On the scale effect of energy conversion in large-scale bulb tubular pump: Characteristics, mechanisms and applications

Bulb tubular pump is a special pump type with considerable application potential in low-head energy engineering. The energy performance of a large-scale bulb tubular pump can only be estimated based on its hydraulic model according to the conventional similarity law. However, engineering experience shows that this approach may cause significant errors, but the reason is still unclear due to the lack of understanding the scale effect of its energy conversion. In this paper, the characteristics, mechanisms and applications of the scale effect of energy conversion in large-scale bulb tubular pump are comprehensively studied. Progressive characteristics of energy conversion are quantified and a special scale effect is found. Hydraulic efficiency and head exhibit a feature of first increasing and then decreasing as the geometric scale increases, and the critical scale is 16. The competition between viscous effect and gravity effect is the origin of this special scale effect. When the bulb tubular pump exceeds a certain size, gravity-induced pressure polarization oscillation and partial flow separation are rapidly highlighted, destroying the hydraulic consistency and leading to a surge in energy loss and hydraulic instability. According to these findings, an improved similarity conversion mode is proposed for the bulb tubular pump, and the hydraulic efficiency drift with the geometric scale is determined by combining the dynamic weights of gravity effect and viscous effect. More importantly, this indicates the existence of a theoretical limit of pump size to balance the economy and safety, which should be paid attention to in energy engineering.