On the Flame Height Definition for Upward Flame Spread
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Date
2007Author
Consalvi, Jean L
Pizzo, Yannick
Porterie, Bernard
Torero, Jose L
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Abstract
Flame height is defined by the experimentalists as the average position of the luminous
flame and, consequently is not directly linked with a quantitative value of a physical
parameter. To determine flame heights from both numerical and theoretical results, a more
quantifiable criterion is needed to define flame heights and must be in agreement with the
experiments to allow comparisons. For wall flames, steady wall flame experiments
revealed that flame height may be defined by a threshold value on the wall heat flux. From
steady wall flame measurements, three definitions of flame height from wall heat flux are
retained: the first is based on the continuous flame while the two others are based on threshold values of 4kW/m^2
and 10kW/m^2. These definitions are applied to determine
flame heights from a two-dimensional time-dependent CFD model used to describe flame
spread along a slab of PMMA. Results show that the predicted flame heights are consistent
with the available data of the literature. Defining flame height by threshold values on the wall heat flux of 4kW/m^2
and 10kW/m^2 allows to correlate the wall heat flux in term of (x-x_p)/(x_fl-x_p), which is the dimensionless characteristic length scale for upward flame spread.
It is also found that the continuous flame is not a characteristic length for the heat transfer
to the unburnt fuel and is not really appropriate to define flame height in upward flame
spread.
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