Abstract
Centrifugal pumps are widely used and are known to be sensitive to inlet air-water two-phase flow conditions. The pump performance degradation mainly depends on the changes in the two-phase flow behavior inside the pump. In the present paper, experimental overall pump performance tests were performed for two different rotational speeds and several inlet air void fractions (αi) up to pump shut-off condition. Visualizations were also performed on the flow patterns of a whole impeller passage and the volute tongue area to physically understand pump performance degradation. The results showed that liquid flow modification does not follow head modification as described by affinity laws, which are only valid for homogeneous bubbly flow regimes. Three-dimensional effects were more pronounced when inlet void fraction increased up to 3%. Bubbly flow with low mean velocities were observed close to the volute tongue for all αi, and returned back to the impeller blade passages. The starting point of pump break down was related to a strong inward reverse flow that occurred in the vicinity of the shroud gap between the impeller and volute tongue area.
Highlights
Centrifugal pumps are widely used in fluid transportation fields, such as energy, chemical, and aerospace, in which always exist gas-liquid two-phase flow phenomena
Bubble diameter increases when αi increases and the flow remains as a bubbly flow regime up to 3% inside the impeller passage
This has been already found by Shao et al [14], but the amount of bubbles was more important for the flow rate with a lesser bubble amount. This corresponds to a smaller reduction in the flow coefficient ratio that was been experimentally observed in Figure 8b compared with Figure 8a
Summary
Centrifugal pumps are widely used in fluid transportation fields, such as energy, chemical, and aerospace, in which always exist gas-liquid two-phase flow phenomena. The handling ability of centrifugal pumps for gas entraining is still confusing for pump designers, which is related to the two-phase flow behaviors inside the impeller and volute passage. Different flow visualization techniques have investigated flow patterns and confirmed that bubble sizes variations are strongly related to inlet void fraction values and rotational speed for a given pump impeller geometry. Most of the centrifugal pump models are based on Eulerian–Eulerian two-phase momentum equations and use several drag force models mainly based on bubble diameter Reynolds number This leads to inaccurate performance prediction at a high gas content. Overall pump performance behavior under different flow rates and controlled inlet air-water two-phase flow conditions was obtained experimentally to cross analyze with local visualization results on the one impeller passage and the volute tongue area. Physics of air-water two phase flow pattern variation were revealed and make contributions to understanding the handling ability of air void in a centrifugal pump
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