Consider a concentric tube annulus for which the inner and outer diameters are 25 and 50 mm. water enters the annular region at 0.03 kg/s and 25°c. if the inner tube wall is heated electrically at a rate (per unit length) of q′ = 3000 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?