Numerical and experimental study on fluid compressibility in a two-stage reciprocating pump-compressor

Abstract

• Liquid piston velocity decreases with increasing pressure due to liquid compressibility. • Liquid piston brings the compression process closer to thermodynamically efficient. • Liquid expansion leads to a decrease of the pump volumetric efficiency. • Pressure ratio has the greatest impact on a decrease of liquid layer height and velocity. Currently, multistage (two and three-stage) reciprocating compressors are used to compress the gas to medium and high pressures. One of the promising operating principles of such machines is a preliminary compression in a single-stage piston compressor and a final gas compression in a working cavity with a liquid piston. The main advantages of a compressor using a liquid piston are intensive cooling of the compressed gas and the absence of leaks and leakages in the cylinder-piston group. This makes it possible to increase the indicated isothermal efficiency and actual volumetric efficiency. The article considered the effect of liquid compressibility in the two-stage piston hybrid positive-displacement machine with a liquid piston. We developed a calculus methodology for the liquid compressibility during gas compression in the second stage of the compression section based on the volume conservation equation and Hooke's law. Numerical simulation showed that the degree of pressure ratio had the greatest impact on the relative decrease in the height and velocity of the liquid layer. We found that the relative decrease in the liquid layer height was within 1%. The relative decrease in the liquid piston velocity was within 5%. We carried out the experiment and validation of the results with a numerical simulation, which reflected the mathematical model validity. The model worked correctly based on an estimated experimental error. The parametric analysis of the test model showed the influence of the main working cavity parameters. The discrepancy in determining the dynamic pressure in the machine cavities conducting theoretical and experimental studies is within 10–15%. The paper analysed the liquid piston compressibility influence on the thermodynamic efficiency of the compression process and compressor performance.