Combustion Characteristics of Materials and Generation of Fire Products

Abstract

Hazards associated with fire are characterized by the generation of calorific energy and products, per unit of time, as a result of the chemical reactions of surfaces and material vapors with oxygen from air. Thermal hazards constitute those scenarios where the release of heat is of major concern. On the other hand, nonthermal hazards are characterized by fire products (smoke, toxic, corrosive, and odorous compounds.) Generation rates of heat and fire products (and their nature) are governed by (1) fire initiation (ignition); (2) fire propagation rate beyond the ignition zone; (3) fire ventilation; (4) external heat sources; (5) presence or absence of fire suppression/extinguishing agents; and (6) materials: (a) their shapes, sizes, and arrangements; (b) their chemical natures; (c) types of additives mixed in; and (d) presence of other materials. In this handbook most of these areas have been discussed from fundamental as well as applied views. For example, the mechanisms of thermal decomposition of polymers, which govern the generation rates of material vapors, are discussed in Chap. 7, generation rate of heat (or heat release rate) from the viewpoint of thermochemistry is discussed in Chap. 5, Flaming ignition of the mixture of material vapors and air is discussed in Chap. 21, and surface flame spread in Chap. 23.

Nomenclature, Greek Letters, Subscripts and Superscripts

A

total exposed surface area of the material (m²)

a _j

mass coefficient for the product yield (g/g)

b _j

molar coefficient for the product yield (g/mol)

B _cr

critical mass transfer number

CHF

critical heat flux (kW/m²)

Ċ ^″_O

mass consumption rate of oxygen (g/m²/s)

Ċ ^″_stoich,O

stoichiometric mass consumption rate of oxygen (g/m²/s)

c _O

mass of oxygen consumed per unit mass of fuel (g/g)

c _P

specific heat (kJ/g/K)

Δc _P

difference between the heat capacities of the extinguishing agent and the fire products (kJ/g/K)

D

optical density (1/m)

E _i

total amount of heat generated in the combustion of a material (kJ)

f _j

volume fraction of a product

fp

fire property

FPI

Fire Propagation Index

FSP _c

convective flame spread parameter

Ġ ^″ _j

mass generation rate of product j (g/m²/s)

Ġ ^″_stoich,j

stoichiometric mass generation rate of product j (g/m²/s)

ΔH _i

heat of combustion per unit mass of fuel vaporized (kJ/g)

ΔH _D

heat of dissociation (kJ/g)

ΔH _g

heat of gasification at ambient temperature (kJ/g)

ΔH _g,con

flame convective energy transfer to the fuel per unit mass of fuel gasified (kJ/g)

ΔH _m

heat of melting at the melting temperature (kJ/g)

ΔH _T

net heat of complete combustion per unit of fuel vaporized (kJ/g)

ΔH _v

heat of vaporization at the vaporization temperature (kJ/g)

ΔH ^*_CO

net heat of complete combustion per unit mass of CO generated (kJ/g)

$\varDelta {H}_{{\mathrm{CO}}_2}^{*}$

net heat of complete combustion per unit mass of CO₂ generated (kJ/g)

ΔH ^*_O

net heat of complete combustion per unit mass of oxygen consumed (kJ/g)

HRP

heat release parameter

h _i

mass coefficient for the heat of combustion (kJ/g)

I/I ₀

fraction of light transmitted through smoke

j

fire product

k

thermal conductivity (kW/m/K)

L _sp

smoke point (m)

l

optical path length (m)

ṁ″

mass loss rate (g/m²/s)

M

molecular weight (g/mol)

m _i

molar coefficient for the heat of combustion (kJ/mol)

ṁ _air

mass flow rate of air (g/s)

${\dot{q}}_e^{{\prime\prime} }$

external heat flux (kW/m²)

${\dot{q}}_f^{{\prime\prime} }$

flame heat flux (kW/m²)

${\dot{Q}}_i^{{\prime\prime} }$

heat release rate per unit sample surface area (kW/m²)

${\dot{Q}}_i^{\prime }$

heat release rate per unit sample width (kW/m)

S

stoichiometric mass air-to-fuel ratio (g/g)

t

time (s)

t _f

time at which there is no more vapor formation (s)

t ₀

time at which the sample is exposed to heat (s)

T

temperature (K)

ΔT _ig

ignition temperature above ambient (K)

TRP

thermal response parameter (kW⋅s^1/2/m²)

u

fire propagation rate (mm/s or m/s)

$\dot{V}$

total volumetric flow rate of fire product-air mixture (m³/s)

Ẇ

total mass flow rate of the fire product-air mixture (g/s)

W _f

total mass of the material lost in the flaming and nonflaming process (g)

W _j

total mass of product j generated in the flaming and nonflaming process (g)

X _f

flame height (m or mm)

X _p

pyrolysis front (m or mm)

X _t

total length available for fire propagation (m or mm)

y _j

yield of product j

Y _j,ex

mass fraction of extinguishing agent

Y _O

mass fraction of oxygen

α

correlation coefficient (nonflaming fire)

β

correlation coefficient (transition region)

ϕ

kinetic parameter for flame extinction

ξ

correlation coefficient (transition region)

Φ

equivalence ratio

χ _ch

combustion efficiency

χ _con

convective component of the combustion efficiency

χ _rad

radiative component of the combustion efficiency

η _j

generation efficiency

κ

ratio between the kinetic parameters for the flame temperature and adiabatic flame temperature

λ

wavelength of light (μm)

σ

Stefan-Boltzmann constant (56.7 × 10⁻¹² kW/m²/K⁴)

$\overline{\tau}$

average specific extinction area (m²/g)

ρ

density (g/m³)

ν _j

stoichiometric coefficient of product j

ν _O

stoichiometric coefficient of oxygen

Ψ _j

stoichiometric yield for the maximum conversion of fuel to product j

Ψ_O

stoichiometric mass oxygen-to-fuel ratio (g/g)

ζ

ratio of fire properties for ventilation-controlled to well-ventilated combustion

ζ _oxid

oxidation zone product generation efficiency ratio

ζ _red

reduction zone product generation efficiency ratio

a

air or ambient

ad

adiabatic

asy

asymptotic

ch

chemical

con

convective

cr

critical

e

external

ex

extinguishment

f

flame or fuel

fc

flame convective

fr

flame radiative

g

gas

g,con

flame convective energy for fuel gasification

i

chemical, convective, radiative

ig

ignition

j

fire product

n

net

0

initial

oxid

oxidation zone of a flame

rad

radiation

red

reduction zone of a flame

stoich

stoichiometric for the maximum possible conversion of fuel monomer to a product

rr

surface re-radiation

s

surface, smoke

vc

ventilation-controlled fire

wv

well-ventilated fire

∞

infinite amount of air

.

per unit time (s⁻¹)

′

per unit width (m⁻¹)

ʺ

per unit area (m⁻²)