Modeling of Carbon Monoxide Dispersion from Vehicle Exhaust in a Partially-Enclosed Roadway Using FDS

The design for a partially enclosed roadway raised concerns regarding the potential for development of hazardous carbon monoxide (CO) concentrations. The proposed roadway section is covered by buildings on three sides with one side open to a river. There were concerns that wind conditions may cause vehicle exhaust to become trapped in the roadway during stalled traffic conditions.

The interaction of wind with the roadway and the local cityscape creates a complex system. The potential for wind conditions that may confine CO to the roadway is difficult to accurately analyze solely based on engineering experience or using simple engineering correlations. Instead, Fire Dynamics Simulator (FDS) version 5.5 was used to model the dispersion of the vehicle exhaust. An FDS model was created that encompassed several hundred meters around the enclosed portion of the roadway up to a height of 80 m. The height was selected to capture the tallest buildings in the domain. Tall buildings surrounding the roadway were included in the model to approximate the wind conditions that would develop as closely as possible within the computational restrictions.

Dispersion of CO from stalled vehicles was modeled under a range of different wind scenarios selected based on a previous pedestrian wind study. Wind speed and direction were varied. Assumptions were made regarding the CO production of each vehicle, exhaust temperature and the density of vehicles in the tunnel.

Based on the FDS model results it was found that the critical limit for 15-minute average CO concentration of 120 ppm was never exceeded at any point. This analysis showed that the natural ventilation of the partially enclosed roadway was sufficient and no additional mechanical exhaust systems were required.