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Lecturer Cairo University, Faculty of Engineering Mechanical Power Engineering

This document discusses key aspects of combustion and flames, including: 1) The main design features of a good combustor for applications like gas turbines and rockets. 2) Ignition and flammability limits - the minimum and maximum concentrations of fuel and oxidant required to sustain combustion. 3) The two main types of flames - premixed and diffusion - and factors that determine whether flames are laminar or turbulent. 4) Additional details on premixed and diffusion flames, including examples and characteristics of each.

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Muhammad Emara
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96 views8 pages

Lecturer Cairo University, Faculty of Engineering Mechanical Power Engineering

This document discusses key aspects of combustion and flames, including: 1) The main design features of a good combustor for applications like gas turbines and rockets. 2) Ignition and flammability limits - the minimum and maximum concentrations of fuel and oxidant required to sustain combustion. 3) The two main types of flames - premixed and diffusion - and factors that determine whether flames are laminar or turbulent. 4) Additional details on premixed and diffusion flames, including examples and characteristics of each.

Uploaded by

Muhammad Emara
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
Available Formats
Download as PDF, TXT or read online on Scribd
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Lecturer

Cairo University, Faculty of Engineering


Mechanical Power Engineering

Contents

1
Applications of flames
Gas turbine Furnaces and boilers stove

Rocket
3

Combustors
Design Features of a Good Combustor
- Must be able to oxidize fuel on a continual basis

- Must provide constant re-ignition

- A reliable fuel injection system

- Enough cooling

- Desirable flame shape

- Low emissions

- Stable combustion
4

2
IGNITION AND FLAMMABILITY LIMITS

- Mixtures of fuel and oxidant do not necessarily react when


mixed together.
-The mixture needs a sufficient amount of energy input
before the reaction becomes self sustaining (electrical spark,
heat addition, radical addition)
- A portion of the mixture must be heated to above the self
ignition temperature (SIT) . A sufficient heat must be generated
at the ignition point to heat the surrounding layers above the
SIT

Homogeneous ignition and point ignitions

Flammability limits

Flame propagation occurs only if the ratio of fuel : oxidant is within


certain limits (flammability limits) Approx. equivalence ratio
(volume) = 50- 300%
Lower flammability limit - is the smallest quantity of
combustible which will support a self propagating flame when
mixed with a air (or other oxidant)

Upper flammability limit - is the highest quantity of combustible


which will support a self propagating flame when mixed with a
air (or other oxidant)
The limits depends on : Ignition temperature , Calorific value, vessel shape,
temperature and pressure, Specific heats of gases, Relative volumes of
diluting gases and reactants,
6

3
Flammability limits
Methane / air Propane / air If the flame reaches the the top of the
tube it is considered a flammable
mixture.
1.8 m

D= 51 mm
7

Ignition and flammability limits

4
Types of Flames

air
 Two basic categories
 Pre-mixed ( fully aerated)
 Diffusion (non aerated)
 Partially aerated
Fuel +air Fuel

 Both characterized as air


Laminar or Turbulent
air
Fuel
9

Types of Flames

 Premixed
 Ex. Bunsen Burner
 Flame moves at fairly low
velocity
 Mechanically create laminar
conditions

 Diffusion
 Ex. Candle Flame
 Fuel: Wax, Oxidizer: Air
 Reaction zone between wax
vapors and air

10

5
Premixed Flame

Flame propagates at a specific speed  Results from gaseous


reactants that are mixed
prior to combustion

 Flame propagates at
low velocities

 Reacts quite rapidly

Example: combustion of gasoline engine


11

Diffusion Flame

 Gaseous reactants
are introduced
separately and mix
during combustion

 Energy release rate


limited by mixing
process

 Reaction zone
between oxidizer and
fuel zone
12

6
Turbulent Flame
Premixed
o Heat release occurs much faster
o Increased flame propagation
o No definite theories to predict
behavior

Diffusion
Can obtain high rates of
combustion energy release per
unit volume
Ex. Diesel Engine
Modeling is very complex, no well
established approach

Spectrum of flame colour

14

7
Flame colour

Black body and soot emissions,


Temperature
Fuel composition (radicals to be formed)
Mixture strength

15

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