22nd Australasian Fluid Mechanics Conference AFMC2020 Brisbane, Australia, 7-10 December 2020 https://doi.org/10.14264/44f591d CFD Analysis of a Pressure Compensator for Variable Displacement Pumps M. Rundo 1 and P. Fresia 1 1 Department of Energy Politecnico di Torino, Turin 10129, Italy Abstract In this paper a 3D CFD model of a three-port pilot valve for the displacement control of positive displacement pumps is presented. The model considers the spool radial clearance and the fillet radii of the metering edges. The valve was tested with imposed positions of the spool for measuring the modulated pressure and the control flow. The spool position was measured by means of a high accuracy contactless transducer. The model was used to determine the relationship between the valve discharge coefficients and the flow number. It was also found that cavitation occurs, leading to a reduction of the discharge coefficient. The developed 3D model can be used for tuning a 0D model of a pump displacement control. Keywords Hydraulic pump; displacement control; discharge coefficient. Introduction In positive displacement machines, such as piston or vane pumps, the flow rate can be continuously modified by means of a displacement control. In pumps with constant pressure compensators, a pilot valve modulates the pressure in a hydraulic actuator mechanically linked to the device for the displacement variation [1]. The system is closed loop controlled, where the linearized coefficients of the valve play the role of gains in the block diagram of the state variable model [2]. The valve design is crucial for both the steady-state and the dynamic performance of the control, in terms of accuracy, transient time and stability [3]. In a 0D model [4], the discharge coefficients of the valve are typically unknown parameters that must be tuned experimentally or by means of CFD simulations [5]. It was demonstrated that the values of such coefficients could discriminate between a condition of stability or instability [6]. Due to the small size of the valve, with flow rates in steady-state conditions lower than 1 L/min, an accurate measurement of the hydraulic characteristics is not an easy task, since the spool position must be determined with a very high precision. Moreover, even the manufacturing tolerances can affect the measurement. In this context, a reliable CFD model of the control valve represents the only solution for determining the discharge coefficients. In literature recent examples of 3D models for the simulation of fluid power direction control valves developed with reliable commercial codes are available. Among the others, Lisowski and Filo [7] determined the relationship between the discharge coefficient and the spool position, while Tamburrano et al. [8] analysed the leakages between the spool and the sleeve in a servovalve with ANSYS Fluent ® . The spin effect on the spool was simulated with PumpLinx ® by Frosina et al. [9]. In the present study a 3D CFD model developed in FloEFD ® is used for determining the discharge coefficients of a hydraulic valve for the displacement control of a piston pump. Both leakages and fillet radii of the spool are considered. Component description The hydraulic scheme of the displacement control used in the present study is shown in figure 1. The pump displacement depends on the equilibrium between the forces exerted by a spring and a hydraulic actuator. The configuration of maximum displacement is obtained when the actuator is connected to the atmospheric pressure and the only force acting on the swash plate is due to the spring. This condition occurs when the three-port valve is at rest and the port A is connected to port T. Figure 1. Hydraulic scheme of the displacement control. When the delivery pressure achieves the setting of the adjustable spring, the spool translates, and the port A is progressively connected to P; at the same time, the flow area A-T is gradually reduced. The consequence is the increment of the pressure in the actuator and the reduction of the pump displacement. After the transient, an equilibrium position of the spool is established, corresponding to a specific value of the pressure in the actuator and of the pump displacement. In figure 2 a section view of the pilot valve is reported: the lower side is connected to the pump casing, while in the upper side an additional control, not used in the current study, is housed. The flow areas are controlled by the lands 1 and 2 respectively for the connection P-A and A-T. The diameter of the holes and of the spool were measured with a resolution of 1 m, while the distance between the external faces of the lands with a precision of 0.01 mm. Ideally, the entire modulation of the flow areas occurs with a spool stroke just higher than 0.1 mm. Figure 2. Section view of the control in the position at rest. The delivery pressure acts always on the left side of the spool thanks to a radial and an axial hole, while the preload of the double spring can be adjusted by means of a screw.