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7/27/2019 ME401 Combustion Intro
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Combustion & Flame
Dr. M. Zahurul Haq
ProfessorDepartment of Mechanical Engineering
Bangladesh University of Engineering & Technology (BUET)Dhaka-1000, Bangladesh
zahurul@me.buet.ac.bdhttp://teacher.buet.ac.bd/zahurul/
ME 401: Internal Combustion Engines
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 ( 20 11) 1 / 21
Combustion Basics Combustion Classification
Combustion
Combustion of fuel-air mixture inside engine cylinder is one of
the processes that controls engine power, efficiency and emissions.
Combustion commonly observed involves flame, which is a thin
region of rapid exothermic chemical reaction.
Flame propagation is the result of strong coupling between
chemical reaction, transport processes of mass diffusion & heat
conduction and fluid flow.
Conventional spark-ignition (SI) flame is a premixed unsteady
turbulent flame, and the fuel-air mixture through which the flamepropagates is in the gaseous state.
Diesel engine (CI) combustion process is predominantly an
unsteady turbulent diffusion flame, and the fuel is initially in the
liquid phase.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2 011 ) 2 / 21
Combustion Basics Combustion Classification
Classification of Flames
1 Premixed Flame: fuel and oxidizer are essentially uniformly
mixed prior to combustion. It is a rapid, essentially isobaric,
exothermic reaction of gaseous fuel & oxidizer, and propagates as a
thin zone with speeds of less than a few m/s.2 Diffusion Flame: reactants are not premixed and must mix
together in the same region where reactions take place. It is
dominated by the mixing of reactants, which can be either laminar
or turbulent, and reaction takes place at the interface between the
fuel and oxidizer.
1 Laminar: flow, mixing and transport are by molecular process.2 Turbulent: flow, mixing and transport are enhanced by
macroscopic relative motion of fluid eddies of turbulent flow.
Steady / UnsteadySolid phase / Liquid phase / Gaseous phase.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 ( 20 11) 3 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
Combustion Stoichiometry
C αH βO γN δ fuel
+ a s (O 2 + 3.76N 2) air
−→ n 1CO 2 + n 2H 2O + n 3N 3 complete combustion product
a s ≡ stoichiometric molar fuel-air ratio
(A/F )s ≡
stoichiometric air-fuel ratio
a s = α +β
4−γ
2=⇒
A
F
s
=
F
A
−1
s
=28.85(4.76a s )
12α + β + 16 γ + 14δ
φ ≡ fuel-air equivalence ratio, simply equivalence ratio
λ ≡ relative air-fuel ratio or excess-air factor
φ = λ−1 =(A/F )s
(A/F )a
=(F /A)a
(F /A)s
: φ =
< 1 : lean mixture
= 1 : stoichiometric mix.
> 1 : rich mixture(A/F )a ≡ actual air-fuel ratio
Homework: Heywood: Ex. 3.1, pp. 69c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2 011 ) 4 / 21
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Combustion Basics Combustion Chemistry & Thermodynamics
Heating Values of Fuels
T
U
H or
Reactants
Products
−(U )V,T o
−(H )P,T o
or
or
T od001
Heating value at constant pressure ≡ Q HV ,P = −(∆H )P ,T o
Heating value at constant volume ≡ Q HV ,V = −(∆U )V ,T o
Q HV ,P − Q HV ,V = −P (V prod − V reac ) = −Ru (n prod − n reac )T o
Ru ≡ universal gas constant (8.314 kJ/kmol-K)c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 ( 20 11) 5 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
T
H Reactants
Products
T o
H2O liq
H2O vap
fuel vap
fuel liq
mf hfg,fuel
mH 2Ohfg,H 2O
d002
Q HHV ,P = Q LHV ,P +
m H 2O
m f
h fg ,H 2O
Q HHV ,P ≡ Higher (Gross) Heating Value
Q LHV ,P ≡ Lower (Net) Heating Value
m H 2O /m f ≡ mass ratio of water produced to fuel burned.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2 011 ) 6 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
Example: Methane-Air Combustion
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∆H R = -802.405 MJ/kmol = -(802.405/16.043) MJ/kg = -50.02 MJ/kg
LHV = −∆H R = 50.02 MJ/kg
Homework: Estimate HHV of CH 4 at constant pressure & at constant volume.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 ( 20 11) 7 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
Adiabatic Flame Temperature
T
U
H
or
Reactants
Products
or
T o T ad
U oR H
oR
or
d003
U oR = U prod (T ad ,V = constant)
H oR = H prod (T ad ,P = constant)
Adiabatic Flame Temperature is the product temperature in an
ideal adiabatic combustion process. Actual peak temperatures in
engines are several hundred degrees less due to:
heat loss from the flame
combustion efficiency is less than 100%: a small fraction of fueldoes not get burned, and some product components dissociate
(endothermic reaction) at high temperatures.c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2 011 ) 8 / 21
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Combustion Basics Combustion Chemistry & Thermodynamics
Typical Equilibrium Combustion Product
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iso-octane, φ = 1.0, 30 bar
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 ( 20 11) 9 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
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c Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 10 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
Major products of lean combustion are H 2O , CO 2, O 2 and N 2;
while, for rich combustion they are H 2O , CO 2, CO , H 2 and N 2.
Maximum flame temperature is at slightly rich condition
(φ 1.05) as a result of both the heat of combustion & heat
capacity of products decaying beyond φ = 1.0.
Between 1.0 φ φ(T max ) heat capacities decays more rapidly
with φ than ∆H c and beyond φ(T max ), ∆H c falls more rapidly
than does the heat capacity.
Increase in temperature promotes dissociation (endothermic)
reactions and increase in pressure decreases dissociation.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2011) 11 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
Combustion Efficiency in ICEs
Exhaust gas of an ICE contains incomplete combustion products
(e.g. CO, H2, unburned hydrocarbon, soot) as well as complete
combustion products (CO2 and H2O). The amounts of incomplete
combustion products are small in case of lean mixture, however
these amounts become more substantial under fuel-rich conditions.
e694
ηc =H R(T o ) − H P (T o )
m f Q HV
ηc ≡ combustion efficiency
T o ≡ ambient temperature
H R ≡ enthalpy of reactants
H P ≡
enthalpy of productsm f ≡ mass of fuel
Q HV ≡ heating value of fuelc Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 12 / 21
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Combustion Basics Combustion Chemistry & Thermodynamics
Data for some Fuels
Fuel Symbol (A/F )s a s LHV T ad ,P SIT 1
(MJ/kg) (K) (K)
Hydrogen H 2(g) 34.01 0.5 119.95 2383 673
Methane CH 4(g) 17.12 2.0 50.0 2227 810
Methanol CH 4O (l) 6.43 1.5 19.9 2223 658
Gasoline C 7H 17(l) 15.27 11.25 44.5 2257 519
Octane C 8H 18(l) 15.03 12.50 44.4 2266 691
Diesel C 14.4H 24.9(l) 14.3 20.63 42.94 2283 483
1Self Ignition Temperaturec Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2011) 13 / 21
Combustion Basics Combustion Chemistry & Thermodynamics
Auto-ignition & Self-ignition Temperature (SIT)
If the temperature of an air-fuel mixture is raised high enough, the
mixture will self ignite without the need of an external igniter. Thetemperature above which this occurs is called the SIT.
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If the mixture temperature is lower than
SIT, no ignition will occur and the
mixture will cool off.
If mixture temperature is above SIT,
self-ignition will occur after a short time
delay called ignition delay (ID).
The higher mixture temperature above
SIT, the shorter will be the ID.
ID depends on initial temperature,
pressure, density, turbulence, swirl,
fuel-air-ratio presence of inert gases, etc.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 14 / 21
Flame
Flame
Flame is the result of the self-sustaining chemical reaction
occurring within a region of space called flame front where
unburned mixture is heated & converted into products.
Flame reaction zone temperatures are around 2800 K. At these
temperatures, flame contains highly reactive atoms & radicals, andtemperature & concentration gradients are set up. Flame
propagation arises from transfer of heat & from diffusion of active
particles from the hot flame to the relatively cold mixture.
As unburned mixture is raised in temperature and reaction rates
accelerate, the rates again accelerating as the concentrations of
active particles increase. Flame speeds are significantly increased
by turbulence which distorts flame front to increase burning areaand enhances heat & mass diffusion.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2011) 15 / 21
Flame Laminar Flame
Laminar Flame Propagation
Laminar burning velocity, S L is an intrinsic property of a
fuel-air mixture. It is defined as ‘the velocity, relative to & normal
to the flame front, with which unburned gas moves into the front
& is transformed to products under laminar flow conditions’.
S L =1
ρu A f
dm b
dt S s = S L + u g
dm b/dt is mass burning rate & A f is flame front surface area.
S s ≡ flame speed: space velocity of flame front normal to itself. It
is not a unique property of combustible fuel-air premixture.
u g ≡ gas expansion velocity & is a function of ρu & ρb .
S L = f ( fuel ,T ,P , φ) ∼= S L,0
T T o
α
P P o
β
c Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 16 / 21
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Flame Laminar Flame
Flame location at
t = t0t = t1
t = t1
t = t1
unburned
unburned
unburned
burned
burned
burned
ug = −S L
S s = S L
S s = 0
S s = S L + ugd004
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2011) 17 / 21
Flame Laminar Flame
Effect of Equivalence Ratio, φ
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c Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 18 / 21
Flame Laminar Flame
Effect of Unburned Mixture Temperature, T u
300 350 400 4500.250.25
0.3
0.4
0.5
0.6
φ φ φ φ = 1.0
T u
(K)
Iso-octane-air mixture
Methane-air mixture
φ φ φ φ = 0.8
Pu
= 0.1 MPa
S L
( m / s )
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α is +ve and close to 2.0. Increase in temperature increases chemical
reaction rates & dissociations. So, more active radicals are produced to
enhance flame propagation.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2011) 19 / 21
Flame Laminar Flame
Effect of Unburned Mixture Pressure, P u
0.10.1 0.2 0.4 0.6 0.8 1.01.00.10.1
0.2
0.3
0.4
0.5
0.6
0.7
φ φφ φ = 0.8
Iso-octane-air mixture
Methane-air mixtureφ φφ φ = 1.0
T u
= 358 K
S L
( m / s )
Pu
(MPa)e730
β is either zero or negative. Increased pressure increases flame
temperature because of less dissociation, and less dissociation meansless active radicals are available to diffuse upstream to enhance flame
propagation.c Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 20 / 21
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Flame Turbulent Flame
Turbulent Flame Propagation
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S T = f ( fuel ,T ,P , φ, turbulence ) =⇒ S T
S L= f (v )
In engines, propagating flame fronts are wrinkled by turbulence which
result in higher burning rate. The effect of turbulence is not always
beneficial as too much turbulence can lead to extinction of flame.
c Dr. M. Zahurul Haq (BUET) Combustion & Flame ME 401 (2011) 21 / 21