Structure of the Rotor Tip Flow in a Highly Loaded Single-Stage Axial-Flow Pump Approaching Stall: Part II — Stall Inception — Understanding the Mechanism and Overcoming Its Negative Impacts

Abstract

When reaching the stall point of an axial-flow pump, the pump head characteristic becomes unstable and the pump head suddenly drops. Before this happens however, at even higher flow rates the NPSH3 and the pump body and shaft vibrations increase dramatically. For effectively increasing the available operating range, it is essential to find a solution for all three problems without reducing the pump efficiency at design. The paper describes an experimental investigation on the outlined subject that gives insight into the flow phenomena leading to stall. Based on this knowledge a very simple type of casing treatment was chosen and investigated. It was found to satisfy all mentioned requirements. Subject to the investigations is a highly loaded axial-flow pump having a nq of 150 (SI units). The overall pump performance was investigated measuring pump head, efficiency, NPSH3, and casing as well as shaft vibrations. Further-more, oil flow pictures taken at the pump casing and at the rotor blades, and video captures of the cavitating core of the tip clearance vortex were analyzed for understanding the flow phenomena leading to stall (see also related paper Part I, Schrapp et al. (2004)). From the video captures it was realized that the behavior of the tip clearance vortex which was found to perform so-called spiral-type vortex breakdown is triggering stall inception in this machine.

Suppression of Performance Curve Instability of an Axial‐Flow Pump by Using a Double‐Inlet‐Nozzle

“…Goltz et al [2][3][4][5][6] showed the flow structures of a single-stage axial-flow pump from stable operation until deep stall. After the onset of stall, a short transient takes place.…”

Düşey Milli Eksenel Bir Pompada Zamana Bağlı Akışın Deneysel Olarak İncelenmesi

“…Design changes at the impeller inlet can be applied to reduce the effect of inlet recirculation and improve the instability in axial pumps, successfully. Instability can be reduced by adding grooves to the inlet (Goltz et al [24][25]) or a double-entrance structure (Flores et al [26], Cao and Li [27]).…”

Experimental and numerical investigation on the effect of tip leakage vortex induced cavitating flow on pressure fluctuation in an axial flow pump

“…Different geometries of the blade tip were employed to analyze the influence on cavitation characteristics. Goltz [14,15] investigated the structure of the rotor tip flow at different operating conditions. The spiral-type vortex breakdown was captured, which led to a rapid expansion of the clearance vortex in stream-wise, span-wise, and pitch-wise direction.…”