Decaying Annular Swirl Flow With Inlet Solid Body Rotation

Abstract

An experimental investigation of a low-speed turbulent swirling flow in a stationary, concentric, annular duct was made. The experiment involved isothermal air as the working fluid in an annulus with a diameter ratio di /d0 = 0.4, an average axial Reynolds number of 72,000, and an average axial velocity of 15 m/s. The swirl profile initially induced at the inlet was of the forced-vortex type. The rate of swirl, or the magnitude of the tangential velocity relative to the axial component, decayed axially from the inlet. Three different swirl rates were considered, one being straight flow. Extensive measurements were made of the velocity field with a cylindrical pressure probe at seven stations located 1.7 to 32.7 equivalent diameters from the entrance. The specific goals were experimental data on the axial decay of angular momentum and inferred values of the effective turbulent tangential viscosity. Results show a uniform axial decay of angular momentum and a profile shape independent of axial location. An empirical model using tangential eddy diffusivities that vary over the cross-section gave the best description of experimental data. The tangential profile shape and tangential viscosity distribution and magnitude did not depend on the initial rate of swirl.