# Constant Volume Chamber (2P)

Chamber with fixed volume of two-phase fluid and variable number of ports

• Library:
• Simscape / Foundation Library / Two-Phase Fluid / Elements

## Description

The Constant Volume Chamber (2P) block models the accumulation of mass and energy in a chamber containing a fixed volume of two-phase fluid. The chamber can have between one and four inlets, labeled from A to D, through which fluid can flow. The fluid volume can exchange heat with a thermal network, such as a network that represents the chamber surroundings, through the thermal port H.

The mass of the fluid in the chamber varies with density, a property that is generally a function of pressure and temperature for a two-phase fluid. Fluid enters when the pressure upstream of the inlet rises above the pressure in the chamber and exits when the pressure gradient reverses. The effect in a model is often to smooth out sudden changes in pressure, much like an electrical capacitor does with voltage.

The flow resistance between the inlet and interior of the chamber is assumed to be negligible. The pressure in the interior is therefore equal to that at the inlet. Similarly, the thermal resistance between the thermal port and interior of the chamber is assumed to be negligible. The temperature in the interior is equal to the temperature at the thermal port.

### Mass Balance

Mass can enter and exit the chamber through ports A, B, C, and D. The volume of the chamber is fixed but the compressibility of the fluid means that its mass can change with pressure and temperature. The rate of mass accumulation in the chamber must exactly equal the mass flow rate through ports A, B, C, and D,

`$\left[{\left(\frac{\partial \rho }{\partial p}\right)}_{u}\frac{dp}{dt}+{\left(\frac{\partial \rho }{\partial u}\right)}_{p}\frac{du}{dt}\right]V={\stackrel{˙}{m}}_{\text{A}}+{\stackrel{˙}{m}}_{\text{B}}+{\stackrel{˙}{m}}_{\text{C}}+{\stackrel{˙}{m}}_{\text{D}}+{ϵ}_{M},$`

where the left-hand side is the rate of mass accumulation and:

• ρ is the density.

• p is the pressure.

• u is the specific internal energy.

• V is the volume.

• $\stackrel{˙}{m}$ is the mass flow rate.

• ϵM is a correction term that accounts for a numerical error caused by the smoothing of the partial derivatives.

Correction Term for Partial-Derivative Smoothing

The block computes the partial derivatives in the mass balance equation by applying the finite-difference method to the tabulated data in the Two-Phase Fluid Properties (2P) block and interpolating the results. The block then smooths the partial derivatives at the phase-transition boundaries by means of cubic polynomial functions. These functions apply between:

• The subcooled liquid and two-phase mixture phase domains when the vapor quality is in the 0–0.1 range.

• The two-phase mixture and superheated vapor phase domains when the vapor quality is in the 0–0.9 range.

The smoothing introduces a small numerical error that the block adjusts for by adding to the mass balance the correction term ϵM, defined as:

`${ϵ}_{M}=\frac{M-V/\nu }{\tau }.$`

where:

• M is the fluid mass in the chamber.

• ν is the specific volume.

• τ is the characteristic duration of a phase-change event.

The block obtains the fluid mass in the chamber from the equation:

`$\frac{dM}{dt}={\stackrel{˙}{m}}_{A}+{\stackrel{˙}{m}}_{\text{B}}+{\stackrel{˙}{m}}_{\text{C}}+{\stackrel{˙}{m}}_{\text{D}}.$`

### Energy Balance

Energy can enter and exit the chamber in two ways: with fluid flow through ports A, B, C, and D, and with the heat flow through port H. No work is done on or by the fluid inside the chamber. The rate of energy accumulation in the internal fluid volume must then equal the sum of the energy flow rates in through ports A, B, C, D, and H,

`$\stackrel{˙}{E}={\varphi }_{\text{A}}+{\varphi }_{\text{B}}+{\varphi }_{\text{C}}+{\varphi }_{\text{D}}+\text{​}{Q}_{\text{H}},$`

where:

• ϕ is energy flow rate.

• Q is heat flow rate.

• E is total energy.

Neglecting the kinetic energy of the fluid, the total energy in the chamber is:

`$E=Mu.$`

### Momentum Balance

The pressure drop due to viscous friction between the individual ports and the interior of the chamber is assumed to be negligible. Gravity is ignored, as are other body forces. The pressure in the internal fluid volume must therefore equal the pressure at ports A, B, C, and D:

`$p={p}_{\text{A}}={p}_{\text{B}}={p}_{\text{C}}={p}_{\text{D}}.$`

### Assumptions and Limitations

• The chamber has a fixed volume of fluid.

• The flow resistance between the inlet and the interior of the chamber is negligible.

• The thermal resistance between the thermal port and the interior of the chamber is negligible.

• The kinetic energy of the fluid in the chamber is negligible.

## Ports

### Conserving

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Two-phase fluid conserving port associated with the chamber inlet.

Two-phase fluid conserving port associated with the second chamber inlet.

#### Dependencies

To enable this port, set the Number of ports parameter to `2`, `3`, or `4`.

Two-phase fluid conserving port associated with the third chamber inlet.

#### Dependencies

To enable this port, set the Number of ports parameter to `3` or `4`.

Two-phase fluid conserving port associated with the fourth chamber inlet. If a chamber has four inlet ports, you can use the block as a junction in a cross connection.

#### Dependencies

To enable this port, set the Number of ports parameter to `4`.

Thermal conserving port through which the fluid in the chamber exchanges heat with a thermal network.

## Parameters

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### Main

Volume of fluid in the chamber. This volume is constant during simulation.

Number of inlet ports in the chamber. The chamber can have between one and four ports, labeled from A to D. When you modify the parameter value, the corresponding ports are exposed or hidden in the block icon.

Inlet area normal to the direction of flow at port A.

Inlet area normal to the direction of flow at port B.

#### Dependencies

To enable this parameter, set Number of ports to `2`, `3`, or `4`.

Inlet area normal to the direction of flow at port C.

#### Dependencies

To enable this parameter, set Number of ports to `3` or `4`.

Inlet area normal to the direction of flow at port D.

#### Dependencies

To enable this parameter, set Number of ports to `4`.

### Effects and Initial Conditions

Thermodynamic variable in terms of which to define the initial conditions of the block.

The value for the Initial fluid energy specification parameter limits the available initial states for the two-phase fluid. When Initial fluid energy specification is:

• `Temperature` — Specify an initial state that is a subcooled liquid or superheated vapor. You cannot specify a liquid-vapor mixture because the temperature is constant across the liquid-vapor mixture region.

• `Vapor quality` — Specify an initial state that is a liquid-vapor mixture. You cannot specify a subcooled liquid or a superheated vapor because the liquid mass fraction is 0 and 1, respectively, across the whole region. Additionally, the block limits the pressure to below the critical pressure.

• `Vapor void fraction` — Specify an initial state that is a liquid-vapor mixture. You cannot specify a subcooled liquid or a superheated vapor because the liquid mass fraction is 0 and 1, respectively, across the whole region. Additionally, the block limits the pressure to below the critical pressure.

• `Specific enthalpy` — Specify the specific enthalpy of the fluid. The block does not limit the initial state.

• `Specific internal energy` — Specify the specific internal energy of the fluid. The block does not limit the initial state.

Pressure in the chamber at the start of simulation, specified against absolute zero.

Temperature in the chamber at the start of simulation, specified against absolute zero.

#### Dependencies

To enable this parameter, set Initial fluid energy specification to `Temperature`.

Mass fraction of vapor in the chamber at the start of simulation.

#### Dependencies

To enable this parameter, set Initial fluid energy specification to `Vapor quality`.

Volume fraction of vapor in the chamber at the start of simulation.

#### Dependencies

To enable this parameter, set Initial fluid energy specification to `Vapor void fraction`.

Specific enthalpy of the fluid in the chamber at the start of simulation.

#### Dependencies

To enable this parameter, set Initial fluid energy specification to `Specific enthalpy`.

Specific internal energy of the fluid in the chamber at the start of simulation.

#### Dependencies

To enable this parameter, set Initial fluid energy specification to ```Specific internal energy```.

Characteristic time to equilibrium of a phase-change event taking place in the chamber. Increase this parameter to slow the rate of phase change or decrease it to speed the rate.

## Version History

Introduced in R2015b

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