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 laboratory of the Grenoble Institute of Technology (Grenoble INP). 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 such as hydropower, hydraulic storage, liquid propulsion, pumping and others.
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 and others. Research is targeted to improve stability and security of the functional domain in critical conditions such as cavitation, multiphase flow and off design behaviors.
In order 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.”|
FIGURE 1 : Components and meshed parts
Design of a swashplate 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 that are communicated with inlet and outlet to distribute the flow, and a rotor, moving 9 pistons in a barrel. The electric and ;mechanical design permits the use of 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 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 in order 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 in order to collect data for design optimization towards better performance. This way the study of the overall behavior of the pump would enable 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.
1. Relative pressure
The pressure cycle seems to be linear as no substantial differences in shape were found between both cases. Risk of cavitation however 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 rotorstator, where regions of liquid at different pressures are put in contact. The main mode of these oscillations corresponds therefore to the product of the rotational speed and the number of pistons. The amplitude also seems to correlate with the pressure difference between inlet and outlet so that linearity holds. These pressure oscillations propagate throughout the whole pump causing unsteady perturbations on the streamlines.
2. Mass flow
Mass flow rate contours show there is a highly complex flow overall, in the stator close to interface region. This is especially important at the cylinders, but also at the inlet and outlet passages, where an undesired backflow region is present and could be removed by improving the design. Case 1 and 2 have been selected so they attain the same mass flow rate. The property of linearity again holds, but oscillations of mass flow 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.
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 is moving at its maximum speed, i.e. at the middle of the stroke. Since torque is directly related to the pressure field, the same comments apply for the comparison of both cases.
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.
With the methodology and simulation tools proposed by NUMECA, CREMHyG has been able to compare performances at different operating conditions of the pump. The prediction of pressure and velocity in the flow through the pump given by OMNIS™/LB offers excellent quality of results while being very fast regarding time constraints. The CFD simulations presented here allowed to validate 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. 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. Amplitude of pressure fluctuations seems to behave linearly with the pressure difference between inlet and outlet. Consequently the cavitation risk increases in the interface region at each piston passage depending on the gradient of pressure. Backflow regions have been identified at inlet and outlet stator and its intensity depends of 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.”|