Effect of annular flow pulsation on flow and mixing characteristics of double concentric jets at low central jet Reynolds number

Abstract This study investigated the effects of annulus pulsation on the flow characteristics and mixing properties of double-concentric jets at a low central jet Reynolds number. The central jet Reynolds number was fixed to Rec = 313. Annulus pulsation was induced by employing a solenoid valve. Streak pictures of instantaneous smoke flow patterns and long-exposure images of jet flows were observed using the laser-light-sheet-assisted smoke flow visualization method. A binary edge-detection technique was used to compute the jet spread widths. Particle image velocimetry (PIV) was utilized to examine the time-averaged velocity vectors, streamline patterns, velocity properties, turbulence intensity distributions, and vorticity contours. A radial dispersion improvement index was defined to quantitatively estimate the mixing property using the tracer-gas concentration detection technique. When the annular flow was pulsated with high amplitudes, it merged toward the central axis owing to an increase in radial momentum. Stagnation points were intermittently witnessed on the central axis. Large jet spread width exhibited, especially at low annulus Reynolds numbers. Turbulence intensities increased drastically in the flow field by the axial elongation of the recirculation region due to the annulus pulsations. The axial elongation of the recirculation region and the intermittent stagnation points led to effective dispersion of the central jet into the annular flow. The radial dispersion improvement index revealed that the mixing capability between the jets could be enhanced up to 80% in the recirculation region by applying annulus pulsations.