Vortex formation at the end of the parallel-plate stack in the standing-wave ther-moacoustic device

This paper investigates vortex formation occurring at the end of a stack of parallel plates, due to an oscillating flow induced by an acoustic standing wave within an acoustic resonator. Particle Image Velocimetry (PIV) is used to quantify the vortex pattern variations within an acoustic cycle phase-by-phase, especially during the "ejection stage" when the fluid flows out of the stack. By changing the intensity of acoustic excitation and the stack configuration (plate thickness and plate spacing), vortex patterns will vary accordingly. The tested plate thicknesses were 0.5, 1, 2, 3, 4 and 5 mm, while the plate spacing varied from about 1.2 to 10.8 mm. Eight different vortex flow patterns were observed and classified, and these eight different vortex patterns showed nine types of transition processes in the "ejection stage". Five most typical vortex transition processes were studied in detail.