Investigation on energy conversion instability of pump mode in hydro-pneumatic energy storage system

The pump mode of hydro-pneumatic energy storage (HPES) system often experiences off-design conditions due to the boundary pressure rises, and the resultant energy conversion instability has an adverse effect on the system operation. However, the evolutionary process of this instability and the corresponding flow events are still not fully understood. Experimental and numerical simulation studies of a centrifugal pump with vaned diffuser were conducted in a wide operating range, and the following notable results were obtained. The energy conversion of pump mode shows three typical stages as the pump head increases, stable work→stumbled work→shaky work, corresponding to the inner flow pattern development of stall-free→stall inception→stall deepening. In this evolutionary process, the attenuation of rotor blade loading gradually spreads from the inlet shroud side to the entire blade. This strengthens the natural adverse potential rothalpy gradient and induces the energetic vortical structures including leading backflow vortices, dynamic stall cells and trailing helical vortices. A strong coupling between large-scale vortical motions and energy conversion is observed and this yields a Logistic growth of the shaft power and pump head fluctuations. The critical condition for the energy conversion instability is the diagnostic function SI = 1, and this is recommended for determining the stability limit of the pump mode.