Effects of pressure ratio allocation on the flow characteristics in a five-stage ionic liquid compressor

Ionic liquid compressors are a new type of hydrogen compressor technology with significant advantages when applied to high-pressure hydrogen refueling stations, where multi-stage compression is an ideal solution. The utilization of a liquid piston introduces complex two-phase flow phenomena within the cylinders, and understanding the characteristics of two-phase flow is crucial for the thermodynamic performance of the compressor. For the multi-stage compressors, pressure ratio allocation has a significant influence on the structural design and energy conversion efficiency of the compressor, however, there is a lack of relevant research in the existing studies. In this paper, the compression cylinder structures of a five-stage ionic liquid compressor under three pressure ratio allocation strategies are designed, and then the two-phase flow characteristics inside the cylinder are numerically investigated. Based on the analysis of the two-phase flow pattern in the cylinder, the minimum initial liquid heights in each stage that ensure the solid piston is not exposed are determined, and the escape characteristics of ionic liquid and the energy efficiency of the ionic liquid compressor were then analyzed. Under the same total pressure ratio and flow rate, when the consumption cost of ionic liquid was considered as a secondary factor, the strategy of equal pressure ratio allocated for five stages was recommended as the compressor efficiency was higher with better heat transfer performance.