Code Manual for CONTAIN 2.0 - Federation of American Scientists

of the gas ahead of the flame front by burned gases, as a result of the well-mixed assumption used
by the intercell flow model.
9.1.4 Bum Rate
This subsection describes how the deflagration bum time is applied to determine the bum rate and
the masses consumed. The total bum time is defined in Equation (9-3).
The basic approach taken to determine the bum rate is to estimate the number of moles of
combustibles and oxygen available to bum over the remainder of the bum time. The number of
moles available to participate in the bum is determined from the number of moles present at a given
point during the bum and by the mole fraction of combustibles or oxygen that must remain at the end
of the bum. The burning rate is then set assuming a steady depletion rate of the most limiting
constituent over the remaining burning time.
The actual processing of the deflagration always begins at the start of a system timestep. Thus, if
one of the ignition criteria described in Section 9.1.1 is met, the cell mixture will begin to bum at
the start of the next system timestep. At the start of the deflagration, the model determines the
minimum mole fraction of combustibles that can remain at the end of the bum. This minimum
unburned mole fraction is defined as xw,fi~~for hydrogen and XCO,fin~ for carbon monoxide.
satisfy the relations:
They
where
x H2,final = Xf xH2,ititid
x co, final = Xf Xcoifiti~
X~ = 1.0- min(max(Combustion Completeness, O), 0.995)
Note X~is the fraction of the combustible gas initially present that does not bum.
(9-11)
(9-12)
(9-13)
The Xw,i~itidparameter is the mole fraction of hydrogen at the start of the bum. The Xcojtitid
parameter is the mole fraction of carbon monoxide gas at the start of the bum. Note that these values
may give a combustible concentration differing from the ignition threshold value because of the
delay in initiating the bum.
The chemical reaction rate is adjusted each bum timestep to account for intercompartment flows and
source injection, so that the bum finishes at the predetermined time with the predicted final mole
fraction of combustible gases. Consumption or production of combustible gases and oxygen by other
phenomenological processes are also accounted for in this adjustment. Note that intercompartment
flow adjustments are only made at the end of the system timestep; however, the bum model
automatically reduces the system timestep to force problem stability and to force flow adjustments
R O 913 6/30/97