Heat transfer enhancement of turbulent channel flow using dual self-oscillating inverted flags: Staggered and side-by-side configurations

Abstract

This study experimentally determined the flapping dynamics of dual self-oscillating inverted flags placed inside turbulent channel flows in side-by-side and staggered configurations and their ability to enhance wall heat removal. Three clearance-distance to channel-width ratios (Gc/W = 0.19, 0.31, and 0.5) and three streamwise-distance to channel-width ratios (Gy/W = 0, 2, and 4) were used to examine distinct flag behaviors. A single flag mounted to the heated wall with various gap clearances was chosen as the benchmark. The flags’ time-varying motions were recorded by a high-speed camera system. Three dynamic regimes were identified on the basis of the flags’ dimensionless stiffness and the channel flow’s Reynolds number: the biased mode, the flapping mode, and the deflected mode. Temperature sensitive paint (TSP) measurements demonstrated that the best cooling enhancement, with a local Nusselt number ratio of over 1.6, was achieved for the single flag system at Gc/W = 0.19. Adding another inverted flag to the side-by-side configuration at Gc/W = 0.19 further enhanced the heat removal performance on both channel walls, and the flapping period increased by nearly 50%. However, placing two side-by-side flags close to each other (Gc/W = 0.31) led to chaotic flapping motions, resulting in diminutive augmentation in heat transfer and an appreciable penalty in pressure drop. In the staggered configuration at Gy/W = 2 and 4, the two inverted flags synchronously flapped with a stable phase difference, and the flapping periods were similar to those of the single flag. The peak Nusselt number ratio was 1.9 for Gy/W = 2, which was attributed to the concerted influence of the staggered inverted flags. The system with staggered flags placed close to the heated wall had a higher thermal enhancement factor than the system with flags mounted in tandem along the channel centerline.