of the channel is not to exceed $4\frac{m}{s}$ and the surface temperature of the channel is to remain under

$50C,$ determine the maximum total power of the electronics components that can safely be mounted

on this circuit board. $($Ans: $Re=1010,T_f=33\mathrm{.}5C,Q_{\text{m}\text{a}\text{x}}=24\mathrm{.}7W)$