Effects of injection flow rate on the thermal performance of the ionic liquid hydrogen compressor

In ionic liquid hydrogen compressors, ionic liquids function as liquid pistons to ensure sealing, lubrication, and direct cooling of hydrogen. However, liquid loss during discharge reduced the in-chamber volume, impairing these functions. Therefore, liquid replenishment is essential during operation to maintain volume stability. Notably, the replenishment process is realised in the form of liquid injection, which also serves as an effective direct cooling method, further bringing the compression process closer to isothermal conditions. This study developed a numerical model of the complete working cycle including the liquid replenishment. The effects of the liquid injection on two-phase distribution and heat transfer were analysed. Multi-cycle simulations were conducted to investigate the influence of different injection velocities on discharge performance, temperature rise, pressure loss and compressor efficiency. Results showed that liquid replenishment significantly affected the two-phase flow characteristics. With an appropriate injection flow rate (0.57 kg/s), the in-chamber ionic liquid mass remained stable at approximately 74.7% of the initial value with the peak hydrogen temperature remaining nearly constant (345.3 K), where the compressor achieved the highest isothermal and volumetric efficiencies of 76.93% and 78.47%, respectively. It was also revealed that inadequate replenishment of 0 or 0.38 kg/s led to higher temperature rise and reduced isothermal efficiency at 62.58% and 71.34%, respectively, while excessive injection of 0.76 kg/s increased the clearance volume and decreased the volumetric efficiency to 72.57%.