GB2113377A

GB2113377A - Improvements in or relating to flame tubes

Info

Publication number: GB2113377A
Application number: GB08201159A
Authority: GB
Inventors: Eric Archie Alexander
Original assignee: Rolls Royce PLC
Current assignee: Rolls Royce PLC
Priority date: 1982-01-15
Filing date: 1982-01-15
Publication date: 1983-08-03
Also published as: GB2113377B

Abstract

A flame tube (16) for a gas turbine engine has a number air inlets (20) which receive air from the engine compressor for combustion and dilution purposes in the flame tube. In order to stabilise the flow through the air inlets, and in particular to reduce or eliminate any tangential or circumferential velocity component, the inlets each have an airflow control (32) which comprises two fences (34) joined by a bridge piece (36). The fences are mounted on opposite sides of the inlet and prevent or reduce the spillage of air over one or other of the inlet sides. In this way, any tendency for asymmetric flow over the inlet sides is reduced, and the circumferential flow which is detrimental to combustor performance is reduced. <IMAGE>

Description

SPECIFICATION Improvements in or relating to flame tubes This invention relates to flame tubes, e.g. for the combusion chambers of gas turbine engines.

There are three main types of combustion chamber for gas turbine engines, the multiple chamber in which a number of tubular flames are enclosed in tubular air casings, the tuboannular, in which the tubular flames are enclosed in an annular air casing, and the annular in which an annular flametube is enclosed is an annular air housing.

In each case, the flame tube has a number of air inlets along the walls through which air flows, this air being used in the combustion process or for diluting the combustion products to the required temperature. One of the major difficulties in achieving an adequate and consistent performance from a gas turbine engine is in directing the airflow from the feed space between the flame tube and the air casing through these air inlets into the flame tube. The flow through the flame tube wall air inlets is generally unstable and the flow pattern symmetry can be affected by relatively small variations in the pressure or velocity distribution of the air feeding the inlets. These maldistributions result in major deviations in performance characteristics such as, exit temperature distribution, wall temperatures and pollutant emissions, notably smoke.This instability in air injection can be triggered by small perturbations which are difficult to detect and trace by current methods, and the deviations in performance can often appear to be random in position and intensity.

There have been a number of proposals made, and introduced in an effort to reduce the instability of air injection into the flame tube. These proposals have included plunged and chuted air inlets, splitters across the air inlets, and reduced air velocities in the feed space, but only limited advances have been made. It now appears that the flow component through these air inlets which has the greatest disturbing effect is that in the tangential or circumferential direction. The variations in the radial or axial components, which affect the flow penetration angle, appear to be less severe in their consequences.

Normally, there is no deliberate attempt to introduce a circumferential velocity component. The feed space flow enters the flame tube through the air inlets with a pronounced radial component some axial velocity, and no intentional circumferential component. In practice, all these components exist and are subject to deviations arising from disturbances in the feed space flow and it is those disturbances in a circumferential direction which appear to have the greatest effect on combustor performance.

The present invention seeks to overcome the problem by reducing the possibility of asymmetry in the circumferential forces acting on the air entering the flame tube by ensuring that the flow of air into the sides of the air inlet is minimised.

The present invention provides a flame tube having a plurality of inlets in its walls, through which inlets a supply of air enters the flame tube from an air feed space, at least some of the holes having airflow control means to at least reduce the airflow spilling over the sides of the respective air inlets, the airflow control means including two fence members extending from the flame tube into the feed space, the fence members being mounted opposite each other on the sides of the air inlet and generally parallel to the direction of air flow in the feed space.

The airflow control means may also include a bridge piece connecting the two fence members, both to reduce the air spillage and to improve the mechanical integrity of the de- vice.

In a particular embodiment of the invention the airflow control means is in the form of an inverted U-shaped strap bridging the air inlet transverse to the air flaw in the feed space.

The present invention will now be more particularly described with reference to the accompanying drawings in which, Figure 1 is a diagrammatic representation of a gas turbine engine, Figure 2 is a diagrammatic view of the combustion chamber of the engine shown in Fig. 1, Figure 3, 4 and 5 are diagrams of the airflow through one of the air inlets of the flame tube shown in Fig. 2, Figure 6 is a view of part of a flame tube according to the present invention and, Figure 7 is a diagram of the airflow into the air inlet shown in Fig. 6.

Referring to Figs. 1 to 5, a gas turbine engine 10 includes an annular combustion chamber 1 2 having an annular air casing 14 and an annular flame tube 16. Air from the engine compressor or compressors (not shown) flows past guide vanes 1 8 and into the flame tube 1 6 through inter alia a number of air inlets 20 via an annular feed space 22.

Compressed air also enters the flame tube through an annular primary air inlet 24 which air is mixed with fuel from fuel injectors 26 and burnt in the primary zone 28, and through cooling air inlets 30 along the walls of the flame tube.

As indicated in Figs. 4, 5 and 6, the air flow VAN into each of the air inlets 20 from the feed space 22 has a substantial radial component VR, an axial component VA and an unintentional tangential or circumferential component Vc. This latter component is the result of asymmetric spillage of air into one side of the air inlet.

Although the mechanism causing the asymmetry and thus the circumferential component Vc is not fully understood, it can be compared with a whirlpool or water leaving a bath through the plug hole. Most of the air approaching an inlet tends to flow straight into the inlet over the upstream edge but, as shown in Fig. 5 the flow is not symmetrical over the opposite side edges of the inlet. For some reason, perhaps due to the generally unstable nature of the flow in the feed space 22, the flow enters over one side edge, but not over the opposite side edge. Instead, there appears to be a circulation around the rim of the inlet and a substantial portion of the inlet flow enters from the side causing the circumferential component Vc.

Referring to Figs. 6 and 7, the present invention comprises the use of an air inlet airflow control means 32, which comprises two fences 34 extending from the flame tube into the feed space, the fences being generally parallel to the notional direction of the air flow in the feed space, and located opposite one another on the sides of an air inlet 20.

The notional direction of flow in the feed space is parallel to the axis of rotation of the engine.

The presence of the fences 34 prevents or at least reduces the spillage of inlet air over one of the sides of the inlet and tend to concentrate the inlet flow over the upstream and downstream edges of the inlet, most of the inlet air flowing over the upstream edge.

In this way, there should be little or no spillage over one or other of the sides of the inlet, thereby reducing or eliminating the component Vc, and improving the consistency of combustor performance. The length of each fence is determined by individual circumstance, but generally will not exceed the radius of the inlet.

In the arrangement shown the fences are joined by a bridge piece 36 to improve mechanical integrity and to reduce the possibility of spillage.

The airflow control means can be applied to all or a selected number of the air inlets, and the use of the invention is not restricted to annular flame tubes, but can be used in tubular flame tubes.

Claims

1. A flame tube having a plurality of inlets in its walls through which inlets a supply of air enters the flame tube from an air feed space, at least some of the inlets having airflow control means to at least reduce the airflow spilling over the sides of the respective air inlets, the airflow control means including two fence members extending from the flame tube into the feed space, the fence members being mounted opposite each other on the sides of the air inlet, and generally parallel to the notional direction of air flow in the feed space.

2. A flame tube as claimed in claim 1 in which the two fences are integral with a bridge piece, the bridge piece being spaced away from the air inlets.

3. A flame tube as claimed in claim 1 in which all the air inlets in the walls of the flame tube have the airflow control means.

4. A flame tube constructed and arranged for use and operation substantially as herein described, and with reference to the accompanying drawings.