Use of Macroscopic Flow Number For Assessing Spaces With Mechanical Ventilation

W. K. Chow

DOI: 10.2190/4RB4-PXJ7-U7CY-M41W

Abstract

The performance of the mechanical ventilation system in large spaces is proposed to be evaluated by macroscopic numbers describing the air flow. This includes the air changes rate per hour, the air change rate per hour per floor area, the momentum jet number, and the nominal time constants. To determine which number is the best choice, experimental studies in nine big waiting halls were performed. the indoor environment in the mechanical ventilated spaces can be assessed by the measured local mean air velocity, carbon dioxide level, and local age of air. Correlations of the measured mean values of local air velocity, carbon dioxide level, and local age of air with those macroscopic flow number are investigated. It is found that the relationships of the air change rate per hour, the air change rate per hour per floor area and the nominal time constants with the mean air speed, the mean carbon dioxide concentration, and the mean local age of air were not clear. A revised momentum jet number taking the height of the diffuser into account is proposed and linear relations can be fitted empirically to correlate it with the mean air speed, carbon dioxide concentration, and the local age of air for these nine sites. It seems that this flow number can be used for evaluating the performance of the ventilation systems.

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