In mechanical engineering there are different phenomena, which have to be tested and monitored to control undesirable effects. Where these phenomena are connected with the emitted noise like in power and process engineering, and working and machinetools, audible sound is very useful tool for non-destructive testing and monitoring. In this paper, our findings connected with transient phenomena, will be presented. The use of audible sound was demonstrated: - in detection of rotating stall and surge at axial and centrifugal turbo machines (fans, compressors and pumps), - in detection of the cavitation in centrifugal pumps and for determination of the net positive suction head (NPSH) critical value, and - in quality control of the gas metal arc welding (GMAW) process. In this paper, all these phenomena are also theoretical demonstrated. In comparison with other methods, the method based on the use of audible sound is cheap, easy and userfriendly. Instead of a special test stand or boring a hole in the machine wall to mount a sensor, only a microphone placed nearby and a computer with a sound card are needed.
B.04 Guest lecture
COBISS.SI-ID: 10982171This work deals with the method for cavitation detection in centrifugal pumps, based on noise and vibration measurement in an audible range, from 20 Hz to 20 kHz. The research showed that this detection method is reliable and efficient. Cavitation phenomenon in the pump causes great increase of noise and vibration level in wide frequency range. At some particular discrete frequencies even higher levels can be achieved. Results showed that this method can be successfully used for detection and monitoring of development cavitation in centrifugal pumps.
D.09 Tutoring for postgraduate students
COBISS.SI-ID: 11791899Cavitation in kinetic pumps can cause deterioration of the hydraulic performance, damage of the pump by pitting and material erosion, and structure vibration and noise. To prevent cavitation in a pump we have to know the beginning and development of the cavitation in the pump. For this purpose, the signal of noise in audible range can be used, among other possibilities. Our experiments have shown that there is a discrete frequency tone or narrowband frequency range within the audible noise spectra, which are in strong correlation with development of the cavitation process in the pump. We found that the amplitudes in the narrowband frequency range are increased by up to 15 dB(A) and more when cavitation is fully developed. Therefore, this characteristic discrete frequency tone or narrowband frequency range can be used to detect the incipient of cavitation and its development. When fully developed cavitation in the pump is detected its prevention has to be done. In order to prevent cavitation in a kinetic pump three new methods were presented and discussed: 1) by injection of the water in the suction nozzle from the pump discharge, 2) by injection of the water in the suction nozzle from a secondary pump or waterworks and 3) by supplying water in the suction nozzle from a suction tank through an additional pipe connected parallel to the main suction pipe. Experimental results have shown that the last two are effective and appropriate for prevention of cavitation in a kinetic pump. The methods are based on activation of a special valve working on the principle of pressure difference before and after the valve, usually when 3% drop of the total delivery head is achieved, i.e. when cavitation is fully developed. In this moment the valve opens the connection to the water supply causing increase of the static pressure in the suction nozzle.
B.04 Guest lecture
COBISS.SI-ID: 12453659Significant share of vacuum cleaner market belong to vacuum cleaners for wet household and professional aspiration. Most important part of a device is wet vacuum cleaner motor. Heavy operation conditions, high performance and long operation live; low mass and smaller dimensions are only few of criteria for new bypass motor development. Special focus is given to low emitted noise and pleasant sound for human ear.This paper present systematic approach to identification of noise sources on existing motor. Research emphasis is focused on understanding the mechanics of noise source generation. New ideas of geometry and dimension optimization are proposed. CFD analyses were focused to find optimal geometry and reduce number of experiments. The presented solutions are foundation for new improved bypass motor.
F.07 Improvements to an existing product
COBISS.SI-ID: 11572507Since firecracker explosions have the characteristics of an impulse noise, they can cause hearing loss, serious personal injuries, fire hazards, annoyance, and even death. Their peak sound pressure levels at a distance of some meters can exceed the limit value of 140 dB, at which hearing protection is necessary. This article deals with some noise aspects resulting from firecracker explosions, which were measured during New Year's Eve. Apart from principal factors influencing the acoustic power of such an explosion, some new statistical aspects are described. A special emphasis is given to the probability distribution function of peak sound pressure levels, originating from a great number of firecracker explosions. Generally, the probability distribution closely follows the Rayleigh distribution, but when the number of explosions in unit time is high enough, it tends to a Gaussian distribution. Such transition is accelerated when reflections are taken into account, since in this case the number of peaks between two sequential zero crossings of sound pressure increases.
F.01 Acquisition of new practical knowledge, information and skills
COBISS.SI-ID: 11151643