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# How is air movement modeled in EnergyPlus and what are typical values?

This is based on another user's question. It has has since been deleted but I think it's worth asking anyway.

Air velocity is a required input for thermal comfort calculations but I'm not clear on how it is entered in EnergyPlus. What type of objects need to be included?

Also, what would typical values be in a domestic situation? I'm looking for references, typical values based on engineering experience (rules of thumb), or ways of measuring/estimating air velocity in existing buildings.

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This subject requires quite a bit of discussion as we often do not give it due diligence in building energy modeling, so here are a few thoughts.

Air movement within a space is not typically modeled in a thermal simulation for energy modeling. Computational fluid dynamics would be required for this level of granularity, and the simulation time is still too large to couple well with hourly simulations effectively. A bulk airflow model (i.e. air flow network) is sometimes used to determine the overall airflow through a space, but this won't tell us much about what is happening regarding air velocities within the space.

The indoor air speed is mostly a function of any ventilation systems within the space. Mechanical ventilation and wind-driven natural ventilation tend to dominate the indoor air flow. It is typical to have supply air diffuser leaving air velocities of 100 - 200 fpm (0.5 - 1.0 m/s). This is far less for displacement ventilation or UFAD. It can be far greater for natural ventilation depending on the wind speeds and aperture design. The terminal velocity of these jets is typically designed to be around 40 - 50 fpm (~ 0.25 m/s). This is the acceptable criteria for the standard comfort conditions within ASHRAE 55.

To measure air velocity measurements needs to be taken from a grid throughout a space. Measurements should be taken over both horizontal and vertical planes within the occupied zone in order to capture any spatial variations. further, both spatial and time averaging should be used since air at low velocities is often susceptible to fluctuations.

The indoor velocity is used to calculate the thermal comfort parameters in EnergyPlus. The user specifies this in the People object under the Air Velocity Schedule Name variable. The air velocity has a significant impact on the overall thermal comfort and must be chosen carefully based on the type of ventilation system provided. Diffuser jet decay and air diffusion models can be used to estimate a general velocity.

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