Analysis of Efficacy of UVGI Inactivation of Airborne Organisms Using Eulerian and Lagrangian Approaches

Ultraviolet germicidal irradiation (UVGI) is increasingly used in health care facilities to kill airborne bacteria and control the spread of airborne infection among occupants. The most widely used form of UVGI is the passive upper-room lamps that generate a horizontal layer irradiance field above the occupied zone. In this study, the efficacy of killing airborne bacteria by upper-room UVGI in a test room is investigated by using computational fluid dynamics (CFD). The killing of airborne bacteria is dependant on the dose the bacteria receive and their susceptibility. In order to calculate the dose, the trajectory of each of the bacteria needs to be predicted. The common practice in CFD is that the flow field is solved using an Eulerian reference system and the particle trajectory is calculated using a Lagrangian reference system from which the particle dose is derived. This approach gives very accurate and detailed information on the history and behavior of each organism. However, in typical experimental procedures, the samples are usually collected when the numbers of the bacteria killed, ventilated, or remaining viable in the room reach steady state and the bacteria killing mechanism is considered as a group activity. This makes comparison of the CFD prediction with experimental data difficult—the Lagrangian reference system is not suitable when the steady-state information regarding the overall behavior of the bacteria in a room at any moment is of interest. In this study, a new approach is proposed to estimate the bacteria inactivation rate in the Eulerian system. For each room condition, both the Lagrangian and Eulerian approaches are utilized to evaluate the UVGI efficacy. The predicted results with the Eulerian approach are compared with available experimental measurements. INTRODUCTION Ultraviolet germicidal irradiation (UVGI) is increasingly used in hospital laboratories and health care facilities to inactivate the airborne bacteria and so control the spread of airborne infection among occupants. The most widely used application of UVGI is in the form of passive upper-room fixtures containing UVGI lamps that provide a horizontal layer of UV energy field above the occupied zone. They are designed to inactivate bacteria that enter the upper irradiated zone, and their efficacy is highly reliant on, among other factors, the flow field conditions in the room. The survival probability of bacteria exposed to UV irradiance depends on the susceptibility of the bacteria in question and the dose in a general form (Federal Register 1993), % Survival = 100 × e–z(dose) , (1)