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Gerotor
pumps are widely used in the automotive industry
for fuel lift, engine oil and transmission systems.
Volumetric efficiency and cavitation damage, are
causes for concern in gerotor pumps with high output
flow. To optimize pump performance and reduce cavitation
damage, it is essential to understand the fluid dynamics
inside the pump.
In a gerotor pump (Fig. 1, below),
the  fluid
is sucked into the inlet port and shifted to the
outlet port. Due to the rotor clearances (Fig. 2),
flow leakage occurs between the high-pressure and
low-pressure sides of the pump. To limit pressure,
excess fluid is re-circulated to the inlet port through
a pressure relief valve. The flow through the rotor
clearances creates high fluid velocity and localized
low-pressure areas, which produce air and vapor bubbles
hence causing cavitation damage and noise.
CFD analysis can be used as a
cost-effective design tool for the optimization of
pump flow performance and reduction of fluid borne
noise [1]. In order to optimize the design of gerotor
pumps, a realistic CFD model is required which takes
into account gear meshing, leakage flow across clearances
and cavitation bubble formation, recompression and
collapse [2,3].
A full 3-D transient model with
moving and deforming boundaries has been developed
specifically for gerotor pumps. This model can predict
cavitation bubble formation, recompression and collapse,
by realistically modeling the dynamics of gear rotation,
meshing and sliding over the inlet and outlet ports,
and flow leakage through the rotor set clearances.
The grid generation and time dependent manipulation
has been carried out using PROSTAR, the pre-processor
of STAR-CD. The mesh motion/rotation and deformation
of the pumping chambers has been defined using a "script".
Arbitrary sliding interfaces have been used to connect
the rotating pumping volumes with the stationary
inlet and outlet ports.
 Extensive
sensitivity studies on grid density and distribution,
and time stepping have been carried out to meet the
accuracy criteria. To be able to incorporate design
improvements from the CFD analysis into the pump
design process, calculation time had to be kept to
within a few hours.
A CFD analysis of the flow inside
gerotor pumps has been conducted for the design optimization
of fuel lift pumps. Similar analyses could be conducted
for gerotor pumps with smaller or larger rotors,
and a wide range of rotor clearances, pump speeds,
and fluid viscosities. A preliminary validation of
the CFD calculations has been carried out in [2].
The CFD results of the average delivery flow rate
and its fluctuation (flow ripple) were compared with
the experimental measurements at the different speeds
and pressures. In general, the CFD results of the
delivery flow rate were in good agreement with the
experimental data and well within the experimental
scatter. The flow ripple is over-predicted due to
the assumption that the flow was incompressible and
because leakage was over-predicted. CFD results for
velocity vectors and pressure distributions are shown
in Figs 4 and 5 at different rotation angles in the
pumping cycle.
A CFD design optimization has
been carried out using a design of experiment (DOE)
method to maximize the pump volumetric efficiency,
and minimize cavitation damage and noise [3]. The
effect of porting geometry and rotor clearances has
been investigated. The DOE analysis of the CFD results
has shown that the most important parameters for
the flow performance are the rotor clearances and
the presence of the inlet grooves. The most important
variables for the flow fluctuations RMS are the tip-to-tip
clearance, the inlet minor sealing angle and the
outlet major sealing angle. Experimental verification
of the CFD optimal design has shown a reduction of
cavitation damage to an acceptable level. Further
development work will be required to include compressible
flow and cavitation in the model.
References
[1] Iudicello, F. and Baseley
S. CFD modeling of the flow control valve in a
hydraulic pump. PTMC 1999, ed. Burrows, C.R. and
Edge, K. A., University of Bath, 1999, pp.297-312.
[2] Iudicello, F. and Mitchell D. CFD modeling of the flow in a gerotor
pump. PTMC 2002, ed. Burrows, C.R. and Edge, K. A., University of Bath,
2002, pp.53-66.
[3] Iudicello, F. CFD modeling and design optimization of a gerotor pump.
Eighth European Congress on Fluid Machinery for the Oil, Gas & Petrochemical
Industry, The Hague, 31 Oct- 1 – 1 Nov 2002.
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Velocity
vectors at different
rotation angles in the pumping cycle
Pressure distributions at different
rotation angles in the pumping cycle
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