A fin includes straight segments and substantially circular arc segments connected with the straight segments in turn along a longitudinal direction such that the arc segments form respective wave crests and wave troughs of the fin. The fin is divided in a lateral direction into first and second end portions and an intermediate portion between the first and second end portions. Each arc segment at least forming the wave troughs in the first end portion is separated from the respective arc segment of the corresponding intermediate portion via a longitudinal slot. A top of each arc segment at least forming the wave troughs in the first end portion is formed with a lateral slot along the lateral direction such that each arc segment at least forming the wave troughs in the first end portion is divided into first and second straight portions separated from each other.