Modeling of High Pressure Radial Piston Pumps

Agarwal, Pulkit. M.S.M.E., Purdue University, December 2014. Modeling of High Pressure Radial Piston Pumps. Major Professor: Andrea Vacca, School of Mechanical Engineering A comprehensive multi-domain simulation tool for investigating the operation of radial piston machines has been developed in the present study. The simulation tool is capable of analyzing the displacing action of the machine parts as well as the power losses occurring in the lubricating interfaces which makes it useful for supporting the design process of radial piston units. The reference machine analyzed in this study is a radial piston pump of rotating cam type design used for high pressure applications. Though the modeling process and calculations in this analysis pertain mostly to this specific design, the concepts involved and numerical procedure can be applied to generic designs of radial piston machines. A lumped parameter based model for complete hydraulic system of the pump has been formulated which can predict the main flow parameters in the pump namely flow rate and pressure at pump outlet. This model can be easily coupled with other hydraulic components present in a circuit to model the systems level performance of the machine. However, an improvement in pump design calls for a detailed investigation of internal components present in the pump specifically the lubricating interfaces present in the pump. The lumped parameter model is capable of generating boundary conditions to simulate the flow behavior in these lubricating interfaces. A separate model for piston-cylinder interface and cam-piston interface has been developed in this study to incorporate the detailed features involved in each of them. The piston/cylinder lubricating interface represents one of the most critical design elements of radial piston machines. The interface performs the functions of a hydrodynamic bearing by supporting the radial loads acting on the piston, seals the high pressure fluid in the displacement chamber and reduces friction between the moving parts. However, operating in the Elastohydrodyamic Lubrication (EHL) regime, it also represents one of the main sources of power loss due to viscous friction and leakage flow. An accurate prediction of instantaneous film thickness, pressure field, and load carrying ability is necessary to create more efficient interface designs. For this purpose, a fully coupled numerical solver has been developed to capture fluid-structure interaction phenomena in the lubricating interface at isothermal fluid conditions. This model considers the piston micro-motion during one complete cycle of pump operation. The radial loads acting on the piston have a significant influence on piston micro-motion and hence the power losses in piston-cylinder interface. These loads are caused majorly by the friction forces existing between the cam and piston. A more accurate evaluation of performance parameters in the piston-cylinder interface can be achieved by calculating the instantaneous friction acting between the cam and piston under lubricating conditions. Different approaches for evaluating this friction coefficient were considered ranging from a simplified assumptions of pure sliding, pure rolling to a detailed analysis of lubricant flow between the cam-piston surfaces. For this purpose, a line contact EHL model was developed that can predict viscous friction forces generated between the cam and piston at changing surface velocities and contact loads. Also, instantaneous pressure field and film thickness can be predicted to a reasonable accuracy. This model is capable of analyzing multiple configurations of cam-piston interface design. The numerical results presented in this thesis provide detailed information of the pump performance parameters at different operating conditions thereby confirming the utility of the simulation tool to support the design process of these units and assist in creation of more energy efficient pumps. Validation of the numerical model developed in this study with experimental results can be a part of future work