A study on intermittency phenomena in the impeller stream via digital particle image velocimetry (DPIV)

Abstract

To characterize intermittency phenomena in the impeller jet, digital particle image velocimetry (DPIV) was used to measure the near-instantaneous flow fields in baffled, stirred reactor, equipped with a Rushton turbine (RT) impeller of large impeller and reactor diameter ratio. From the transient velocity fields obtained by DPIV, characterized by a chaotic movement of the jet axis in a fan section in the plane of laser sheet, effective intermittency phenomena in the impeller stream were identified for lower rotational speed. To gain insight into the nature of the intermittency phenomena, the impeller discharge angle corresponding to the near-instantaneous velocity field was calculated and its time series were constructed and analyzed via frequency count and power spectral density function (PSD). The macro-instability (MI) behavior of the reactor was investigated and its presence in the bulk flow was verified via spectral analysis of time series of the spatially averaged vorticity for lower rotational speed (namely 30, 60 and 120 min −1). Coherent spectral analysis was introduced to determine the cause of the intermittency phenomenon in the impeller stream as well as the relationship between the intermittency phenomenon and MI in the bulk flow. Results show that intermittency in the impeller stream is linearly related to MI in the reactor. The influence of MI should not be neglected in study of impeller stream intermittency phenomena.