### 9.1.1. Setting Up HVAC Simulations

This section discusses how to set up various physical processes, CFD features, and components involved in HVAC simulations.

#### 9.1.1.1. Buoyancy

Most HVAC cases involve flow that is affected by buoyancy. Buoyancy can be activated on the Basic Settings tab of the Domain details view.

Two buoyancy models are available: Full and Boussinesq. These models are automatically selected according to the properties of the selected fluid(s).

- The Full buoyancy model is used if fluid density is a function of temperature and/or pressure (which includes all ideal gases and real fluids). In this case, a Buoyancy Reference Density must be set as the expected average density of the domain.

- The Boussinesq model is used if fluid density is not a function of temperature or pressure. In this case, a Buoyancy Reference Temperature must be set as the expected average temperature of the domain.

When fluid properties are functions of pressure, the absolute pressure is used to evaluate them. The calculation of absolute pressure requires a Buoyancy Reference Location to be defined, preferably by specifying coordinates.

When modeling fire, it is recommended that you choose a compressible fluid because density variations will be significant. An incompressible fluid should be chosen only if density variations are small (a few percent or less).

#### 9.1.1.2. Thermal Radiation

To set the radiation model for a fluid domain, visit the Fluid Models panel for that domain and set the following:

##### 9.1.1.2.1. Thermal Radiation Model

For HVAC studies, select either Monte Carlo or Discrete Transfer. If directed radiation is to be modeled, Monte Carlo must be used.

##### 9.1.1.2.2. Spectral Model

Select either Gray or Multiband. Spectral bands are used to discretize the spectrum and should therefore be able to adequately resolve all radiation quantities that depend on wavelength (or frequency or wave number). For HVAC, two bands will usually suffice.

##### 9.1.1.2.3. Scattering Model

A scattering model should not be used if you are modeling clear air. The isotropic scattering model should be used if you are modeling air that contains dust or fog.

To set up radiation for a solid domain, visit the Solid Models panel for that domain (each solid domain must be set up separately). The only radiation model available for solid domains is Monte Carlo.

Note: If any solid domain uses the Monte Carlo radiation model (that is, if it uses radiation at all), then all fluid domains using a radiation model must use the Monte Carlo model.

The material used in a domain that transmits radiation has radiation properties that specify Absorption Coefficient, Scattering Coefficient, and the Refractive Index. These properties may be edited in the Materials details view.

Note: Radiation modeling cannot be used with Eulerian multiphase simulations.

Thermal radiation properties are specified on the Boundary Details panel for each boundary of a domain that transmits radiation. For opaque surfaces, the properties that must be specified are: Emissivity and Diffuse Fraction. For inlets, outlets, and openings, you may specify either the Local Temperature or an External Blackbody Temperature.

The Monte Carlo and Discrete Transfer models allow radiation sources to be specified on the Sources panel for any subdomain or wall boundary. For subdomains, radiation sources per unit volume are specified; for boundaries, radiation fluxes are specified. Radiation sources may be directed or isotropic. Multiple isotropic sources and up to one directed source may be specified for any given wall boundary or subdomain.

Material properties related to radiation, thermal radiation properties for boundaries, and source strengths can be specified as expressions that depend on one or more of the built-in variables: Wavelength in Vacuum (or wavelo), Frequency (or freq), Wavenumber in Vacuum (or waveno).

A domain representing an opaque solid should not have a radiation model set. The boundaries of radiation-transmitting domains that interface with such a solid domain should be specified as opaque.

External windows of a room can be modeled as solid domains which interface with the room (air) domain; they may also be modeled as an external boundary of the room domain. In either case, the exterior boundary must be modeled as an opaque wall. A diffuse and a directed radiation source emitted from the opaque surface can be used to simulate sunlight. In order to simulate the motion of the sun, the direction vector for directed radiation can be specified by CEL expressions that depend on time (t). Radiation escaping through a window can be modeled by specifying a non-zero emissivity (to cause radiation absorption) and either:

- Specifying a heat transfer coefficient via a CEL expression that accounts for the thermal energy lost

- Specifying a fixed wall temperature.

When using solid domains that transmit radiation, a spectral radiation model is recommended. If a simulation contains no solid domains that transmit radiation, a gray radiation model can be used for rough calculations but a spectral model should be used for more detailed modeling.

#### 9.1.1.3. CHT (Conjugate Heat Transfer) Domains

CHT domains are solid domains that model heat transfer. In CFX, all solid domains must model heat transfer, and are therefore CHT domains. If you do not want to model heat transfer in a particular region, do not assign the mesh for that region to any domain.

Boundaries between domains that model heat transfer have temperatures and thermal fluxes calculated automatically, and should not have thermal boundary conditions specified. External boundaries (which can represent solids that are not explicitly modeled) require the specification of a thermal boundary condition.

Boundary conditions other than thermal boundary conditions (for example, wall roughness) may be specified on the boundaries of a fluid domain that interface with a solid domain.

Sources of thermal energy and/or radiation can be added to a subdomain of a CHT domain.

#### 9.1.1.4. Mesh Quality

Ensure that wall boundary layers have adequate mesh resolution. This is important regardless of the type of wall heat transfer: adiabatic, specified temperature, specified heat flux, or heat transfer coefficient.

The mesh resolution in a boundary layer affects the prediction of convective heat transfer and the temperature gradient near the wall. For walls without a specified temperature, the temperature gradient near the wall affects the calculated wall temperature and, consequently, the amount of radiation emitted (provided that the emissivity of the wall is non-zero).

#### 9.1.1.5. Fans

Fans should be represented by momentum sources if they are embedded in the domain. Fans can also be represented by an inlet or outlet boundary condition or both.

#### 9.1.1.6. Thermostats

A Thermostat can be defined using a User Fortran routine. Refer to the HVAC tutorial for details.

#### 9.1.1.7. Collections of Objects

If your HVAC simulation models a large number of people/equipment/items, consider volumetric sources of heat, CO2, and resistances.

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