Effects of longitudinal vortices on turbulent junction flow heat transfer

Turbulent junction flow is a vortex system upstream of a blockage embedded in a turbulent boundary layer, which leads to augmented heat transfer near the blockage. Vortex legs wrap around the blockage and travel downstream as longitudinal vortices. In multistage axial turbines or multiple row tube heat exchangers, these longitudinal vortices can collide with junction flow vortices further downstream. Furthermore, these devices often augment mixing and increase heat transfer with high freestream turbulence. A comprehensive study on how longitudinal vortices affect junction flow heat transfer for different Reynolds numbers, freestream turbulence values, and longitudinal vortex configurations was performed. Longitudinal vortices were generated upstream of a symmetric airfoil using delta winglet vortex generators. Spatially resolved endwall heat transfer coefficients upstream and around the junction were measured using infrared thermography. Time-averaged flowfield measurements of the incoming longitudinal vortices were taken using stereo particle image velocimetry. Longitudinal vortices provided limited heat transfer augmentation around the junction and did not disrupt the time-averaged horseshoe vortex. These interactions were identical at high turbulence, but longitudinal vortices were less effective at augmenting heat transfer, likely due to an increase in vortex wandering. Vortex wandering also led to a broadening in lateral heat transfer profiles.