Abstract
An experimental study was conducted to understand the intermittent heating behavior downstream of a gaseous line burner under forced flow conditions. While previous studies have addressed time-averaged properties, here measurements of the flame location and intermittent heat flux profile help to give a time-dependent picture of downstream heating from the flame, useful for understanding wind-driven flame spread. Two frequencies are extracted from experiments, the maximum flame forward pulsation frequency in the direction of the wind, which helps describe the motion of the flame, and the local flame-fuel contact frequency in the flame region, which is useful in calculating the actual heat flux that can be received by the unburnt fuel via direct flame contact. The forward pulsation frequency is obtained through video analysis using a variable interval time average (VITA) method. Scaling analysis indicates that the flame forward pulsation frequency varies as a power-law function of the Froude number and fire heat-release rate, . For the local flame-fuel contact frequency, it is found that the non-dimensional flame-fuel contact frequency remains approximately constant before the local Rix reaches 1, e.g., attached flames. When Rix>1, decreases with local as Rix flames lift up. A piece-wise function was proposed to predict the local flame-fuel contact frequency including the two Rix scenarios. Information from this study helps to shed light on the intermittent behavior of flames under wind, which may be a critical factor in explaining the mechanisms of forward flame spread in wildland and other similar wind-driven fires.
Highlights
Wind-driven fires have been studied extensively over the past few decades, there are still significant gaps in understanding, especially applied to wind-driven fires resembling a line fire configuration
Unlike previous studies on flame frequencies (Hamins et al, 1992; Cetegen and Ahmed, 1993), which focused on puffing under stagnant conditions, this paper investigated the flame pulsation frequency under a cross-wind flow
Experiments were conducted on a variety wind-driven line fires where the intermittent behavior of the flame was studied
Summary
Wind-driven fires have been studied extensively over the past few decades, there are still significant gaps in understanding, especially applied to wind-driven fires resembling a line fire configuration. An intermittent “puffing” phenomenon has been extensively studied using pool fires where the puffing frequency of the flame has been found to be well correlated with the diameter of the fire source (Grant and Jones, 1975; Hamins et al, 1992; Hu et al, 2015) These experiments on buoyant plumes suggest that puffing is primarily the result of a buoyant flow instability, which involves the strong coupled interaction of a toroidal vortex formed a short distance above a fuel or burner surface (Cetegen and Ahmed, 1993). 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 play a role in generating intermittent motions within the flame
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