A counterflow, concentric tube heat exchanger is used to cool the lubricating oil for a large industrial gas turbine engine. The flow rate of cooling water through the inner tube (D = 25 mm) is 0.2 kg/s, while the flow rate of oil through the outer annulus (Do= 45 mm) is 0.1 kg/s. The oil and water enter at temperatures of 100 and 30C, respectively. The outlet temperature of the oil is 60C. (a) Determine the outlet temperature of cooling water [degC]. (b) Determine the long-mean-temperature difference [degC]. (c) Determine Re, Nu, and h; for the cooling water at the inner tube. (d) Determine Re, Nu, and ho for the cooling water at the outer annulus. (e) How long must the tube be made if the outlet temperature of the oil is to be 60C? Nusselt number for fully developed laminar flow in a circular annulus with one surface insulated and the other at constant temperature. (from Kays and Perkins 1972) D/D Nu Nu 0 3.66 0.05 17.46 4.06 0.10 11.56 4.11 0.25 7.37 4.23 0.50 5.74 4.43 =1.00 4.86 4.86