Fire whirls due to surrounding flame sources and the influence of the rotation speed on the flame height

Abstract

In this paper, we use numerical simulation and laboratory experimental observation to show that fire whirls can be generated spontaneously through the interaction between a central flame and surrounding organized or randomly distributed flames. The momentum of the air stream entrained by the main flame decreases as it crosses a surrounding flame, so that the main flame rotates if surrounding flames are arranged in such a way as to block the passage of the air stream directed towards the centre of the main flame and to favour flows in a particular circumferential direction. An analysis is performed to study the role of the rotation speed in the flame height. It is found that the flame height initially decreases to a minimum owing to the inflow boundary layer wind reducing the initial vertical velocity of gas for low rotation speed and to entrainment enhancement reducing the rising time, and then it increases owing to the pressure reduction at the centre of the rotating vortex and entrainment suppression extending the rising time.

Fire Whirls

“…A fire whirl is not necessarily composed of swirling flames within the vortex column (Countryman 1971), as many cases formed from hot gases downwind of large fires have also been reported (Figure 1b) (Zhou & Wu 2007, McRae et al 2013. Hence, given the collective body of evidence on fire whirls, they can be classified in two main types: on source and off source.…”

“…Fire whirls with flame heights between 0.1 and 1.0 m are defined as small scale (Snegirev et al 2004) and have been abundantly studied both experimentally and numerically (Emori & Saito 1982;Battaglia et al 2000a,b;Snegirev et al 2004;Hassan et al 2005;Zhou & Wu 2007;Chuah et al 2009;Lei et al 2015a;Lei & Liu 2016). Whirls with flame heights between 1 and 10 m are categorized as medium scale, whereas whirls of the order of tens to hundreds of meters in height are categorized as large scale (Snegirev et al 2004, Hartl 2016.…”

“…Type I whirls are most like the enclosed fire whirls discussed in the following section, as both are on source and quasi-steady. This type could also be formed by an array of multiple fires with or without cross flow (Figure 3b), where any asymmetry in the flow or geometry may cause or amplify the swirl that generates the whirl (Zhou & Wu 2007). This behavior is thought to have been observed following the deliberate firebombing of Dresden and Hamburg during World War II (Soma & Saito 1991).…”

“…(e) Formation of a fire whirl due to the interaction of multiple fire sources with cross flow (Liu et al 2007). ( f ) Fire whirl formation from multiple fires without wind (Zhou & Wu 2007). ( g) Generation of whirling columns of hot gases over an L-shaped fire source with 1-cm width through a 0.2-m/s cross flow (Kuwana et al 2013).…”

Experimental Research on Burning Rate, Vertical Velocity and Radiation of Medium-Scale Fire Whirls

“…The burning behaviors including burning rate, flame height and flame temperature were mainly investigated by experimental researchers, such as Emmons and Ying [7] by rotating screen-flame model, Soma and Saito [2] and Kuwana et al [3] utilizing bench-scale model by scaling laws, and Lei et al [8] by fixed frame-flame model. In recent years, numerical simulations [9,10] have also been widely used for solving the Navier-Stokes or vorticity equations of fire whirl, in order to interpret the detailed fluid flow structure. However, considerable simplified sub-models were generally utilized to describe the chemical and physical processes for numerical modeling.…”