• The fin role of the untwisted tape cannot be negligible in loose- and snug- fitting with tube. • Secondary flow induced by the tape reduces the fin role of the tape in loose- and snug- fitting with tube • Reduction of fin role is attributed to alter fluid-tape and tape-wall heat transfer. • Impacts of geometric and flow parameters on the fin role are analyzed. • Twisted tape fitting types alter hardly affect the average performance. In the study of heat transfer enhancement using twisted tape inserts, a notable experimental phenomenon that both snug-fitting and loose-fitting twisted tapes achieve nearly identical heat transfer enhancement within tubes has been observed. A comprehensive explanation for this phenomenon remains absent. This paper tries to fill this gap by systematically investigating the collective influence of the twisted tape's structural properties, material characteristics, and fluid dynamics on heat transfer mechanisms under both snug-fitting and loose-fitting conditions. A primary focus is placed on the effect of the twisted tape's secondary flow on its fin role. The results reveal that for untwisted tape, the fin role is pronounced, the tape-to-tube gap significantly impacts thermal performance, leading to substantial differences between loose-fitting and snug-fitting configurations. For twisted tape insert, the secondary flow induced by the twisted tape attenuates its fin role in both loose-fitting and snug-fitting configurations, while factors intensifying secondary flow (e.g., increased Re , decreased Pr , higher twist ratio) diminish the fin role, parameters enhancing conductive heat transfer of tape (e.g., increased tape thickness, higher thermal conductivity) enhance the fin role slightly, notably, the local Nu on the tube wall is affected by the fitting type, but the average Nu remains almost insensitive. This work provides theoretical insights into the complex heat transfer mechanisms in tubes with twisted tape insert.
Liu et al. (2026) studied this question.