Title: Investigation of different liquid replenishment schemes on the performance of the ionic liquid compressor

Authors: Yi Jin; Yi Guo; Yuming Tang; Xueyuan Peng; Zhenjun Ma

Addresses: School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China ' School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China; State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China ' School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China ' School of Energy and Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China; State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, 710049, China ' Sustainable Buildings Research Centre, University of Wollongong, 2522, Australia

Abstract: In ionic liquid hydrogen compressors, the liquid provides functions of sealing, piston lubrication, and hydrogen cooling. However, some liquids are expelled with hydrogen during the discharge, leading to a gradual decrease in liquid volume. Liquid replenishment is therefore necessary to maintain an adequate liquid level for effective sealing and lubrication, while also enhancing cooling efficiency and assisting the compression process closer to isothermal conditions. In this study, a transient two-phase flow model was developed to evaluate the thermal performance of the compressor under three different liquid replenishment schemes. The volume of fluid method was employed to track the interface between hydrogen and ionic liquids. Meanwhile, the two-phase flow and heat transfer were simulated over a complete compression cycle. Results showed that replenishment improved compression performance, achieving a minimum polytropic index of 1.145 and a maximum isothermal efficiency of 86.06%. However, replenishment occupied part of the suction stroke and increased liquid discharge, reducing volumetric efficiency, with the lowest value of 69.23%. Among the three schemes, the cylinder-top replenishment method demonstrated an outstanding trade-off between isothermal efficiency (83.51%) and volumetric efficiency (89.92%), making it a promising strategy for practical hydrogen refuelling applications.

Keywords: hydrogen; liquid piston; two-phase flow; isothermal process; CFD.

DOI: 10.1504/IJHM.2026.152976

International Journal of Hydromechatronics, 2026 Vol.9 No.1, pp.28 - 56

Received: 23 Jun 2025
Accepted: 19 Aug 2025
Published online: 17 Apr 2026 *

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