ME401 Combustion Intro

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

[email protected]://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.


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




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


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


Example: Methane-Air Combustion

e716

∆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.



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




Typical Equilibrium Combustion Product

e714

iso-octane, φ = 1.0, 30 bar



e717

c Dr. M. Zahurul Haq (BUET) Combustion & Flame M E 401 (2011) 10 / 21


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 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




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


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.

e732

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.


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.


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




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


Flame Laminar Flame

Effect of Equivalence Ratio, φ

e713


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 )

e731

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.


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



Flame Turbulent Flame

Turbulent Flame Propagation

e728

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.


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ME401 Combustion Intro