Variable orifice in an isothermal axial-piston machine
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The Valve Plate Orifice (IL) block models a crescent-shaped opening between moving pistons and a pump in an axial-piston machine. The rotating pistons periodically connect to the pump intake or discharge through the orifice plate. You can connect two valve plate blocks to each cylinder of an axial-piston pump to represent both the pump inlet and outlet slots.
A rotating cylinder, with one crescent slot, is connected to the pump intake at port A and pump discharge at port B. These points are connected to the plate between the Pressure carryover angle and radians (180 degrees) from each other. The plate rotation angle is set by the signal at port G. The cylinder position angle, γ, is the sum of the position signal, G, and the initial angle offset, the Phase angle, γ0:
γ is always between 0 and 2π. For any combined signal and offset larger than 2π rad, γ is maintained at 2π, and for any combined signal and offset lower than 0 rad, and γ is maintained at 0. To change the initial position of the orifice relative to the slot, you can adjust the Phase angle parameter.
Axial-Piston Machine With Five Pistons
The numbers in the diagram indicate the components of an axial-piston machine:
Valve plate orifice
The cylinder rotational alignment with the slot is described by the following angles:
Cylinder angle at rotation onto slot, γ1:
Cylinder angle at complete rotation onto slot, γ2:
Cylinder angle at rotation beyond slot, γ3:
Cylinder angle at complete rotation beyond slot, γ4:
Ψ is the Pressure carryover angle. This angle represents the average angular distance the piston travels during its pressure transition period from a closed to opened slot.
r is the half of the Cylinder orifice diameter.
R is the Cylinder block pitch radius.
Orifice area calculations during cylinder motion
The transition opening area, which is the opening between the cylinder rotation angles γ1 and γ2, is calculated as:
The transition closing area, which is the opening between the cylinder rotation angles γ3 and γ4, is calculated as:
where the opening and closing parameters are:
The area between γ2 and γ3 is and the area between γ4 and γ1 is The maximum opening of the orifice is
The flow through a valve plate orifice is calculated from the pressure-area relationship:
Cd is the Discharge coefficient.
Aorifice is the area open to flow.
is the average fluid density.
Δp is the pressure drop over the valve, PA – PB.
The critical pressure difference, Δpcrit, is the pressure differential associated with the Critical Reynolds number, Recrit, which is the point of transition between laminar and turbulent flow in the fluid:
A— Liquid port
Entry point to the orifice.
B— Liquid port
Exit point from the orifice.
G— Cylinder rotation angle, rad
Cylinder rotation angle in radians, specified as a physical signal..
Cylinder block pitch radius— Cylinder pitch radius
0.05 m(default) | positive scalar
Pitch radius of the rotating cylinder.
Cylinder orifice diameter— Diameter of the cylinder slot
2.5e-3 m(default) | positive scalar
Diameter of the cylinder slot.
Pressure carryover angle— Pressure transition distance
0.06 rad(default) | scalar
Average angular distance the piston travels during the pressure transition period from a closed to opening slot.
Phase angle— Initial plate offset angle
0 rad(default) | scalar
Initial plate offset angle. The total angle between the plate and slot is the sum of the Phase angle and the Pressure carryover angle, Ψ.
Leakage area— Gap area when in fully closed position
1e-10 m^2(default) | positive scalar
Sum of all gaps when the valve is in a fully closed position. Any area smaller than this value is maintained at the specified leakage area. This contributes to numerical stability by maintaining continuity in the flow.
Discharge coefficient— Discharge coefficient
0.64(default) | positive scalar
Correction factor that accounts for discharge losses in theoretical flows.
Critical Reynolds number— Upper Reynolds number limit for laminar flow
150(default) | positive scalar
Upper Reynolds number limit for laminar flow through the orifice.