Influence of ceiling fan's speed and direction on efficacy of upperroom, ultraviolet germicidal irradiation: Experimental

Abstract

a b s t r a c tIncreasing a ceiling fan's speed from its lowest setting of 61 rpm, which resulted in 0.77 m 3 /s of airflow, to its highest setting of 176 rpm, which resulted in 2.5 m 3 /s of airflow, or having the fan blow either upward or downward had no statistically significant effect on the efficacy of upper-room ultraviolet germicidal irradiation (UVGI). This outcome suggests that air circulation due to the ceiling fan was sufficient and that any additional increase would not improve efficacy. Numerous experimental studies on upper-room UVGI in which fans were used to provide air mixing have been published. However, none have quantified the air movement produced by these fans or described their tests in sufficient detail to allow results to be compared to predictions using computational fluid dynamics (CFD). The present work provides the required information. In addition to the usual boundary conditions needed for CFD, we made experimental measurements of UV susceptibility of the microorganisms used in the upper-room UVGI tests. We measured UV susceptibilities for Mycobacterium parafortuitum and Bacillus atrophaeus spores to be 0.074 and 0.018 m 2 /J, respectively. In a previous publication, we reported the spatial distribution of fluence rate, which is also needed for predicting efficacy from CFD. In a companion paper referred to as Part II, upper-room UVGI efficacy was predicted by both Eulerian and Lagrangian CFD and compared to the experimental results from the present study.

A validated numerical investigation of the ceiling fan's role in the upper-room UVGI efficacy

“…The twenty z-values were introduced, and the stochastic inactivation algorithm of the Lagrangian method was deployed to determine the number of surviving microorganisms at exhaust and the fraction remaining according to equation (5). For each simulated case, Eulerian and Lagrangian predictions of fraction remaining are compared over the full susceptibility range to the perfect mixing regime and to the experimental data for the two pathogens [19] with their mean and 95% confidence intervals.…”

“…This study examines the upper-room UVGI efficacy in an experimental chamber located at the Harvard School of Public Health, described, along with the experimental methods, in Rudnick et al [19]. The room had dimensions of 4.6 Â 2.97 Â 3.05 m, and is shown with the computational mesh in Fig.…”

“…Measurement data were collected for two microorganisms: Bacillus atrophaeus, with UV susceptibility of 0.018 ± 0.0017 m 2 /J, and Mycobacterium parafortuitum, with UV susceptibility of 0.074 ± 0.011 m 2 /J [19]. Microorganisms were nebulized into the room through a spherical injector of 75 mm diameter, located in the center of the room 1.5 m above the floor with a supply rate of 12 L/ min.…”

“…The fluence rate data for this particular lamp fixture and experimental setup are available in the literature [19]. The fluence rate data were interpolated using the software built-in interpolation methods, but without much effect from the interpolation.…”

“…An analytical expression for the fraction remaining in case of perfect mixing was developed and used for comparison with the simulated cases. Predictions were compared to experimental measurements for two microorganisms with about a four-fold difference in UV susceptibility [19]. It is important to notice that the Computational Fluid Dynamics (CFD) predictions presented here had been conducted before these experimental results became available.…”