BladderAccumulator

Description

Model of a gas-loaded accumulator with a bladder separating the media. Fluid inertia or the stiffness of the bladder is not considered. The gas is modelled as an ideal gas.

The parameters Biasing pressure, Nominal volume and Dead volume describe the accumulator in the unconnected state. At the beginning of a simulation, the state variables of the accumulator are computed depending on the parameter Initial pressure of the hydraulic volume and Isothermal start.

If the bladder expands to its maximum volume, only the Dead volume remains in the accumulator. Due to the typically small dead volume, a high mathematical stiffness of the accumulator can result. This behavior might require choosing a correspondingly small timestep.

The parameter Polytropic exponent characterizes how pressure changes affect the volume and stiffness of the gas bladder. If Polytropic exponent is set to unity, the change of state within the gas is isothermal. This implies perfect heat transfer between the gas in the bladder and the environment such that no change of temperature occurs in the gas. This situation is encountered in hydraulic accumulators if the load cycles of the liquid are lengthy.

If the Polytropic exponent is set to the isentropic exponent (1.4 for air), the gas undergoes an adiabatic change of state which means that no heat exchange with the environment can occur. This is typically the case for very short load cycles.

If Isothermal start is set to true, it is assumed that the change from biasing pressure to the initial pressure happened at constant temperature. If Isothermal start is set to false, a polytropic process incorporating the parameter Polytropic exponent is assumed.