CFD Analysis of Air Change Rate (ACR) and Space Volume on Contaminant Dilution

ASHRAE Annual Virtual Conference, 2020

Author:

Kishor Khankari

Abstract:

The ventilation airflow rates are often specified in terms of Air Changes per Hour (ACH). Such specification requires higher airflow rates for larger spaces irrespective of the strength of contaminants. This study with the help of CFD analyses evaluates the impact of space volume on the spread and distribution of contaminant. It indicates that the specification of the same ACH for all space volumes can make smaller spaces vulnerable for a higher dose of contaminant exposure. This study shows that by proportionately adjusting the ACH with the space volume can maintain the similar levels of occupant exposure across the space volumes and limit the spread of contaminants in smaller spaces. These analyses indicate that the exposure levels in the breathing zone of occupants can be significantly higher than those predicted by the space averaged “well-mixed” conditions. The Spread Index analyses indicate that with the constant ACH approach and due to reduced supply airflow rate the contaminants would reach higher concentrations and spread extensively in smaller spaces than larger spaces. It further indicates that with the volume adjusted ACH approach such spread can be significantly reduced for smaller spaces and equalized across the space volumes. These analyses were performed for identical configurations of supply and return diffusers without adjusting their number, location, and throws. As a result in the case of volume adjusted ACH slightly higher exposure levels and contaminant spread are predicted in the breathing zone of larger spaces. It should be noted that each space is unique, and therefore, HVAC configuration should be optimized by preforming such CFD analyses to maximize the ventilation effectiveness before increasing the supply airflow rates.