Three-dimensional flow structures influence the heat exchanger’s performance. In this study flow visualization experiments were performed in six scaled-up models of a louvered fin heat exchanger with round tubes. The models have a staggered tube 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. For small Reynolds numbers no horseshoe vortices are developed in front of the tubes and the recirculation regions downstream the tubes are small. As the Reynolds number is increased, the horseshoe vortices become larger and stronger. The recirculation bubbles grow until they cover the entire back of the tube. When the Reynolds number is further increased, the recirculation region becomes unsteady. At the same Reynolds number the vortex strength and the number of vortices in the second tube row is larger than in the first tube row. Reducing the fin pitch suppresses the vortex and wake development. Further it was found that the first unsteady flow patterns appear in the wake of the heat exchanger and these instabilities move upstream with increasing Reynolds number. The onset of unsteadiness is postponed to higher Reynolds numbers when the fin pitch or louver angle is reduced.

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