Design
The design work not only includes standard mechanical
ventilation processes, but also the following processes for natural
ventilation:
Single-zone model / multi-zone model
The zone model is based on the
abstraction of a building as a flow network consisting of zones
and flow elements. The zones represent a volume in the building
that can be described in terms of pressure, temperature and foreign
matter concentration.
These zones and nodes are linked by flow elements which represent
the airflow rate based on pressure difference. Flow elements can
be doors, air intake louvres and gravity roof ventilators, or the
components in a simple ventilation system such as ducts, grilles
and connectors.
It is also possible to take the airflow pressure characteristics
of fans into account. Air pressure coefficients combine the pressure
at the external nodes with the current airspeed. Not only the effects
of wind, but also the level of air exchange due to chimney effects
(as a result of differences in the density of the air in the building
and external air) can be calculated. The requirements of mass conservation
in each zone are used to solve non-linear equations for each timeframe.
This dimensioning process enables,
- the calculation of any number of vents,
- the variation of all relevant parameters
for these vents, such as installation height, geometric or aerodynamic
surface, pressure loss coefficient (or drag coefficient) and local
wind pressure coefficient,
- the incorporation of values for outside
air temperature, wind direction and wind speed,
- an assessment of the effects of mechanical
ventilation or air extraction on natural ventilation.
Generally, precise data can be obtained in the
dimensioning process on air temperature in the work zone, exhaust
air temperature, mass air flows and air velocity, as well as the
neutral plane, depending on the values of the above parameters.
CFD - Computational Fluid Dynamics
CFD software enables the computer simulation of flow patterns and
the associated physical phenomena, such as heat and mass transfer
in industrial ventilation components and systems.
The applications for CFD extend
from modelling laminar and turbulent internal and external air flows,
though the calculation of temperatures, including heat exchange
and radiation, to the acquisition of data on of mixing processes,
mass transfer and the expansion of concentration fields, such as
smoke spread and fire simulation.
In order to use CFD models, it is
initially necessary to prepare a three-dimensional geometry of the
building, including all details that are relevant for air flows.
The resulting volume is then divided up into a finite number of
small reference units (cells). The number of cells required depends
on the size and complexity of the building (and can vary between
20,000 and several million cells).
The final step is to solve balance
equations in respect of momentum, energy, mass and, where applicable,
turbulence and gas type for each individual cell. Since the conditions
in each of the cells are influenced by the neighbouring cells, the
ensuing equation system is iterative.
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