COMBUSTION ENGINEERING - LTT -Herzlich willkommen … · Chemical Reaction Kinetics 4. ... pure fuel non-premixed flame Ox F Ox ... Air plane turbine Modern gas turbine Non-Premixed

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COMBUSTION ENGINEERING

Credits to Profs. F. Beyrau (OvGU), F. Dinkelacker (Leibniz Universität Hannover), A.

Leipertz (Erlangen)

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Combustion Engineering

Benoit Fond, Junior Professor

[email protected]

G10/R119

Website for slides

http://www.ltt.ovgu.de/Lehre.html

mailto:[email protected]

http://www.ltt.ovgu.de/Lehre.html

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Content of Lecture

1. Phenomenology of Combustion

2. Thermodynamic Fundamentals

3. Chemical Reaction Kinetics

4. Ignition and Ignition Limits

5. Laminar Flame Theory

6. Turbulent Combustion

7. Pollutants of Combustion

8. Combustion of Liquid and Solid Fuels

9. Numerical Simulation

10. Measurement Techniques of Combustion Processes

11. Applied Aspects of Turbulent Combustion

12. Technical Burner Systems

13. (Internal Combustion Engines)

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"Fascination of Fire"

Fire has always been a fascinating phenomenon!

It also provides more then 90% of the worldwide energy support today

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Content

1. Phenomenology of Combustion

• Combustion Technology - Why ?

• Complexity of Combustion

• Characterising Concepts

• Four Functional Process Steps of Combustion

- Excursion: How to extinguish a fire ?

• Laminar Flames - Turbulent Flames

• Premixed Flames – Non-Premixed (Diffusion) Flames

• First Comparison

• Examples of Flames and Combustion Systems

• Purpose of Combustion

• Summary

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Why Combustion Technology

Combustion is one of the oldest technologies of mankind

• Fire for heating, to protect from animals

• Clearing of forest

• Food preparation

• Metal processing

• Weapon technology : Incendiary devices

Combustion has two sides:

• Technology to use

• Destruction by Fire

Greek Mythology: Prometheus brought fire to mankind. But his "boss" (the highest god Zeus) feared the increase of human power. Therefore he punished Prometheus, chained him to a rock, where an eagle picks his liver.

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• Development of Industry:Significant Progress from Energy- and Combustion Technology:

• Steam engine

• Power plant

• Process engineering

• Internal engines

• Gas turbines

• Jet propulsion

• Transportation systems

(Steam engine, Railway, Road traffic, Aviation, Space ?)

• Note: More than 90% of worldwide use of energy is connected

with combustion !!!

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Modern Combustion Technology for :

• Increase of Efficiency (natural resources are limited)

• Reduction of pollutants (poisonous,carcinogen, change of climate etc.)

• Noise abatement

• Reduction of size of burning chamber (e.g. airplane + automobile engines)

Keywords are for example:

"Drei-Liter-Auto" - Three liter per 100 km

"ULEV" - Ultra Low Emission Vehicle

"ZEV" - Zero Emission Vehicle

"Single-Digit NOx" - (< 10 ppm NOx)

Diesel truck without particle filter

Source : US Environmental Protection Agency

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Pratt & Whitney

PW4000

Turbofan Engine

e.g. Boeing 747-400

Airbus A310-300

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• Size of flame and combustion chamber?

• How much fuel and air, respectively?

• Is the fuel consumption reasonable? (efficiency, rate of conversion)

• Safety

• Pollutant- emissions

Heat-

exchanger

Flame

Air

Fuel

Inappropriate

flame sizeBrennkammer

Brenner

Tasks for combustion technology

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Traditional

• Experience

• Trial-and-error method

• Design from global computations

Heat-

exchanger

Flame

Air

Fuel

• heat- and mass-transport owing

to convective flows

• diffusion

• vaporization

• reaction

• radiation, etc. ...

increasingly

interdisciplinary

task

Modern approach

• Computation based on local physical and

chemical Processes:

Tasks for combustion technology

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Complexity of Combustion

Combustion:

"Transformation of chemical bound energy into heat"

Typical

• Fuel and oxidizer react together• Oxidizer O2 (Air). • Explosives and solid rocket propellant contains O2 in chemical bound

form (Monergole).

• Energy release (exothermic reaction)

• Reaction often is very "fast"

• Many reaction steps.

e.g. CH4 + 2O2 -> CO2 + 2H2O is an oversimplification

• Heat and mass transport is significantly involved.

Combustion is complex, still not fully understood !!

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Complexity of Combustion

Where flame is bright?

Note: Luminescence of flame is secondary process, not necessary

definition for reaction zone

(also "flameless oxidation" is possible)

Exposure time

1/8 sec 1 sec 8 sec

Where is the reaction zone ?

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Four Functional Steps

Four functional process steps for combustion (gaseous fuel):

(1) Mixing of fuel and oxidizer

(2) Heat up, that reaction can start (Ignition)

(3) Combustion reaction with heat release

(4) Heat utilization

Feed back

External

Ignition

Combustion is a

self stabilizing

process

Self

Ignition

First characterization

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How to extinguish a flame ?

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Four Processes:

(1) Mixing of fuel and oxidizer

(2) Heating to ignite

(3) Combustion reaction with heat release

(4) Heat utilization

Feed back

How to extinguish a flame ?

Stop fuel supply

(e.g., forest fires,

clear forest aisle)Stop air supply

(Inert extinguisher

e.g. Halon, CO2)

Remove heat

to stop ignition (water;

metal grid)

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Characterizing Concepts

Typical times: Mixing 0,1 - 10 sec

Reaction 10-3 sec

Often mixing dominates combustion

Often mixing supported by convective flow:

either laminar or turbulent flow

Laminar flame: Flowfield independent of time

Turbulent flame: Flowfield depends on time

combustion laminar for 0)(

)()( e.g.

tT

tTTtT

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Essential characterization:

laminar

and

turbulent flame

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2 fundamental types of flames

Non-premixed flame:

Fuel + Ox. come together in reaction zone

Premixed flame:

Fuel + Ox. mixed before reaction

Note 1: Detailed analysis shows that even in premixed flames diffusion is an

essential phenomenon. Thus name "diffusion flame" is too simplified; better is "non

premixed flame").

Note 2: Intermediate types possible "partially premixed flames"

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Premixed flame

Fuel

Stoichiome-

tric Surface

Luminous

zone

(yellow)

Air Air

Non-premixed flame

F.+ Air

Flame front

(blue)

Post-

oxidation

(low blue)

Laminar Flame Theory

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stoichiometrically

premixed flame

pure

fuel

non-premixed

flame

F OxOx

flame front/

reaction zone

F+ OxF + Ox

(F-rich)

F OxOx

(1.)

partially premixed

flame

Tube Burner / Bunsen Burner

Laminar Flame Theory

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Flame Types

Premixed Flame Nonpremixed Flame

Partially Premixed

Flame

Photos by Dr. F. Dinkelacker, Erlangen, 2005

Butane/Air

Fuel flow rate is hold constant

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laminar turbulent

Non-

Premixed

(Diffusion-)

flame

Premixed

flame

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laminar

Candle

gas stove

(part. premixed)

Porous burner

turbulent

Fire,

Industrial burner,

Air plane turbine

Modern gas

turbine

Non-

Premixed

(Diffusion-)

flame

Premixed

flame

Important characterization of flames:

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Examples for Combustion Systems

Fuel

Luminous

zone

(yellow)

Air Air

The candle flame as classical

example of laminar

non-premixed (diffusion) flamme

Wick

Candle Flame

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Gas stove burner, partly premixed flame with air

intake inside venturi injector

(from Günther)

Bunsen burner, can be modified between

premixed (blue) and non-premixed (yellow) flame

Gas stove burner / bunsen burner

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Rotary furnace for production of cement (length about 30 m)Turbulent long diffusion flame, radiative heat transfer

(from Görner)

Cement production

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Pratt & Whitney

F100-PW-229 Engine

Military jet engine with afterburner

CompressorBurning

chamberTurbine

Afterburner with

flame stabilization

Jet engine

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Modern gas turbine with annular burning chamber for premixed combustion

Siemens V84.3A

Gas turbine

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Oil heating furnace

Biomass Heater

(Guntamatic Powerchip)

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First comparitive discussion:

Laminar -->Turbulent Flames: Mixing increases

Combustion faster, concentrated

Nonpremixed Flame: Quite stable combustion, "secure"

Premixed Flame: Controlled reaction possible:

NOx reduction

Soot reduction

But danger of flash back

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Further characteristics concerning the temporal behaviour of

combustion

• Stationary CombustionCombustion field remains (on average) stable

• Instationary CombustionLocation of (average) combustion field changes with time

)()(

Combustionent for turbul e.g.

tTTtT f(t)T

T

:ryinstationa

in timeconstant :stationary

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laminar

Candle

Lighter

Gas stove

(Part.

Premixed)

turbulent

Woodfire

Jet Engine

Modern gas

turbine

Non-

prem.-

flame

Prem.-

flame

laminar

Droplet

ignition

Ignition

turbulent

Diesel engine

(with direct

injection)

Spark Ignition

engine

Stationary Instationary

Stationary and Instationary Flames

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Otto engine (SI) with port fuel injection Instationary turbulent premixed combustion

Diesel engine with direct injection Instationary turbulent non-premixed

combustion

Internal combustion engines

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Purpose of Combustion

Primarily chemical energy is transformed to heat. This can be used for different

purposes

Purpose Examples

Heat for heating system Heating burner (Oil, Gas, Solids)

Heat for high temperature processing Cement furnace

Melting furnace

Electricity Boiler (Coal, Oil, Gas) - Rankine

Stationary gas turbine - Brayton

Mech. power, e.g. for traffic Internal combustion engine

Jet engine

Chemical decomposition Waste incineration

Light, "Comfort" Candle

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Quelle: Martin GmbH

Example: waste incineration

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Flares – for the controlled

combustion of excess fuel

(safety reasons)

Sooting (1st Generation)

Quelle: Internet

2nd Generation

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Summary

• Summary:

• Combustion technology - one of the most important technologies

• Most important tasks for combustion technology today are

pollutant reduction and an increasing efficiency

• Characterizing Concepts

• 4 functional process steps of combustion

• Characteristics:

• Laminar - Turbulent Flames

• Diffusion Flame - Premixed Flame

• Stationary - Instationary Combustion

• Purpose of Combustion

• Heat, Power, Light, Chemical processing and decomposition, ...

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Combustion Literature

English:

• Turns, S. R. "An Introduction to Combustion: Concepts and Application", McGraw-Hills

2011 (quite new, relatively good, ca. 60E)

• Warnatz, J., Maas, U., Dibble, R. "Combustion", Springer, 2006

(Basic Processes, Kinetics, Modelling, ca. 80E)

• Kuo, K. "Principles of Combustion", J. Wiley 1986

(Detailed Theory)

• Lewis, v. Elbe "Combustion, Flames and Explosions of Gases", 3. Auflage 1986,

Academic Press (a "classical" book)

Peters, N. : "15 Lectures on laminar and turbulent combustion", Aachen, 1992

http://www.itm.rwth-aachen.de (theoretical orientation)

German:

• Warnatz, J., Maas, U., Dibble, R. "Verbrennung", 3. Auflage, Springer 2001, 40 €

• Günther, R. "Verbrennung und Feuerungen", Springer 1974

(Technische Aspekte, Viele Brennerformen, Theorie tw. veraltet, ca. 40 €)

• Görner, K. "Technische Verbrennungssysteme", Springer 1991

(Grundlagen, Simulation, Kohleverbrennung, ca. 65 €)

• Merker, Schwarz, Stiesch, Otto "Verbrennungsmotoren - Simulation der Verbrennung

und Schadstoffbildung", 2. Auflage, Teubner 2004, 40 €

COMBUSTION ENGINEERING - LTT -Herzlich willkommen … · Chemical Reaction Kinetics 4. ... pure fuel non-premixed flame Ox F Ox ... Air plane turbine Modern gas turbine Non-Premixed

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