Thermal and airflow input

The choice of IDA ICE zone models affects the CFD boundary input

Energy zone model is the most basic model for CFD input. The energy model does not include view factors for long wave radiation. The room is represented with a single air temperature value.

Climate zone model in the dynamic IDA ICE simulation utilize objects that offer view factors for long wave heat radiation, but the room air temperature is still one single value.

Stratified climate zone model provides a higher level of detail for CFD input for surface temperatures and exhaust air temperatures. The room temperature is calculated for a number of horizontal layers. When multiple exhaust terminals are modeled, the exhaust air temperature shown in the CFD tab for boundary input is a weighted average. If only one exhaust terminal is modeled, the temperature value from that terminal is used directly. 

Surface temperatures will be transferred to the CFD tab from the dynamic simulation. In the CFD simulation, these surface temperatures may then vary due to features that 'anchor' them to an outside temperature using specialized functions.

CFD variable 'Age of air' includes bugs in certain cases

The CFD variable 'Age of air' is incorrectly calculated for recirculation airflow units and for airflow through openings between zones. The issue is that the air age is reset to zero upon re-entry into the CFD domain.

NB! Initial vs. calculated heat balance in the CFD simulation

The initial heat balance in the CFD simulation uses the exhaust air temperature/temperatures as an input. If the exhaust air temperature/temperatures calculated in the CFD differs from this input value, the resulting heat balance will deviate from that of the IDA ICE simulation.

Radiation not included in IDA ICE CFD simulations

There is no model for thermal radiation in IDA ICE CFD. Instead, radiation heat affects the surface temperatures that serve as boundary conditions for the CFD simulation.

Assign objects a CFD role

• For every geometric object (extruded parts, OBJ objects etc.) in the CFD domain, open it and assign it to a specific CFD role.
• NB! In current version the sum of ideal heating and ideal cooling is always assigned only to one geometry. The workaround is to use "Equipment" as room heaters (e.g. for cases with several radiators).
• Multiple objects can be assigned as an adiabatic obstruction.

​The three internal gains light, occupants and equipment can be assigned to different 3D objects. In other words, occupants objects may have varying MET levels, and equipment objects may emit different amounts of heat. For every internal gain there needs to be an object of it in the zone as well as a 3D object to assign it with for the CFD calculation.

Not assigning CFD role to internal gains

When internal gains are not assigned a specific CFD role, the CFD extension automatically distributes the heat flux within a default geometric volume, which differ between objects;

Not assigning CFD role to ideal heater or ideal cooler

If no CFD role is assigned to an ideal heater or cooler, the convective heating or cooling will be evenly distributed across the mesh cells of the CFD domain.

Airflow input

• ​Under General tab -> Zones -> Terminals, some of the key data is presented in a table, which gives an overview.
• Airflows in IDA ICE CFD will be the selected time and date (instant) of the dynamic simulation.
• Each air terminal has an "Air source" assigned to it. In the zone form under "Air handling units" new air handling units can be added. Once added, it will be available in the air source drop-down menu.

Exhaust airflow input

Each exhaust terminal has a defined air source, an AHU. For each AHU in the zone the airflow is distributed using the "Terminal area" in the General tab for the exhaust terminal. For example, if there is two exhaust terminals with the same air source and their respectively Terminal area is 0.1 m² and 0.4 m² (sum 0.5 m²), their airflows will be 20 % respectively 80 % of the air source. 

Supply airflow input

• Distribution parameter of supply airflow:
  • "Design air flow":
    Used when the supply airflow is not evenly distributed between different supply terminals. Consider it as terms of a sum, e.g. if terminal A has value 2 and terminal B has value 1, one third (1/3) is supplied by terminal B. Entering actual airflow is also an option.
  • "Design air velocity":
    Velocity (m/s) = "Design air flow" / "Terminal area"
    NB! The velocity is only updated when the form is closed and re-opened or switching between tabs. Please also note that the "Design air velocity" is not affected if geometry is enabled under the CFD tab. 
• Supply terminal, Basic: 
  • Set inflow angles in the CFD tab of the air supply terminal. Use 3D tab to visualize the effects.

• Supply terminal with enabled geometry will apply angle and azimuth. But in current CFD version no arrows are displayed in the 3D tab when the geometry is enabled.

Please note that not all geometric representations are compatible with every type of inlet supply air angle input. 
A list related to this will be published here.

Example of using angle and azimuth:

CO₂ sources

In IDA ICE 5.2, CO₂ production can be assigned to both occupants and equipment. For occupants, CO₂ generation is calculated as a function of the MET value and the associated schedule with profile(s). For equipment, CO₂ generation is calculated as a function of the “CO₂ per unit” parameter and the associated schedule with profile(s).

• Ambient CO₂ is according to value in System parameters.
• Only CO₂ emissions are calculated, CO₂ capture (negative emissions) is not possible to calculate.
• CO₂ is calculated as a volume fraction, but is treated as a separate species.
• The CO₂ emission is produced in a volume near the IDA ICE object of the internal gain, it is not connected to a geometry object that is assigned a CFD role. Open the floor surface of the zone to edit the placement of the IDA ICE object.
• CO₂ emissions can not be connected to a custom control, only regular schedules with profiles. 
• Please note:
  • In tab Simulation, setting internal gains to a value lower than 100 % will not have an effect on the CO₂ emissions.
  • Before starting your CFD simulation, to include CO₂ emissions in your simulation, you need to enable "Solve for CO2" by opening "Solver options" in tab CFD. 
  • In post-processing, the value of CO₂ for sensors in the backend graph, is excluding ambient CO₂ emission levels.

Water radiator

A water radiator is inserted via the Palette (to the left) using drag and drop when a surface of a zone is opened.

NB! In some cases, the convective heat output of the water radiator object may be unrealistic. It is important that parameters are reasonable and reflect realistic conditions, especially geometric dimensions and design power. Verify your input in the table "Boundary input and sources" in the CFD tab. This section of the workflow is described on the page titled Boundary condition input.

Using a room unit in the floor or ceiling, in a single-zone model for CFD

Example: Floor heating in a single-zone model for CFD

By default in IDA ICE an adiabatic floor slab with no connection to ambient is connected to its own ceiling construction. This might cause problems when modelling floor slab in IDA ICE CFD, because unwanted heating might be transferred to the ceiling. 

By modelling a zone above the floor heating zone for CFD simulation the ceiling is connected to the new zone and floor heating is isolated to the floor.

Active beam

To add an active beam, open a zone ceiling and drag and drop the object "Active beam". Size it with its properties and CFD tab. In the CFD tab the "THERM-BEAM-XY-1" has supply areas in four directions, and the "THERM-BEAM-YY-1" has supply areas in two directions. The geometry for CFD needs to be enabled by ticking the checkbox "Enable CFD representation". 

Q = Heat flux from IDA ICE simulation
V1 = Supply air volume flow
T1 = Supply air temperature
V2 = Induced air volume flow
T2 = Average temperature of induced air
IR = Induction Ratio = V2/V1
V3 = V1*(IR+1)

rho = f(T) Ideal gas, using constant reference pressure
T2' = T2+Q/(rho2*V2*cp)

T3 = (rho1*V1*T1+rho2*V2*T2')/(rho1*V1+rho2*V2)

Sensors as measuring probes

Insert sensor points to evaluate convergence.
Open floor of the zone (e.g. from 3D view -> Right-click floor -> Open).

• In "Palette" to the left, drag and drop "Sensor" and place it.
  • Double-click the sensor and enter z value - height above the floor.
• Ignore "Measured signal" - it's irrelevant to CFD simulation.

The Introduction Exercise - Thermal and airflow input

Assign all parts a CFD role

• In the 3D tab, click the button "z+". This will cut the view horizontally and the inside is now accessible.
• Double-click the occupant and assign its CFD role to "Zone.Occupants". Press "OK".

• Double-click the convector and assign its CFD role to "Zone.Ideal coolers and heaters". Close the window.

• Left-click the floor and click the blue text to the left "Open zone" under Properties.

• Under "Internal gains":
  • Double-click occupants and set MET value to 1.2 (126 W).

• Under "Internal gains":
  • Right-click Equipment object and press "Delete".
  • Leave Light object as it is. There is no geometry created for it in this example. Modelled this way will let it emit heat but it will be evenly distributed over the zone surfaces.

Air flows and convector

• In the zone form set the Supply and Return air to 2.5 l/(s,m²).
• Double-click the Ideal heater and set its value for "Long wave radiation fraction" to 0.1.

• Click the blue radio button "Terminals".
  • Double-click the Supply terminal.
    • In tab "General", set Angle to 45° and Azimuth to 90°. Close the window.

• Close the zone form.
• Look at the arrow direction of the Supply terminal in the 3D tab.

Adding sensors

• Double-click the floor in the 3D view and open the "Palette" to the left.
• Drag and drop two sensors to the floor, placement according to image.

• Left-click each sensor, and set its z value to 1 respectively 2 m, 2 m for the one nearest the window.
• Ignore the "Measured signal" input.