Large Eddy Simulation of Fire Smoke Re-circulation in Urban Street Canyons of Different Aspect Ratios

Abstract

Critical re-circulation phenomenon happens when a fire occurs in an urban street canyon, in which the uprising smoke plume is re- circulated back into the base of the street canyon by the wind-induced flow. This happens when the inertia of the wind flow with a certain so-called critical re-circulation velocity balances the buoyancy force of the fire smoke. The fire smoke dispersion and critical re- circulation wind velocity have been revealed in previous works for urban street canyons with aspect ratio (W/H) of unity. This paper presents an investigation of the effect of street canyon aspect ratio on the above behaviour. Large Eddy Simulation (LES) was performed by Fire Dynamics Simulation (FDS) for a 5 MW fire corresponding to the Heat Release Rate of a burning car. The simulated street canyon has height of 18 m and different width of 9 m, 12 m, 15 m, 18 m, 21 m, 24 m, 27 m, 30 m and 36 m respectively, with aspect ratios varied from 0.5 to 2.0. The smoke soot and CO concentration distribution were presented for different street canyon aspect ratios. It was found that the variation of critical re-circulation wind velocity with street canyon aspect ratio can be divided into three zones. The critical re-circulation wind velocity firstly increases at the first zone, then remains constant at the second zone, and finally increases again at the third zone with the increasing of the street canyon aspect ratio. This special behaviour was explained based on the flow characteristics in the street canyon.