Flow Phenomena and Time Resolved Concentration Measurements in an Axial Flow Mixer

Experiments were conducted to better understand the flow physics associated with axial flow mixers in pipes. Specifically, the dependence of the downstream mixing evolution on the velocity ratio of the secondary to primary stream was explored. Experiments were conducted in a 25.4 mm diameter water pipe flow loop (25,700 ≤ RD ≤ 28,500), in which a fluorescein dye was coaxially injected. The injection tube diameter was 1.5 mm. Three velocity ratios, VR = 0.5, 1.0 and 2.0 were explored, where VR = Vjet/Vmain. The present results indicate that the effects of velocity ratio on the mean concentration are primarily evident in the near-field flow downstream of the injector, while concentration variance measurements indicate a primary influence at intermediate axial locations. Analysis of higher order moments and flow visualizations suggest that these influences are associated with the injected flow conditions. Two-dimensional LIF analysis of the coherent jet breakup region showed an instability in this transition related to injector flow Reynolds number. The present concentration measurements do not indicate the exponential variance decay commonly used for modelling mixing in pipes. Far field data exhibit low wavenumber motions as predicted by the recent theory of Guilkey et al. (1997).