GB2079441A - Evaporative mixers - Google Patents

Abstract

An evaporative fuel/air mixer for avoidance of unwanted exhaust products, includes a means 16 for supplying substantially inert hot gas, means 18 for injecting fuel into the inert hot gas, and means 14 for mixing the fuel and hot gas with a cold air supply. The inert hot gas can be taken from a first combustion zone 11, of a combustion chamber 10, in which a rich mixture is burnt. <IMAGE>

Description

SPECIFICATION Combustion apparatus The present invention relates to evaporative fuel/air mixers, particularly such mixers for use in gas turbine engines.

An important consideration in the design of combustion systems for gas turbine engines is the avoidance of unwanted exhaust products such as Nitrous oxides, carbon monoxide and unburned hydrocarbons. The importance of efficient and clean combustion is continually increasing as liquid fuel becomes scarcer and more expensive and as legislation is introduced in an attempt to improve the environment. It is therefore important to aim for a uniform mixture of fuel and air. With a non-uniform mixture some combustion will inevitably occur at unfavourable conditions, and variation of the ratio of fuel to air supplied will only alter the zones of combustion in which various unfavourable conditions occur.

In practice the provision of a completely uniform mixture is extremely difficult to achieve. In systems where fuel is sprayed into a combustion chamber the rate of evaporation of a fuel droplet will depend on the region of the chamber into which it is projected, and many droplets will not have evaporated before entering a combustion zone. Systems relying on fuel evaporating into an airstream before being introduced into the combustion chamber also have problems.

The volume required to ensure complete evaporation of the fuel and mixing of the resultant gas is inordinately large unless heat is supplied to the process.

However, when heat is supplied the mixture prior to entering the combustion chamber is dangerously prone to "flash-back" - that is a spread of the flame front upstream from the combustion chamber - with potentially catastrophic results.

According to the present invention an evaporative fuel/air mixer includes means for supplying substantially inert hot gas, means for injecting fuel into the inert hot gas, and means for mixing the fuel and inert hot gas with a cold air supply.

With this arrangement fuel droplet evaporation is enhanced without the danger of premature ignition.

In a gas turbine engine having a mixer according to the invention the inert gas supply is preferably taken from the gases downstream of the flame front, whilst the colder supply is from the compressor. In some cases it may be desirable or necessary to cool the inert hot gas to some extent before injecting fuel into it.

One known type of combustion chamber which is designed to avoid combustion at stoichiometric conditions (where a very hot flame producing nitrous oxides is obtained) has a first combustion zone where the mixture is rich.

The remaining fuel and combustion products then pass to a second combustion zone where more air is added and the resultant weak mixture is burnt. This type of combustion chamber is particularly suitable for use with the present invention, and the inert hot gas supply can be tapped from the first combustion zone and, if cooling is required, can be passed through the air supply to the second combustion zone.

The means for mixing the fuel and inert hot gas with the cold air supply preferably use the type of distributtor known as a pepperpot - a spherical or conical plate with a plurality of holes therein. The pepperpot is preferably situated at a position in the cold air supply conduit which is at a lower pressure then the pressure in the first combustion zone.

One embodiment of the invention will now be described, by way of example only, with reference to the accompanying diagrammatic drawings, of which: Figure 1 is an elevation, in section, of a combustion chamber embodying the invention, and Figure 2 is a detail of a distributor for mixing the fuel and hot inert gas with the cold air supply.

A combustion chamber 10 (Figure 1) has a first zone 11 and a second zone 12. Air from, for example, the compressor of a gas turbing engine (not shown) is fed into the first zone 11 from a conduit 13 into which a mixture of fuel and hot inert gas is sprayed through a "pepperpot" distributor (Figure 2)14 having a spherical or conical head with a plurality of holes 15 therein. A pipe 16 leads from the first zone 11 to the pepperpot 14. The pepperpot distributor 14 is positioned at a position of minimum area in the conduit 13. The pipe 16 may pass through a conduit 17 which carries air from the compressor to the second zone 12. Afuel supply pipe 18 leads into the pipe 16, and an igniter 19 is positioned in the first zone 11.

In operation fuel is supplied through the supply pipe 18 in proportions such that the mixture of fuel, inert gas and air entering the first chamber 11 is rich (that is, combustion will leave fuel present when all oxygen has been used up). The mixture is ignited by the igniter 19. From the first zone 11 the mixture 0 fuel and combustion gases passes to the second zone 12 where air from the conduit 17 is added to give a weak mixture in which the remaining fuel is burnt.

Hot inert gas tapped from the first zone 11 is drawn through pipe 16 to the pepperpot 14, as a result of the pressure difference between the positions of pepperpot 14 and the first combustion zone 11. Because of its heat, fuel inserted therein will quickly evaporate, and because of the inertness there is no danger of spontaneous combustion. By spraying the mixture of fuel and inert gas through the many small holes 15 of the pepperpot 14 into air in the conduit 13 a very even mixture of fuel and air is achieved in a short length of conduit 13 before entering the first zone 11.

It may, in some cases, be required to cool the hot inert gases in pipe 16 to some extent before adding fuel. A convenient method of cooling is by passing pipe 16 through conduit 17, using a heat exchanger (not shown) in conduit 17 if necessary.

It will be realised that, for ease of description, Figure 1 has been much simplified, and that a practical installation will differ considerably from that illustrated. It will also be realised that although the invention has been illustrated and described with reference to a "two-zone" type combustor, it can also be used with more conventional systems. In such cases the hot inert gas may be tapped, for example, from downstream of a turbine, and may not need cooling.

Gases passing along pipe 16 may contain some unburntfuel, which will be no disadvantage. It should be realised that the term "inert" as used in this specification does not mean that the gas will contain no oxygen at all which would be almost impossible in the type of equipment described. The oxygen level will, however, be insufficient to support combustion. Also, whilst transfer of gas along pipe 16 is described as being induced by a pressure difference between pepperpot 14 an zone 11, transfer might be effected or assisted by pumping means.

Claims (8)

1. An evaporative fuel/air mixer including means for supplying substantially inert hot gas, means for injecting fuel into the inert hot gas, and means for mixing the fuel and hot gas with a cold air supply.

2. Afuel/air mixer as claimed in Claim 1 wherein the fuel and hot gas is mixed with the cold air supply using a distributor of the type known as a pepperpot.

3. Acombustion chamber having a fuel/air mixer as claimed in Claim 1 or in Claim 2 wherein the inert hot gas is taken from gases downstream of a flame front.

4. A combustion chamber as claimed in Claim 3 having a first combustion zone in which a rich mixture is burnt and from which hot inert gas is tapped, and a second combustion zone in which a weak mixture is burnt.

5. A combustion chamber as claimed in Claim 3 or in Claim 4wherein the pressure at the point where the fuel and hot gas supply and the cold air supply meet is lower than the pressure at the point from which the hot gas supply is taken.

6. A combustion chamber substantially as herein described with reference to the accompanying Figure 1.

7. A combustion chamber substantially as herein described.

8. A gas turbine engine including a combustion chamber as claimed in any one of Claims 3 to 7.