In this study, we improved a reported mathematical model to predict the UV irradiance of multiple upper-room ultraviolet germicidal (UR-UVGI) fixtures system and then incorporated an improved drift flux model to study the disinfection efficacy of the system. The effect of operating UR-UVGI fixture number and average UV irradiance were investigated under well-mixed ventilation scheme. Results show that higher disinfection efficacy was found in four UR-UVGI fixtures system whose irradiance distribution is much more symmetrically. Being different from the reported results obtained with winter condition, when apply four UR-UVGI fixtures system on a ventilated room, higher volume average irradiance will be the better disinfection efficacy for the microorganisms Z value less than 0.2m~2J~(-1). When the microorganisms Z value is higher than 0.2m~2J~(-1), the optimal volume average irradiance should be 0.4-0.6Wm~(-2) for a well-designed UR-UVGI system.
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