Influence of the fin spacing on the flow field near the tube-fin junction in louvered fin compact heat exchangers

Three-dimensional flow effects influence the heat exchanger’s performance. In this study flow visualization experiments were performed in two enlarged models of a louvered fin heat exchanger with round tubes. The models have a staggered tube layout and differ only in their fin spacing. A water tunnel was designed and built and the flow visualizations were carried out using dye injection. It was found that the development of the horseshoe vortex system upstream the tube and the wake zone downstream was clearly affected by the Reynolds number and the fin spacing. At small Reynolds number no horseshoe vortex was formed in front of the tube and a wake zone with two recirculation bubbles was developed downstream the tube. As it was increased, a horseshoe vortex was formed, while the wake zone grew, opened and finally became unsteady. This transition in flow behaviour happened at lower Reynolds numbers for the largest fin spacing. Thus, a reduction of the fin spacing results in a dissipation of turbulent and vortical motion by mechanical blockage and skin friction. Next, the critical Reynolds numbers at which the inlet and outlet louvers start to shed vortices was determined for the different tube rows. The initial instability occurred in the wake of the exit louver of the third tube row. When increasing the Reynolds number the instabilities propagated upstream into the heat exchanger. Increasing the fin spacing enhanced the propagation of instabilities