Design and Development of Gerotor Pump using CFD analysis
Modern day automotive research is focused on reducing cost of new vehicle without compromising the quality. So it is imperative that an automotive supplier makes use of advanced evaluation methods to develop new products.
Internal combustion engine is the heart of a vehicle and its performance depends on its subsystems. For higher engine efficiency , lubrication plays a vital role. Without proper lubrication, engine components may damage and lead to poor performance.
To pursuit consistent output from the engine, design engineers face many challenges in designing the subsystems. One of the challenges is to design proper lubrication system, especially the oil pump. In modern vehicle, the generated rotor (Gerotor) oil pump drives the lubrication system to next level in order to meet the latest trend of engine design.
The Gerotor pump is an internal gear type-rotary positive displacement pump. It takes power from engine crankshaft. Now a days it is widely used in automotives for oil and fuel supply because of its unique working capabilities, simplicity , flexibility in design and ease of manufacturing.
Significant amount of money and time are spent for developing such pump by means of experimental and analytical techniques. In order to minimize the duration for developing such a product, numerical techniques like CFD plays a crucial role, since it helps simulate the actual physics to study the results with less dependency on experimental test setups
Now let’s look into the approach used to automate the pump analysis using CFD, to imulate flow through Gerotor oil pump to attain the flow requirements of the engine.
Fluid Domain preparation strategy
As the first step of CFD analysis, raw 3D CAD model needs to be pre-processed. Fluid flow path from the CAD model has to be extracted with acceptable accuracy. This should be studied thoroughly and modified considering the constraints of meshing and solver activities. For example, to run CFD analysis of a Gerotor pump, narrow chamfers and corners in the model has to be simplified to improve the mesh quality without compromising with the flow physics.
Tetrahedral mesh are used for the ports (static zones) of the pump. Pure Hexahedral elements are used to generate structured mesh of rotor core (refer fig. 02, 03) for the fluid flowing gap between inner and outer gear teeth.
To simulate transient CFD analysis of the Gerotor pump, interfaces are formed to simulate sliding effect of the rotor inside the port cavity. UDF (user defined function program) is used to move the nodes of hexahedral elements of the rotor core to simulate the uneven rotation of inner gear and outer gear
For pre-processing, Ansys ICEM-CFD can be used for generating tetrahedral and hexahedral mesh.
Solution Method Solver Setting
Transient analysis is carried out in this case to capture time based variation in the performance parameters. Pressure based constant model is used since oil behaves like an incompressible fluid under observatory operating conditions. For simplicity, temperature of the oil is assumed constant.
To model turbulence, k-e model with standard wall function is used. PISO velocity coupling with PRESTO algorithm for pressure is used for quick convergence. To model cavitation and to evaluate suction time, multiphase model is preferred with liquid and vapour mixture fluid properties. To solve the case, Ansys FLUENT CFD tool was used.
Pertaining to boundary conditions, inlet and outlet were kept under atmospheric conditions with the speed of inner rotor in rpm taken into consideration.
Pump Performance Prediction
CFD analysis helps to validate critical performance parameters namely flow rate, pressure head and shaft torque required at different operating conditions.
To generate performance curve of the pump, power and efficiency parameters can be numerically calculated using observed values of torque required by inner rotor and flow rate of the pump, which can be further validated with experimental data.
CFD simulation can be used to study the performance of different pump designs, under various operating conditions. Such prediction helps the designers to pick the best performing model from the bunch of analyzed models even without making prototypes and going through tedious time and money consuming conventional procedures.
NPSH and Flow Rate Prediction
From the CFD results, NPSH (Net positive suction Head) can be predicted (refer fig. 06). Reliability of the results has been verified by comparing with the experimental data.
Leakage and Shaft Torque
Simulation of Gerotor pump helps predicting torque, cavitation, pressure acting on the seal and forces on the shaft. Pure hexahedral mesh shown in Figure. 02 is used to capture rotor core in a Gerotor pump to predict volumetric results like flow rate and prediction of leakage with better accuracy.
Under different operating conditions, considering different speeds and pressures, key performance parameters namely leakage and shaft power are also investigated. Tip-to-tip clearance and leakage paths (refer fig. 05) which will have impact on volumetric efficiency of the pump can also be studied.
In Gerotor pumps, flow rate is directly proportional to pump speed. But as speed increases, cavitation may arise. CFD simulation can be used to identify threshold pump speed at which cavitation starts to limit the loss in flow rate.
Suction Time Prediction
We can also use CFD simulation to find the time taken to suck the oil from the oil sump inlet to the delivery point outlet (refer fig. 09). The case is setup using multiphase model. Results with suction flow animation that can help engineer to develop optimized model.
- Maintaining the micron range of clearances between the rotating parts.
- Capturing the physics between rotors with collapsible mesh and solving using UDF’s (user define functions).
- Accurate modeling is required to capture the gaps in assembly in order to predict the accurate volumetric efficiency.
- To provide acceptable mesh, density is critical within the gaps.
Hence it is proposed to design, develop a Gerotor oil pump using CFD techniques. The CFD model is useful for concept considerations in an early stage of product development. The model can be easily integrated with the other associated hydraulic systems to the pump. The key performance parameter of Gerotor pump like mass flow rate and outlet pressure with respect to speed can be easily predicted with acceptable accuracy. From the above study, complex pumps like Gerotor can be developed as per Industrial Standards.