GB2038948A - Gas Turbine By-pass Jet Engines - Google Patents

Abstract

In a three-stage gas turbine by- pass type jet engine, comprising LP, IP, HP compressors 11, 14, 17, combustion equipment 22 and HP, IP, LP turbines 18, 15, 12, air from the LP compressor flows through by-pass passage 19a, 20 to mix with the engine exhaust gases (upper part of the Figure) or to discharge direct to atmosphere (lower part of the Figure). Some air from the IP compressor 14 flows through a second by-pass passage 20, 21 so by-passing the HP compressor, combustion equipment, HP and IP turbines to mix with combustion gas discharging from the IP turbine 15, the combustion gas and by-pass air mixture passing through the LP turbine 12. <IMAGE>

Description

SPECIFICATION Improvements in Three-Stage Gas Turbine Engines The present invention relates to three-stage gas turbine engines having a low, an intermediate and a high pressure stage each mounted on an individual shaft.

It has always been one of the major aims in gas turbine design to optimise the specific fuel consumption. It is well known that it is beneficial, in terms of specific fuel consumption, to have what is known as 'by-pass' engines, where some of the air after passing through a compressor is diverted around the rest of the engine. It has been common for by-pass engines to be of the 2 spool type having what are known as a low pressure shaft and a high pressure shaft. In these air from the low pressure (upstream) compressor is divided into 2 portions, a by-pass portion and a core portion, the core portion of which is further compressed by the high pressure compressor, and is used to support combustion in combustion chambers after which it passes through first a high pressure turbine and then a low pressure turbine. More recently what are known as fan jet engines have been developed.These have one row of low pressure compressor blades of considerably greater radius than the others, which impels a stream of air through an annular duct defined between an outer casing and a core engine casing. Fan jet engines frequently have 3 shafts, a low compression, a medium compression and a high compression.

It is well known in the development of by-pass engines that the higher the by-pass ratio (that is, the ratio between the masses of unfuelled and fuelled air flows) the better the specific fuel consumption. However, the extent to which the by-pass ratio can be increased is limited by several factors, the first important factor being the relative shrinking of the low pressure trubine flow as by-pass ratio is increased. Also, geometrically, it is inevitable that low pressure turbine tip speeds will be considerabiy lower than the fan tip speed.

There comes a point, therefore, where a given bypass ratio can only be attained by increasing the numbers of low pressure turbine rows, or by incorporating a gear-box in the low pressure system. Both these solutions result in added complication, cost, and weight.

According to the present invention a threestage gas turbine engine having a low pressure shaft, a medium pressure shaft and a high pressure shaft, each with a compressor and a turbine, and in which some air from the low pressure compressor by-passes the rest of the engine, includes a by-pass downstream of a medium pressure compressor whereby some air after compression by the medium pressure compressor by-passes the high pressure stages, combustion chamber and medium pressure turbine but mixes with combustion gases which have left the combustion chamber and passed through the high and medium pressure turbines to pass through the low pressure turbine. By this means the by-pass ratio of an engine can be increased without the requirement for additional low pressure turbine stages or gear-box.

Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing, which shows in section two forms of engine embodying the invention, one on either side of the centre-line.

A gas turbine engine has 3 shafts, a low compressor shaft 10 joining a low pressure compressor 11 and a low pressure turbine 12, a medium pressure shaft 13 joining a medium pressure compressor 14 to a medium pressure turbine 15, and a high pressure shaft 16 joining a high pressure compressor 1 7 to a high pressure turbine 1 8. The low pressure compressor 11 has a fan which impels a flow of air through an annular duct defined between an outer casing 1 9(a) upper figure or 1 9(b) lower figure and a centre casing 20.Air is also compressed by the low pressure compressor 11 and passed to the medium pressure compressor 14, whence after being further compressed part of the air is passed down an annular passage between the centre casing 20 and an inner casing 21 whilst the rest is passed to the high pressure compressor 1 7. Air passed through the high pressure compressor 1 7 is fed to combustion chambers 22 where fuel is added and ignited, after which the combustion gases pass through the high pressure and medium pressure turbines 18, 1 5. The combustion gases and medium pressure air which has passed between the inner and centre casings 21, 20 then mix and pass through the low pressure turbine 12 and thence either to atmosphere through a nozzle 26 (lower figure) or to mix in a mixer 23 with fan air which has passed between outer casing 19(a) and centre casing 20 and thence through a nozzle 24 to atmosphere (top figure). In the engine illustrated in the lower figure fan air exhausts to atmosphere through an annular nozzle 25. It will be realised that the numbers of compressor and turbine rows on each shaft will be dependent on the various by-pass ratios (that is of low pressure and medium pressure air), the overall by-pass ratio, and various other design factors.

Claims

1. A three-stage gas turbine engine having a low pressure shaft, a medium pressure shaft and a high pressure shaft, each with a compressor and turbine, and in which some air from the low pressure compressor by-passes the rest of the engine, including a by-pass downstream of a medium pressure compressor whereby some air after compression by the medium pressure compressor by-passes the high pressure stages, combustion chamber and medium pressure turbine but mixes with combustion gases which have left the combustion chamber and passed through the high and medium pressure turbines to pass through the low pressure turbine.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (3)

**WARNING** start of CLMS field may overlap end of DESC **. SPECIFICATION Improvements in Three-Stage Gas Turbine Engines The present invention relates to three-stage gas turbine engines having a low, an intermediate and a high pressure stage each mounted on an individual shaft. It has always been one of the major aims in gas turbine design to optimise the specific fuel consumption. It is well known that it is beneficial, in terms of specific fuel consumption, to have what is known as 'by-pass' engines, where some of the air after passing through a compressor is diverted around the rest of the engine. It has been common for by-pass engines to be of the 2 spool type having what are known as a low pressure shaft and a high pressure shaft. In these air from the low pressure (upstream) compressor is divided into 2 portions, a by-pass portion and a core portion, the core portion of which is further compressed by the high pressure compressor, and is used to support combustion in combustion chambers after which it passes through first a high pressure turbine and then a low pressure turbine. More recently what are known as fan jet engines have been developed.These have one row of low pressure compressor blades of considerably greater radius than the others, which impels a stream of air through an annular duct defined between an outer casing and a core engine casing. Fan jet engines frequently have 3 shafts, a low compression, a medium compression and a high compression. It is well known in the development of by-pass engines that the higher the by-pass ratio (that is, the ratio between the masses of unfuelled and fuelled air flows) the better the specific fuel consumption. However, the extent to which the by-pass ratio can be increased is limited by several factors, the first important factor being the relative shrinking of the low pressure trubine flow as by-pass ratio is increased. Also, geometrically, it is inevitable that low pressure turbine tip speeds will be considerabiy lower than the fan tip speed. There comes a point, therefore, where a given bypass ratio can only be attained by increasing the numbers of low pressure turbine rows, or by incorporating a gear-box in the low pressure system. Both these solutions result in added complication, cost, and weight. According to the present invention a threestage gas turbine engine having a low pressure shaft, a medium pressure shaft and a high pressure shaft, each with a compressor and a turbine, and in which some air from the low pressure compressor by-passes the rest of the engine, includes a by-pass downstream of a medium pressure compressor whereby some air after compression by the medium pressure compressor by-passes the high pressure stages, combustion chamber and medium pressure turbine but mixes with combustion gases which have left the combustion chamber and passed through the high and medium pressure turbines to pass through the low pressure turbine. By this means the by-pass ratio of an engine can be increased without the requirement for additional low pressure turbine stages or gear-box. Some embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawing, which shows in section two forms of engine embodying the invention, one on either side of the centre-line. A gas turbine engine has 3 shafts, a low compressor shaft 10 joining a low pressure compressor 11 and a low pressure turbine 12, a medium pressure shaft 13 joining a medium pressure compressor 14 to a medium pressure turbine 15, and a high pressure shaft 16 joining a high pressure compressor 1 7 to a high pressure turbine 1 8. The low pressure compressor 11 has a fan which impels a flow of air through an annular duct defined between an outer casing 1 9(a) upper figure or 1 9(b) lower figure and a centre casing 20.Air is also compressed by the low pressure compressor 11 and passed to the medium pressure compressor 14, whence after being further compressed part of the air is passed down an annular passage between the centre casing 20 and an inner casing 21 whilst the rest is passed to the high pressure compressor 1 7. Air passed through the high pressure compressor 1 7 is fed to combustion chambers 22 where fuel is added and ignited, after which the combustion gases pass through the high pressure and medium pressure turbines 18, 1 5. The combustion gases and medium pressure air which has passed between the inner and centre casings 21, 20 then mix and pass through the low pressure turbine 12 and thence either to atmosphere through a nozzle 26 (lower figure) or to mix in a mixer 23 with fan air which has passed between outer casing 19(a) and centre casing 20 and thence through a nozzle 24 to atmosphere (top figure). In the engine illustrated in the lower figure fan air exhausts to atmosphere through an annular nozzle 25. It will be realised that the numbers of compressor and turbine rows on each shaft will be dependent on the various by-pass ratios (that is of low pressure and medium pressure air), the overall by-pass ratio, and various other design factors. Claims

1. A three-stage gas turbine engine having a low pressure shaft, a medium pressure shaft and a high pressure shaft, each with a compressor and turbine, and in which some air from the low pressure compressor by-passes the rest of the engine, including a by-pass downstream of a medium pressure compressor whereby some air after compression by the medium pressure compressor by-passes the high pressure stages, combustion chamber and medium pressure turbine but mixes with combustion gases which have left the combustion chamber and passed through the high and medium pressure turbines to pass through the low pressure turbine.

2. A three-stage gas turbine engine as claimed in claim 1 wherein gas from the low pressure turbine and by-pass air from the low pressure compressor are mixed before passing to atmosphere.

3. A three-stage gas turbine engine substantially as herein described with reference I the accompanying drawings.