Lefebvre A.H., Ballal D.R. Gas Turbine Combustion: Alternative Fuels and Emissions
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210 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
the rst case the bottleneck on performance is in phase 1, while in the second
it is in phase 2 or phase 3. Similarly, a failure to achieve in ight the alti-
tude relighting performance predicted from rig tests on a single segment of
a multitube combustor, can usually be attributed to a breakdown of phase 3.
Thus, recognition of the multiphase nature of the ignition process can use-
fully assist combustion chamber development.
5.16.1 Factors influencing Phase 1
Survival of the kernel of hot gas created by the spark depends entirely on
whether or not the rate of heat release by combustion within the kernel
exceeds the rate of heat loss to the surroundings by radiation and turbulent
diffusion.
The rate of heat release is governed mainly by the effective fuel/air ratio
adjacent to the plug, which should be close to stoichiometric, and by the
size and temperature of the kernel, which are, in turn, determined by the
energy and duration of the spark. The rate of heat loss from the kernel is
largely dictated by the local conditions of velocity and turbulence and by the
quantity of excess fuel present in the ignition zone. This phase of the ignition
process is also strongly affected by the design of the igniter plug—ush re or
sunken re, by its location, and by the extent to which the plug tip protrudes
through the liner wall.
5.16.2 Factors influencing Phase 2
The location of the igniter is also important in phase 2 because it determines
whether the hot kernel is entrained into the primary-zone reversal or is
swept away downstream. This phase is also governed by all the factors that
Igniter
1
2
3
Figure 5.39
Three-phase nature of the ignition process.
Combustion Performance 211
control ame stability. Thus, an increase in pressure and/or temperature, or
a reduction in primary-zone velocity, or any change in fuel/air ratio toward
the stoichiometric value, all of which are benecial to stability, will also
improve phase 2.
5.16.3 Factors influencing Phase 3
The location of the interconnector tubes is of prime importance. Ideally,
the tube entrance should coincide with the region of highest gas temperature
in the liner, whereas the tube exit should be sited so as to ensure that the
issuing hot gas ows directly into the recirculation zone of the adjacent liner.
Care should be taken to minimize the ow of lm-cooling air over the ends
of the tube because this could interfere with the ow of hot gas and, more
important, seriously reduce its temperature.
Phase 3 is aided by the use of interconnectors in which the ow area is
made large in order to facilitate the passage of ame, and whose length is
kept short to minimize heat loss by external convection to the annulus air.
Basic data on quenching effects of relevance to interconnector performance
is contained in Reference [73].
5.17 Methods of Improving Ignition Performance
If the ignition performance of a combustion chamber is unsatisfactory, the
rst step is to nd out in which phase the bottleneck is arising. This informa-
tion can be obtained quite readily by examining the position of the ignition
loop in relation to the stability limits.
Since the ow properties that control stability also exercise a similar inu-
ence on ignition behavior, it might be expected that ignition and stability
limits should coincide. Stability limits, however, relate essentially to burn-
ing conditions and high metal temperatures, whereas ignition is inevitably
associated with cold liner walls and comparatively high heat losses. For this
reason, the two limits can never be the same, but the object of ignition devel-
opment is to ensure that they are separated only by the effects of heat loss.
If the ignition loop lies well inside the stability loop, this indicates that
the limitation on ignition performance is arising in phase 1. This may be
checked by changing the spark energy, which should produce a correspond-
ing change in the ignition loop. If the ignition and stability loops lie in close
proximity, the bottleneck on performance is almost certainly in phase 2.
These points are illustrated in Figure 5.40. Failure in phase 3 is indicated
when the maximum relighting altitude is signicantly less than the value
predicted from rig tests carried out on a single liner.
212 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
5.17.1 Correlation of experimental Data
From an analysis of lean lightup data acquired from a large number of aircraft
combustion chambers [12–17], the following equation for lean lightup fuel/
air ratio, q
LLO
, was derived [11].
q
B
V
m
PT
D
LLO
c
Ar
=
3
15
3
2
300
.
exp( /)λ
rrr
H
,
(5.29)
where B is a constant whose value depends on the geometry and mixing
characteristics of the combustion zone and also on the amount of air
employed in primary combustion.
For each combustor, a value of B was chosen for insertion into Equation 5.29
that gave the best t to the experimental data. These values are listed in
Table 5.1. The level of agreement between predicted and measured values
of lean lightup fuel/air ratio, as illustrated for three aircraft combustors in
Figures 5.41 through 5.43, is considered satisfactory, especially in view of the
well-known lack of consistency that is usually associated with experimental
data on spark ignition.
It is of interest to note that Equation 5.29 is virtually identical to Equation 5.27
except for a higher pressure dependence;
P
3
15.
vs.
P
3
13.
.
Fuel/air ratioFuel/air ratio
P
3
, T
3
constant
Failure in
phase 1.
Failure in
phase 2.
Air mass flow rate
Ignition
requirement
Stability
Stability
Ignition
Ignition
Figure 5.40
Curves illustrating the two main types of ignition failure.
Combustion Performance 213
20
F 100
All fuels
Standard day (T = 288 K)
Cold day (T = 244 K)
m
.
A
= 0.48 kg/s
10
0
010
q
LLO
(predicted), g/kg
q
LLO
(measured), g/kg
20
Figure 5.41
Comparison of measured and predicted values of q
LLO
for an F 100 combustor. (From Lefebvre,
A.H., Journal of Engineering for Gas Turbines and Power, 107, 24–37, 1985. With permission.)
40
J 85
All fuels
30
P
3
= 37.5–152 kPa
T
3
= 304–447 K
m
.
A
= 1.39–4.94 kg/s
20
10
0
0 10 20
q
LLO
(predicted), g/kg
q
LLO
(measured), g/kg
30 40
Figure 5.42
Comparison of measured and predicted values of q
LLO
for a J 85 combustor. (From Lefebvre, A.H.,
Journal of Engineering for Gas Turbines and Power, 107, 24–37, 1985. With permission.)
214 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
Nomenclature
A
f
ame area, m
2
A
ref
combustor reference area, m
2
B mass transfer number, or constant in Equation 5.29
B
a
aerodynamic blockage
B
g
geometric blockage of ameholder
b temperature dependence of reaction rates in Equation 5.5
C
p
specic heat at constant pressure, J/kg K
D
c
characteristic dimension of ameholder, m
D
o
initial drop diameter, also Sauter mean diameter, m
D
ref
maximum diameter or width of combustor casing, m
E
min
minimum ignition energy, mJ
f
c
fraction of total combustor air employed in combustion
f
f
fraction of fuel vaporized within combustion zone
f
pz
fraction of total combustor air employed in primary-zone combustion
H lower specic energy of fuel, J/kg
k thermal conductivity, J/msK
l turbulence scale, m
m
mass ow rate, kg/s
n number of drops, or reaction order
13
12
11
J 79–17C
P
3
= 101 kPa
m
∙
A
= 3.18 kg/s
T
3
= 238–278 K
T
F
= 238–278 K
10
9
8
7
6
5
4
456789
q
LLO
(predicted), g/kg
q
LLO
(measured), g/kg
10 11 12 13
Figure 5.43
Comparison of measured and predicted values of q
LLO
for a J 79 combustor. (From Lefebvre, A.H.,
Journal of Engineering for Gas Turbines and Power, 107, 24–37, 1985. With permission.)
Combustion Performance 215
P pressure, Pa (kPa in Equations 5.19, 5.20, 5.27, and 5.29)
q fuel/air ratio by mass
q
ref
reference dynamic pressure, Pa
R gas constant (286.9 m
2
/s
2
K)
Re drop Reynolds number (u′ D
o
/ υ
g
)
SMD Sauter mean diameter, m or µm
S
T
turbulent ame speed, m/s
T temperature, K
U velocity, m/s
U
j
jet velocity, m/s
U
ref
combustor reference velocity, m/s
u′ RMS component of uctuating velocity, m/s
V volume, m
3
w gutter width, m
ΔP
L
liner pressure differential, Pa
ΔT temperature rise due to combustion, K
ϕ equivalence ratio
λ evaporation constant, m
2
/s (mm
2
/s in Equation 5.20)
ν kinematic viscosity, m
2
/s
µ dynamic viscosity, kg/ms
η
c
combustion efciency
η
θ
combustion efciency (reaction-controlled)
η
e
combustion efciency (evaporation-controlled)
η
m
combustion efciency (mixing-controlled)
ρ density
Subscripts
A air
F fuel
c combustion zone value
g gas
ad adiabatic
eff effective value
LBO lean blowout value
LLO lean lightup value
o initial value
oυ overall value
p preheater value
pz primary-zone value
ref reference value
st stoichiometric value
WE weak extinction value
3 combustor inlet value
∞ ambient value
216 Gas Turbine Combustion: Alternative Fuels and Emissions, Third Edition
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