Energy conversion loss mechanisms in a mixed-flow pump-as-turbine influenced by guide vane shedding vortices

Vertical mixed-flow PAT (VMFPAT) systems, as a cost-effective solution for renewable energy recovery in small- and medium-scale power generation, exhibit significant challenges to the stability and efficiency of energy conversion influenced by the guide vane shedding vortices (GSV) within the system. However, the underlying mechanisms linking the flow disturbances induced by GSV to the instability of energy conversion remain unclear. This study systematically investigated the spatiotemporal evolution of GSV, along with their influence on energy conversion loss mechanisms. The main findings are as follows: A mapping relationship between unsteady vortex shedding and energy fluctuations was established, revealing a strong negative correlation between vortex intensity and power output. Within the impeller region, the GSV is divided by the rotating blades into upstream (USV) and downstream (DSV) shedding vortices. Owing to the discrepancy between the migration velocity of the high-pressure quasi-steady region and the sliding velocity of the USV, two distinct energy attenuation modes are induced. By applying the synchrosqueezed wavelet transform (SWT) to compress the frequency content of total pressure fluctuations, the time–frequency resolution was enhanced, thereby capturing high-energy-density spectral fluctuations associated with these attenuation modes. Furthermore, a time-averaged background power field model was constructed as a baseline to account for the effects of vortex evolution disturbances, enabling the quantification of energy losses induced by GSV interference. The maximum power loss attributable to GSV reached up to 6.82 % of the positive power. These findings offer theoretical insights for vortex control and the enhancement of energy conversion stability in VMFPAT systems.