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

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.