HEAT TRANSFER CHARACTERISTICS OF INTERNALLY FINNED TUBE

This paper describes R&D study of an internally finned tube aiming at an application to small-scale smoke-tube boilers. An enhancement of heat transfer of smoke tubes provides the possibility of size reduction of boilers, and in addition, a significant contribution toward efficient use of energy, e.g. through latent-heat-recovery technology. Developed internally finned tube consists of multiple aluminum plate fins aligned in parallel across the tube cross-section. Experimental investigation was conducted with a wind tunnel, and local heat transfer coefficients were measured. Various types of fin geometry were examined, and the split-fin type with baffle plates gave very high heat transfer performance. The enhancement in heat transfer was mainly caused by the edge effect and resultant suppression of boundary layer development by splitting fins in several units as well as the mixing and/or agitation of the gas stream across the cross-section. The number of heat transfer unit, NTU, being actually a heat transfer rate, reaches around 15 times of the bare tube without internal fin. This demonstrates high potential of the developed internally finned tube for an application to industrial smoke-tube boilers.