Combustion apparatus

Abstract

A combustor having an array of dilution holes it provided with an asymmetrically located trim port for injection a controlled volume of air into the combustor. The controlled volume mixes with fuel rich air near the combustor wall and improves the dilution of the combustion gases. The improved dilution reduces the formation of NOx.

Description

This invention relates to combustion apparatus and particularly combustion apparatus in which dilution air is supplied to quench combustion products within a combustor.

Modern emission requirements set specific targets for the amount of pollutants that may be ejected from combustion apparatus. One undesirable pollutant is NO_x. NO_x is produced particularly at high temperature conditions where there is a slightly weaker than stoichiometric mixture of air and fuel.

It is an object of the present invention to seek to provide an improved combustion apparatus giving reduced emissions.

According to a first aspect of the invention there is provided combustion apparatus comprising a combustion envelope bounding a combustion volume the combustion volume having an upstream end and a downstream end, the combustion envelope having a plurality of rows of dilution ports for the provision of dilution air which mixes in use in a midstream flow with hot combustion gas from a fuel injector, wherein each row extends in a circumferential direction and is spaced from an adjacent upstream or downstream row, the ports of each row being offset circumferentially from the dilution ports of an adjacent row, characterised in that at least one trim port is provided upstream of one or more of the rows of dilution ports for supplying in use a flow of trim cooling air into the combustion volume which mixes with hot combustion gas between the midstream flow and an envelope cooling film attached to a wall of the envelope.

Preferably the trim port is sized to limit the degree of penetration of the trim cooling air into the combustion volume.

The degree of penetration is preferably less than one third of half the radial height of the combustion volume.

The trim port may be between 0.35 and 0.7 of the diameter of one of the dilution ports.

Preferably the axis of the trim port is located downstream and circumferentially offset from a dilution port in one or more of the plurality of rows.

Preferably the trim port is positioned to permit the flow of trim cooling air to be conveyed by swirl within the combustor to a location downstream and circumferentially aligned with a dilution port.

The combustion volume may be annular.

Preferably the combustion envelope has a fuel injection location, wherein a dilution port in the most upstream row is circumferentially aligned with the fuel injection location and the trim port is associated with the circumferentially aligned trip port.

The fuel injection location may be in an upstream wall of the combustion envelope.

Preferably the combustion apparatus has two rows of dilution ports.

According to a second aspect of the invention there is provided a method of improving combustion within a combustion envelope bounding a combustion volume the combustion volume having an upstream end and a downstream end and the combustion envelope having a plurality of rows of dilution ports, characterised in that the method comprises providing a trim port wherein dilution air is supplied through the dilution ports and mixed with hot combustion gas in a midstream flow, wherein trim cooling air is supplied through the trim ports and mixed with hot combustion gas upstream of one or the rows of dilution ports and outside the midstream flow.

Preferably swirl within the combustor conveys the trim cooling air to a location downstream and circumferentially aligned with a dilution port.

The combustion apparatus may comprise a combustion envelope bounding a combustion volume the combustion volume having an upstream end and a downstream end, the combustion envelope having a plurality of rows of dilution ports for the provision of dilution air into the combustion volume, wherein each row extends in a circumferential direction and is spaced from an adjacent row in a direction perpendicular to the direction in which each row extends, wherein the dilution ports of each row are offset circumferentially from the dilution port of an adjacent row, characterised in that a trim port is provided between the first row from the upstream end and the second row.

Embodiments of the invention will now be described by way of example only, with reference to the accompanying drawings, in which:

FIG. 1 depicts a segment of an annular combustor in accordance with the invention.

FIG. 2 depicts air flow through the dilution ports and trim ports.

FIG. 1 depicts a portion of an annular combustor 2. The combustor is used in conjunction with a fuel injector (not shown) that introduces fuel into the upstream end of the combustor. The fuel injector is preferably of the type known as a “rich burn” injector which means that it although it uses air to atomise fuel the majority of the air required for combustion is provided through dilution ports 4, 6 on the radially inner and radially outer walls of the annular combustor.

The walls of the combustor themselves are kept cool by a film of air that is effusion fed onto the wall by anangled hole that extends through the wall or a tile attached to the wall depending on the architecture of the combustor. As the cooling air is fed to the wall at an angle the air attaches to the wall to form a film that isolates the wall from hot combustion gases. The film is thin and remains attached to the wall till it is replenished by a new film from effusion cooling holes further downstream. Although the volume of air used for wall cooling is relatively small, once the film detaches from the wall it mixes with hot combustion gases and can help lower the temperature of the main gas flow through the combustor.

A plurality of fuel injectors are mounted to the head end of the combustor and are circumferentially spaced from one another at a uniform spacing.

The dilution ports 4, 6 are also arranged in circumferentially extending rows with the ports through the radially inner wall and the radially outer walls being axially and circumferentially aligned. The ports in each row are equally spaced and the ports of adjacent rows are offset circumferentially by half the port pitch. There are twice as many dilution ports (per row) as there are injectors, with the arrangement being that the first row of dilution ports spaced from the injector has half its dilution ports arranged to be circumferentially aligned with the axis 8 of the injectors with the remaining half to be circumferentially intermediate the injector axes.

Accordingly, the configuration of ports results in a series of discreet currents of air 10, 12 that penetrate into the combustion space. The flows coalesce 14 near the central region of the combustor when measured between the radially inner and radially outer walls such that the partially combusted, fuel rich mixture coming from the injectors is diluted and further mixed with air flowing through the first (primary) row of ports and then further diluted and mixed with air flowing through the second (secondary) row of ports. The combustion products can be diluted in a controlled manner along the length of the combustor in order to minimise both smoke emissions and NO_x production.

The streams of air entering the combustor through the dilution ports have a relatively high momentum ratio so that they can penetrate to the combustor's mid stream. This maximises rapid dilution of the hot, fuel rich mixture in the central region of the combustor but the strong currents of cooling gad flowing through the dilution ports at a position close to the combustor walls interact poorly with the hot gas in this region. Poor interaction can result in little dilution in the axial plane which can persist downstream as streaks of hot gas that subsequently promotes NO_x formation in the combustor. The hot streaks also contribute to an increase in the near wall radial temperature profile at the combustor exit. Since the combustion gases are fed directly to the turbines significant air is required to control the temperature to maintain a high turbine life.

To improve the mixing of the fuel rich gases between the midstream flow and the cooling film attached to the walls of the combustor a trim ports 18 are provided. As minimum NO_x formation is typically required, both the radially inner and radially outer combustor walls are provided with trim ports to enable mixing both in radially inner and radially outer locations.

The function of the trim port is to introduce a controlled amount of air 16 into the combustion volume in the vicinity of the combustor walls between the boundary cooling film and the combustor's midstream that is diluted by the dilution air entering the combustor through the dilution ports 4, 6. This is typically a region from the wall cooling film to up to a third of half the combustor height. The relatively small volume of air entering through the trim port does not have the momentum required for it to penetrate to the centre of the combustor and is therefore fed to the near wall vicinity of the combustor to dilute and cool the hot gases at this location.

The degree of penetration of the air and the placement of its injection into the combustion volume is important as it controls the impact the air has on reducing local gas temperature. If the penetration is too strong the jet will penetrate too deeply into the combustor volume and merge with the main primary port flow as it mixes out in the mid-section of the combustor. If the penetration is too weak then the trim flow will be absorbed into the near wall cooling film and not contribute to dilution until much further downstream.

It is desirable that the trim port is located close to the primary ports which are preferably at the same circumferential location as the fuel injectors. The exact location of the trim port will depend on the specific application and combustor geometry. The lower momentum of the air entering through the trim port enables swirl in the combustor to carry the air that entered through the trim port circumferentially as it passes axially down the combustor. Thus the swirl can carry the trim port air to pass downstream of the first row dilution port that lies directly in front of the fuel injector.

The optimum mixing point for the trim port air to mix with the hot gas outside the core flow is just upstream of and between the secondary ports.

For an annular combustor the location for the trim port is typically found on a line prescribed from a point between adjacent primary ports and the point at which the projected injector axis crosses the circumferential line midway between the centre of the primary port row and the centre of the secondary port row. Because the trim port air requires a short axial distance to mix and spread the trim port is preferably located either at the same axial spacing as the first row of dilution ports or just downstream of these ports.

Although the trim ports can be located upstream of the primary dilution ports and still permit the swirl to carry the input air to the desired mixing location there is a danger that the input air may become fouled within the dilution air which will adjust its path and can result in the trim air being entrained in the core flow and failing to provide cooling to the hot gas in the near wall area. A greater than desirable circumferential spacing between the trim port and primary dilution port may be required to ensure such entrainment does not occur.

It will be appreciated that where the swirl direction is in a different direction the line 20 may be a mirror of its marked position when reflected about the injector axis. A different swirl direction may be achieved by different fuel injector architectures.

Preferably the trim port diameter is between 0.35 and 0.7 of the diameter of the corresponding primary port.

In addition to a direct impact on NO_x control through reducing near wall gas temperatures, the invention is also of assistance by improving the homogenisation of the near wall radial temperature profile at the combustor exit. The more homogeneous temperatures allow the amount of air used at this location to be reduced when compared with similar combustors without the trim port. This reduction in the use of air at this point allows it to be input into the combustor further upstream where it will have a greater effect on NO_x formation.

Although the invention has been described with respect to annular combustors it is equally applicable to can, or other forms of combustors having dilution ports.

Claims

1. Combustion apparatus comprising

a combustion envelope bounding a combustion volume the combustion volume having an upstream end and a downstream end,

the combustion envelope having a plurality of rows of dilution ports for the provision of dilution air which mixes in use in a midstream flow with hot combustion gas from a fuel injector,

wherein each row extends in a circumferential direction and is spaced from an adjacent upstream or downstream row, the ports of each row being offset circumferentially from the dilution ports of an adjacent row,

characterised in that at least one trim port is provided upstream of one or more of the rows of dilution ports for supplying in use a flow of trim cooling air into the combustion volume which mixes with hot combustion gas between the midstream flow and an envelope cooling film attached to a wall of the envelope.

2. Combustion apparatus according to claim 1, wherein the trim port is sized to limit the degree of penetration of the trim cooling air into the combustion volume.

3. Combustion apparatus according to claim 2, wherein the degree of penetration is less than one third of half the radial height of the combustion volume.

4. Combustion apparatus according to claim 1, wherein the trim port is between 0.35 and 0.7 of the diameter of one of the dilution ports.

5. Combustion apparatus according to claim 1, wherein the axis of the trim port is located downstream and circumferentially offset from a dilution port in one or more of the plurality of rows.

6. Combustion apparatus according to claim 1, wherein the trim port is positioned to permit the flow of trim cooling air to be conveyed by swirl within the combustor to a location downstream and circumferentially aligned with a dilution port.

7. Combustion apparatus according to claim 1, wherein the combustion volume is annular.

8. Combustion apparatus according to claim 1, wherein the combustion envelope has a fuel injection location, wherein a dilution port in the most upstream row is circumferentially aligned with the fuel injection location and the trim port is associated with the circumferentially aligned trip port.

9. Combustion apparatus according to claim 8, wherein the fuel injection location is in an upstream wall of the combustion envelope.

10. Combustion apparatus according to claim 8, wherein the combustion apparatus has two rows of dilution ports.

11. A method of improving combustion within a combustion envelope bounding a combustion volume the combustion volume having an upstream end and a downstream end and the combustion envelope having a plurality of rows of dilution ports, characterised in that the method comprises providing a trim port wherein dilution air is supplied through the dilution ports and mixed with hot combustion gas in a midstream flow, wherein trim cooling air is supplied through the trim ports and mixed with hot combustion gas upstream of one or the rows of dilution ports and outside the midstream flow.

12. A method according to claim 11, wherein swirl within the combustor conveys the trim cooling air to a location downstream and circumferentially aligned with a dilution port.