GB2209205A - A fuel vaporiser - Google Patents

Abstract

A form of T-shaped fuel vaporiser for a gas turbine engine is described in which the vaporiser mixture outlets are defined by a generally oval shaped cap joined to one end of a vaporiser stem. The outlets are defined by a cap rim and part of the wall of the stem so that the latter is washed on both sides by the fuel/air mixture. The arrangement eliminates hot spots which tend to occur with T-shaped vaporisers on the inside bends of the cross-piece and outlets, but without sacrificing the desirable flow distribution characteristics of a T-shaped vaporiser. <IMAGE>

Description

A FUEL VAPORISER The invention relates to a fuel vaporiser, particularly for a liquid fuelled gas turbine engine.

Fuel is inducted into the combustion chamber of a liquid fuelled gas turbine engine, generally, either by means of a high pressure spray atomiser or by means of a fuel vaporiser. The present invention is concerned with the latter.

The vaporiser delivers a mixture of fuel and primary air to the interior of a combustion chamber in which a plurality of vaporisers are mounted. Fuel is pumped through a supply pipe into the interior of the vaporiser where it mixes with the forced airstream from a compressor. This mixture is burned in the chamber in continuous combustion. Radiation from the combustion process heats the vaporiser structure and pipes and assists in vaporising the fuel for complete mixing of the fuel and primary air. Further secondary air from the compressor is inducted into the combustion chamber.

This air being relatively highly compressed is at an increased temperature and further assists pre-heating of the vaporiser structure.

Even distribution of fuel mixture is important to achieve proper efficient combustion. To achieve this it is known to use a vaporiser having a T-shaped structure in which primary air enters through a single circular stem, of substantially constant cross-sectional area, containing a fuel feed. The resulting fuel-air mixture exits in a reverse flow direction through twin circular outlets spaced equally on either side of the stem. The cylindrical portions comprising the inlet and outlets are joined by a connecting cross-piece. - The total cross-sectional area of the outlets, is approximately equal to the cross-sectional area of the inlet.

Difficulty is experienced, however, in cooling the faces of the inlet stem, outlets and the cross-piece in the region of the inner walls of the curved sections.

Heat is normally absorbed from these areas by fuel washing on the inside of the walls. But hot spots have long proved difficult to eliminate in the aforementioned regions where separations of the main flow from the vaporiser walls can lead to at least partially stagnant regions of flow. In these regions the rates of transfer of heat from the walls to the fuel/air mixture is significantly reduced. At higher engine pressure ratios the consequent problems of overheating of these inner faces is increasingly troublesome, conventional cooling arrangements prove inadequate, and eventually oxidation of the vaporiser walls can occur. Previous attempts to overcome the difficulty include a design of a vaporiser having a mushroom shaped head or cap.Reversal of the fuel/air mixture flow by impingement on the inside of a mushroom-shaped or part-spherical end cap cooled the structure adequately but the annular outlet formed around the rim of the cap produced less advantageous mixture distribution and less efficient combustion.

The main object of the present invention is to prevent formation of hot spots on the fuel vaporiser whilst retaining the preferred fuel distribution properties possessed by the T-shaped vaporiser.

According to the present invention there is provided a fuel vaporiser having a generally T-shaped form comprising a stem housing an inlet pipe for connection to a fuel supply and a cap having a generally oval shaped rim which for part of its length in the region of its minor axis is joined to the downstream end of the stem and which for the remainder of its length is spaced apart from the outer wall of the stem thereby forming at least one rearward facing outlet orifice the inner edge of which is defined by the wall of the stem.

Preferably the oval cap is located concentrically with the longitudinal axis of the stem so as to form two outlet orifices on opposite sides of the stem, the total combined area of which is substantially equal to the cross-section area of the stem.

Further features of the invention will be apparent from an exemplary embodiment which will now be described, by way of example only, with reference to the accompanying drawings, in which: Fig 1 shows a sectional elevation on BB of Fig 2 of a capped fuel vaporiser, and Fig 2 shows a sectional view on AA through the cap of the vaporiser of Fig 1.

Referring first to Fig 1, air enters the vaporiser through a tubular stem 1 from the left in the plane of the drawing. To the right hand end of stem 1 there is attached a cap 2, which faces leftwards. Air travelling through stem 1 towards the right impinges on the inner surface of cap 2, its direction of flow is thereby reversed and issues through outlet orifices 4 and 5 on either side of stem 1.

The tubular stem 1 has a circular section towards the left in Fig 1 and this is gradually transformed to a roughly square or rectangular section in the region of the cap 2, as will be seen in the sectional view of Fig 2 on the plane AA of Fig 1. Also in the transverse plane AA the cap 2 has a generally oval section. That is, in the plane of the elevation of Fig 2 the major or vertical dimension of the cap is substantially greater than the minor or horizontal dimension, and the latter is equal to the corresponding width of one end of the stem 1. The cap 2 and stem 1 may be formed as separate components and attached one to the other or, alternatively, they may be formed integrally, for example, as a single casting.

Preferably, the cap 2 is located concentrically with the longitudinal axis of the stem 1 so that the rim 3 of the cap cooperates with the wall of the stem, towards one end thereof, to define a pair of outlet orifices 4 and 5 on opposite sides of the stem. The wall of the stem forms, in the drawing, the substantially straight inner edges of two roughly D-shaped outlets on opposite sides of a roughly square-sectioned stem. The total outlet area, that is, the combined area of both outlets, is approximately equal to the transverse area of the said square section of the stem. In the embodiment being described the transverse section of stem 1 is transformed from circular at its left end to roughly square at its right hand. The area of the section remains substantially constant along the length of the stem.

Fuel is introduced to the vaporiser through fuel supply pipe 6 and bifurcated head 7 disposed on the axis of the stem towards its upstream end ie to the left in Fig 1. Fuel sprays out of the bifurcated head 7 into the forced airstream flowing through stem 1.

The fuel emerges in fine droplets which mix with the primary airflow and vaporise on their path through the stem 1 and cap 2. Some of the droplets impinge on the inner walls of stem 1, on the end face of cap 2 and on both sides of the stem wall in the region of the outlet orifices 4 and 5.

Combustion apparatus within which the subject vaporiser finds use typically comprises an air-jacketed combustion chamber having at its upstream end a base plate upon which a plurality of such vaporisers is mounted, with inlets for fuel and primary air. A high pressure ratio compressor provides the primary air stream through the vaporiser stems and a secondary air flow around the combustion chamber which enters through a multiplicity of ducts and orifices in the chamber walls to further mix with the primary mixture from the vaporisers.

The final mixture supports a continuous combustion process in the chamber. Intense burning takes place in the region downstream of the vaporisers although ignition commences close to the vaporisers which can be heated to relatively high temperatures. Generally fuel washing on the interior of the vaporiser absorbs, by its latent heat of vaporisation, substantial amounts of heat from the body of the vaporiser. This helps cool the structure and to keep the vaporiser temperature within acceptable working limits.

However, in T-shaped vaporisers hot spots have been found to occur in the region of the inside radii of the transverse duct, downstream end of the stem and the flow reversing outlet orifices. In known T-shaped vaporisers of the type referred to the outlet orifices hitherto have been defined by short, rearward facing, open-ended cylinders spaced apart from the main stem by short lengths of transverse ducting. This arrangement gives rise to hot spots on the inside of the bends thus formed where damage to the vaporiser material is found to occur. In the arrangement presently being described such regions have been deleted without deleterious effects on fuel/air mixture distribution. In the vicinity of the end of the stem 1 enclosed by cap 2 fuel droplets wash both sides of the stem walls and thereby impart greater cooling to the stem extracting heat from the stem by latent head of vaporisation. The twin outlets on diametrically opposite sides of the main stem retain the desired mixture distribution characteristics sought.

In the described example means is provided for diverting mixture issuing from the outlet orifices away from the stem. This means comprises a ramp, generally indicated at 8 in Fig 1, preferably this ramp consists of an annular shaped boss formed integrally with the stem. The inclined side of the ramp 8 faces towards the outlet orifices 4 and 5 and cooperates with the rim 3 of the cap 2 to turn the direction of mixture flow radially outwards away from the stem 1.

Claims (8)

1. A liquid fuel vgporiser of generally T-shaped form comprising a stem housing an inlet pipe for connections to a fuel supply and a cap having a generally oval shaped rim which for part of its length in the region of its minor axis is joined to the downstream end of the stem and which for the remainder of its length is spaced apart from the outer wall of the stem thereby forming at least one rearward facing outlet orifice the inner edge of which is defined by the wall of the stem.

2. A vaporiser as claimed in claim 1 wherein the oval cap is located concentrically with the longitudinal axis of the stem so as to form two outlet orifices on opposite sides of the stem.

3. A vaporiser as claimed in either claim 1 or claim 2 wherein the transverse cross-sectional area of the stem is substantially constant along its length.

4. A vaporiser as claimed in any preceding claim wherein the total outlet area is substantially equal to the transverse area of stem.

5. A vaporiser as claimed in any preceding claim wherein the stem has a substantially rectangular transverse section at least in the outlet region.

6. A vaporiser as claimed in any preceding claim further comprising means external of the stem for diverting away from the stem mixture issuing from the or each outlet orifice.

7. A vaporiser as claimed in claim 7 wherein the diverting means comprises a ramp formed on the exterior of the stem and having an inclined surface which cooperates with the cap rim forming an outlet orifice to divert mixture issuing from said orifice.

8. A vaporiser substantially as described with reference to the figures of the accompanying drawing.