Question $2$

Combustion gases passing through a $3-cm-$internal$-$diameter

circular tube are used to vaporize waste water at atmospheric

pressure. Hot gases enter the tube at $250C$ and $1$ atm at a

mean velocity of $5\frac{m}{s},$ and leave at $150C.$ If the average

heat transfer coefficient is $120\frac{W}{m^2}K$ and the inner surface

temperature of the tube is constant at $110C,$ determine:

i$.$ The rate of heat transfer $(W).$

ii$.$ The tube length $($m$).$

iii. The rate of evaporation of water $(k\frac{g}{h}r)$

Use the following properties for air at the average temperature of $200C$ :

$=0\mathrm{.}7662k\frac{g}{m^3}$;$c_p=1023\frac{J}{k}g*K$

The latent heat of vaporization of water at $1$ atm and $100C$:$h_{fg}=2257k\frac{J}{k}g$