N. Matsuda et al., Investigation of low frequency fluctuations in flow field in agitated vessel from visualization and LDV measurement, KAG KOG RON, 27(2), 2001, pp. 259-264
The velocity in an agitated vessel with three kinds of impeller (paddle, pi
tched blade paddle, and Rushton turbine) was measured with two-dimensional
laser doppler velocimetry (LDV) and the flow was visualized in order to inv
estigate 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 signi
ficant velocity fluctuation in the lower frequency region, the frequency of
which, f(u), corresponds to 20-30 times the blade passage frequency. and i
s proportional to the rotational speed in the range 0.667-2.00 s(-1). The p
roportional coefficients between the frequency and the rotational speed are
specific to the impellers and depend on the type, the number trf blades, a
nd the set-up position of the impellers. There are significant velocity flu
ctuations 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 po
sition of the impellers. It is found from the flow visualization that the f
requency of the fluctuation, f(u), corresponds to that of the flow which cr
osses the impeller shaft, and the low frequency fluctuation in the flow cor
responds to that of the change in the large-scale flow pattern caused from
the blade passage.