Natural Ventilation Options

Advanced tab on Model Options dialog.

Model holes (including holes in virtual partitions)

You can optionally switch off all holes in the model including those holes generated by defining partitions as virtual partitions. In this case the building is modelled as if no holes were defined.

Tip: Unchecking this data can be a quick way to model all virtual partitions in the model as standard partitions.

Calculated Natural Ventilation

Wind Factor

See Natural ventilation in HVAC model data.

Discharge coefficient for open windows and holes

Enter the discharge coefficient for all open windows and holes. The default of 0.65 is a good general purpose setting.

 

ClosedTheory explaining the discharge coefficient

 

From the IEA Annexe 20 document Air flow Through Large Openings in Buildings

 

The EnergyPlus Airflow Network model is based on the following empirical powerlaw relationship between the flow and the pressure difference across a crack or opening in the building envelope:

 

 

The volumetric flow rate, Q [m3/s], is a simple function of the pressure drop, ∆P [Pa], across the opening. A common variation of the powerlaw equation is:

 

 

where the mass flow rate, F [kg/s], is a simple function of the pressure drop. A third variation is related to the orifice equation:

 

 

where:

 

Cd = discharge coefficient, and

A = orifice opening area.

 

Theoretically, the value of the flow exponent, n, should lie between 0.5 and 1.0. Large openings are characterized by values very close to 0.5, while values near 0.65 have been found for small crack-like openings.

 

Suggested values for discharge coefficient

The literature suggests a large range of different values for discharge coefficient (symbol Cd often used in orifice equations). For example IEA Annexe 20 Air flow Through Large Openings in Buildings indicates that discharge coefficients reported in the literature vary between 0.3 and 0.8 and that it is not understood what causes the differences.

 

Other sources recommend a value close to 0.6. For example the CONTAMW 2.0 User Manual states that the discharge coefficient, is related to the dynamic effects and is typically close to 0.6 for an orifice and slightly higher for other openings in buildings.

 

ASHRAE propose the following correlation based on interzone temperature differences:

 

Cd = 0.4 + 0.0045 δT

 

for the range δT 0.5 to 40.

 

Given other uncertainties in natural ventilation calculations (wind pressure coefficients, effective areas of real-world openings and crack flows etc), using a discharge coefficient between 0.60 and 0.65 should provide sufficient accuracy.

Modulation of Openings

See Natural ventilation in HVAC model data.

Scheduled Natural Ventilation

Airflow through internal openings

When using Scheduled natural ventilation, windows, vents, doors and holes in internal partitions can be modelled by mixing a predefined amount of air from one zone with the adjacent zone by checking the Airflow through internal openings checkbox and entering the Airflow rate per opening area. This inter-zone airflow is transferred equally in both directions and is modelled using the EnergyPlus ZoneMixingoption.

 

See also: