Time Resolved Concentration Measurements in an Axial Flow Mixer

Experimental results are reported providing information on the downstream mixing evolution in axial pipe flow mixers where a scalar is introduced into the pipe via a coaxial injection tube. 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 scalar concentration statistics are mainly evident in the first several outer pipe diameters downstream. In the far field, velocity ratio effects are shown to be insignificant on the concentration statistics. All cases showed a similar trend of an initial increase in variance at the centerline as the injected fluid begins mixing with the main pipe flow. This is followed by a region of rapid “exponential-like” decay, followed by a much slower decay rate after approximately 50 pipe diameters. Space-time correlations of the scalar concentration between far field locations verify the low wavenumber motions as predicted by the recent theory of Kerstein and McMurtry [A. Kerstein and P. McMurtry, “Low-wave-number statistics of randomly advected passive scalars,” Phys. Rev. E 50, 2057 (1994)], and are consistent with the slower than exponential downstream mixing rate.