Investigation of low frequency fluctuations in flow field in agitated vessel from visualization and LDV measurement

The velocity in an agitated vessel with three kinds of impeller (paddle, pitched blade paddle, and Rushton turbine) was measured with two-dimensional laser doppler velocimetry (LDV) and the flow was visualized in order to investigate the relation between the low frequency fluctuation in the velocity and the flow in the vessel. The velocity time series obtained was analyzed with statistical stochastics and with the power spectrum. There is a significant velocity fluctuation in the lower frequency region, the frequency of which, f(u), corresponds to 20-30 times the blade passage frequency. and is proportional to the rotational speed in the range 0.667-2.00 s(-1). The proportional coefficients between the frequency and the rotational speed are specific to the impellers and depend on the type, the number trf blades, and the set-up position of the impellers. There are significant velocity fluctuations in the very low frequencies region, the frequencies of which, f(L) and f(S), correspond to 100-300 times the blade passage frequency and are proportional to the rotational speed. The proportional coefficients depend on the type and the number of blades, and depend slightly on the set-up position of the impellers. It is found from the flow visualization that the frequency of the fluctuation, f(u), corresponds to that of the flow which crosses the impeller shaft, and the low frequency fluctuation in the flow corresponds to that of the change in the large-scale flow pattern caused from the blade passage.