In conclusion, the blade passing frequency of the freezer fan and the 10th normal mode of the fan louver are extremely close to each other.
Using this modification, the 10th normal mode of the fan louver moved to 157.7 Hz in the frequency domain, and none of the submodes overlapped with the blade passing frequency. Thus, the interaction between the structure and the cavitywas reduced.
(3) The blade passing frequency
is the dominant frequency of the pressure fluctuations in the volute casing except the vicinity of the volute tongue for all operating cases, and the dominant one near the volute tongue is the blade passing frequency
at the design point and 0~0.5 times the blade passing frequency
at other off-design points.
In the center region, the fluid flow is quite free from constraint; the flow pattern agrees with the diameter mode theory, so the dominant frequency is 2 times the impeller blade passing frequency. Ming demonstrated that the natural frequency of zero nodal diameter of RCP impellers under wet mode is 552 Hz, which is very close to 550 Hz .
And Figure 13 indicates that the blade passing frequency is the dominant frequency for all operating conditions.
The other frequencies are the impeller blade passing frequency and harmonics.
(2) The diametrical modes theory and the calculation results demonstrated that the dominant frequency of the monitors that are close to the boundary is blade passing frequency, that is, 275 Hz, and for those monitors which are in the center region, the dominant frequency is 550 Hz.
This difference is important in the monitoring of RSI in large hydraulic turbomachinery because the lower harmonics of blade passing frequency
are affected by the bearing response.
The excitation frequencies of propeller fluctuating thrust are tonal at the blade passing frequency. Obviously, the critical speed corresponding to the 2nd natural frequency is above the rotational speed.
For longitudinal vibration existing in service, the steady response of thrust bearing induced by thrust load at blade passing frequency with respect to various spring constants of thrust bearing is depicted in Figure 13.
Concentrating at the blade passing frequency and considering the existing understanding of square functional relationship between propeller steady thrust and shafting rotating speed, the fluctuating thrust is still set as a fixed proportion 5% of steady thrust at different rotational speeds.
Then, the magnification of thrust load at the blade passing frequency is easy to understand.