Fuel vaporizer

A fuel vaporizer is formed from two concentric tubes. A mixture of fuel and air having a high proportion of fuel passes along the annular space between the two tubes while air alone passes along the inner tube.

FIELD OF THE INVENTION 
This invention relates to a fuel vaporizer, particularly a fuel vaporizer 
for use in a gas turbine engine. 
DESCRIPTION OF THE PRIOR ART 
The prior art will be described with reference to the accompanying 
diagrammatic FIG. 1 which is a cross-section of a known fuel vaporizer 1 
which is located in a fuel burning region 2 of a gas turbine engine 
bounded by a wall 3. The fuel vaporizer 1 comprises a hollow tube of 
circular cross-section and of a substantially U-shape penetrating the wall 
3 at one end 6. 
The pressure within the fuel burning region 2 is arranged to be lower than 
on the other side of the wall 3 so that air outside the fuel burning 
region 2 passes through the fuel vaporizer 1 and into the fuel burning 
region 2. A fuel injector 4 sprays fuel denoted by arrows 5 into the 
airstream passing through the fuel vaporizer 1. 
The fuel burning region 2 is hot and this heat is transferred to the fuel 
vaporizer 1, the fuel vaporizer 1 in turn transfers this heat to the fuel 
5 passing through it. As a result the tubular wall of the fuel vaporizer 1 
is cooled below the temperature of the fuel burning region 2 and the fuel 
5 is vaporized so that a mixture of air and fuel vapour leaves the free 
end 7 of the fuel vaporizer 1 and is then burnt in the fuel burning region 
2. 
It is generally desirable to increase the burning temperature, that is the 
temperature within the fuel burning region 3, within gas turbine engines 
to increase their efficiency, and this produces a problem. 
As the burning temperature increases the operating temperature of fuel 
vaporizer 1 increases until it is so weakened by temperature that it can 
no longer withstand the mechanical stresses upon it and breaks up. This 
can be prevented by increasing the amount of fuel being injected into the 
airstream by the fuel injector 4, this increases the amount of cooling of 
the fuel vaporizer 1, lowering its temperature further below that of the 
fuel burning region 2 and thus preventing its break up. 
Unfortunately, despite the fact that the total ratio of fuel to air within 
the fuel burning region can be adjusted to any desired value it has 
nevertheless been found that if the ratio of fuel to air in the mixture 
leaving the fuel vaporizer 1 is too high black smoke will be emitted by 
the engine. This is often undesirable for visibility and environmental 
reasons. 
Thus the problem may be encountered that if the engine is not to emit black 
smoke its burner temperature must be limited and the engine's efficiency 
is as a result reduced. 
BRIEF SUMMARY OF THE INVENTION 
The present invention was intended to produce a fuel vaporizer at least 
partially overcoming this problem. 
This invention provides a fuel vaporizer comprising a fuel vaporizer 
comprising a first tube, a second tube and fuel injection means, the 
second tube being located within the first tube and defining a space 
between the two tubes to allow a first gas flow to flow in the space 
between the two tubes and a second gas flow to flow within the second 
tube, the two tubes and the fuel injection means being arranged so that, 
in use, the first gas flow contains a higher proportion of fuel than the 
second gas flow. 
This solves the problem by permitting it to operate at a low overall fuel 
air ratio while allowing adequate cooling. 
Preferably the second gas flow contains substantially no fuel.

DETAILED DESCRIPTION 
A fuel vaporizer 8 is within a fuel burning region 2 bounded by a wall 3. 
This is a burner can within a gas turbine engine. A fixed end 12 of the 
fuel vaporizer 8 is secured about the perimeter of a hole 13 in the wall 
3. The pressure within the fuel burning region 2 is less than the air 
pressure on the other side of the wall 3, as a result air passes through 
the fuel vaporizer 8 into the fuel burning region 2. 
The fuel vaporizer 8 is formed by an outer circular tube 9 and a coaxial 
circular inner tube 10, defining between them an annular channel 11. Six 
fins 15 linking the two tubes 9 and 10 are spaced around the annular 
channel 11 and extend parallel to the axis of the tubes 9 and 10. The fuel 
vaporizer 8 has three distinct sections starting from the fixed end 12, 
there is a first, entry, section 16 comprising the outer tube 9 only, a 
second, main, section 17 comprising both the outer and inner tubes 9 and 
10 respectively, and a third, exit, section 18 again comprising the outer 
tube 9 only and terminating in a free end 19. 
A fuel injector 14 passes along the axis of the outer tube 9 and terminates 
in a plurality of fuel spraying nozzles 20 within the first, entry, 
portion 16 of the fuel vaporizer 8. 
The fuel spray nozzles 20 spray fuel into the airflow passing through the 
fuel vaporizer 8 in the directions of the arrows 22. These directions are 
not parallel to the axis of the outer tube 9 and are arranged so that all 
of the liquid fuel passes into the annular channel 11 between the inner 
and outer tubes 10 and 9 and effectively no liquid fuel passes into a 
circular channel 21 inside the inner tube 10. In practice a very small 
amount of fuel will inevitably pass along the circular channel 10, but 
this amount is negligible. As a result the ratio of fuel to air passing 
down the annular channel 11 in the second, main, section of the vaporizer 
8 is very high, a mass ratio of 2 parts air to one part fuel being used. 
There must be a fuel air mixture in this annular passage 11 however, if 
pure fuel were to be used it would be likely to be "cracked" by the heat 
of the vaporizer outer tube 9 producing gummy or solid deposits which 
would disrupt or block fuel flow. 
As the fuel air mixture passes along the annular channel 11 of the fuel 
vaporizer 8, the fuel absorbs heat from the vaporizer 8 and is vaporized, 
this heat absorption acting to cool the vaporizer 8. The fins 15 support 
the inner tube 10 within the outer tube 9, conduct heat so that the inner 
tube 10 acts as a heat sink for the outer tube 9 and ensures that the fuel 
air mixture remains evenly distributed around the annular channel 11. 
On exit from the second, main, section of the vaporizer 8 the fuel air 
mixture coming from the annular channel 11 contains a very high proportion 
of fuel vapour while the air coming from the circular channel 21 contains 
virtually no fuel vapour. In the third, exit, section of the vaporizer 8 
the outer tube 9 reduces in radius towards the free end 19 of the 
vaporizer 8 to cause mixing of the airstreams from the annular passage 11 
and the circular passage 21 so that a homogenous mixture of fuel vapour 
and air at a mass ratio of 5 or more parts air to 1 part fuel vapour is 
emitted from the free end 19 of the vaporizer 8 into the fuel burning 
region 2 and is burnt. 
The very high ratio of fuel to air in the annular channel 11 adjacent to 
the outer tube 9 causes a high rate of cooling of the outer tube 9, 
allowing the vaporizer 8 to endure higher ambient temperatures within the 
fuel burning region 2, however the lower ratio of fuel to air in the 
mixture passing out of the vaporizer 8 into the fuel burning region 2 
avoids the emission of black smoke that would otherwise be produced by 
such fuel air ratios. 
An alternative form of fuel vaporizer 23 is shown in FIG. 4. Where the 
previous vaporizer 8 was substantially "U" shaped the vaporizer 23 is 
substantially "W" shaped. Each of the tubes 9 and 10 is bifurcated in the 
region 24 to produce a vaporizer 23 having a single fixed end 12, first, 
entry, section 16, two, second, main, sections 17 and two third, exit, 
sections 18 and free ends 19. 
A problem which can arise in the vaporizer design shown in FIG. 4 is that 
the flow of fuel air mixture in the annular channel 11 can stagnate. This 
reduces the cooling in the stagnated areas and as a result local 
overheating or blockage by carbon deposits can occur in the stagnated 
regions causing thermal fatigue and reducing the life of the vaporizer 23. 
It can also be difficult to ensure that equal quantities of fuel pass down 
each arm of the vapouriser to ensure equal cooling of the two arms. 
A vaporizer design overcoming this problem is shown in FIG. 5. The 
vaporizer 25 is substantially the same as the vaporizer of FIG. 4, having 
an outer tube 9 and an inner tube 10 both bifurcated in a region 24 to 
give a single fixed end 12, a first, entry section 16, two second, main, 
sections 17 and two third, exit sections 18 and free ends 19. The circular 
channel 21 within the inner tube 10 is unchanged. The annular channel 11 
between the inner tube 10 and the outer tube 9 is divided into a number of 
separate channels 26 by 8 strakes 27. 
Initially, where the tubes 9 and 10 are single sixteen strakes 27 divide 
the annular channel 11 into sixteen equally sized passages 26. Eight of 
these passages 26 carry the fuel air mixture into one bifurcation of the 
annular channel 11 while eight carry the fuel air mixture into the other 
bifurcation. In the main regions 17 of the vaporizer each of the two 
annular channels 11 is divided into eight equally sized passages 26 by 
eight strakes 27. In the bifurcation region 24, the strakes 27 are 
arranged so that the passages 26 are volute. 
A weir 28 is arranged around the inner circumference of the outer tube 9 in 
front of the ends of the strakes 27. The weir 28 causes turbulence in the 
flow of fuel and fuel vapour along the inner surface of the outer tube 9 
to ensure that the fuel is distributed evenly among all of the passages 
26. 
This arrangement ensures good cooling of all of the vaporizer 25. 
In the exit sections 18 of the vaporizer 25 the outer tubes 9 and the inner 
tubes 10 reduce in diameter, this ensures good mixing of the relatively 
strong fuel air mixture from the annular passage 11 and the relatively 
weak fuel air mixture from the circular passage 21 before the fuel is 
burnt. 
The weir 28 could be replaced by other turbulence producing structures such 
as pedestals. 
The passages 26 are of differing lengths and so cool different amounts of 
the surface of the vaporizer 25, additionally the heating experienced by 
the vaporizer 25 will vary from point to point on its surface, so the 
amount of heat energy which must be absorbed by the fuel air mixture of 
each of the channels 26 will be different. As a result it may be preferred 
to vary the cross sectional areas of the passages 26 along their lengths, 
give them different entry areas, or arrange that fuel air mixtures of 
different strengths flow along them in order to ensure satisfactory 
cooling at all points. 
It will be understood that vaporizers employing the invention can be formed 
with any number of free ends 19 and any number of branchings such as that 
at 24. 
The two tubes 9 and 10 need not be coaxial and could be of any convenient 
cross-sectional shape or could even have a varying cross-section along 
their lengths. 
In the arrangement of FIGS. 2 and 3 it would be possible to not use the 
fins 15 but to rely on the shaping of the tubes 9 and 10 and the use of 
structures on the surfaces of the tubes 9 and 10 or passing across the gap 
between them to control the movement of fuel and fuel vapour in the 
passage 11 between the two tubes 9 and 10. 
In some situations it may be advantageous to place structures in the exit 
section 18 of the vaporizer 8,23,25 or to alter the shape of the tube 9 to 
aid mixing of the airstreams from the annular passage 11 and circular 
passage 12, structures may also be provided within the passages 11 and 12 
to encourage such mixing. It may also be advantageous to place structures 
in the entrance section 16 of the vapouriser 8,23,25 to aid formation of 
the two separate fuel air mixtures. 
It may also be advantageous to provide structures in the entry section 16 
of the vapouriser 8,23,25 and in the annular passage 11 to encourage local 
mixing of air and fuel in order to improve the rate of cooling and heat 
transfer. These structures or additional structures in the entry section 
16 of the vapouriser 8,23,25 and annular passage 11 could also be used to 
ensure that the fuel from the nozzles 20 is distributed correctly around 
the annular channel 11. 
The two tubes 9 and 10 need not be rigidly connected but could be arranged 
to have some play between them in order to reduce thermally induced 
stresses on the vapouriser 8,23,25. 
If desired the two tubes could be formed of different materials. 
The number of fins 15 used to support the inner tube 10 could of course be 
varied as required, or support structures of other types, such as rods 
linking the two tubes 9 and 10, could be used. 
It may be found advantageous in some circumstances to have some of the fuel 
passing down the circular channel 21, this will still work provided the 
ratio of fuel to air in the annular channel 11 is higher than the ratio of 
fuel to air in the circular channel 21. 
The best ratios of fuel to air in the annular channel 11 and for the 
vaporizer 8,23,25 as a whole will of course vary depending on the 
characteristics of the engine and its fuel. 
It is not essential for the fuel vapour and air mixture leaving the 
vaporizer 8,23,25 to be homogenous, provided that before the mixture is 
burnt sufficient mixing takes place to reduce the highest fuel vapour to 
air ratios in the mixture below the level at which smoke is produced.