Gas turbine engine combustion system

#The invention is concerned with a gas turbine engine combustion system and with means for mixing fuel and air in a gas-fuelled engine, particularly gas turbine engines using gas fuel of low calorific value.

#Fuel-air mixing means (burners) to provide the combustible medium for gas turbine engine operation take many and varied forms according to manufacturer preference. A manufacturer may become expert in a particular burner type and wherever possible will adapt that type of burner to suit the engine duty, for example to burn unusual or particular kinds of fuel.

#The present applicant has already devised a combustion system which incorporates a burner of the radial inflow swirler type. It is sometimes desirable to able to burn a fuel gas of low calorific value (LCV fuel), from say a coal gasification process. Difficulties in the use of such fuel include the volume of fuel required for a given power output being comparatively large in relation to the volume of air when compared to, for example, high calorific value (HCV) liquid fuels. Between these extremes, there are significant differences in respect of, among other things, fuel injection position, direction of flow and flow rates in order to achieve best mixing of air and fuel. Also, where an LCV fuel has a relatively high flame speed, flame speed being the rate at which a flame will propagate in a mixture (which is fast for example where it contains a high proportion of hydrogen), there is higher risk of fuel pre-ignition. When this occurs in parts of the burner not intended to accept a flame, damage may be caused to components of the burner.

#The invention will be described by reference to the following drawings, in which:

#FIG. 1 shows a section through a burner and combustion chamber assembly fitted with an inflow swirler of the type utilized by the invention;

#FIG. 2 is an enlarged view on section A—A of FIG. 1, showing a swirler according to the invention in more detail;

#FIG. 1 illustrates a section through a known type of burner and combustion chamber assembly for a gas turbine engine, where burner head 1 with air/fuel mixing swirler 2 is attached to the upstream end of a combustion chamber comprising in flow series a combustion pre-chamber 3 and a combustion main chamber 4. It will be seen that the pre-chamber 3 is of appreciably smaller diameter and cross-sectional area than the main chamber 4, and there is a short transition region where the chamber diameter flares outwardly from the pre-chamber to the main chamber. A conduit 5 is provided for LCV gas fuel supply to the burner. Arrows 6, 7 and 8 respectively indicate the direction of air flow to the burner swirler inlet, the fuel-air mixture for combustion and the combustion products themselves, which products pass through the engine turbine section downstream (not shown) to do work and then are exhausted to atmosphere. The main combustion region within the combustion chamber is indicated at 9.

#In the enlarged view on section A—A of FIG. 1 shown in FIG. 2, the swirler element 2 includes a plurality of swirler vanes 10, six such vanes being shown for purposes of illustration. Air supply passages 11 are defined between adjacent vanes and the inflowing air passes through these to enter the pre-chamber at it's the outer periphery. Each vane 10 is formed with a fuel outlet passage 14, a restriction 13, which in this embodiment comprises a portion of passage having a narrower width than fuel outlet passage 14, and an LCV fuel gas port 12 (shown as a dashed circle), which is connected to conduit 5 through a gallery or other form of connection within burner head 1 (FIG. 1). The fuel passages 14 formed in the vanes 10 and the air passages 11 formed between the vanes extend inwardly from the outer periphery of the swirler at inclined angles compared to the radial direction of the swirler. As a result, the outlet ends of both sets of passages emerge at a radially inner portion of the swirler so as to lie tangentially to a notional circle 15 (shown dashed) concentric with the swirler and pre-chamber. Hence, the air and fuel enter the pre-chamber with a swirling motion about its longitudinal centerline, which encourages good fuel-air mixing and helps to stabilize combustion in the main chamber.

#It may be convenient to note at this point that “radial inflow swirler” is a term of art which includes swirlers of this type, since the air and fuel inflows through the swirler passages have components of velocity in the radial direction.