This model describes the one-directional pressure driven air
flow through a crack-like opening, using the equation

V̇ = k
Δp^{m},

where *V̇* is the volume flow rate, *k* is a flow
coefficient and *m* is the flow exponent. The flow coefficient
is

k = L
C_{D,Rat} Δp_{Rat}^{(0.5-m)}
(2/ρ_{0})^{0.5},

where *L* is the effective air leakage area,
*C*_{D,Rat} is the discharge coefficient at the
reference condition, *Δp*_{Rat} is the pressure drop
at the rating condition, and *ρ*_{0} is the mass
density at the medium default pressure, temperature and
humidity.

The effective air leakage area *L* can be obtained, for
example, from the ASHRAE fundamentals (ASHRAE, 1997, p. 25.18). In
the ASHRAE fundamentals, the effective air leakage area is based on
a reference pressure difference of *Δp*_{Rat} = 4 Pa
and a discharge coefficient of *C*_{D,Rat} = 1. A
similar model is also used in the CONTAM software (Dols and Walton,
2002). Dols and Walton (2002) recommend to use for the flow
exponent *m=0.6* to *m=0.7* if the flow exponent is not
reported with the test results.

#### References

**ASHRAE, 1997.** *ASHRAE Fundamentals*, American
Society of Heating, Refrigeration and Air-Conditioning Engineers,
1997.
**Dols and Walton, 2002.** W. Stuart Dols and George N.
Walton, *CONTAMW 2.0 User Manual, Multizone Airflow and
Contaminant Transport Analysis Software*, Building and Fire
Research Laboratory, National Institute of Standards and
Technology, Tech. Report NISTIR 6921, November, 2002.

- June 24, 2018, by Michael Wetter:

Removed parameter `lWet`

as it is only used to compute
the Reynolds number, and the Reynolds number is not used by this
model. Also removed the variable `Re`

for the Reynolds
number.

Changed base class to remove the parameters `A`

and
`CD`

which are not used by this model.

This change is non-backward compatible.

This is for IBPSA,
#932.
- May 30, 2018, by Michael Wetter:

Improved documentation for IBPSA,
#546.
- April 24, 2017, by Filip Jorissen:

Corrected error in computation of `A`

which was
`A=CD/CDRat * L * dpRat^(0.5-m)`

rather than
`A=CDRat/CD * L * dpRat^(0.5-m)`

.

See #743.
- January 22, 2016, by Michael Wetter:

Corrected type declaration of pressure difference. This is for
#404.
- October 8, 2013 by Michael Wetter:

Changed the parameter `useConstantDensity`

to
`useDefaultProperties`

to use consistent names within
this package. A conversion script can be used to update this
parameter.
- July 20, 2010 by Michael Wetter:

Migrated model to Modelica 3.1 and integrated it into the Buildings
library.
- February 10, 2005 by Michael Wetter:

Released first version.

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