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

In this paper, mixing between the fluid from a 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 a mean Reynolds number between 100 and 250. The flow rate ratio between the primary and secondary flow is 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 the mixing channel. This mixing enhancement method is shown to be effective with a mixing degree as high as 0.9 achieved at a mean Reynolds number of about 166. A combination of different mixing 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.