Consider a concentric tube annulus for which the inner and outer diameter are 25 and 50 mm. Water enters the annular region at 0.04 kg/s and 25?C. If the inner tube wall is heated electrically at a rate per unit length of q′ = 4000 W/m, while the outer tube wall is insulated, how long must the tubes be for the water to achieve an outlet temperature of 85?C? What is the inner tube surface temperature at the outlet,where fully developed conditions may be assumed?