3. 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 (Di=25 mm) is 0.2 kg/s. The flow rate of hot oil through the outer annulus (Do=45 mm) is 0.1 kg/s. The convection coefficients associated with the oil and water flow are ho=40 W/m2K and hi=2250 W/mK, respectively. The oil and water enter at temperatures of 100 and 30C, respectively. What is the required tube length (m) for an oil outlet temperature of 60C? (30p) Properties: Water: cp=4178J/kgK, =725x10-6 Ns/m2, k=0.625 W/mK, Pr=4.85 cp=2131 J/kgK Oil: