Thrust
Area 3
Noise
and vibration, leakage, contamination and human factors Project
3C
CFD simulation of
cavitating flows - 3D
URANS Multiphase CFD Model of
Axial Piston Pump Project Lead
Dr.
S.H Frankel
Professor
School
of Mechanical Engineering
Purdue
University, West Lafayette
Pump
Specification Displacement 75 cc
RPM 3000
Pressure Drop 200 Bar
Swash Plate Angle 17 Deg
Achievements
1)
Simulated and Validated 3d
Flow and cavitation behavior in piston pump at above boundary
conditions
2) Simulation provides the insight into pressurizations physics , flow
ripple and cavition behavior in pump
3) Implicit scheme with Higher time step size is used which lowers the
turn around time for one rotation (approx 1 Day per rotation on single
CPU 2GB RAM machine)
CFD model Details
Hydraulic
Axial piston pump is a device
which displaces hydraulic oil from low pressure side to high pressure
side. CFD
model consist of End case, valve
plate and Cylinder block cavities. The cylinder block cavity consist of
9 pistons
with fixed
and deforming
control volumes.Cylinder block cavity and pistons rotates about
the axis of piston. Swash plate causes piston
to reciprocate resulting in hydraulic oil displaced to high pressure
side.
Hence control
volume in
the cylinder block cavity
continuously changes with time which requires deforming Grid to
perform this simulation. The animation shows the dynamic grid motion
Pump
Pressure
Animation At Top Dead
center (TDC), Pressure increases suddenly
from a lower value (20 Bar) to higher value (238 Bar) as the cylinder
block rotates from low
pressure side (End of suction) to high pressure side (start of
delivery) at pre compression grooves in
valve plate . On the other hand at Bottom Dead
center (BDC), pressure drops suddenly from
high pressure (238 Bar) to suction pressure (20 Bar) when the cylinder
block goes from high pressure side (end of delivery) to low pressure
side (start of suction) at the
relief groove in valve plate . This is because of the reverse flow from
high pressure side to low pressure side during cylinder block
transition at TDC/BDC
Pressure
Low pressure side In
low pressure side as the piston moves away from the valve plate as the
cylinder rotates towards TDC, a cavity is formed in the cylinder
block, resulting in flow of hydraulic oil from inlet port to the
cylinder block. This expansion of the cylinder creates continuous
suction. When the cylinder is in transition
state between two adjacent valve plate ports, the suction is greatly
enhanced due to reduction in flow area. The Second and third ports are
bigger than the first and last ports. This means that more hydraulic
oil will flow into the cylinder when the cylinder is in contact with
these ports. Moreover pressure in the end case is slightly
effected by the reverse flow of hydraulic oil during cylinder
transition at BDC Pressure
High pressure side In
high
pressure side as the piston moves towards the valve plate as the
cylinder rotates towards BDC, it does work against the system
resistance (load of 220 Bar) which increases the pressure at the piston
wall resulting in flow of hydraulic oil from displacement chamber
to outlet port. This compression caused by piston translation is
greatly enhanced when cylinder block is in transition state
between two adjacent valve plate ports due to reduction in flow area.
The
Second and third port are bigger than the first and last port. This
means that more hydraulic oil will flow out of the cylinder when the
cylinder is in
contact with these ports Cylinder block filling In low
pressure side as the piston moves away from the valve plate, hydraulic
oil flows from inlet port to fill displacement chamber cavities. This
filling process is three dimensional which can be seen from animation
due to the presence of a large eddy inside the cavity. The flow is
aligned to bottom of the cylinder cavity at the start of suction and
aligns itself more in the radial direction because of centrifugal force
due to rotation of cylinder block (see Cylinder block at 1/2 piston
stroke). Also more hydraulic oil is drawn into the cylinder block
cavity through second and third ports than from first and last
ports because of more contact area
Cylinder block discharge In low
pressure side as the piston moves towards the valve plate, hydraulic
oil flows out of displacement chamber cavities to outlet port. In
animation, the flow looks uniform and is aligned in the radial
direction because of rotation of cylinder block cavity (see Cylinder
block at Maximum stroke). Also
more hydraulic oil goes out of the cylinder block cavity through
second and third ports than from first and last ports because of
more
contact area
Cylinder block at
Maximum Stroke In low
pressure side at the start of suction, due to rotation of cylinder
block about pump axis, flow is aligned to the bottom of
the wall and in the outward radial direction . In high pressure side,
the animation shows uniform flow with slight alignment of the flow in
the radial direction. The same can be seen from the below animation Cylinder block at
1/2 piston stroke
Reverse flow from Valve plate to
Cylinder block during transition (TDC and BDC) Hydraulic oil is predominantly
incompressible and pressure disturbances
travel at very high speed in incompressible
fluids.The sudden increase and decrease
of Pressure at TDC and BDC in the cylinder block is
because of the reverse flow of oil from high pressure side to low
pressure side. When cylinder block is at TDC, reverse flow occurs
from first port on
the valve plate from TDC in high pressure side to cylinder block cavity
at TDC. On
the other hand when the cylinder block is at BDC , the cylinder block
still
contains high pressure fluid in the dead volume which again leads
to
reverse flow from first port on the valve plate from BDC in high
pressure side through the relief groove in valve plate to low
pressure in the case. This reverse flow leads to a high
velocity jet (Mach Number 0.6) which creates very low
pressure region and regions of cavitation inception. The velocity
vectors can be seen from the below animation Reverse
flow from Valve plate to Cylinder block during transition (TDC and BDC)
- Velocity
vector
Cavitaion in
Valve plate Cavitation in
valve plate is caused by the reverse flow of jet moving at very high
velocity (0.6 Mach) as seen in the animation. This jet reduces
the absolute pressure at pre compression amd relief groovs locations below the vapour
pressure of hydraulic oil leading to inception of
cavitation. The vapour bubble formed
moves from high pressure side to cylinder block cavity at TDC / BDC and
to the end case in the low pressure side and dissipates very
quickly
click here for results
Contact
Information
Dheeraj
Saxena
Graduate
Student
Maurice
J. Zucrow Labs.,
1003 Chaffee Hall, Purdue University
West Lafayette, IN 47907
Phone: (765) 494 0075
Fax: (765) 494 0530
Email: dsaxena@ecn.purdue.edu
