Chlorine gas release in urban area : Calculations and consequences through CFD modelling and comparison with standard softwate

This paper focuses attention on emergency management associated a chlorine gas release in urban area. In particular we want to show how the complexity of the geometry of a city could affect the distribution of a cloud of toxic substance. For this reason, it was simulated by the CFD code Fluent a release of chlorine and has compared the result with a simulation carried out with standard code for risk analysis, in our case PHAST. The study of release of hazardous substance is characteristic of risk analysis. The release may be caused by accident event involving plant or material transported by roadway and railway. Substances transported by road or train can produce effects greater than for plant installations, because the location of these substances can pass through the high population density areas as in the urban centre. the past few years, after the attack of the twin towers, the substances transported by road are used in terrorist attacks in sensitive areas like the center of a city (Lisi R., et al, 2007; Maschio G., Milazzo M.F., 2008) The gas release is characterized by the degree of atmospheric turbulence. This parameter affects the mixing of cloud with the surrounding air. The atmosphere turbulence is caused by friction between the ground and air. So the parameter that characterizes the turbulence and therefore the dispersion is the ground roughness. Ground roughness is determined by number and size of roughness elements present in an area. In conventional codes for the study of risk analysis, such as PHAST (DNV Software), the ground roughness makes homogeneous the ground, and the code don’t consider the contribution of the singular parts that constituent the ground. The study area appears to plane geometry or two-dimensional. In the case of urban areas and thus made complex by the presence of many buildings, the gas dispersion is affected, there may be channels or points of stagnation gas. For this reason we studied a release of chlorine in three-dimensional urban environment with code CDF Fluent (Scargiali F. et al, 2008; Di Sabatino et al, 2008) In particular we studied a release of chlorine gas. This substance is widespread and used; for this reason transport of chlorine by road or rail is common. In fact, chlorine is one of the toxic substances used for terrorist attacks.