HEAT-TRANSFER ENHANCEMENT IN ANNULAR CHANNELS WITH HELICAL AND LONGITUDINAL FINS

Heat exchange in an annulus with and without fins was investigated. Two helical fin arrangements and a comparable area, multiflight, longitudinal fin design were compared to the plain annulus without fins. All experiments were done in the same shell, hence the same volume space, derived from a commercial heat reclaimer unit. Film heat transfer coefficients (h) for the annularflow were calculated from experimental data. Our results show that enhancement in annular heat transfer coefficient for both helical arrangements (conducting and nonconducting fin) was 40-50%, thus the film coefficient was essentially the same on both fin and curved surfaces. The film coefficients were also the same for the longitudinal fins and the plain annulus cases, but the enhancement was 260%, much higher than expected. We believe the reasons for this are the short L/D of the annulus coupled with a predominant entrance/exit ''cross-flow'' effect; thus, short annular exchangers of this type may give very high enhancements as a general rule.