A CFD-Based Decision Matrix for Evacuation Planning: Minimizing Exposure to Hazardous Chemicals

Abstract

The extent of casualty and property loss due to chemical accidents depends on how well the emergency action plan was established in advance and how fast the warning notice and evacuation orders are given to the public. Assistant methods for the establishment of protective action plans have been developed for several decades. However, the currently developed decision trees are complicated, so they may require a detailed analysis, and previous decision matrices do not consider the indoor and outdoor concentration directly and hence do not allow a change in evacuation order. In this study, five key parameters, report time, toxic cloud arrival/removal time and indoor/outdoor concentration, are selected for the evacuation decision, and the effectiveness of leakage and wind speed on five parameters is investigated. CFD simulations are performed for the various values of mass flow rate and wind speed. Near the release point of toxic gas, the maximum concentration is unaffected by wind speed, but the mass flow rate significantly influences it at low wind speeds. In the far field, the maximum concentration decreases with increasing wind speed. The termination time for shelter-in-place, suggesting a shift to evacuation, decreases with both higher mass flow rate and wind speed. For smaller mass flow rates (m˙=0.1kg/s), indoor concentration exceeds outdoor levels after 25.9 min, while for larger mass flow rates (m˙=2.0kg/s), this time shortens to 15.2 min. Increasing wind speed from 0.5 m/s to 5.0 m/s decreases the equilibrium concentration from 13.9 ppm to 3.4 ppm and reduces the escape time from 48.9 min to 16.0 min. Overall, higher mass flow rates and wind speeds shorten the equilibrium and escape times, improving toxic cloud removal efficiency. Based on the simulation results, a new evacuation decision matrix is developed which minimizes the total exposure concentration. This study provides the proper evacuation time along distance which eventually prevents traffic congestion because of the simultaneous escape rush.