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Translational Mechanical Converter (MA)

Interface between moist air and mechanical translational networks

  • Translational Mechanical Converter (MA) block

Libraries:
Simscape / Foundation Library / Moist Air / Elements

Description

The Translational Mechanical Converter (MA) block models an interface between a moist air network and a mechanical translational network. The block converts moist air pressure into mechanical force and vice versa. You can use it as a building block for linear actuators.

The converter contains a variable volume of moist air. The pressure and temperature evolve based on the compressibility and thermal capacity of this moist air volume. Liquid water condenses out of the moist air volume when it reaches saturation. The Mechanical orientation parameter lets you specify whether an increase in the moist air volume inside the converter results in a positive or negative displacement of port R relative to port C.

The block equations use these symbols. Subscripts a, w, g, and d indicate the properties of dry air, water vapor, trace gas, and water droplets, respectively. Subscript ws indicates water vapor at saturation. Subscripts A, H, and S indicate the appropriate port. Subscript I indicates the properties of the internal moist air volume.

m˙Mass flow rate
ΦEnergy flow rate
QHeat flow rate
pPressure
ρDensity
RSpecific gas constant
VVolume of moist air inside the converter
cpSpecific heat at constant volume
hSpecific enthalpy
uSpecific internal energy
xMass fraction (xw is specific humidity, which is another term for water vapor mass fraction)
yMole fraction
φRelative humidity
rHumidity ratio
rdMass ratio of water droplets to moist air
TTemperature
tTime

The net flow rates into the moist air volume inside the converter are

m˙net=m˙Am˙condense+m˙wS+m˙gS+m˙d,evapΦnet=ΦA+QH+ΦS(1λd)m˙condensehdm˙w,net=m˙wAm˙condense+m˙wS+m˙d,evapm˙g,net=m˙gA+m˙gSm˙d,net=m˙dA+m˙dS+λdm˙condensem˙d,evap

where:

  • m˙condense is the rate of condensation.

  • m˙d,evap is the rate of water droplet evaporation.

  • Φcondense is the rate of energy loss from the condensed water.

  • λd is the value of the Fraction of condensate entrained as water droplets parameter.

  • ΦS is the rate of energy added by the sources of moisture and trace gas. m˙wS and m˙gS are the mass flow rates of water and gas, respectively, through port S. The values of m˙wS, m˙gS, and ΦS are determined by the moisture and trace gas sources connected to port S of the converter.

Water vapor mass conservation relates the water vapor mass flow rate to the dynamics of the humidity level in the internal moist air volume

dxwIdtρIV+xwIm˙net=m˙w,net

Similarly, trace gas mass conservation relates the trace gas mass flow rate to the dynamics of the trace gas level in the internal moist air volume

dxgIdtρIV+xgIm˙net=m˙g,net

The water droplets mass conservation equation relates the water droplet mass flow rate to the entrained water droplet dynamics in the internal moist air volume

drdIdtρIV+rdIm˙net=m˙d,net.

Mixture mass conservation relates the mixture mass flow rate to the dynamics of the pressure, temperature, and mass fractions of the internal moist air volume:

(1pIdpIdt1TIdTIdt)ρIV+RaRwRI(m˙w,netxwm˙net)+RaRgRI(m˙g,netxgm˙net)+ρIV˙=m˙net

where V˙ is the rate of change of the converter volume.

Finally, energy conservation relates the energy flow rate to the dynamics of the pressure, temperature, and mass fractions of the internal moist air volume:

(cpIRI+rdcpd)VρIdTIdt+uaIm˙MA,net+(uwIuaI)m˙w,net+(ugIuaI)m˙g,net+hdm˙d,net=ΦnetpIV˙

The equation of state relates the mixture density to the pressure and temperature:

pI=ρIRITI

The mixture specific gas constant is

RI=xaIRa+xwIRw+xgIRg

The converter volume is

V=Vdead+Sintdintεint

where:

  • Vdead is the dead volume.

  • Sint is the interface cross-sectional area.

  • dint is the interface displacement.

  • εint is the mechanical orientation coefficient. If Mechanical orientation is Pressure at A causes positive displacement of R relative to C, εint = 1. If Mechanical orientation is Pressure at A causes negative displacement of R relative to C, εint = –1.

If you connect the converter to a Multibody joint, use the physical signal input port p to specify the displacement of port R relative to port C. Otherwise, the block calculates the interface displacement from relative port velocities. The interface displacement is zero when the moist air volume inside the converter is equal to the dead volume. Then, depending on the Mechanical orientation parameter value:

  • If Pressure at A causes positive displacement of R relative to C, the interface displacement increases when the moist air volume increases from dead volume.

  • If Pressure at A causes negative displacement of R relative to C, the interface displacement decreases when the moist air volume increases from dead volume.

The force balance on the mechanical interface is

Fint=(penvpI)Sintεint

where:

  • Fint is the force from port R to port C.

  • penv is the environment pressure.

Flow resistance and thermal resistance are not modeled in the converter:

pA=pITH=TI

When the moist air volume reaches saturation, condensation may occur. The specific humidity at saturation is

xwsI=φwsRIRwpwsIpI

where:

  • φws is the relative humidity at saturation (typically 1).

  • pwsI is the water vapor saturation pressure evaluated at TI.

The rate of condensation is

m˙condense={0,if xwIxwsIxwIxwsIτcondenseρIV,if xwI>xwsI

where τcondense is the value of the Water vapor condensation time constant parameter.

The rate of evaporation is

m˙d,evap=xwsIxwIxwsIτevaprdIρIV,

where τevap is the value of the Water droplets evaporation time constant parameter.

Assumptions and Limitations

  • The converter casing is perfectly rigid.

  • Flow resistance between the converter inlet and the moist air volume is not modeled. Connect a Local Restriction (MA) block or a Flow Resistance (MA) block to port A to model pressure losses associated with the inlet.

  • Thermal resistance between port H and the moist air volume is not modeled. Use Thermal library blocks to model thermal resistances between the moist air mixture and the environment, including any thermal effects of a chamber wall.

  • The moving interface is perfectly sealed.

  • The block does not model the mechanical effects of the moving interface, such as hard stops, friction, and inertia.

Examples

Ports

Input

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Input physical signal that passes the position information from a Simscape™ Multibody™ joint. Connect this port to the position sensing port p of the joint. For more information, see Connecting Simscape Networks to Simscape Multibody Joints.

Dependencies

To enable this port, set the Interface displacement parameter to Provide input signal from Multibody joint.

Output

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Physical signal output port that measures the rate of condensation that leaves the system. This value does not include the portion of condensation that is entrained as water droplets.

Physical signal output port that outputs a vector signal. The vector contains the pressure (in Pa), temperature (in K), moisture level, and trace gas level measurements inside the component. Use the Measurement Selector (MA) block to unpack this vector signal.

Conserving

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Moist air conserving port associated with the converter inlet.

Thermal conserving port associated with the temperature of the moist air mixture inside the converter.

Mechanical translational conserving port associated with the moving interface.

Mechanical translational conserving port associated with the converter casing.

Connect this port to port S of a block from the Moisture & Trace Gas Sources library to add or remove moisture and trace gas. For more information, see Using Moisture and Trace Gas Sources.

Dependencies

This port is visible only if you set the Moisture and trace gas source parameter to Controlled.

Parameters

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Main

Select the relative orientation of the converter with respect to the volume of moist air inside the converter:

  • Pressure at A causes positive displacement of R relative to C — Increase in the moist air volume results in a positive displacement of port R relative to port C.

  • Pressure at A causes negative displacement of R relative to C — Increase in the moist air volume results in a negative displacement of port R relative to port C.

Select method to determine displacement of port R relative to port C:

  • Calculate from velocity of port R relative to port C — Calculate displacement from relative port velocities, based on the block equations. This is the default method.

  • Provide input signal from Multibody joint — Enable the input physical signal port p to pass the displacement information from a Multibody joint. Use this method only when you connect the converter to a Multibody joint by using a Translational Multibody Interface block. For more information, see How to Pass Position Information.

Translational offset of port R relative to port C at the start of simulation. A value of 0 corresponds to an initial moist air volume equal to Dead volume.

Dependencies

Enabled when the Interface displacement parameter is set to Calculate from velocity of port R relative to port C.

  • If Mechanical orientation is Pressure at A causes positive displacement of R relative to C, the parameter value must be greater than or equal to 0.

  • If Mechanical orientation is Pressure at A causes negative displacement of R relative to C, the parameter value must be less than or equal to 0.

The area on which the moist air exerts pressure to generate the translational force.

Volume of moist air when the interface displacement is 0.

Cross-sectional area of the converter inlet, in the direction normal to the air flow path.

Select a specification method for the environment pressure:

  • Atmospheric pressure — Use the atmospheric pressure, specified by the Moist Air Properties (MA) block connected to the circuit.

  • Specified pressure — Specify a value by using the Environment pressure parameter.

Pressure outside the converter acting against the pressure of the converter moist air volume. A value of 0 indicates that the converter expands into vacuum.

Dependencies

Enabled when the Environment pressure specification parameter is set to Specified pressure.

Moisture and Trace Gas

Relative humidity above which condensation occurs.

Characteristic time scale at which an oversaturated moist air volume returns to saturation by condensing out excess moisture.

Characteristic time scale at which water droplets evaporate to vapor.

Fraction of the condensate in the moist air that is entrained as water droplets.

This parameter controls visibility of port S and provides these options for modeling moisture and trace gas levels inside the component:

  • None — No moisture or trace gas is injected into or extracted from the block. Port S is hidden. This is the default.

  • Constant — Moisture and trace gas are injected into or extracted from the block at a constant rate. The same parameters as in the Moisture Source (MA) and Trace Gas Source (MA) blocks become available in the Moisture and Trace Gas section of the block interface. Port S is hidden.

  • Controlled — Moisture and trace gas are injected into or extracted from the block at a time-varying rate. Port S is exposed. Connect the Controlled Moisture Source (MA) and Controlled Trace Gas Source (MA) blocks to this port.

Select whether the block adds or removes moisture as water vapor or water droplets.

Dependencies

To enable this parameter, set Moisture and trace gas source to Constant.

Select how the block adds or removes water vapor. If you clear this check box, the enthalpy of the added or removed humidity corresponds to the enthalpy of water vapor, which is greater than that of liquid water.

If you select this check box, the enthalpy of the added or removed humidity corresponds to the enthalpy of liquid water, which is less than that of water vapor. When water vapor is added, it evaporates from liquid and the latent heat is contributed by the connected fluid volume. When water vapor is removed, it condenses to liquid and the latent heat is released to the connected fluid volume.

Dependencies

To enable this parameter, set Moisture and trace gas source to Constant and Moisture added or removed to Water vapor.

Water vapor or water droplets mass flow rate through the block. A positive value adds moisture to the connected moist air volume. A negative value extracts moisture from that volume.

Dependencies

To enable this parameter, set Moisture and trace gas source to Constant.

Select a specification method for the moisture temperature:

  • Atmospheric temperature — Use the atmospheric temperature, specified by the Moist Air Properties (MA) block connected to the circuit.

  • Specified temperature — Specify a value by using the Temperature of added moisture parameter.

Dependencies

To enable this parameter, set Moisture and trace gas source to Constant.

Enter the desired temperature of added moisture. This temperature remains constant during simulation. The block uses this value to evaluate the specific enthalpy of the added moisture only. The specific enthalpy of removed moisture is based on the temperature of the connected moist air volume.

Dependencies

To enable this parameter, set Added moisture temperature specification to Specified temperature.

Trace gas mass flow rate through the block. A positive value adds trace gas to the connected moist air volume. A negative value extracts trace gas from that volume.

Dependencies

To enable this parameter, set Moisture and trace gas source to Constant.

Select a specification method for the trace gas temperature:

  • Atmospheric temperature — Use the atmospheric temperature, specified by the Moist Air Properties (MA) block connected to the circuit.

  • Specified temperature — Specify a value by using the Temperature of added trace gas parameter.

Dependencies

To enable this parameter, set Moisture and trace gas source to Constant.

Enter the desired temperature of added trace gas. This temperature remains constant during simulation. The block uses this value to evaluate the specific enthalpy of the added trace gas only. The specific enthalpy of removed trace gas is based on the temperature of the connected moist air volume.

Dependencies

To enable this parameter, set Added trace gas temperature specification to Specified temperature.

Initial Conditions

Moist air pressure at the start of the simulation.

Priority the solver assigns to the Initial pressure parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Initial moist air temperature.

Priority the solver assigns to the Initial temperature parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Method to specify the initial moist air humidity.

Relative humidity in the moist air at the start of the simulation. The relative humidity is the ratio of the water vapor partial pressure to the water vapor saturation pressure, or the ratio of the water vapor mole fraction to the water vapor mole fraction at saturation.

Dependencies

To enable this parameter, set Initial humidity specification to Relative humidity.

Priority the solver assigns to the Initial relative humidity parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial humidity specification to Relative humidity.

Specific humidity in the moist air at the start of simulation. The specific humidity is the mass fraction of water vapor to the combined total mass of water vapor, trace gas, and dry air.

Dependencies

To enable this parameter, set Initial humidity specification to Specific humidity.

Priority the solver assigns to the Initial specific humidity parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial humidity specification to Specific humidity.

Mole fraction of the water vapor in the moist air channel at the start of simulation. The water vapor mole fraction is relative to the combined molar quantity of water vapor, trace species, and dry air.

Dependencies

To enable this parameter, set Initial humidity specification to Mole fraction.

Priority the solver assigns to the Initial water vapor mole fraction parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial humidity specification to Mole fraction.

Humidity ratio in the moist air channel at the start of the simulation. The humidity ratio is the ratio of the mass of water vapor to the mass of dry air and trace gas.

Dependencies

To enable this parameter, set Initial humidity specification to Humidity ratio.

Priority the solver assigns to the Initial humidity ratio humidity parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial humidity specification to Humidity ratio.

Wet-bulb temperature at the start of the simulation. The block uses this value to calculate humidity.

Dependencies

To enable this parameter, set Initial humidity specification to Wet-bulb temperature.

Priority the solver assigns to the Initial wet-bulb temperature parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial humidity specification to Wet-bulb temperature.

Measurement type of trace gas.

Amount of trace gas in the moist air by mass fraction at the start of the simulation. The mass fraction is relative to the combined total mass of water vapor, trace gas, and dry air.

The block ignores this parameter if the Trace gas model parameter in the Moist Air Properties (MA) block is None.

Dependencies

To enable this parameter, set Initial trace gas specification to Mass fraction.

Priority the solver assigns to the Initial trace gas mass fraction priority parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial trace gas specification to Mass fraction.

Amount of trace gas in the moist air channel by mole fraction at the start of the simulation. The mole fraction is relative to the combined molar total of water vapor, trace gas, and dry air.

The block ignores this parameter if the Trace gas model parameter in the Moist Air Properties (MA) block is None.

Dependencies

To enable this parameter, set Initial trace gas specification to Mole fraction.

Priority the solver assigns to the Initial trace gas mole fraction priority parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Dependencies

To enable this parameter, set Initial trace gas specification to Mole fraction.

Initial mass ratio of water droplets to moist air.

Priority the solver assigns to the Initial mass ratio of water droplets to moist air priority parameter when initializing the block.

Set this parameter to High to define your initial conditions. You may need to set this parameter to Low or None if this initial condition conflicts with the initial conditions of another block.

Extended Capabilities

C/C++ Code Generation
Generate C and C++ code using Simulink® Coder™.

Version History

Introduced in R2018a

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