Chemical Engineering Science, Vol.132, 32-45, 2015
Mass transfer and shear rate on a wall normal to an impinging circular jet
The electrodiffusion technique and the time-resolved tomographic PIV were used in impinging jet issued from a convergent conical nozzle at a Reynolds number of 2450 based on the nozzle diameter d and the jet exit velocity. The relative distance from the nozzle to the impinging wall was equal to h/d =2. The experimentally gained velocity fields provides information on the organization of coherent flow structures, which play a role in the wall shear rate and the mass transfer phenomenon at the impinged wall. Instantaneous wall shear rate and local mass transfer were measured at an impinging wall using platinum electrodes with a diameter of 0.5 mm flush mounted into a platinum disc electrode with a diameter of 49.5 mm. The small electrodes were used alternately for the measurement of wall shear rate and mass transfer. The proposed correction factor on the inertia of the concentration boundary layer, which is the ratio of the corrected fluctuations of wall shear rate to their primary values, along with the analysis of the signals history, provide valuable information on the state of the hydrodynamic boundary layer. The mean wall shear rate and local mass transfer exhibit their maximum values at the normalised radial positions r/d=0.6-0.7. There are the interactions of the primary Kelvin-Helmholtz (K-H) vortices with smaller structures issued from the breaking-down of these vortices, both rotating in the same direction. Counter-rotating secondary vortices form between the wall and primary K-H vortices at a radial distance about r/d = 1.5. They are present in a wide region (r/d similar to 1.5 to 2.5); around r/d = 1.5 they are very close to the wall leading to a clear periodicity of wall shear rate, and at r/d = 2.5 they are farther above the wall since no periodicity of the wall shear rate was observed. The local mass transfer rates were integrated and compared with the global mass transfer measured by a platinum disc electrode. (C) 2015 Elsevier Ltd. All rights reserved.