ignition process in small jet engines

20
IGNITION PROCESS IN SMALL JET ENGINES By Y. Levy, V. Sherbaum, V. Nadvany, Y. Nekhamkin Faculty of Aerospace Engg, Technion

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Page 1: IGNITION PROCESS IN SMALL JET ENGINES

IGNITION PROCESS IN

SMALL JET ENGINES

By Y. Levy, V. Sherbaum,

V. Nadvany, Y. Nekhamkin

Faculty of Aerospace Engg,

Technion

Page 2: IGNITION PROCESS IN SMALL JET ENGINES

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.

Page 3: IGNITION PROCESS IN SMALL JET ENGINES

Vaporizer advantages:

Low cost.

Modest fuel-pump pressure requirements.

Low soot formation.

Page 4: IGNITION PROCESS IN SMALL JET ENGINES

Schematic drawing of a small gas turbine

with vaporizer

Page 5: IGNITION PROCESS IN SMALL JET ENGINES

How does the vaporizer work?

Heat

Page 6: IGNITION PROCESS IN SMALL JET ENGINES

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.

Page 7: IGNITION PROCESS IN SMALL JET ENGINES

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.

Page 8: IGNITION PROCESS IN SMALL JET ENGINES

The experimental vaporizer design

With

assisting air

Typical

VaporizerInclined

impactor

Page 9: IGNITION PROCESS IN SMALL JET ENGINES

Example of atomization data measurements

extracted from PDA system

Page 10: IGNITION PROCESS IN SMALL JET ENGINES

Fuel Pressure = 1.5 bar, Air velocity 20m/s.

Standard Modified

Page 11: IGNITION PROCESS IN SMALL JET ENGINES

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

Page 12: IGNITION PROCESS IN SMALL JET ENGINES

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

Page 13: IGNITION PROCESS IN SMALL JET ENGINES

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)

Page 14: IGNITION PROCESS IN SMALL JET ENGINES

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.

Page 15: IGNITION PROCESS IN SMALL JET ENGINES

Igniter position

Page 16: IGNITION PROCESS IN SMALL JET ENGINES

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

Page 17: IGNITION PROCESS IN SMALL JET ENGINES

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)

Page 18: IGNITION PROCESS IN SMALL JET ENGINES

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

Page 19: IGNITION PROCESS IN SMALL JET ENGINES

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

Page 20: IGNITION PROCESS IN SMALL JET ENGINES

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.