Study of junction flows in louvered fin round tube heat exchangers using the dye injection technique

Abstract

Detailed studies of junction flows in heat exchangers with an interrupted fin design are rare. However, understanding these flow structures is important for design and optimization purposes, because the thermal hydraulic performance of heat exchangers is strongly related to the flow behaviour. In this study flow visualization experiments were performed in six scaled-up models of a louver fin round tube heat exchanger. The models have three tube rows in a staggered layout and differ only in their fin spacing and louver angle. A water tunnel was designed and built and the flow visualizations were carried out using dye injection. At low Reynolds numbers the streakline follows the tube contours, while at higher Reynolds numbers a horseshoe vortex is developed ahead of the tubes. The two resulting streamwise vortex legs are destroyed by the downstream louvers (i.e. downstream the turnaround louver), especially at higher Reynolds numbers, smaller fin pitches and larger louver angles. Increasing the fin spacing results in a larger and stronger horseshoe vortex. This illustrates that a reduction of the fin spacing results in a dissipation of vortical motion by mechanical blockage and skin friction. Furthermore it was observed that the vortex strength and number of vortices in the second tube row is larger than in the first tube row. This is due to the thicker boundary layer in the second tube row, and the flow deflection, which is typical for louvered fin heat exchangers. Visualizations at the tube-louver junction showed that in the transition part between the angled louver and the flat landing a vortex is present underneath the louver surface which propagates towards the angled louver.

Performance enhancement of a louvered fin heat exchanger by using delta winglet vortex generators

“…Huisseune et al [9] explained that this is partly because the distance between the fin leading edge and the tube in the first tube row is shorter than the distance between the exit louver and a downstream tube. However, the main reason for the difference in horseshoe vortex development between the first and the downstream tube rows is caused by the flow deflection which affects the pressure distribution [9,10]. The horseshoe vortices wrap around the front half of the tube, but they also do not persist far downstream as they are destroyed by the louvers in the same tube row.…”

“…The flow deflection also affects the local pressure distributions upstream of the tubes of the downstream tube rows and thus influences the horseshoe vortex development at these locations. The horseshoe vortex system in the downstream tube rows of the louvered fin heat exchangers is clearly stronger than the horseshoe vortex system in the first tube row [9,10]. Louvered fins improve the heat exchanger performance by altering the main flow characteristics.…”

“…In this case the junction flows are more complex than in flat tube heat exchangers due to the formation of horseshoe vortices upstream of the tube and wake zones downstream. The horseshoe vortex development, wake zones and flow deflection in round tube louvered fin heat exchangers was previously investigated, both experimentally [9] and numerically [10]. It was found that the flow deflection is affected both by the tubes in the same tube row (intra-tube row effect) and by the tubes in the upstream tube rows (inter-tube row effect).…”

Influence of the louver and delta winglet geometry on the thermal hydraulic performance of a compound heat exchanger

“…Next to the reduced impact of the artificially created vortices by the delta winglets, the naturally occurring horseshoe vortices are also suppressed for the smallest fin pitch. This was observed during visualization experiments in a water tunnel [49]. The absence of horseshoe vortices also lowers the average Colburn j-factor.…”