Quantification of liquid mixing enhanced by alternatively pulsed injection in a confined jet configuration

In this paper, mixing between the fluid froma primary planar jet and two surrounding secondary planar jets which are pulsated out-of-phase is studied experimentally. Solenoid values are used to control the flow injection into the mixing channel with pulse-width modulation. The experiments are conducted using water at a range of pulsation frequency, two duty cycles (25 and 50%) and amean Reynolds number between 100 and 250. The flowrate ratio between the primary and secondary flowis kept as unity.Both particle-image velocimetry and planar laser-induced fluorescence techniques are used to visualise the flow patterns and to quantify the mixing degree in themixing channel. Thismixing enhancementmethod is shown to be effective with amixing degree as high as 0.9 achieved at amean Reynolds number of about 166.A combination of differentmixing mechanisms is found at play, including sequential segmentation, shearing and stretching, vortex entrainment and breakup. At a given Reynolds number, an optimal frequency exists which scales approximately with a Strouhal number (St = fh/U) of unity. Furthermore, at a given mean Reynolds number a lower duty cycle is found to produce a better mixing due to a resultant higher instantaneous Reynolds number in the jet flow. © 2011 The Visualization Society of Japan.