The excitation force of the impeller includes contributions by its
blades, and changing the excitation frequency requires a change in
BPF. As noted above, it is impractical to change the motor frequency,
so changing the BPF essentially requires changing the
number of impeller blades. This study proposed a noise reduction
measure after comparing the results for using only 4 or 12 impeller
blades at both the top and bottom of the suction housing. Mesh
models for these designs are shown in Fig. 20 (compare to
Fig. 13); in the analyses using these models, the same field point
was used as in the previous analysis.Fig. 21 shows the FRFs
obtained at field point 25 for the 4- and 12-blade models. Comparison
of these two graphs shows that their size, frequency, and
phase information are similar. However, a closer look reveals that
the natural frequency has moved to a low frequency. With this
movement of the natural frequency, the natural frequency in the
analysis is increasing and the natural frequency becomes dense
especially at 2000 Hz and above. Fig. 22 shows that the portion,
where the impeller occupies in the vertical cross section of the suction
housing, has been removed with 360 rotation. It is thus
assumed that the impeller is innumerable. A look at the FRF illustrated
in Fig. 23 reveals that a number of the natural frequencies of
2000 Hz or higher are densely gathered. The actual suction housing
has many more blades, 52. Thus, the similar results as above are
expected. Thus, the similar results as above are
expected. Thus, reducing the number of impeller blades is proposed as a noise reduction measure.