Air at 1 atm and 10 °C flows across a bank of tubes 15 rows high and 5 rows deep at a velocity of 7m/s measured at a point in the flow before the air enters the tube bank. The surfaces of the tubes are maintained at 65°C. The diameter of the tubes is 2.54cm; they are arranged in an in-line manner so that the spacing in both the normal and parallel directions to the flow is 3.81cm. The exit temperature of air is 20°C。(1atm、10°C的空气流经一15行5列的顺排管束,进入管束前流速为7m/s。管束表面温度为65°C。管外径2.54cm。管束垂直和平行流动方向间距为3.81cm。出口温度为20°C。)求Nu。