Chapter 8, Problem 8.97P

Consider a concentric tube annulus for which the inner and outer diameters are 25 and $\text{5}0\text{mm}$ . Water enters the annular region at $\mathrm{0.0.}\text{4kg}/\text{s}$ and $\text{25}°\text{C}$ . If the inner tube wall is heated electrically at a rate (per unit length) of $q\text{'}=4000\text{W/m}$ , while the outer tube wall is insulated how long must the tubes be (or the water to achieve an outlet temperature of $\text{85}°\text{C}$ ? What is the inner tube surface temperature at the outlet, where fully developed conditions may be assumed?