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Gas-Charged Accumulator - Simulate hydraulic accumulator with gas as compressible medium

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Accumulators

Description

This block models a gas-charged accumulator. The accumulator consists of a precharged gas chamber and a fluid chamber connected to a hydraulic system. The chambers are separated by a bladder, piston, or any kind of elastic diaphragm.

If the fluid pressure at the accumulator inlet becomes higher than the precharge pressure, fluid enters the accumulator chamber and compresses the gas, thus storing hydraulic energy. A drop in the fluid pressure at the inlet forces the stored fluid back into the system.

Normally, pressure in the gas chamber is equal to that of the fluid chamber. But if pressure at the accumulator inlet (p) drops below the accumulator's precharge value (p_pr), the gas chamber gets isolated from the system with the inlet valve. In this case, pressure in the gas chamber remains constant and equal to the precharge value, while pressure at the inlet depends on pressure in the system to which the accumulator is connected. If pressure at the inlet builds up to the precharge value or higher, the chambers start interacting again. The accumulator is described with the following equations:

where

`V_F`	Fluid volume
`V_A`	Accumulator capacity
`p`	Inlet gauge pressure
`p_pr`	Precharge pressure
`k`	Specific heat ratio
`q`	Volumetric flow rate
`t`	Time

Basic Assumptions and Limitations

The model is based on the following assumptions:

The gas compression is determined on the basis of the thermodynamics of ideal gases.
The process is assumed to be polytropic.
No loading on the separator, such as inertia, friction, and so on, is considered.
Fluid compressibility is not taken into account.

Dialog Box and Parameters

Capacity: Accumulator capacity. The default value is 0.008 m^3.
Preload pressure (gauge): Precharge gauge pressure. The default value is 1e6 Pa.
Specific heat ratio: Specific heat ratio (adiabatic index). No units. The default value is 1.4. To account for heat exchange, you can set it within a range between 1 (isothermal process) and 1.4 (adiabatic process).
Initial volume: Initial volume of fluid in the accumulator. This parameter specifies the initial condition for use in computing the block's initial state at the beginning of a simulation run. For more information, see Computing Initial Conditions. The default value is 0.

Global Parameters

Atmospheric pressure: Absolute pressure of the environment. The default value is 101325 Pa.

Ports

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

The flow rate is positive if fluid flows into the accumulator.