# How EnergyPlus calculates Richardson number for zone flow regime

According to the Engineering reference of EnergyPlus, it calculates the Richardson number to determine the zone flow regime, thus determining which internal CHTC algorithm should be used during the simulation.

The Richardson number can be formulated by:

Where *Gr* and *Re* mean the Grashof number and Reynolds number, respectively. *g* means the gravitational acceleration, *β* is the thermal expansion coefficient, ΔT is the temperature difference between surface and indoor air, ** u** is the air velocity. LGr and LRe are the characteristic length for Gr and Re calculation.

**The key point is which ' u' is used for Ri calculation?** As the room air is usually assumed to be perfectly-mixed in EnergyPlus. Imagine that I have a mechanical ventilation system that supplies fresh air with 0.1 m3/s to the zone and it corresponds to about 10 air change rates per hour for the zone.

@feilongdang is "CHTC" short for "convective heat transfer calculation"? Also, the Richardson number section in the Engineering Reference appears to specifically focus on the

internal surfaceconvection coefficient calculation when using theAdaptive Convection Algorithmoption. Is this what you intend to do?@Aaron Boranian.Yes, Thank you! CHTC is short for convective heat transfer coefficient. What I want to investigate is how does EnergyPlus calculate the Richardson number to determine the zone flow regime so that the Adaptive algorithm can select the appropriate internal CHTC algorithm. The key is which

air velocityis adopted for the Richadson number calculation. I set a ventilation (10-air change rate per hour) for a room, which should be forced or mixed convection , but the Adaptive Convection algorithm only regards the zone air flow regime as "simple buoyancy" pattern.