G The elemental unit of an air heater consists of a long circular rod of diameter D, which is encapsulated by a finned sleeve and in which thermal energy is generated by ohmic heating. The N fins of thickness t and length L are integrally fabricated with the square sleeve of width w. Under steady-state operating conditions, the rate of thermal energy generation corresponds to the rate of heat transfer to airflow over the sleeve. (a) Under conditions for which a uniform surface temperature Ts is maintained around the circumference of the heater and the temperature T and convection coefficient h of the airflow are known, obtain an expression for the rate of heat transfer per unit length to the air. Evaluate the heat rate for Ts 300C, D 20 mm, an aluminum sleeve (ks 240 W/m K), w 40 mm, N 16, t 4 mm, L 20 mm, T 50C, and h 500 W/m2 K.