Flow dynamics and mass transfer in impinging circular jet at low Reynolds number. Comparison of convergent and orifice nozzles

Electrodiffusion and Particle Image Velocimetry (PIV) measurements were made in two impinging jets, the first exiting from a convergent nozzle and the second from a square-edged orifice nozzle having same exit diameter. The Reynolds number, based on nozzle diameter and exit bulk-velocity, was equal to 1360 in each flow. The wall-shear rate and mass transfer generated by orifice impinging jet on a flat plate are up to 42% and 18% respectively higher than in its counterpart convergent nozzle jet. The transfer feature on the impingement surface has a close relationship with near field flow dynamics, itself affected by flow conditions at the nozzle exit. The orifice jet flow generates larger in size, well-defined and vigorous primary Kelvin–Helmholtz (K–H) structures with comparison to the convergent nozzle jet. These differences were associated with differences in initial velocity profiles and in the resulting flow development. The vena contracta in the orifice jet generates a thinner shear layer and an increase of the exit mean velocity relative to exit bulk-velocity. Organized structures in the two flows are educed using DMD and POD applied to PIV measurements. The dominant frequency of each flow related to the K–H instability, captured using DMD analysis of PIV fields, is also obtained by studying energy spectra of electrodiffusion signals. A quantitative measure of the kinetic energy (KE) distribution in different POD modes reveals that for reconstruction of 80% of the total KE, 7 modes are required for the orifice impinging jet and 42 modes for the convergent impinging jet. This comparison confirms the larger degree of flow organization in orifice jet before impinging on the target wall and explains its performance in the resulting mass transfer on the wall surface.