Fluid macro-instability is a large scale low frequency transient quasi-periodic phenomenon existing in stirred vessel

and affects fluid energy and mass transfer behavior. To reveal the nature of unsteady flow in a stirred vessel with double rigid-flexible impeller

tap water was used as working fluid

and wavelet analysis and flow field visualization were adopted to analyze the influence of double rigid-flexible impeller on the frequency of macro-instability. The difference between double rigid and double rigid-flexible impellers was analyzed in terms of mixing performance. The frequency of macro-instability increased linearly with agitation speed. At agitation speed 180 r·min-1

macro-instability frequency values of rigid-flexible impeller were 0.5096 Hz and 0.3459 Hz with off-bottom clearance 0.33T and 0.5T

respectively. Compared with double rigid impeller system

mixing time was shortened by about 22.5% and 35%

respectively. But macro-instability with off-bottom clearance 0.25T disappeared and bands phenomenon appeared. Shortening the off-bottom clearance could reduce the regular area. Double rigid-flexible combination impeller regulated fluid macro-instability and strengthened energy transfer behavior

so as to shorten mixing time and improve mixing efficiency of the fluid.