Post on 27-Jul-2020
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−
= =+ −