Experimental and Numerical Investigations on the Cavitation Phenomenon in a Centrifugal Pump

Centrifugal pumps play an important role in engineering applications since they are commonly used in industrial and residential systems, covering wide range of flow rates. Improving the performance of turbomachines such as the centrifugal pumps can be difficult to achieve, since the flow is turbulent with unsteady behaviour and cavitation. Cavitation is a complex phenomenon that is commonly considered as one of the main causes of deterioration in pump performance. Diagnosing cavitation and detecting its level of severity are essential for maintaining the pump’s reliability. Continuous condition monitoring of the pump is important to increase its operational life, decrease maintenance costs and hence, enhance the reliability of the pump. Early detection of cavitation can also improve the pump’s life expectancy by adopting various preventative actions. In this research, the first technique used for detecting cavitation is Computational Fluid Dynamics because it can provide suitable visualisation and reasonably accurate information, regarding the behaviour of fluid flow in the pump. In this work, both qualitative and quantitative analyses were carried out through a wide range of operating conditions and different geometrical configurations of a centrifugal pump under single-phase and cavitation conditions. Both, global and local flow field characteristics were investigated for better understanding. For qualitatively analysis, contours of static pressure and velocity magnitude under single-phase conditions and vapour volume fractions contour under cavitation conditions were adopted. On the other hand, the head and pressure variation in both time and frequency domains were analysed for qualitative analysis. The results showed that, as the pump rotational speed, number of impeller blades, and the outlet impeller diameter increase the head of the pump increases as well as the occurrence of cavitation. Based on the extensive numerical investigations for variety of operational and geometrical parameters, novel semi-empirical correlations under single-phase and cavitation conditions for the pump head and power coefficients were developed. Developments of aforementioned relations were carried out using multiple regression analysis technique. The second and third research areas consist of an extensive experimental analysis on the effects of operating conditions on the pump performance to predict cavitation using vibration and acoustic signature analyses. Detailed experimental investigations were carried out for the detection and diagnosis of cavitation, with the aid of sophisticated equipment and sensors. The condition monitoring was experimentally carried out in both, time and frequency domains analyses. Time domain method was applied to analyse the vibration and acoustic signals in time waveform analysis (TWFA). These signatures were analysed using different statistical parameters such as peak, root mean square (RMS), peak-to-peak and variance. In addition, transforming and analysing these signals in frequency domain was made by using Fast Fourier Transform technique. Analyses of these signals in frequency domain were also carried out using different statistical parameters such as mean and RMS features under wide various frequency ranges. The results revealed that using cavitation detection index (CDI) was a powerful technique, which can be used in both time and frequency domains for detecting cavitation and comparing the sensitivity of the vibration and acoustic techniques in estimating earlier stage of cavitation. Moreover, vibration technique was more sensitive to detect different levels of cavitation, especially inception of cavitation as compared to acoustic technique. This research has also found that the range of frequency between 0Hz to 15kHz was more sensitive to detect cavitation in the pump at the early stages. However, further investigation indicated that a frequency range of 1Hz to 2kHz was also effective on predicting the cavitation. Based on these findings, it can be suggested to use low range of frequency sensors (accelerometer and microphone) to capture the cavitation phenomenon instead of higher range of frequency, which are more expensive.

In addition, it was found that all three techniques adopted in this investigation such as; CFD, vibration and acoustic techniques are well capable to analyse cavitation behaviours under different operating conditions. Moreover, it was observed that the numerical results and vibration technique can detect the inception of cavitation within a pump earlier than the acoustic technique. The results also revealed that, the combined use of these techniques (numerical and experimental) could increase the reliability. The combined method can be a considered to be a robust method, which can provide detailed information about the performance of the pump and detection/diagnosis of cavitation within a centrifugal pump. Hence, this will assist in prolonging the life of the pump and protect the system from emergency shutdown.