Constant Volume Hydraulic Chamber

Hydraulic capacity of constant volume

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Hydraulic Elements

Description

The Constant Volume Hydraulic Chamber block models a fixed-volume chamber with rigid or flexible walls, to be used in hydraulic valves, pumps, manifolds, pipes, hoses, and so on. Use this block in models where you have to account for some form of fluid compressibility. You can select the appropriate representation of fluid compressibility using the block parameters.

Fluid compressibility in its simplest form is simulated according to the following equations:

Vf=Vc+VcEp

q=VcEdpdt

where

qFlow rate into the chamber
VfVolume of fluid in the chamber
VcGeometrical chamber volume
EFluid bulk modulus
pGauge pressure of fluid in the chamber

If pressure in the chamber is likely to fall to negative values and approach cavitation limit, the above equations must be enhanced. In this block, it is done by representing the fluid in the chamber as a mixture of liquid and a small amount of entrained, nondissolved gas (see [1, 2]). The mixture bulk modulus is determined as:

E=El1+α(papa+p)1/n1+αpa1/nn·(pa+p)n+1nEl

where

ElPure liquid bulk modulus
pαAtmospheric pressure
αRelative gas content at atmospheric pressure, α = VG/VL
VGGas volume at atmospheric pressure
VLVolume of liquid
nGas-specific heat ratio

The main objective of representing fluid as a mixture of liquid and gas is to introduce an approximate model of cavitation, which takes place in a chamber if pressure drops below fluid vapor saturation level. As it is seen in the graph below, the bulk modulus of a mixture decreases at ppa, thus considerably slowing down further pressure change. At high pressure, p>>pa, a small amount of nondissolved gas has practically no effect on the system behavior.

Cavitation is an inherently thermodynamic process, requiring consideration of multiple-phase fluids, heat transfers, etc., and as such cannot be accurately simulated with Simscape™ software. But the simplified version implemented in the block is good enough to signal if pressure falls below dangerous level, and to prevent computation failure that normally occurs at negative pressures.

If pressure falls below absolute vacuum (–101325 Pa), the simulation stops and an error message is displayed.

If chamber walls have noticeable compliance, the above equations must be further enhanced by representing geometrical chamber volume as a function of pressure:

Vc=πd2/4·L

d(s)=Kp1+τsp(s)

where

dInternal diameter of the cylindrical chamber
LLength of the cylindrical chamber
KpProportionality coefficient (m/Pa)
τTime constant
sLaplace operator

Coefficient Kp establishes relationship between pressure and the internal diameter at steady-state conditions. For metal tubes, the coefficient can be computed as (see [2]):

Kp=dEM(D2+d2D2d2+ν)

where

DPipe external diameter
EMModulus of elasticity (Young's modulus) for the pipe material
νPoisson's ratio for the pipe material

For hoses, the coefficient can be provided by the manufacturer.

The process of expansion and contraction in pipes and especially in hoses is a complex combination of nonlinear elastic and viscoelastic deformations. This process is approximated in the block with the first-order lag, whose time constant is determined empirically (for example, see [3]).

As a result, by selecting appropriate values, you can implement four different models of fluid compressibility with this block:

  • Chamber with rigid walls, no entrained gas in the fluid

  • Cylindrical chamber with compliant walls, no entrained gas in the fluid

  • Chamber with rigid walls, fluid with entrained gas

  • Cylindrical chamber with compliant walls, fluid with entrained gas

The block allows two methods of specifying the chamber size:

  • By volume — Use this option for cylindrical or non-cylindrical chambers with rigid walls. You only need to know the volume of the chamber. This chamber type does not account for wall compliance.

  • By length and diameter — Use this option for cylindrical chambers with rigid or compliant walls, such as circular pipes or hoses.

The block has one hydraulic conserving port associated with the chamber inlet. The block positive direction is from its port to the reference point. This means that the flow rate is positive if it flows into the chamber.

Basic Assumptions and Limitations

  • No inertia associated with pipe walls is taken into account.

  • Chamber with compliant walls is assumed to have a cylindrical shape. Chamber with rigid wall can have any shape.

Dialog Box and Parameters

Dialog Box and Parameters

Parameters Tab

Chamber specification

The parameter can have one of two values: By volume or By length and diameter. The value By length and diameter is recommended if a chamber is formed by a circular pipe. If the parameter is set to By volume, wall compliance is not taken into account. The default value of the parameter is By volume.

Chamber wall type

The parameter can have one of two values: Rigid or Compliant. If the parameter is set to Rigid, wall compliance is not taken into account, which can improve computational efficiency. The value Compliant is recommended for hoses and metal pipes, where compliance can affect the system behavior. The default value of the parameter is Rigid. The parameter is used if the Chamber specification parameter is set to By length and diameter.

Chamber volume

Volume of fluid in the chamber. The default value is 1e-4 m^3. The parameter is used if the Chamber specification parameter is set to By volume.

Chamber internal diameter

Internal diameter of the cylindrical chamber. The default value is 0.01 m. The parameter is used if the Chamber specification parameter is set to By length and diameter.

Cylindrical chamber length

Length of the cylindrical chamber. The default value is 1 m. The parameter is used if the Chamber specification parameter is set to By length and diameter.

Static pressure-diameter coefficient

Coefficient Kp that establishes relationship between pressure and the internal diameter at steady-state conditions. The parameter can be determined analytically or experimentally. The default value is 1.2e-12 m/Pa. The parameter is used if Chamber wall type is set to Compliant.

Viscoelastic process time constant

Time constant in the transfer function relating pipe internal diameter to pressure variations. With this parameter, the simulated elastic or viscoelastic process is approximated with the first-order lag. The parameter is determined experimentally or provided by the manufacturer. The default value is 0.01 s. The parameter is used if Chamber wall type is set to Compliant.

Specific heat ratio

Gas-specific heat ratio. The default value is 1.4.

 Restricted Parameters

Variables Tab

Use the Variables tab to set the priority and initial target values for the block variables prior to simulation. For more information, see Set Priority and Initial Target for Block Variables.

Global Parameters

Parameters determined by the type of working fluid:

  • Fluid bulk modulus

  • Nondissolved gas ratio — Nondissolved gas relative content determined as a ratio of gas volume to the liquid volume.

Use the Hydraulic Fluid block or the Custom Hydraulic Fluid block to specify the fluid properties.

Ports

The block has one hydraulic conserving port associated with the chamber inlet.

References

[1] Manring, N.D., Hydraulic Control Systems, John Wiley & Sons, New York, 2005

[2] Meritt, H.E., Hydraulic Control Systems, John Wiley & Sons, New York, 1967

[3] Holcke, Jan, Frequency Response of Hydraulic Hoses, RIT, FTH, Stockholm, 2002

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