ASHRAE+62.1-2022+(1) - Page 045
Extracted Content
(This is a normative appendix and is part of the standard.)
NORMATIVE APPENDIX C ZONE AIR DISTRIBUTION EFFECTIVENESS: ALTERNATIVE PROCEDURES
This appendix provides a procedure for determining zone air distribution effectiveness ( Ez ) for all system types.
Informative Note: Table 6-4 provides default values of Ez that are permitted to be used for the air distribution configurations described in the table. The reference Ez value of 1 is typical of ideal mixing in the zone. The strategy of removing contaminants or displacing contaminants from the breathing zone may result in an effective Ez value greater than unity, which is typical of stratified systems.
C1. ZONE AIR DISTRIBUTION EFFECTIVENESS
Zone air distribution effectiveness shall be calculated in accordance with Equation C-1:
Ez = ( Ce - Cs )/( C - Cs ) (C-1)
where
Ez = zone air distribution effectiveness
C = average contaminant concentration at the breathing zone
Ce = average contaminant concentration at the exhaust
Cs = average contaminant concentration at the supply
C1.1 Personalized Ventilation Systems. For the purpose of calculating zone air distribution effectiveness for personalized ventilation systems, the breathing zone shall be 9 ft [2] (0.8 m [2] ) centered on each occupant with a height of 4.5 ft (1.4 m) from the floor.
C2. MODELED AIR DISTRIBUTION SYSTEM
C2.1 Computational Model. The computational fluid dynamics model for calculating zone air distribution effectiveness shall be in accordance with the following subsections.
C2.1.1 Computational Domain. The computational domain shall comprise all sensible heat sources, all major obstructions to airflow, and all air distribution devices. The calculation domain shall include all boundary walls.
C2.1.2 Solution Variables. Analysis shall include the solutions for fluid flow, heat transfer, and chemical species transport. The buoyancy (gravitational) effects shall be included in the calculation procedure.
C2.1.3 Boundary Conditions. Sensible heat sources shall be permitted to be modeled as volumetric heat sources to allow the air to pass through the source or as hollow blocks (no mesh inside) specified with either heat flux or constant temperature on the surfaces of the blocks. Boundary walls shall be modeled as adiabatic (zero heat flux), specified heat flux, or specified temperature boundary.
C2.1.4 Species Transport. The sources shall be modeled as volumetric source or a boundary flux with known generation rate with zero release velocity. The analysis shall be performed with a uniformly distributed source at the breathing zone level of the occupants. All the boundary walls shall be modeled as impermeable to the chemical species.
Informative Note: The species modeled should be a tracer gas, such as CO2. Discretion is left to the modeler to determine the appropriate model depending on the design compounds in the zone.
C2.1.5 Turbulence Model. Reynolds (ensemble) averaging turbulence models shall be used.
Informative Note: Renormalization group and realizable k- models meet the requirements of this section. C2.1.6 Computational Mesh. A fine mesh shall be generated near the sensible heat sources, such as occupants and computers, to resolve the thermal plume surrounding these sources. The fine mesh shall be generated on all supply air and return air locations.
C2.1.7 Solution Convergence. The solution convergence levels shall include the monitoring of relevant physical quantities, such as temperature or species concentration, at strategic locations. The globally scaled residuals shall be decreased to10 [–3] for all equations except the energy and species equations, for which the residuals shall be decreased to 10 [–7] . The mass and energy balance shall be calculated up to at least four (4) decimal places.
Informative Note: Review of the thermal comfort of occupants in the computational model may be desirable.
ANSI/ASHRAE Standard 62.1-2022 43