Safe operation of kinetic pumps, as liquid movers, can be threatened by cavitation phenomenon in, amongst others. Cavitation is the Achilles' heel of kinetic pumps. It can cause deterioration of the hydraulic performance, damage of the pump by pitting and material erosion, and structure vibration and noise. Cavitation can appear within the entire range of operating conditions, therefore it must by all means be prevented. To prevent cavitation in a pump we have to know the beginning and development of the cavitation in the pump. For this purpose, the emitted noise in the audible range can be used, amongst other possibilities. Experiments have shown that there is a discrete frequency tone within the audible noise spectra, which is in strong correlation with development of the cavitation process in the pump. Therefore, the discrete frequency tone can be separated from the noise spectra of a cavitating pump and used to detect the incipient of cavitation and its development as well as to prevent the onset of the cavitation process in the pump, by means of initiating an alarm, shutdown, or control signal via an electrical control system.
COBISS.SI-ID: 10815771
Measurement of noise and vibration signal in audible frequency range to detect cavitation in centrifugal pumps is rather unknown technique. There were already some studies performed on this technique and they showed quite good results. Due to many factors that influence the quality of the measurement, an uncertainty analysis should be performed. This paper deals with estimation of a measurement uncertainty for different kinds of measurement ways to detect the cavitation in a centrifugal pump with noise and vibration signal in audible frequency range from 20 Hz to 20 kHz. Especially the measurement uncertainties for cavitation detection in broad frequency range and at a discrete frequency were analyzed. Results showed that this technique is reliable despite many possible influences on uncertainty.
COBISS.SI-ID: 11830043
The main noise source in heating, ventilation, and air conditioning systems is usually a ventilating fan. Noise, generated by the ventilating fan is transmitted through the duct into the living and working environment. A typical fan noise spectrum consists of a broadband noise, which is superimposed with pure tones. Different methods are available to reduce a transmission of such noise from the ventilating fan into the living and working environment. In this article it is demonstrated how a feedforward active noise control system can be implemented together with a side branch resonator. Effectiveness of the feedforward active noise control system depends on the quality of a reference signal, which should be in a perfect correlation with the primary noise. An acoustic feedback is the main problem of feedforward active noise control systems in ducts. A combined method uses a single loudspeaker to work as a dipole source and a side branch resonator to reduce the acoustic feedback. A side branch resonator reduces noise transmission in a narrowband frequency range as well. In this article, a theoretical background of a dipole source with a side branch resonator is presented, along with some measurement results and simulations of active noise control.
COBISS.SI-ID: 11839771
Noise, generated by a centrifugal blower, can be divided according to its origin, into aerodynamically induced noise and vibration induced noise. The contribution of the individual noise source to the total emitted noise is hard to determine, but it is crucial for the design of noise reduction measures. In order to reduce the noise of the centrifugal blower in a broad range of operating conditions, an identification of noise sources needs to be performed. An analysis of the most important noise origin in a centrifugal blower presented in this article was performed by measurements of the transfer function between noise and vibration, under different types of excitation. From the analyses one can conclude that the dominant noise source of a centrifugal blower can be attributed to the aerodynamically generated noise which exceeds the vibration induced noise for more than 10 dB in a broad frequency range.
COBISS.SI-ID: 11736347
Cavitation within a pump causes structural vibration with noise, among other by-products. Experiments have shown that there is a discrete frequency or broadband peak within the audible noise spectra, which is in strong correlation with the development of the cavitation process in the pump. Furthermore, the peak of the discrete frequency, or broadband peak, coincides with the net positive suction head (NPSH) critical value, which corresponds to a 3% drop in the total delivery head. Therefore, the discrete frequency tone can be used to detect the incipience of cavitation and its development as well as to determine the NPSH required or critical value with in situ operation of a pump. In this study, we wanted to clarify the mechanism of noise generation, which is responsible for the discrete frequency component and on which cavitation has an important effect. For this purpose, three different measurement methods were used: the first is based on measurement of the sound pressure level (SPL) in the surrounding air, the second is based on measurement of the underwater acoustics and the third is based on measurement of the structural vibration. Experiments have shown that the characteristic discrete frequency tone, which is in close correlation with the cavitation process, is a result of structural vibrations (modes) or resonances caused by implosion of bubbles and bombardment of the inner surfaces of the pump.
COBISS.SI-ID: 10947099