This example 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.
The model can be simulated in two modes: predefined system inputs or manual system inputs. For predefined system inputs, the control settings for the HVAC system is specified in a signal builder block. For manual system inputs, the control settings can be adjusted at run time using the dashboard controls.
The example HVAC component library is built on a custom moist air domain using the Simscape™ language. The across variables are pressure, dry-bulb temperature, and water vapor mass fraction. The through variables are total mass flow rate, energy flow rate, and water vapor mass flow rate. The moist air is assumed to be a perfect gas. Additional moisture may be added to any moist air volume. When a moist air volume reaches saturation, liquid water condenses and is extracted out of the volume.
This figure shows a psychometric chart for the moist air inside the cabin. It is based on the ASHRAE Psychometric Chart No. 1 and is used to convey thermodynamic properties of moist air, such as dry-bulb temperature, web-bulb temperature, relative humidity, humidity ratio, and enthalpy. The trajectory of the state of the cabin moist air during simulation is plotted on the chart.