Ventilation

For Airflow Networks, infiltration and exfiltration are simulated via two methods, the Effective Leakage Area or via determining the characteristics of airflow through a Crack.

Effective Leakage Area

The effective leakage area (ELA) component is utilised to describe surface air leakage, consisting of five separate elements. The connection between pressure and airflow can be expressed in the following manner:

$$m = ELA \cdot Cd \sqrt{2\rho \cdot (\Delta P_r)^{0.5} - n(\Delta P)^n}$$

$˙m$ = Air mass flow rate [kg/s]

$ELA$ = Effective leakage area [m$2$]

$ρ$ = Air density [kg/m$3$]

$ΔPr$ = Reference pressure difference [Pa]

$ΔP$ = Pressure difference across this component [Pa]

$Cd$ = Discharge coefficient [dimensionless]

$n$ = Air mass flow exponent [dimensionless]

Air Mass Flow Exponent

This numeric field is designated for inputting the pressure difference exponent. The acceptable range for the exponent lies between 0.5 and 1.0, with a default value preset at 0.65.

Leakage Area

This numeric field allows input of the effective leakage area in square meters. It serves to characterize openings for infiltration calculations as per the ASHRAE Handbook of Fundamentals and must be greater than zero.

Discharge Coefficient

This numerical field is utilized to input the discharge coefficient, a dimensionless value typically greater than zero. The default value is set to 1.0.

Reference Pressure Difference

This numerical field serves to input the reference pressure difference in pascals [Pa]. The value should exceed zero, with a default setting of 4.0 Pa.

Air Mass Flow Exponent

This numeric field is designated for inputting the pressure difference exponent. The acceptable range for the exponent lies between 0.5 and 1.0, with a default value preset at 0.65.

Crack

This method delineates the characteristics of airflow through a crack and the corresponding measurement circumstances. It employs the power law form, which expresses airflow through the crack as a function of the pressure difference across it:

$$Q = (\text{CrackFactor}) \cdot CT \cdot CQ (\Delta P)^n$$

Q = air mass flow (kg/s)

C$Q$ = air mass flow coefficient (kg/s-Pa$n$ @ 1 Pa)

C$T$ = reference condition temperature correction factor (dimensionless)

$ΔP$ = pressure difference across crack (Pa)

n = air flow exponent (dimensionless)

$$CT = \left[ \left( \frac{\rho_o}{\rho} \right)^{n-1} \left( \frac{\nu_o}{\nu} \right)^{2n-1} \right]$$

$ρ$ = Air density at the specific air temperature and humidity ratio conditions [kg/m$3$]

$ν$ = Air kinetic viscosity at the specific air temperature condition [m$2$/s]

$ρo$ = Air density at the reference air conditions [kg/m$3$]

$Cequ$ = Air kinetic viscosity at the reference air temperature [m$2$/s]

Air Mass Flow Exponent

This numeric field is designated for inputting the pressure difference exponent. The acceptable range for the exponent lies between 0.5 and 1.0, with a default value preset at 0.65.

Air Mass Flow Coefficient at Reference Conditions

This refers to the value $𝜈𝑜νo​$ in the crack airflow equation. It is expressed in units of kg/s at 1 Pa. The value must exceed zero.

Reference Temperature

The temperature in degrees Celsius at which the Surface Crack Data were acquired. The default value is set to 20°C.

Reference Humidity Ratio

This denotes the humidity ratio in kgWater/kgDryAir at which the Surface Crack Data were collected. The default value is 0 kgWater/kgDryAir.

Reference Barometric Pressure

This indicates the barometric pressure in pascals (Pa) at which the Surface Crack Data were recorded. The default value is set to 101325 Pa.