Predicting worker exposure--the effect of ventilation velocity, free-stream turbulence and thermal condition.

Three-dimensional computational fluid dynamics (CFD) simulations were used to predict the flow field and resulting worker exposures when toxic airborne contaminants were released into the wake region of a mannequin that had its back to the airflow while holding the source of airborne contaminants. The effects of ventilation velocity, free-stream turbulence, and various thermal conditions on fluid flow and exposure levels were evaluated. The results showed good agreement between predicted and experimental concentrations at the mouth at a broad range of airflow velocities when the mannequin was both heated and unheated. When the mannequin was unheated, the exposure level decreased as the ventilation velocity increased. The expectation that buoyancy provided by the heat from the mannequin would be most important at very low velocities and decreasingly important at high velocities was proved true for both the predicted and observed exposures. The result was that when the mannequin was heated to normal human body temperatures, exposure levels had an inverted V relationship with velocity. These findings are important, since they call into question the common practice of modeling human exposures with mannequins at ambient temperatures. In addition, free-stream turbulence could be used to reduce worker exposure to airborne pollutants as suggested by the simulations. CFD enabled a detailed investigation of the effect of particular factors for exposure predictions in a cost-effective way.