Modeling Moisture and Trace Gas Levels

Moist Air Source Domain

Unlike the moist air domain, the moist air source domain is a separate domain for modeling moisture and trace gas levels in moist air systems.

Regular connection ports (boundary nodes) of the Moist Air library blocks belong to the moist air domain. These ports are usually named A or B.

Blocks with a finite moist air volume also contain an internal node, which provides the pressure, temperature, moisture level, and trace gas level inside the component. This internal node connects to the moist air source domain. The corresponding port is named S.

For example, these are the ports of a Pipe (MA) block.

A and B ― Moist air conserving ports associated with the boundary nodes. Use these ports to connect this block to other blocks in a moist air circuit.
S ― Moist air source conserving port associated with the internal node. Use this port to model humidity, water droplet, and trace gas levels, by connecting it to a block in the Moisture & Trace Gas Sources library. This port is hidden by default. The Moisture and trace gas source parameter controls visibility of port S and provides options for modeling moisture and trace gas levels.
H ― Thermal conserving port associated with the temperature either of the air mixture inside the block or of a block element, such as pipe wall.
W and F ― Physical signal output ports.

Blocks in the Moisture & Trace Gas Sources library, which inject or extract water vapor, water droplets, and trace gas, also have a port S that belongs to the moist air source domain. This naming convention helps to distinguish the two different domain types. For more information, see Using Moisture and Trace Gas Sources.

Trace Gas Modeling Options

The Moist Air Properties (MA) block provides three ways to model the amount of trace gas in the air mixture of the connected circuit:

None — No trace gas is present. The moist air mixture consists only of dry air and water vapor.
Track mass fraction only — The trace gas level can be nonzero and vary during simulation. However, the amount of trace gas is assumed to be small enough to have a negligible impact on the fluid properties of the moist air mixture.
Track mass fraction and gas properties — The trace gas level can be nonzero and vary during simulation. The fluid properties of the moist air mixture depend on the amount of trace gas in the mixture.

Trace Gas — None

If you set the Trace gas model parameter of a Moist Air Properties (MA) block to None, the moist air mixture in the connected circuit consists only of dry air and water moisture. Any nonzero values of trace gas level in parameters, variable targets, and inputs of the blocks connected to the circuit are ignored. These include, for example, the amount of trace gas in a reservoir, the initial amount of trace gas in a moist air volume, and all trace gas sources. The block equations listed on the block reference pages are simplified by replacing the trace gas mass or mole fraction terms in mixture properties calculations with 0.

You do not have to enter any fluid properties associated with trace gas, and there is no run-time trace gas properties table lookup. The underlying system equations are also simplified for increased run-time efficiency.

Trace Gas — Track mass fraction only

If you select the Track mass fraction only option, this means that you want to keep track of varying amounts of trace gas in the model, but consider its impact on the mixture properties negligible. Use this option if you expect the amount of trace gas in the mixture to be very small.

You must specify the trace gas diffusivity in air (for the smoothed upwind scheme) and the gas constant, but do not need to enter any other fluid properties associated with trace gas, and there is no run-time trace gas properties table lookup. Therefore, this option is also useful if you do not have property data for the trace gas.

Trace gas properties in block equations are replaced with the corresponding properties of dry air. There are no other equation simplifications.

Trace Gas — Track mass fraction and gas properties

If you select the Track mass fraction and gas properties option, this means that you want to keep track of varying amounts of trace gas, including its impact on the mixture properties. This is the most general and complete option. You must provide the properties of trace gas. All equations listed on the block reference pages assume this option.

Modeling Entrained Water Droplets

Since R2024b

When you select Enable entrained water droplets in the Moist Air Properties (MA) block connected to your moist air network, the blocks in the network model water droplets suspended in the moist air flow. The blocks also track the amount of water droplets and convect them downstream.

If you clear the Enable entrained water droplets check box, the model ignores any nonzero values for water droplets in the parameters and inputs of blocks connected to the network. The block equations listed on the block reference pages replace the water droplet terms in calculations with 0.

Water Droplet Modeling Assumptions

When you enable water droplet modeling, the blocks in the network assume that:

The entrained water droplets are the same temperature as the moist air.
The water droplet specific enthalpy is included in the energy balance equations. If the network includes more water droplets, the temperature changes less easily because of the additional heat capacity from the droplets.
Water droplets do not impact other properties such as density, dynamic viscosity, thermal conductivity, and the Prandtl number.
The amount of water droplets is small relative to the amount of moist air.
For blocks that model fluid volume:
- If the moist air is above saturation or if the surface is cold enough, the water vapor can condense. Use the Fraction of condensate entrained as water droplets parameter in each block to specify the amount of condensate that converts to water droplets. The remaining condensate leaves the flow.
- If the moist air is less than saturation and water droplets are present, then the water droplets re-evaporate into the moist air.

Using Moisture and Trace Gas Sources

Moisture and trace gas can be injected into blocks with finite moist air volume. (For a complete list of these blocks, see Blocks with Moist Air Volume.) For example, adding moisture to a Constant Volume Chamber (MA) block can represent respiration of occupants in a room. Moisture and trace gas can also be extracted from these blocks. For example, removing trace gas from a Pipe (MA) block can represent a filter that extracts CO2 from moist air flow.

You can use the blocks in the Moisture & Trace Gas Sources library to inject or extract water vapor, water droplets, and trace gas. The moisture and trace gas sources can be connected only to the conserving port S of the blocks with moist air volume, which is the port associated with the moist air source domain. For more information, see Moist Air Source Domain.

All Moist Air library blocks with a finite moist air volume have a Moisture and trace gas source parameter, which controls the visibility of port S and provides 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. This option provides an easy way to model a constant rate of change for moisture and trace gas levels directly inside the component, without connecting additional blocks. It is equivalent to connecting an external constant source.
Controlled — Moisture and trace gas are injected into or extracted from the block at a time-varying rate. Port S is exposed. Connect the Moisture Source (MA) and Trace Gas Source (MA) blocks to this port. You can also use this option to connect multiple moisture and trace gas sources to the same block with moist air volume, to represent different effects. For example, a Constant Volume Chamber (MA) block can have all of these sources connected to its port S:
- A Moisture Source (MA) block with the Moisture added or removed parameter set to Water vapor, representing water vapor input or output.
- A second Moisture Source (MA) block with the Moisture added or removed parameter set to Water droplets, representing water droplet input or output.
- A Trace Gas Source (MA) block, representing trace gas input or output.

If no water vapor, water droplets, or trace gas is injected to or extracted from a block with moist air volume, set its Moisture and trace gas source parameter to None to hide the unused port S.

Note

Using moisture source blocks to remove moisture is a different effect than condensation. Condensation occurs independently whenever φ_w ≥ φ_ws. For more information, see Saturation and Condensation.

Measuring Moisture and Trace Gas Levels

The Sensors sublibrary of the Moist Air library contains the regular sensor blocks, similar to the ones found in other domains. The Moisture & Trace Gas Sensor (MA) block lets you measure the moisture level and trace gas level in a moist air network, upstream of the measured node. These blocks can be connected only to boundary nodes (regular moist air conserving ports). Therefore, they cannot measure moist air properties at internal nodes representing a moist air volume.

To measure the amount of moisture and trace gas in a moist air volume, along with the pressure and temperature, each block with a finite internal moist air volume has a physical signal port F, which outputs a vector physical signal in base SI units:

% Moist air volume measurements
F == [value(p_I, 'Pa'); value(T_I, 'K'); RH_I; x_w_I; y_w_I; HR_I; x_g_I; y_g_I; x_d_I];

where:

p_I is the pressure of the moist air volume, in Pa
T_I is the temperature of the moist air volume, in K
RH_I is the relative humidity of the moist air volume
x_w_I is the specific humidity of moist air volume
y_w_I is the water vapor mole fraction of the moist air volume
HR_I is the humidity ratio of the moist air volume
x_g_I is the trace gas mass fraction of the moist air volume
y_g_I is the trace gas mole fraction of the moist air volume
x_d_I is the mass ratio of water droplets to moist air of the moist air volume

Use the Measurement Selector (MA) block to unpack the vector signal and reassign units to pressure and temperature values. Connect the Measurement Selector (MA) block to the physical signal output port F of a block with finite internal moist air volume to access the data.