A Global Relation of Fire Smoke Re-Circulation Behaviour in Urban Street Canyons

Abstract

Street canyon, formed by buildings along two sides of a road, is an important and ubiquitous architectural element in the metropolises. When a fire occurs in an urban street canyon, a critical serious phenomenon is found to happen where the uprising fire smoke is re-circulated back into the base of the street canyon by the wind flow beyond a critical velocity. Theoretical analysis is derived based on Froude number (Fr), by balancing the inertial force of the wind flow to the buoyancy strength of the fire smoke. It is found that the critical re-circulation wind velocity is proportional to an integrated global parameter Large Eddy Simulations (LES) are carried out by Fire Dynamics Simulator (FDS). The critical Froude number (Fr) is found to be about 0.7~0.8. The simulation results are well collapsed by the generalized theoretical relation. It is further revealed that the non-dimensional critical re-circulation wind velocity against aspect ratio of street canyons in the skimming flow pattern (W/H < 1.43) falls into two behavioural regimes, where it firstly increases then remains constant with the increase in street canyon aspect ratio (W/H) with a turning point at W/H = 1. A global non-dimensional relation is finally achieved for the critical re-circulation wind velocity (u), fire heat release rate ( ) and its height (H) to the top of the street canyon as well as the street canyon aspect ratio (W/H).

Numerical Simulation of the Smoke Recirculation Behavior in Street Canyons with Different Aspect Ratios and Cross-Wind Conditions

“…The decrease in the critical velocity for different street widths could be effectively proposed through a series of simple equations, such as , displayed in the right corner of Figure 7 . It is seen that the β coefficients are all close to −0.3, which is consistent with previous investigations by Hu [ 31 ], where . In the meantime, for an ideal street canyon with an aspect ratio of n = 1, the critical recirculation velocity is shown to be almost constant at around 2.8–2.9 m/s (see the ellipse in Figure 7 ); hence, the average critical recirculation velocity for ideal street canyons is suggested to be 2.84 m/s.…”

“…Then, a uniform inlet velocity boundary condition was set on the left side of the simulation domain as representative of the ambient cross-wind. The configuration abides by some previous works by Hu [ 20 , 21 , 31 ], Zhang [ 26 , 30 ] and Pesic [ 23 ], but the wind profile was not considered in these works. The wind direction was perpendicular to the building, but the velocity of the cross-wind was altered step by step in the simulation to search for the critical smoke recirculation velocity of the street canyon, which was determined visually from the smoke dispersion throughout the simulation time period.…”

“…Considering the different flow patterns due to various aspect ratio conditions, when a fire occurs in the street canyon, the aspect ratio effect on the smoke dispersion could be another issue to be addressed. The aspect ratio effect on the critical wind velocity and CO concentration was discussed by Zhang [ 30 ] and Hu [ 31 ]. It was found that the critical wind velocity does not fall into a simple linear correlation; however, the building height did not surpass 24 m in their models, which might not be enough.…”

The investigation of street canyon flow field and pollutant diffusion under the coupling effect of dynamic air flow field and roof angle

“…[6][7][8] The airflow structure and pollutant distribution can be affected by many factors, including meteorological conditions, building structure, traffic flow, the effects of canopy properties, location and shape of pollution sources and so on. [9][10][11][12][13] The building structure is the main factor that could affect the airflow structure and pollutant distribution. 14 Therefore, in order to find a way to reduce the damage of air pollution to the human body, the study of the influence of building structure characteristics of street canyon on airflow structure and pollutant distribution is necessary.…”

Computational Assessment of the Hazardous Release Dispersion from a Diesel Pool Fire in a Complex Building’s Area

“…Several studies in an open space pool fire accident exist [29][30][31][32][33][34][35], as well as for a pollutant dispersion in a street canyon [17,18,36,37]. However, only a few of them focus on the study of a fire accident inside a street canyon [21,[38][39][40][41][42]. The pollutant dispersion into an array of cubes has been studied experimentally and computationally [10,43,44]; however, there is a lack of documentation for a fire incident inside an array of cubes.…”