Local flame attachment and heat fluxes in wind-driven line fires

Abstract

A detailed experimental investigation of turbulent diffusion flames under forced flow was conducted to study local heat fluxes to a nearly adiabatic surface downstream of a gaseous line burner. A variety of ambient wind velocities and fuel flow rates were employed to study different fire scenarios modeling the dynamics of wind-driven fire spread as found in wildland, mine or tunnel fires. The downstream heat flux distribution was correlated as a piecewise function with the Richardson number in two regimes, the first with higher heat fluxes, where the flame remained attached the downstream surface (attached region) and the second with a steeper decay of heat fluxes (plume region). Analysis of the heat flux distribution revealed that local heat fluxes roughly reach a maximum where the Richardson number equaled unity. This was thought to be a good marker of the regime where the flame detaches from the surface, e.g. where buoyancy from the flame overcomes inertial forces from the oncoming flow. This observation was further corroborated by analysis of side-view images of the flame, which showed the attachment location was linearly correlated with the location where the Richardson number equaled unity. The results from this study suggest that local heat flux values reach a maximum at the transition between a momentum-dominated (attached, wind-driven) to buoyancydominated (plume or fire) regime in forced flow scenarios. The results have interesting implications to the problem of flame attachment, which is known to accelerate fire spread in both inclined and wind-driven fire scenarios.

Impact of flat roof–integrated solar photovoltaic installation mode on building fire safety

“…This is an important issue affecting the uncertainty in the determination of the flame extension geometry. Since there are also inherent oscillations for the visible extended flame front beneath the panel, the image average method introduced in Section 2 was adopted to get the mean flame extension geometry as was done in literature . Figure exemplifies the typical flame extension probability contour diagrams with = 13 kW and h = 10 cm for different panel tilt angles.…”

“…Since there are also inherent oscillations for the visible extended flame front beneath the panel, the image average method introduced in Section 2 was adopted to get the mean flame extension geometry as was done in literature. [29][30][31][32] Figure 4 exemplifies According to Zhang et al, 37 the flame extension length is related to the HRR of the confined flame:…”

Prediction of Three-Dimensional Downward Flame Spread Characteristics over Poly(methyl methacrylate) Slabs in Different Pressure Environments

“…For an inclined surface, the gravitational acceleration in calculating the Grashof number must be replaced by its streamwise component $g\cos \mathrm{sans-serif\theta }$ [3,33,34,35,36,37]. As a result, the modified Grashof number $G{r}^{\ast }$ is given by $G{r}^{\ast }=\frac{g\cos \mathrm{sans-serif\theta }\times \mathrm{sans-serif\psi }false(T_f-T_{p}false)L^3}{v^2}$ where $\mathsf{\psi }$ is the volume thermal expansion coefficient; L is characteristic length along fire plume, which can be estimated as the fire length of unit width, $1/\sin false(\mathsf{\alpha }/2false)$ in this study; and $v$ is kinematic viscosity.…”

An Experimental Study of Intermittent Heating Frequencies From Wind-Driven Flames

“…In wind-driven fires, the pulsation of the flame is not expected to scale with burner size in the same way as fires under stagnant conditions as wind plays a significant role in the fire behavior, too. These fires have already been shown to be strongly influenced by a competition between upward buoyant forces from the flame and forward momentum from the wind (Tang et al, 2017a), suggesting a combination of these forces will also play a role in generating intermittent motions within the flame.…”

“…In our previous work (Tang et al, 2017a), the local total heat flux distribution on the downstream surface of winddriven line fires was investigated and a local Richardson number [Ri x = Gr x / Re 2 x , describing flame buoyancy over wind momentum (Subbarao and Cantwell, 1992;Johnson and Kostiuk, 2000)] was employed to scale measured non-dimensional heat fluxes. Inertial forces would be expected to dominate the flame behavior when Ri x <0.1, and buoyant forces when Ri x > 10.…”