Exploration of transient flow perturbations in a bulb turbine diffuser using proper orthogonal decomposition.

This article focuses on the flow dynamics responsible for sharp efficiency drops observed in some low-head hydraulic turbines, specifically analyzing the BulbT bulb turbine model. Using time-resolved particle image velocimetry (TR-PIV) and pressure measurements combined with spatial proper orthogonal decomposition (POD), the study identifies two distinct types of transient flow separations occurring on opposite sides of the draft tube past the best efficiency point (BEP). One type involves an asymmetric acceleration of the core flow near the turbine’s axis linked to upper wall separations in the conical diffuser, while the other corresponds to lower wall separations near the trumpet section, each affecting pressure recovery differently. The research reveals a strong history effect where one type of separation tends to persist over multiple events, contributing to hysteresis in efficiency measurements, and highlights that upper separations increasingly dominate the efficiency drop at higher flow rates. These findings provide new insights into the complex transient flow behaviors underlying efficiency losses and offer guidance for future numerical modeling and turbine design improvements.