PURPOSE: To increase heat transfer rate by a method wherein the side surface of a flute is worked so as to be wavy recess and projection proceeding to the lengthwise direction of a tube, the recess and projection of the wavy forms of neighboring flutes are synchronized mutually, the flow passage of fluid is bent in zig zag and mixing is promoted.

CONSTITUTION: Radial dotted lines show the flat parts 102 of the side surfaces of flutes while radial full lines show the crest lines 102a and the trough lines 102b of the wavy surface. The height of the wave is highest at the outer ends of the flutes and lowest at the inner ends of the same. The recesses and the projections of the wavy forms of neighboring flutes are synchronized so that the flow passage of fluid is bent while keeping an uniform width substantially. A distance between planes including the crest line 102a and the trough line 102b of the neighboring flute respectively is designed so that the distance at the outer end C1 is smaller than the same at the inner end C2. The configurations of waves are regulated so that the curvature of the passage of the fluid is larger at the outer end and smaller at the inner end in such a manner whereby the flow speed of the fluid at the inner and outer ends may be unified as much as possible, the performance of whole heat transfer area may be increased in average and mixing as well as turbulent flow may be promoted by the curvature substantially whereby heat transfer rate may be increased remarkably.