CREMHyG Analyzes Transient Flow in a Multi-Piston Pump Design

Author: Claude Rebattet, Head of CREMHyG laboratory, University of Grenoble Alpes, France

The Grenoble Hydraulic Machinery Research and Testing Centre (CREMHyG) is a Grenoble Institute of Technology (Grenoble INP) laboratory. The turbomachinery testing platform CREMHyG’s main areas of activity focus on hydroelectric energy and associated machines (pumps, turbines, and pump turbines). They test hydraulic applications, including hydropower, storage, liquid propulsion, pumping, etc.

CREMHyG Lab collaborates with industrial companies to develop the future of hydroelectric energy. The experimental platform is equipped with large facilities of up to 300 kW to perform contractual testing under the International Electrotechnical Commission (IEC) standard of reduced models of pumps, turbines, and reversible pump turbines and with smaller ones of up to 20 kW for research and training. The experiments and tests focus on steady and unsteady flows through rotating machinery: Francis turbines, reversible PSPs, axial inducers, centrifugal or piston pumps, etc. Research is targeted to improve the stability and security of the functional domain in critical conditions such as cavitation, multiphase flow, and off-design behaviors.

To compare simulation and test, a first demonstrator of a piston pump developed for drilling applications has been scaled ¼ to be powered by a 10 kW motor. We present the first investigation on the simulations performed with NUMECA’s Lattice Boltzmann solver OMNIS/LB.

“ CREMHyG decided to use NUMECA’s Lattice Boltzmann solver OMNIS/LB, as they needed a solution well adapted for complex geometries and complicated displacements.”

Design of a Swashplate Axial Piston Pump 

Figure 1. Components and meshed parts of the axial piston pump

One of the objectives is to design a swashplate axial multi-piston pump without check valves to distribute the flow to pistons. The pump consists of two parts: a stator, composed of two conducts communicated with an inlet and outlet to distribute the flow, and a rotor, moving nine pistons in a barrel. The electric and mechanical design permits using variable speeds and inclinations of the plate to optimize flexible discharge and starting and stopping operations without torque overloads or strong pressure peaks. Each piston interacts with the stator. The rotor-stator interface connects through a contact surface between the rotating barrel and static valve plate.

Simulation of a Swashplate Axial Piston Pump

The main objective of the fluid simulation is to analyze flow circulation and pressure transients to prevent mechanical solicitations linked to alternate displacement of pistons and rotor-stator interaction in the interface volume. The following locations have been analyzed: cylinders, interface stator-rotor (stator side), stator, outlet pipe, and inlet pipe. 

Benchmark on Flow Performance Analysis

In this benchmark, CREMHyG is seeking to analyze the flow performance through the axial multi-piston pump to collect data for design optimization toward better performance. In this way, the study of the overall behavior of the pump would enable an understanding of the design impact on the performance. To do so, CREMHyG has decided to use NUMECA’s Lattice Boltzmann solver OMNIS/LB, as they needed a solution well adapted for complex geometries and complicated displacements.

From a preliminary CAD design of the pump and a simulation model taking into account the displacement of all the pistons (rotation and translation), the CFD model has been used to analyze the unsteady flow through this complex geometry; to identify pressure fluctuations and distribution at different stations, and to estimate global performances concerning torque and power.

To understand the behavior of the piston pump, two different regimes and outlet pressures have been simulated under the same discharge flow rate:
» Case 1: Low outlet pressure and rotational speed, high stroke.
» Case 2: High outlet pressure and rotational speed, low stroke.

And two flow variables have been analyzed:
» Relative pressure: pressure difference between inlet and outlet.
» Mass flow rate.

Results

1. Relative pressure

The pressure cycle appears to be linear as no substantial differences in shape were found between both cases. The risk of cavitation is larger in Case 2, where the lowest pressure is found at the beginning of the intake stroke. Pressure oscillations are generated at the interface rotor-stator, where liquid regions are in contact at different pressures. Therefore, the main mode of these oscillations corresponds to the product of the rotational speed and the number of pistons. The amplitude also seems to correlate with the pressure difference between the inlet and outlet, so linearity holds. These pressure oscillations propagate throughout the pump, causing unsteady perturbations on the streamlines.

2. Mass flow

Mass flow rate contours show a highly complex flow in the stator close to the interface region. This is especially important at the cylinders and inlet and outlet passages, where an undesired backflow region is present and could be removed by improving the design. Cases 1 and 2 have been selected to attain the same mass flow rate. The linearity property again holds, but mass flow oscillations at the cylinders only occur in Case 2 (higher pressure and lower stroke) at the beginning of the intake stroke. The behavior for the rest of the cycle follows a smooth sinusoidal curve. The summation of the mass flow rate at all cylinders produces a fairly uniform flow from the inlet to the outlet. 

 3. Torque

The contribution of each piston to the torque during the intake stroke is negligible (red curve). Only during the exhaust stroke, the piston exerts a substantial force against the pressurized liquid. Maximum torque is achieved when the piston moves at its maximum speed, i.e., in the middle of the stroke. Since torque is directly related to the pressure field, the same comments apply while comparing both cases.

4. Power

Total hydraulic power has been obtained from the product of the torque and the rotational speed. Since Case 2 has a larger rotational speed, the power is also larger.

Conclusions

With the methodology and simulation tools proposed by NUMECA, CREMHyG has been able to compare performances at different pump operating conditions. The prediction of pressure and velocity in the flow through the pump given by OMNIS/LB offers excellent results while being very fast regarding time constraints. The CFD simulations presented here allowed us to validate the preliminary design and to better define the experimental setup (location of pressure taps).

Flow at the pump is very complex because of the sequential movement of the pistons. The rotor-stator interface, without ball valves, is specific to this type of pump and produces pressure waves generated when regions of very different pressure are connected instantly. The amplitude of pressure fluctuations seems to behave linearly with the pressure difference between the inlet and outlet. Consequently, the cavitation risk increases in the interface region at each piston passage, depending on the pressure gradient. Backflow regions have been identified at the inlet and outlet stators, and their intensity depends on flow conditions. Torque and power show that flow perturbations are globally not affecting power stability. Thanks to OMNIS/ LB, pump developers can get good indications to optimize their designs based on transient analyses, therefore being able to minimize operational risks.

"Thanks to OMNIS/LB, pump developers can get good indications to optimize their designs based on transient analyses, therefore being able to minimize operational risks.”