Lecture 6. Laminar Non-premixed Flames

part 1

X.S. Bai Laminar Non-premixed Flames

Space shuttle

TNF in rocket engines

X.S. Bai Laminar Non-premixed Flames

Airbus A380 engine

Rolls Royce Trent 900

X.S. Bai Laminar Non-premixed Flames

Gas turbine combustor

X.S. Bai Laminar Non-premixed Flames

Laminar diffusion flame

O

F

O

U x

Pe 50 Pe 50 Pe 200 Pe 1

Pe = UdD Diffusion coefficient

Diameter of the fuel jet Peclet number

X.S. Bai Laminar Non-premixed Flames

Fuel bed

Photo taken in November 2008

X.S. Bai Laminar Non-premixed Flames

Candel flame

air

Fuel

X.S. Bai Laminar Non-premixed Flames

Non-premixed flames

•  Diesel engine flames •  Industrial furnace firing solid fuels •  Aircraft engines •  Gas turbines •  Space rocket engines •  Fires

•  Hot combustion temperature – not good for NOx control •  Fuel rich combustion – not good for soot and particle control

•  Safety is high – aeronautical and aerospace engines

X.S. Bai Laminar Non-premixed Flames

Content

•  Structures of laminar non-premixed flames •  Mixture fraction •  Burke-Schumann flame-sheet model •  Jet diffusion flames •  Laminar diffusion flame athigh mixing rate

X.S. Bai Laminar Non-premixed Flames

Experimental observations of laminar non-premixed flames

X.S. Bai Laminar Non-premixed Flames

Piloted jet flame: Sandia, Delft TU, TU Darmstadt, TNF workshop

X.S. Bai Laminar Non-premixed Flames

Counterflow diffusion flame

streamline

air flame front

stagnation

plane fuel x

X.S. Bai Laminar Non-premixed Flames

Structure of non-premixed flames

-0.2

0

0.2

0.4

0.6

0.8

1

1.2

-15 -10 -5 0 5 10 15 20

O2 CH4 H2O

mas

s fra

ctio

ns

x mm

X.S. Bai Laminar Non-premixed Flames

Structure of non-premixed flames

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

-7 -6 -5 -4 -3 -2 -1 0 1

OH CH3 H

ma

ss fr

act

ion

s

x mm

X.S. Bai Laminar Non-premixed Flames

Structure of non-premixed flames

•  Fuel rich zone: between the fuel stream and the reaction zone

•  Reaction zone: between the fuel rich zone and oxygen rich zone

•  Oxygen rich zone: between the oxygen stream and reaction zone

X.S. Bai Laminar Non-premixed Flames

Laminar diffusion flames

•  Laminar non-premixed flame: fuel and air supplied to combustor separately

•  Fuel diffuses to the reaction zone from the fuel rich side •  Oxygen diffuses to the reaction zone from the fuel lean side •  Products diffuses from the reaction zone to the fuel and oxygen

stream •  Temperature (energy) diffuses from the reaction zone to the fuel

and oxygen streams •  Chemical reactions occur as soon as the fuel and oxygen mix

•  Diffusion is slower than chemical reaction •  The process depends thus on the mixing rate by diffusion •  Non-premixed flames are also called diffusion flames

X.S. Bai Laminar Non-premixed Flames

Mathematical description of laminar non-premixed flames

X.S. Bai Laminar Non-premixed Flames

Mixture fraction

F inlet 1 A inlet 2

m1 m2

Mixture fraction is defined as the ratio of the total sum of the mass of materials that are originated from the fuel stream to the total mass. Using Z to denote mixture fraction Z = mass originated from the fuel / total mass - Mixture fraction is conserved during combustion

1

1 2

mZm m

=+

1, 1

st

A st

Z ZZZ Z

φφ

φ γ−

= =+ −

11st

A

Zγ

=+

X.S. Bai Laminar Non-premixed Flames

Conserved scalar

•  A scalar that is not changed during chemical reactions –  It may change as a result of flow mixing

•  Element mass fraction is a conserved scalar –  YC, YO, YH, YN

•  Mixture fraction is a conserved scalar –  Mass originated from the fuel does not change due to mass

conservation law •  Equivalence ratio is not a conserved scalar

–  Both mass of fuel and mass of oxidizer can change during chemical reactions

•  One can trace back the equivalence ratio of the original unburned mixture using mixture fraction

1, 1

st

A st

Z ZZZ Z

φφ

φ γ−

= =+ −