The efficiency ηf of a straight fin with a rectangular cross section is determined by measuring three temperatures: the temperature ϑ0 at the fin base, the temperature ϑh at the top of the fin and the...

The efficiency ηf of a straight fin with a rectangular cross section is determined by measuring three temperatures: the temperature ϑ0 at the fin base, the temperature ϑh at the top of the fin and the temperature ϑS of the fluid which surrounds the fin. Calculate ηf for ϑ0 = 75.0 ◦C, ϑh = 40.5 ◦C and ϑS = 15.0 ◦C.