Chapter 8, Problem 8.62P

Water at a flow rate of $\text{<mover accent="true"> <mo> m </mo> <mo> ˙ </mo> </mover> =0}\text{.215}\text{kg/s}$ is cooled from 70°C to 30°C by passing it through a thin-walled tube of diameter D = 50 mm and maintaining a coolant at $T_{\infty }=15°\text{C}$ in cross flow over the lube.

(a) What is the required tube length if the coolant is air and its velocity is $\text{V= 20 m/s}$ ?

(b) What is the tube length if the coolant is water and $\text{V = 2 m/s}$ ?