Flow instability and energy performance of a coastal axial-flow pump as turbine under the influence of upstream waves

In recent years, the utilization of the pumps as turbines (PAT) has been proven to be a feasible alternative for making good use of the flood and residual waters in regions with abundant rains. Nevertheless, in the areas along rivers and coasts, PAT flow instability caused by water waves, has presented itself as a considerable threat to its safe and efficient operations. Therefore, in order to investigate the effect of periodic waves on a coastal PAT system's evolutional flow state, the rms is defined to comprehensively study their pressure pulsation characteristics in three cases, while the entropy production method is used to analyze the energy loss characteristic. Herein, Case 1 is no wave condition, while Case 2 and 3 are waves with large and small periods, respectively. Both numerical and experimental results in terms of the machine's external performance characteristics were in good agreement. The periodic waves produced in upstream reservoir changed the pressure pulsation pattern in flow pipes, thereby enhancing the fluctuation of PAT external performance characteristics. In particular, the average growth rate of rms value in the PAT section for Case 2 (35.1%) is obviously larger than that in Case 3 (16.5%), due to the large wave height in Case 2. In addition, the effect of periodic waves in upstream reservoir decreases with the propagation distance. Hence, the growth rates of total entropy production (TEP) and rms values declined sharply along the flow direction. Besides, compared with the PAT efficiency pulsation amplitude under no-wave condition (2.6%), the fluctuation amplitudes in Case 2 and 3 are significantly increased by upstream waves, i.e., 9.3% and 8.6%, respectively. This study's findings can serve a useful reference for the achievement of safe and stable operations of PAT stations along rivers or coastal regions.