Consider a $10-$m$-$long smooth rectangular tube, with a$=50$mm and b$=25$mm$,$ that is maintained at a constant surface temperature.

Liquid water enters the tube at $20\backslash$deg C with a mass flow rate of $0\mathrm{.}027$k$($g$)/($s$).$ Determine the tube surface temperature necessary to heat

the water to the desired outlet temperature of $80\backslash$deg C$.(\backslash$Nu $=3\mathrm{.}39)$