# Numerical Simulation Study on the Volumetric Efficiency Loss of CO2 Swash Plate Axial Piston Pumps
## 1 Introduction  
CO2 emissions lead to global climate change. Carbon Capture, Utilization, and Storage (CCUS) technology is the only technological option to achieve low carbon impacts in the oilfield drive sequestration process by burying CO2 in the ground. This reduces the emission of greenhouse gases and, at the same time, increases the oilfield's recovery rate [1, 2, 3]. Swashplate axial piston pumps, by virtue of their structural advantages in the field of high-pressure fluid transmission, are widely used in systems that require high pressures and high flow rates. The performance of the piston pump determines the stability of the whole hydraulic system; therefore, research on the efficiency of the piston pump is crucial. Many international scholars have conducted research on the performance of piston pumps and three friction pairs (the port pair, piston pair, and slipper pair). [PERSON] [4] analyzed test data from nine groups of working conditions representing the actual problem of low efficiency of a dense-phase CO2 pump in order to examine the pump's performance. Their results showed that medium compression accounted for a large proportion of the pump's volume loss. However, the model was of a reciprocating pump, not an axial piston pump. [PERSON] et al. [5] studied the flow characteristics of a double-row axial piston pump anddetermined the influence of structural parameters on the flow. Their results showed that the flow pulsation of the piston pump was small when there were five pistons inside and outside. The check valve, oil suction valve, and dead zone volume of the plunger cavity were the main factors affecting the peak flow of the plunger pump. [PERSON] et al. [6] pointed out that friction pair clearance is the key factor affecting the efficiency of a high-pressure, large-displacement piston pump. They established a thermodynamic model of the piston pump friction pair and a piston pump test system. Their results showed that the model could accurately predict the piston pair temperature. [PERSON] et al. [7,8] theoretically analyzed the leakage loss and friction loss of three friction pairs in a piston pump and obtained a new performance formula. Through experimental verification, the accuracy of the model was proved. Their results showed that when the operating speed was 300-2700 rpm and the pressure was 2-30 MPa, the accuracy of the model was higher. [PERSON] et al. [9,10] introduced a mathematical model of the leakage and pressure distribution for each friction pair in a piston pump. When the model was combined with CFD and experiments, the results showed that the model could predict the output flow and pressure pulsation of the piston pump. Prior performance research on the port pair includes that by ye [PERSON] et al. [11], who used a multi-objective genetic algorithm to optimize the sealing performance of a port pair. Their results showed that the lubrication performance of the optimized liquid film was improved. [PERSON] et al. [12] used a multi-objective genetic algorithm to optimize the structure of the cylindrical damping groove, in view of the influence the valve plate damping groove's structure has on the output flow of the piston pump. [PERSON] et al. [13] analyzed the influence of the cylinder tilt angle, clearance, working speed, and other parameters on the pressure distribution, leakage, and torque of the secondary liquid film of the port. [PERSON] et al. [14] solved the Reynolds equation for the liquid film of the port pair and compared and analyzed the lubrication characteristics and pressure distribution characteristics of the wedge-shaped liquid film. [PERSON] et al. [15] pointed out that the liquid film temperature of the piston pump port pair affects the lubrication and friction characteristics, and they established an experimental platform to test the influence of different swashplate inclinations and working pressures on the liquid film temperature.