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A second parameter for the Baker-Strehlow-Tang (BST) approach is the reactivity
of the released gas. Reactivity ratings used by Zeeuwen and Wiekema (1978) classify
reactivity based upon chemical laminar burning velocity. Low reactivity has velocities
less than 45 cm/sec. High reactivity applies to those chemicals with burn velocities
greater than 75 cm/sec and anything in between is labeled medium reactivity. Most of the
flammable chemicals in ALOHA do not have reactivity values. As a default (reactivity
not known), the model uses medium reactivity.
Based upon values for congestion and reactivity, BST estimates turbulent flame
speed in Mach number. The model uses recent Mach values (Pierorazio et al, 2005) that
are higher than those found in earlier publications. These newer values are based upon
larger scale experiments and are claimed to better represent results typical of an industrial
plant incident. Note, in certain cases, the tables predict that there will be a deflagration to
detonation transition (DDT). Based upon a recommendation from the external review
team, the model uses the same Mach number as in the hard ignition detonation scenario.
reactivity\congestion
low medium high
high 0.36 DDT DDT
medium 0.11 0.44 0.5
low 0.026 0.23 0.34
Table 2 : Revised Baker-Strehlow-Tang flame speeds (Mach number)
The Baker-Strehlow-Tang-model uses a set of empirically determined graphs of
normalized overpressure versus normalized distance with a different graph for different
flame speeds. The model does a curve fit to these graphs, using a function of the form
y = D if x < x 0
else y = A ⋅ B 1/ x x C
where A, B, C, D, and x0 are constants. The table gives values of these constants for
various flame Mach numbers
constants\Mach €
0.2 0.35 0.7 5.2
A 0.0335 0.1041 0.3764 0.2932
B 0.8359 0.8642 0.7439 1.399
C -1.1192 -1.0568 -1.2728 -1.1591
D 0.065 0.22 0.65 20
x0 0.35 0.32 0.3 0.16
Table 3 : Curve fit constants for various Mach numbers for use in the BST method
The normalized distance, x, is defined as
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