Analysis of the isothermal transition process in wide aspect ratio rectangular channels

Abstract

This work is aiming at extending the understanding of the transition process from a laminar gas flow to a turbulent gas flow in wide aspect ratio rectangular channels under isothermal conditions. The role of the inlet geometry and its impact on the transition process have been studied. Three inlet geometries have been used to influence the transition process, each geometry having a different critical Reynolds number at which the flow is becoming turbulent. The geometries are named bellmouth, square edged and undulated geometry. Multiple measurement techniques were applied: Laser Doppler Anemometry was applied to measure the inlet boundary conditions in terms of mean and fluctuating velocity field statistics. Static pressure measurements were applied to measure the friction diagram in between 2000 < ReDh < 10,000. Double frame Particle Image Velocimetry was applied to measure velocity correlations between the stream-wise velocity component and the velocity component perpendicular to the wall and its connection to the friction diagram. Furthermore, Time Resolved Particle Image Velocimetry was applied to visualize the generation of coherent vortical structures originating at the inlet into the rectangular channel.