GB2049915A

GB2049915A - Gas turbine combustor

Info

Publication number: GB2049915A
Application number: GB8011541A
Authority: GB
Original assignee: English Electric Co Ltd
Current assignee: English Electric Co Ltd
Priority date: 1979-04-06
Filing date: 1980-04-08
Publication date: 1980-12-31

Abstract

A device for improving the efficiency of combustion in a gas- turbine swirl combustor comprises a radial swirler having a cone, pointing in the direction of flow, fitted around the fuel inlet nozzle 4, thus forming an inner wall 6 for the annular air inlet 5. The cone is fitted with projecting pins 3 at points around the surface which pins project into the air flow and cause fine scale turbulence. In a second embodiment (Fig. 2, not shown), for an axial air inlet arrangement, the fuel nozzle is surrounded by, or formed in the shape of, a dilating core so that the shear layer developing over the conical surface undergoes considerable extra 'straining' due to a positive pressure gradient, dilation, and transverse curvature. Fine scale turbulence is again obtained. <IMAGE>

Description

SPECIFICATION Gas turbine combustor The present invention relates to gas turbine combustors particularly, but not exclusively, of the 'swirl' combustor type, and means for enhancing the mixing of fuel and air within the combustion chamber.

The efficiency and uniformity of combustion is strongly influenced by the extent to which the fuel and air (the reactants) are mixed within the chamber. Localized hot zones within the combustion chamber and consequent inefficient smoke emission can result from insufficient mixing of the reactants.

It is the object of the present invention to increase mixing of the reactants to reduce hot zones within the combustion chamber.

According to the present invention, a gas turbine combustor comprises a combustion chamber, a fuel injector projecting into said combustion chamber and a charge air inlet concentric with said fuel injector, said charge air inlet having an inner wall which is so shaped or adapted as to enhance turbulence in a recircultion zone between the fuel path in the central region of the chamber and the surrounding air flow path.

According to one aspect of the present invention, the inner wall may be provided with a multiplicity of discontinuities small in relation to the dimensions of the combustion chamber recirculation zone. Preferably the discontinuities are pins extending into the charge air flow path, the pins forming a substantially regular array around the inner wall.

The inner wall may be conical and directed towards the combustion chamber the array of pins comprising rings of pins concentric with the axis of the conical wall, successive rings being staggered around the cone to provide a greater uniformity of the discontinuities around the cone.

According to a second aspect of the present invention, the inner wall may consist of a centre body of conical form, diverging into the combustion chamber so as to generate a turbulent boundary layer around the centre body. The turbulent boundary layer eventually detaches from the rear of the centre body and enhances mixing in an annular region between the fuel path in the central region of the chamber and the surrounding airflow path.

Two embodiments of the invention will now be described with reference to the accompanying drawings in which: Figure 1 is a section through a combustor, in accordance with the first embodiment of the invention; and Figure 2 is a section through a combustor, in accordance with the second embodiment of the invention.

Referring to Figure 1, a combustor 1 has a combustion chamber 2, an annular air inlet 5 and a fuel injector 4 positioned concentrically within the inlet 5. The fuel injector 4 protrudes into the chamber 2.

Air Is fed into the chamber via the inlet 5 from behind the fuel injector 4, the flow path being initially radial to the axis of the fuel injector and ,axially swirling thereafter.

The air flow path is generally annular within the combustion chamber 2, and surrounds the fuel path. Combustion air is also fed into the chamber directly, from ports 7 downstream from the fuel injector 4, thus assisting the estabiishment of a recirculation zone downstream from the fuel injector 4 and tending to complete the combustion.

In accordance with the present invention, the inlet 5 has an inner wall 6 concentric with the axis of the fuel injector 4. This inner wall is conical in shape, and points towards the combustion chamber 2. The wall 6 has pins 3 extending into the charge airflow path and generally destabilising the flow. The pins 3 are arranged in concentric rings around the conical inner wall 6, the rings being staggered so as to expose each pin to the flow, and as far as possible providing a uniform field of turbulence around the wall.

The induced turbulence comprises additional small scale vortices within the annular flow path inside the combustion chamber, and this has the effect of increasing the transport of fuel particles across the boundary between the fuel path and the charge air flow path. The probability of a given fuel particle experiencing a higher local air velocity is thereby increased. mixing of the reactants increased, and as a result localised hot zones within the chamber 2 will be reduced. The content of both oxide nitrogen and smoke emission is thereby improved.

The recirculation zone is not substantially altered since the turbulence induced is on a scale, typically 1/100, of the dimensions of the zone.

Thus the turbulence is superimposed on an established re-circulation zone structure, and no major redesigning of the combustor is required.

By way of an example, using a combustion chamber flame tube designed to accommodate about 3lb/sec. flow in the inlet 5, the inlet air temperature being typically 2000 C, and the outlet exhaust gases having a temperature of 9700 C, a smoke particle reduction from 6T3 to 5 on the Bacharach scale (or 12.5bug to 5y9 on the gravimetric scale) was obtained with pins 1.27 cm. in height. Whilst this result is modest in terms of the visuai threshold of smoke, the result is an improvement since there are no delecterious effects elsewhere in the comoustor.

Clearly, alternative patterns of pins on the conical inner wall 6, or pins of differing length, can be used depending on the particular type of combustor.

Referring to Figure 2, which shows an alternative embodiment of the invention, a combustor 11 has a combustion chamber 22 and an air inlet 25. The latter has a concentric inner wall 26 of conical shape directed away from the combustion chamber 22.

A fuel injector 24 concentric with the inlet 25, extends inside the conical inner wall 26, the 'base' of the inner wall cone 26 extending into the combustion chamber 22.

The fuel injector 24 is connected to the outer wall of the inlet 25 by means of radially mounted swirler vanes 29.

The charge air flows into the combustion chamber 22 from the inlet 25, as well as from ports 27 feeding air directly into the chamber 22 downstream from the inlet 25, both flow paths establishing a recirculation zone of air and fuel (from the fuel injector) as before.

The conical inner wall 26 dilates the charge air flow in the inlet.25, so that the flow experiences 'straining' in addition to a rise in pressure. These effects accelerate the development of the boundary layer on the inner wall 26, so that transition to turbulent flow is more readily assured, even at moderately low free stream unit Reynolds numbers (eg 1 04/cm). By virtue of the form of the centre body, the charge air flow entering the combustion chamber will now contain a residual sheer stress field assisting the development of fine scale flow instabilities, and therefore turbulence, at the boundary of the inlet flow path and the fuel path. This enhances the mixing of reactants in a similar manner and preferably on a similar scale, as that described with reference to Figure 1. Thus, as before, turbulence is superimposed on an established recirculation zone structure, no major redesigning of the combustor being required.

Claims

1. A gas turbine combustor comprising a combustion chamber, a fuel injector projecting into said combustion chamber and a charge air inlet concentric with said fuel injector, said charge air inlet having an inner wall which is so shaped or adapted as to enhance turbulence in a recirculation zone between the fuel path in the central region of the chamber and the surrounding air flow path.

2. A gas turbine combustor according to Claim 1, wherein said inner wall is provided with a multiplicity of discontinuities small in relation to the dimensions of the combustion chamber recirculation zone.

A gas turbine combustor according to Claim 2, wherein said discontinuities are provided by pins extending into the charge air flow path, the pins forming a substantially regular array around the inner wall.

4. A gas turbine combustor according to Claim 3, wherein said inner wall is conical and is directed towards the combustion chamber, said array of pins comprising rings of pins concentric with the axis of the conical wall, successive rings being staggered around the cone to provide a greater uniformity of said discontinuities around the cone.

5. A gas turbine combustor according to Claim 1, wherein said inner wall is of conical form diverging into the combustion chamber so as to generate a turbulent boundary layer around the centre body.

6. A gas turbine combustor according to Claims 1--4 substantially as hereinbefore described with reference to Figure 1 of the accompanying drawings.

7. A gas turbine combustor according to Claims 1 and 5 substantially as hereinbefore described with reference to Figure 2 of the accompanying drawings.