Effect of flame zone depth on the correlation of flame length with fireline intensity

Background: Previously established correlations of flame length L with fireline intensity I_B are based on theory and data which showed that flame zone depth D of a line fire could be neglected if L was much greater than D.

Aims: We evaluated this correlation for wildland fires where D is typically a non-negligible proportion of L (i.e. roughly L/D < ~2).

Methods: Experiments were conducted to measure flame length L from line-source fires using a gas burner where I_B and D were controlled independently (0.014 ≤ L/D ≤ 13.6).

Key results: The resulting correlation showed D significantly reduced L for a given I_B over the entire range of observations and was in accord with independent data from spreading fires. Flame length is reduced because the horizontal extent of deep flame zones entrains more air for combustion than assumed by theory involving only the vertical flame profile.

Conclusions: Analysis suggested that the noted variability among published correlations of L with I_Bmay be partly explained by varying L/D ratios typical of wildland fires.

Implications: Fire behaviour modelling that relies on correlations of L with I_B for scaling of heat transfer processes would likely benefit by including the effects of D.