Laminar and turbulent flow development study in a rectangular duct with 180{degree sign} sharp bend by using stereo particle image velocimetry and liquid crystal thermography measurements

This work presents a detailed insight into the flow progression and surface heat transfer distribution across the sharp 180{degree sign} bend of a two-pass rectangular duct for laminar ( Re = 800) and turbulent ( Re = 6500) in-flow conditions. Stereoscopic Particle Image Velocimetry (SPIV) as well as 2D2C (2-dimensional and 2-component) PIV measurements, and Liquid crystal thermography (LCT) techniques are appropriately used for flow and heat transfer characterization across the complete sharp 180{degree sign} bend. The centrifugal instabilities arise due to the sharp bend, induces the secondary flows in the form of counter-rotating vortices pairs commonly known as Dean vortices. These secondary vortices play a significant role in the localized laminar-turbulent transition and turbulence augmentations for laminar and turbulent inflow conditions. Subsequently, quantitative analysis shows that complete 180{degree sign} turning of flow resulted in intense augmentation of spatially averaged turbulence quantities. RMS fluctuations in the transverse direction VT|rms increase by 298% and 186% for respective flow conditions. Augmentation of ~ 287% (laminar) and 260% (turbulent) in the wall-normal RMS fluctuations ( VN|rms) are observed. These augments in transverse and wall-normal velocity fluctuations result in a very sharp amplification of spatially averaged turbulent kinetic energy ( k), i.e., 1825% for inlet laminar and 928% for inlet turbulent flow regimes.