ignition process in small jet engines
TRANSCRIPT
IGNITION PROCESS IN
SMALL JET ENGINES
By Y. Levy, V. Sherbaum,
V. Nadvany, Y. Nekhamkin
Faculty of Aerospace Engg,
Technion
Difficulties in the design of small jet engine
injectors:
1.Low fuel flow rate.
2.Low a nozzle pressure drop.
3.Need for simple configurations.
Vaporizer advantages:
Low cost.
Modest fuel-pump pressure requirements.
Low soot formation.
Schematic drawing of a small gas turbine
with vaporizer
How does the vaporizer work?
Heat
Main drawbacks:
Very poor atomization at start – up (when the
vaporizer is cold).
Delicate heat balance
Existing Solutions:
Dual fuel supply systems
Torch igniter
Pyrophoric fuel.
All these solutions complicates the fuel system.
The objective of the present work:
Investigate influence of the aerodynamic and
design parameters of combustor (air velocity,
air velocity profile at the vaporizer exit, the
droplets size and flux distributions, shape of
impactor, the igniter location) on the ignition
performances and based on these results to
minimize needs of ignition supporting
systems.
The experimental vaporizer design
With
assisting air
Typical
VaporizerInclined
impactor
Example of atomization data measurements
extracted from PDA system
Fuel Pressure = 1.5 bar, Air velocity 20m/s.
Standard Modified
150
200
250
300
350
-3 -2 -1 0 1 2 3
SM
D, m
icro
ns
P = 0.5 bar bar
150
200
250
300
350
-3 -2 -1 0 1 2 3
Relative radius r/r0, Y axis
SM
D, m
icro
ns
Droplets SMD distribution along the
vaporizer radius, (Z = 30 mm, b =120 deg).
.
Vair=45 m/s
Vair=0
0
0.005
0.01
0.015
0.02
0.025
0.03
-3 -2 -1 0 1 2 3
Pw =2 bar
1 bar0.5 bar
0
0.01
0.02
0.03
0.04
0.05
0.06
0.07
-3 -2 -1 0 1 2 3
Relative radius, r /r0, x direction
Pw =2 bar
0.5 bar1 bar
Liquid flux distribution along the vaporizer radius,
X direction Z = 30 mm, β = 120 deg.
Vair = 0
Vair = 45 m/s0
0
0.2
0.4
0.6
0.8
1
1.2
-1 0 1
measured
values
a)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
-1 -0.75 -0.5 -0.25 0 0.25 0.5 0.75 1
Dimensionless radius, r/r0
CFD Test
Air velocity distribution at the vaporizer exit
(a)-along y-axis, b)-along x-axis).
b)
Ignition Low Boundary (one fuel nozzle)
.
.
.
.
.
.
.
. . . .
Air flow rate, kg/s
b= b=
plane atomizer
Limiting ignition point
Successful ignition tests were achieved under the following conditions:
- fuel pressure – 1.5 - 6 bar
- fuel/air ratio - 0. 01 – 0.1 (for 3 vaporizers)
- averaged air velocity over the liner cross section – 4.5 - 24 m/s
- air temperature - 293 K (ambient - 285 K).
- fuel temperature - 283 K.
Igniter position
Schematic picture of the igniter location and ignition region
(top –front view, bottom - 3-D picture). The existing igniter is
located closer to bottom side.
Existing igniter
location
Tested ignition region
b)
a)
c)
Computer simulation of the temperature distribution at the vaporizer
region (a) - at the vaporizer wall, b) – at the section along the
combustor, c) – at the cross section of the combustor).
b)
vaporizer Air, fuel
Jet of pyrophoric
fuel
Hot air-fuel
mixture Recirculation
zone
Small holes at
the vaporizer
housing
Schematic drawing of possible vaporizer design for ignition
improvement by pyrophoric fuel application.
Some remarks about pyrophoric ignition
Existing vaporizer does not provide spray.
Almost all liquid flows through the vaporizer
outlet as non-atomized continuous fluid.
Modified impact atomizers demonstrated better
atomization quality.
The successful spark ignition over a wide area of the thermodynamic parameters was demonstrated. The igniter location zone and optimal igniter position was found.
CONCLUSIONS
It has been found that successful ignition depends strongly on the local droplets diameter and on the air velocity locus at the ignition point and can be achieved even for large average droplets SMD values.