Computational and experimental comparison of tube wall heat transfer augmented by winglets in louvered fin heat exchangers

Louvered fins are used in compact heat exchangers to increase heat transfer by interrupting thermal boundary layer growth thereby increasing the convective heat transfer coefficients and reducing the air side resistance. Recently, it has been experimentally shown that heat transfer along the tube wall can be augmented by the placement of delta winglets on the louvers at an angle to the flow. The focus of this combined experimental and computational study is to determine the effect of realistic winglets on tube wall heat transfer. Comparisons of the computational simulations were made to the experimental results, which were obtained using a twenty times scaled model. Winglet performance characteristics were studied on solid louvers and pierced louvers whereby the latter simulates what would occur for a manufactured louver having a winglet. For a solid louver having a winglet, the tube wall heat transfer augmentation was found to be as high as 5.4%. Pierced louver cases were observed to produce slightly higher heat transfer augmentations than solid louver cases. Computational results suggest that the mechanism behind tube wall heat transfer augmentation is flow redirection and not winglet induced vortices.