Controlled Mass Flow Rate Source (MA)

Generate time-varying mass flow rate

Libraries:
Simscape / Foundation Library / Moist Air / Sources

Description

The Controlled Mass Flow Rate Source (MA) block represents an ideal mechanical energy source in a moist air network. The mass flow rate is controlled by the input physical signal at port M. The source can maintain the specified mass flow rate regardless of the pressure differential. There is no flow resistance and no heat exchange with the environment. A positive mass flow rate causes moist air to flow from port A to port B.

The equations describing the source use these symbols.

c_p	Specific heat at constant pressure
h	Specific enthalpy
h_t	Specific total enthalpy
$\dot{m}$	Mass flow rate (flow rate associated with a port is positive when it flows into the block)
p	Pressure
ρ	Density
R	Specific gas constant
s	Specific entropy
T	Temperature
Φ_work	Power delivered to the moist air flow through the source

Subscripts A and B indicate the appropriate port.

Mass balance:

$\begin{array}{l} {\dot{m}}_{A} + {\dot{m}}_{B} = 0 \\ {\dot{m}}_{w A} + {\dot{m}}_{w B} = 0 \\ {\dot{m}}_{g A} + {\dot{m}}_{g B} = 0 \end{array}$

Energy balance:

$Φ_{A} + Φ_{B} + Φ_{w o r k} = 0$

If the source performs no work (Power added parameter is set to None), then $Φ_{w o r k} = 0$ .

If the source is isentropic (Power added parameter is set to Isentropic power), then

$Φ_{w o r k} = {\dot{m}}_{A} (h_{t B} - h_{t A})$

where

$\begin{array}{l} h_{t A} = h_{A} + \frac{1}{2} {(\frac{{\dot{m}}_{A}}{ρ_{A} S_{A}})}^{2} \\ h_{t B} = h_{B} + \frac{1}{2} {(\frac{{\dot{m}}_{B}}{ρ_{B} S_{B}})}^{2} \end{array}$

The mixture-specific enthalpies, h_A = h(T_A) and h_B = h(T_B), are constrained by the isentropic relation, that is, there is no change in entropy:

$\int_{T_{A}}^{T_{B}} \frac{1}{T} d h (T) = R \ln (\frac{p_{B}}{p_{A}})$

The quantity specified by the input signal of the source is

${\dot{m}}_{A} = {\dot{m}}_{s p e c i f i e d}$

Assumptions and Limitations

There are no irreversible losses.
There is no heat exchange with the environment.

Examples

Vehicle HVAC System

Models moist air flow in a vehicle heating, ventilation, and air conditioning (HVAC) system. The vehicle cabin is represented as a volume of moist air exchanging heat with the external environment. The moist air flows through a recirculation flap, a blower, an evaporator, a blend door, and a heater before returning to the cabin. The recirculation flap selects flow intake from the cabin or from the external environment. The blender door diverts flow around the heater to control the temperature.

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Aircraft Environmental Control System

Models an aircraft environmental control system (ECS) that regulates pressure, temperature, humidity, and ozone (O3) to maintain a comfortable and safe cabin environment. Cooling and dehumidification are provided by the air cycle machine (ACM), which operates as an inverse Brayton cycle to remove heat from pressurized hot engine bleed air. Some hot bleed air is mixed directly with the output of the ACM to adjust the temperature. Pressurization is maintained by the outflow valve in the cabin. This model simulates the ECS operating from a hot ground condition to a cold cruise condition and back to a cold ground condition.

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Ports

Input

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M — Mass flow rate control signal, kg/s
physical signal

Input physical signal that specifies the mass flow rate of the moist air through the source.

Conserving

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A — Source inlet
moist air

Moist air conserving port. A positive mass flow rate causes moist air to flow from port A to port B

B — Source outlet
moist air

Moist air conserving port. A positive mass flow rate causes moist air to flow from port A to port B.

Parameters

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Power added — Select whether the source performs work
`Isentropic power` (default) | `None`

Select whether the source performs work on the moist air flow:

Isentropic power — The source performs isentropic work on the moist air to maintain the specified mass flow rate. Use this option to represent an idealized pump or compressor and properly account for the energy input and output, especially in closed-loop systems.
None — The source performs no work on the flow, neither adding nor removing power, regardless of the mass flow rate produced by the source. Use this option to set up the desired flow condition upstream of the system, without affecting the temperature of the flow.

Cross-sectional area at port A — Area normal to flow path at port A
`0.01 m^2` (default)

Area normal to flow path at port A.

Controlled Mass Flow Rate Source (MA)