GB2041193A

GB2041193A - Gas turbine engine combustion apparatus

Info

Publication number: GB2041193A
Application number: GB8003443A
Authority: GB
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1979-02-20
Filing date: 1980-02-01
Publication date: 1980-09-03
Also published as: GB2041193B

Abstract

A combustion chamber of a gas turbine engine has an upstream variable rate diffuser for controlling the airflow into first and second annular ducts 34, 38 which partially surround the combustion chamber 23, primary and dilution air flowing into the combustion chamber from the first annular duct 34 and bypass air flowing into the combustion chamber from the second annular duct 38. The variable rate diffuser comprises a primary duct 12a and a downstream fence 16 located in a secondary duct 14 which has a bleed duct 18, the outlet of the primary duct being smaller in diameter than the inlet to the first annular duct 34. In operation by varying the bleed rate the rate of diffusion into the combustion chamber can be varied and if the bleed rate is reduced to zero at low power conditions, vitiated air can be drawn from the combustion chamber along the second annular duct 38 and returned to the combustion chamber at its upstream end to reduce emissions of NOx. <IMAGE>

Description

SPECIFICATION Improvements in or relating to gas turbine engine combustion apparatus This invention relates to the control of airflow in gas turbine engine combustion chambers for the purpose of reducing harmful exhaust emissions.

The problem of exhaust gas emissions basically manifest itself as an excess of oxides of nitrogen (now) at high power operation and an excess of carbon monoxide (CO) at low power operation. Techniques have been produced which reduce the NOX emissions, but these have tended to increase the CO emissions and vice versa.

The problem originates in the primary zone of the combustor where rapid reaction rates are achieved by using near-stoichiometric mixtures of fuel and air, as a result of which, the temperatures generated are sufficiently high to promote the formation of NOx. The maximum temperature can be reduced by operating at an off-stoichiometric mixture strength, the condition of mixture strength being known as the equivalence ration ( which is defined as the ratio of fuel to air fractions between the operational and the stoichiometric conditions.

The effect of operating the primary zone at off stoichiometric condition is that the formation of NOX can be significantly reduced providing that do is greater than about 1.2 or less than about 0.8. The fuel rich solution leads to a large combusion chamber and for aeroengines and their industrial derivatives it is necessary to take the fuel lean solution which in itself has adverse effects when the engine is operating at part load conditions. There is a tendency for + and the compressor delivery air temperature to drop, resulting in the emission of large quantities of CO and the likelihood of combustion instability.

One solution to this problem is to use staged combustion in which low power requirements are met entirely by a first stage combustor running at near stoichiometric conditions, whilst at greater loads, the first stage is used as a torch to ensure stability in a larger, second stage combustor, operating at a lower equivalence ratio. This type of system can be difficult to manufacture, requires a large number of fuelling points and fuel injection systems using pre-mixing and pre-vapourising techniques may be required to make combustion adequately homogeneous.

Another solution is to vary the primary zone equivalence ratio by controlling the amount of air flowing into it, and a method and apparatus utilising a fluidic control of air distribution is disclosed in our UK Patent Application No. 37326/76.

In that patent application there was disclosed a combustion chamber having first and second air inlet means and a variable rate diffuser upstream of both said inlet means, the first air inlet means comprising a first annuls duct defined by part of the wall of the combustion chamber and an intermediate cas ing, the combustion chamber having air inlets for the flow of primary and dilution air from the first annular duct, the second air inlet means comprising a second annular duct surrounding the first annular duct, the second annular duct including a part of the wall of the combustion chamber having air inlets for the flow of bypass air, the variable rate diffuser comprising vortex generating means and a variable rate air bleed, the variable rate diffuser receiving a supply of air and delivering said air to said inlet means in a ratio dependent on the rate of diffusion therein.

The variable rate diffuser typically comprises a primary duct receiving a supply of air located within a secondary duct, a fence in the secondary duct downstream of the outlet of the primary duct and a bleed duct in the secondary duct upstream of the fence.

It has now been found that by modifying the variable rate diffuser so that the outlet diameter of the primary duct of the variable rate diffuser is smaller than the diameter of the first air inlet means of the combustion chamber, and operating at low and zero bleed conditions the bypass flow can be reversed so that vitiated air can be drawn back into the combustion chamber and this latter process is known to reduce the emissions of NO The present invention will now be more particularly described with reference to the accompanying drawings in which: Figure 1 shows a combustion chamber as disclosed in our co-pending application no.

37326/76, Figure 2 shows a combustion chamber according to the present invention and, Figure 3 is a graph showing the relationship between bleed flow rate and bypass flow.

Referring to Figs. 1 and 2 a variable rate diffuser is attached to combustion apparatus 22, the diffuser comprising a parallel-walled primary duct 1 2 located in a secondary duct 14, an annular fence 1 6 which constitutes a vortex generating means fixed in the secondary duct downstream of the outlet of the primary duct, and a bleed duct 1 8 in the secondary duct. A capture tube 20, which can be larger, smaller or equal in diameter to the primary duct 1 2 is positioned in the secondary duct to receive air from the primary duct and together with the secondary duct forms part of the combustion apparatus shown in Figs. 1 and 2.

The combusion apparatus 22 comprises a combustion chamber 23 defined by a liner 24, having a fuel supply 26, a primary air inlet 28, intermediate air inlets 30 and dilution air inlets 32. A first annular duct 34 formed by an intermediate casing 36 around the combustion chamber is attached to the capture tube 20 and encloses the primary, intermediate and dilution air inlets 28, 30 and 32 respectively. A second annular duct 38 is formed by an outer casing 40 surrounding the casing 36 and includes bypass parts 42 for spillage air, the casing 40 being attached to the secondary tube 14.The spillage air is not intended to be part of the combustion process but serves to cool the combustion chamber walls and the dilution air inlets are included in the duct 34 since the spillage air may not be at a sufficient pressure to obtain adequate penetration of the dilution air into the combustion chamber.

Referring more particularly to Fig. 2, the modification according to the invention comprises modifying the primary duct of the variable rate diffuser so that instead of being the same diameter as the capture tube 20, it is a smaller diameter and acts as an ejector 1 2a Alternatively the ejector 1 2a can be a parallel walled tube shown in chain lines in Fig. 2, similar to the tube 1 2 in Fig. 1 but of smaller diameter than the capture tube 20. The remainder of the diffuser 10 and combustion apparatus 22 are described with reference to Fig. 1.

When the combustion apparatus is operating at full power, the bypass flow through ports 42 can be reversed by reducing the bleed through duct 18 to zero and the ejector 1 2a will function to draw vitiated air back along the duct 38 to be injected into the combustion chamber 23.

The process of re-cyling vitiated air is known to reduce the emissions of NOx and therefore the modification according to the invention should be useful in reducing these emissions. Further, this hot vitiated air could be used to promote pre-vapourisation of the fuel.

Referring to Fig. 3 which shows the relationship between the flow along the bypass 38, expressed as a percentage of the total mass flow entering the combustion apparatus 22 and the bleed flow expressed as a percentage of mainstream flow bled out through the diffuser, it can be seen that by reducing the bleed flow rate, the bypass flow direction can be reversed bringing the diluted products of combustion back into the combustion chamber primary zone.

Claims

1. A combustion chamber having first and second air inlet means and a variable rate diffuser upstream of both said air inlet means, the first air inlet means comprising a first annular duct defined by part of the wall of the combustion chamber and intermediate casing, the combustion chamber having inlets for the flow of primary and dilution air from the first annular duct, the second air inlet means comprising a second annular duct, the second annular duct including a part of the wall of the combustion chamber having air inlets for the flow of bypass air, the variable rate diffuser comprising a primary duct arranged to receive a supply of compressed air, the primary duct being located in a secondary duct, a fence in the secondary duct downstream of the outlet of the primary duct and a bleed duct in the secondary duct upstream of the fence, the outlet of the primary duct being smaller in diameter than the diameter of the first air inlet means of the combustion chamber.

2. A combustion chamber constructed and arranged for use and operation substantially as hereinbefore described and as shown in Figs. 2 of the accompanying drawings.