A thick-walled steel pipe (k = 60 W/m ∙ K) carrying hot water is cooled externally by a cross-flow airstream at a velocity of 20 m/s and a temperature of 25°C. The inner and outer diameters of the pipe are Di = 20 mm and Do = 25 mm, respectively. At a certain location along the pipe, the mean temperature of the water is 80°C. Assuming the flow inside the tube is fully developed with a Reynolds number of 20,000 find the heat transfer rate to the airstream per unit pipe length.