Investigations on centrifugal pumps under air entrainment conditions

Centrifugal pumps are widely used in process industry and power engineering, e.g. in refineries as feeding pumps or in power plants in cooling circuits. In nuclear power plants they are operated in various places and particularly in safety-related functions, like emergency core cooling. Though simple in design centrifugal pumps offer advantages, like high efficiency and low energy consumption, smooth and steady operation and high reliability. Gas entrainment as well as internal steam generation by cavitation is known as being detrimental and critical to the operation of centrifugal pumps, since they were primarily designed for single phase operation. Gas entrainment may for instance occur in situations, where water is conveyed from a reservoir with a shallow liquid height. There hollow vortices may form as a consequence of low liquid level and pre-existing fluid swirling. Particularly, such a situation may be prevailing in nuclear power plants, e.g. when emergency cooling water is taken from a liquid reservoir, like the condensation chamber.
Presence of gas in pumps may lead to abrasion at impeller blades, strong vibrations with damaging of bearings and loss of cooling for shaft and bearings and early fatigue as a consequence. At least the presence of gas will lead to decreasing pump performance even to the point of abrupt collapse of flow rate. Up to now, the effects of air entrainment have been investigated exemplarily under various operating conditions [Caruso et al., 2013], [Kimura et al., 2008], but the characteristics of gas phase fraction accumulation inside centrifugal pumps, particularly in the impeller and nearby the shaft seal region, are insufficiently understood. The presented work contributes quantitative measurements, visualizations and analyses of gasliquid phase distributions to the fundamental understanding of the effects of gas entrainment in centrifugal pumps. Advanced tomographic measuring methods with high spatial and temporal resolution were applied to investigate the two-phase distribution in the impeller region of an industrial centrifugal pump and a miniature centrifugal pump.
The presented investigations are founded by the German Federal Ministry of Education and Research (BMBF) under the funding code 02NUK023.