Effect of Combustion Conditions on Species Production

Abstract

A complete compartment fire hazard assessment requires a knowledge of toxic chemical species production. Although combustion products include a vast number of chemical species, in practical circumstances the bulk of the product gas mixture can be characterized by less than 10 species. Of these, carbon monoxide (CO) represents the most common fire toxicant (see Chap. 63). Over half of all fire fatalities have been attributed to CO inhalation [1, 2]. Concentrations as low as 4000 ppm (0.4 % by volume) can be fatal in less than an hour, and carbon monoxide levels of several percent have been observed in full-scale compartment fires. A complete toxicity assessment should not only include the toxicity of CO but also include the synergistic effects of other combustion products, such as elevated CO₂ and deficient O₂ levels.

Nomenclature and Subscripts

B _i

yield coefficients of species i

C

stoichiometric molar ratio of water to carbon dioxide

C _j

volume concentration of fuel j when fuel stream is stoichiometrically mixed with oxidant stream

C _p

heat capacity of products of complete combustion, (kJ/g ⋅ mol K)

${\boldsymbol{D}}_{{\mathbf{O}}_{\mathbf{2}}}$

mass depletion of oxygen per gram of fuel burned (g/g)

E

energy released per kg of oxygen consumed

F

normalized yield or generation efficiency

ΔH _c,j

heat of combustion of the species j, (kJ/g ⋅ mol)

j

fuel species of interest

k

maximum theoretical yield

L _f,tip

length of flame tip for flame extending down a corridor ceiling

M

molecular weight

m _a

mass of air

ṁ _a

mass flow rate of air

m _f

mass of fuel

ṁ _f

mass loss rate of fuel

${\ddot{\boldsymbol{m}}}_{\boldsymbol{f}}$

derivative of the fuel mass loss rate

ṁ _exhaust

mass flow rate out of the layer

n

molar quantity

n _prod

number of moles of products of complete combustion per mole of reactants (stoichiometric mixture of fuel and oxidant streams)

Q

ideal heat release rate

r

stoichiometric fuel-to-air ratio

r _a

stoichiometric air-to-fuel ratio

${\boldsymbol{r}}_{{\boldsymbol{O}}_{\mathbf{2}}}$

stoichiometric fuel-to-oxygen ratio

T

temperature

T _SL,j

adiabatic flame temperature at the stoichiometric limit for fuel species j (K)

T _o

temperature of the gas mixture prior to reaction (K)

t

time

t _r

residence time of gases in the upper layer

τ_SS

steady-state time ratio

V _ul

volume of the upper layer

X

mole fraction

${\boldsymbol{X}}_{{\boldsymbol{i}}_{\mathbf{dry}}}$

dry mole fraction of species i (H₂O removed from sample)

${\boldsymbol{X}}_{{\boldsymbol{i}}_{\mathbf{wet}}}$

wet mole fraction of species i

Y

yield (g/g) also refers to ${D}_{{\mathrm{O}}_2}$

${\boldsymbol{Y}}_{{\mathbf{O}}_{\mathbf{2}},\mathrm{air}}$

mass fraction of oxygen in air

z

distance between the bottom of the compartment outflow and the ceiling in the adjacent space

γ

dimensionless layer depth in adjacent space $\left(\gamma =\delta /\mathrm{z}\right)$

δ

layer depth in the adjacent space

ϕ

equivalence ratio

ϕ _c

compartment equivalence ratio

ϕ_cv

equivalence ratio defined per a specified control volume

ϕ _p

plume equivalence ratio

ϕ_ul

upper-layer equivalence ratio

ρ_ul

density of the upper layer

A

air

f

fuel

CO

carbon monoxide

O₂

oxygen

CO₂

carbon dioxide

H₂O

water

H₂

hydrogen

THC

total unburned hydrocarbons

resid,C

residual carbon

${\boldsymbol{X}}_{{\boldsymbol{i}}_{\mathbf{wet}}}$

wet gas concentration with water in the mixture

${\boldsymbol{X}}_{{\boldsymbol{i}}_{\mathbf{dry}}}$

dry gas concentration with no water in the mixture