On near‐wall jets in a disc‐like gas vortex unit

Abstract

in Wiley Online Library (wileyonlinelibrary.com) To clarify the three-dimensional (3D) structure of near-wall jets observed in disc-like gas vortex units (GVUs), experimental and numerical studies are performed. The experimental results are obtained using stereoscopic particle image velocimetry (PIV), laser doppler anemometry, pressure probes and surface oil flow visualization techniques. The first three techniques have been used to investigate the bulk flow hydrodynamics of the vortex unit. Surface oil flow visualization is adopted to visualize streamlines near the end-walls of the vortex unit. The surface streamlines help to determine the azimuthal and radial velocity components of the radial near-wall jets. Simulations of the vortex unit using FLUENT V R v.14a are simultaneously performed, computationally resolving the near-wall jet regions in the axial direction. The simulation results together with the surface oil flow visualization establish the 3D structure of the near-wall jets in GVUs for the first time in literature. It is also conjectured that the near-wall jets develop due to the combined effect of bulk flow acceleration and swirl. The centrifugal force diminishes in the vicinity of the end-walls. The radially inward pressure gradient in these regions, no longer balanced by the centrifugal force, pushes gas radially inward thus developing the near-wall jets.

An experimental and numerical study of the suppression of jets, counterflow, and backflow in vortex units

“…A smaller slot angle also results in the presence of smaller (or no) jets (and consequently, smaller counterflow region) in the unit, which subsequently reduces or eliminates the counterflow region. A detailed computational analysis of this phenomenon has already been by provided by Niyogi et al Less inclined inlet slots give smaller intensity jets. The smaller the counterflow region, the faster it disappears.…”

“…Swirl decay due to the exhaust wall friction generates an adverse pressure gradient along the exhaust line resulting in an extended backflow. The latter is discussed in more detail by Niyogi et al…”

“…This research has led to demonstrating various remarkable flow features in these devices. In gas vortex units (GVUs), primary and secondary flow patterns like jets, recirculation, and backflow were observed . For gas–solid vortex units (GSVUs), minimum and maximum solids capacities have been predicted based on a dimensional analysis of various data from literature.…”

“…The primary flow feature is the formation of localized, near‐wall jets in the gas flow upon exiting the inlet slots . The jets are a result of the acceleration of radially converging gas flow approaching the central exhaust of the vortex unit.…”

On the mechanisms of secondary flows in a gas vortex unit

“…The axisymmetric assumption is considered valid, based on previous work, where it was shown that the use of multiple equidistant injection slots (36) uniformly distributes the gas in the GVU and makes the flow nearly axisymmetric even in the vicinity of the injection slots. 29 The no-slip boundary condition is imposed at the end-walls of the GVU. The Reynolds Averaged Navier-Stokes (RANS) turbulence modeling approach is adopted, as the main goal of the present work is to obtain ensemble-averaged GVU hydrodynamics.…”