EP2342499A2

EP2342499A2 - Single can combustor gas turbine with bifurcated hot gas header and dilution air differentiated flow

Info

Publication number: EP2342499A2
Application number: EP09774735A
Authority: EP
Inventors: Stefano Tiribuzi
Original assignee: Enel Produzione SpA
Current assignee: Enel Produzione SpA
Priority date: 2008-11-05
Filing date: 2009-11-05
Publication date: 2011-07-13
Anticipated expiration: 2029-11-05
Also published as: EP2342499B1; WO2010052662A2; IT1391548B1; WO2010052662A3; ITFI20080211A1

Abstract

A gas turbine with a single can combustor and a gases bifurcated collector, wherein the combustor (5) comprises a gas chamber (7) whose axis is substantially orthogonal to the main machine axis (X) and delimited by a wall (9) obtained on which are primary holes (15), through which the air required to complete the combustion - coming from a compressor (2) - passes, and holes for dilution air (16a, 16b) near the outlet section of the gas chamber wherein the latter is introduced into the gases bifurcated collector (18, 20) whose mean lying plane is substantially orthogonal to the main axis (X), the dilution air holes (16a) obtained around the direction of the main axis (X) have a larger section with respect to the dilution air holes (16b) obtained around the direction orthogonal to said main axis (X). Thus, improved homogenization of the temperature of the hot gases entering into the turbine is attained.

Description

SINGLE CAN COMBUSTOR GAS TURBINE WITH BIFURCATED HOT GAS HEADER AND DILUTION AIR DIFFERENTIATED FLOW

DESCRIPTION Field of the Invention

The invention generally refers to the field of gas turbine and more precisely to the combustion section of a gas turbine. In particular, the invention refers to the combustion section of a gas turbine made up of a single can combustor combined with a hot gas bifurcated collector. Background of the Invention

The gas turbines, widely used in various industrial sectors, are essentially made up of three parts: the compressor, the combustion section and the turbine, also called expander. The compressor sucks external air and compresses it. The compressed air passes in the combustion section, where the fuel is injected and the combustion reaction occurs. The hot gases thus generated pass in the expander where they actuate the turbine rotor generating mechanical power greater than the one used for compressing air, thus providing some power useful for external actuations.

The compressor and turbine rotors are usually keyed on a single shaft whose axis represents the main axis of the machine (machine axis). The combustion section, regardless of the various types corresponding to different geometric configurations, may be divided into four main parts: the plenum, into which the compressed air coming from the compressor flows, one or more burners, one or more combustion chambers and one or more collectors. Each burner injects fuel and ensures flame anchorage and stability. The combustion reaction occurs in the combustion chamber, where the burners pipes lead, and the flow of hot gases as generated is obtained under the best conditions for turbine inlet. Lastly, the collector is used to channel the hot gases developed in the combustion chamber towards the turbine inlet, connecting the end section of the chamber with the annular- shaped inlet section of the turbine. The assembly is housed in a suitably shaped air box which serves to ensure the maintenance of the fluids under pressure and the mechanical resistance of the component. The assembly of burners, combustion chambers and parts of the air box accommodating them forms the combustor. The assembly of the combustion chambers and the collector/s form a single pipe referred to as flame pipe. The flame pipe may have different configurations which characterise the type of combustor. The flame pipe configuration of interest for the present invention is that of a flame pipe made up of a single gas chamber (single can combustor) combined with a bifurcated collector. The gas chamber is usually made up of a cylindrical-shaped metal basket, laterally delimited by a cylindrical jacket and closed at one of its ends by a conical cap. The single burner this type of combustor is equipped with is arranged at the centre of the cap. The basket is in turn housed in an external casing for accommodating said air box, made up of an external cylinder and a sealing cover. The two metal cylinders, i.e. that of the gas chamber jacket and that of the air box, are coaxial. The oxidizing air feeding the burner ascends in the annular volume comprised between the two cylinders (ascent ring). The flame generated in the combustor is of the diffusion type, wherein only a small amount of oxidising air is introduced into the combustion chamber through the burner, while most of the oxidizing air passes from the ring to the combustion chamber through various openings formed on the cylindrical side wall of the basket. Starting from the end where the burner is located, these passages initially comprise a series of holes through which the air required to complete the fuel combustion (primary holes) passes and, at the end, near the basket outlet, they comprise a series of holes serving to reduce and homogenize the hot gas temperature (dilution holes). Alongside these holes the basket surface is provided with a close distribution of variously shaped slots where the air required to cool the metallic surface passes. Therefore, only a small amount of oxidising air raising the ascent ring reaches the burner, whereby the mixture in the initial part of the combustion chamber (dome) is over- stoichiometric, i.e. it is a fuel-rich but air-poor mixture..

Regarding the field of application of the present invention, the combustor is located on the side of the machine axis, with the basket axis (combustor axis) preferably oriented perpendicularly relative to the machine axis. Thus this leads to a non-axial-symmetrical configuration of the gas turbine assembly. This is reflected in the fact that the flow of the main fluids are non -symmetrical relative to axis of the turbine machine, wherein the main fluids are the oxidizing air upstream of the flame and the combustion gases downstream thereof. As a matter of fact, the oxidising air flows along the compressor with an axial-symmetrical distribution around its axis and with a direction essentially parallel to the machine axis, then it must be diverted and channelled towards the side of the machine where the combustor is located. The air raises up the ring that surrounds the combustion chamber, with a direction parallel to the combustor axis, and thus transverse with respect to the machine axis, until it reaches the burner arranged at distal position with respect to the machine axis. Subsequently, the gases, whose flow is initially oriented parallel to the combustor axis, but this time directed towards the machine axis, must be reoriented once again in the axial direction of the machine and distributed axial-symmetrically around the main axis before finally expanding along the turbine.

The connection of the flows of the main fluids between the various sections of the turbine machine and the reorientation of such flows occurs in a component of the combustion section, located between the compressor body and the turbine body, hereinafter referred to as "insert", in that it allows inserting the combustor into the main body of the machine. The insert comprises an external component and an internal component, which delimit two volumes arranged within each other. The compressed air coming from the combustor flows into the volume delimited by the external component, which is of a spheroid shape, and thus is called bulb. Then the compressed air, after changing direction, exits to ascend again along the annular pipe of the combustor. The second component forms another volume insert, that is delimited by the bifurcated collector. The bifurcated collector comprises two parts: one cylindrical part which is fitted to the cylindrical mouth of the combustion chamber, and the other made up of two branches, having a usually decreasing transverse section, which bifurcate moving around the two sides of the shaft of the gas turbine and meeting at the opposite end, thus acquiring a general torus-shaped configuration. The torus is oriented in such a manner that one of its sides faces the annular inlet of the turbine. This side of the torus is in turn open with an outlet ring connected with that of the turbine, in such a manner that the flow of the gases is distributed along the entire perimeter of the ring of the first stage of the turbine. Each of the two branches of the collector distributes the gases along a half-perimeter of the turbine inlet ring.

With this type of combustor it is difficult to achieve good homogeneity of the temperature of the gases entering into the turbine. As a matter of fact, the temperature distribution along the outlet ring depends on the distribution in the circular collector inlet section and on the distance that the single flows of the gases cover between these two sections. The gas temperature distribution in the collector inlet section coincides with the temperature of the combustion chamber outlet section. The latter is usually greater at the centre, due to the numerous air inlets that flow towards the flame through lateral jets present through the basket jacket. These passages are distributed in an averagely uniform manner around the perimeter of the combustion chamber and this determines a central circular section of gases much hotter with respect to a peripheral zone of gases, which are cooler due to greater dilution thereof. Due to the particular configuration of the bifurcated collector, the central hotter part of the flow of gases impacts against the internal surface of the torus-shaped distributor of the collector and thus it is diverted towards the turbine ending up concentrated in the narrow section of the turbine inlet ring directed towards the insert side of the combustor. The thermodynamic cycle of the gas turbines is of the internal combustion open type, known as Brayton/Joule cycle. The main fluid of the cycle is gaseous and it is initially made of ambient air, upstream of the combustion section, and subsequently of combustion gases, downstream of the flame. The cycle is open, in that it is closed through the ambient atmosphere from which the oxidizing air is taken and into which the gases are discharged. The power developed by a gas turbine increases proportionally with the flow rate of the gases and their medium temperature. Efficiency increases proportionally with the increase of the ratio between the maximum and minimum values reached by the temperature of the main fluid along the cycle. The minimum temperature of the cycle is that of the air taken from the external environment and it does not depend on the machine, while the maximum temperature of the cycle is the average temperature achieved by the gases in the section where their expansion in the turbine starts. The temperature of the gases in this section is not perfectly homogeneous, but it has differences between one point and another and the value considered to determine the efficiency of the gas turbine is the medium one weighed on the local mass flow rate. It depends on various factors and mainly on the ratio between the calorific value of the injected fuel and the mass flow rate of the combustion air, i.e. on the average enrichment rate of the mixture.

Since the increase of power and energy efficiency is linked to the medium value of the temperature of the combusted gases, it is advantageous to raise such value as most as possible. However, such raising of the value is limited by technological needs due to the resistance of the materials impacted by the hot gas flow. The most thermally stressed materials are those that form the walls of the flame pipe and the blades of the first stages of the turbine, whereby localized cooling systems are used to reduce the temperature achieved by the most exposed material. Regardless of such expedients it is necessary to limit the overall mixture enrichment, in such a manner that the maximum gas temperature does not exceed the technological limits of the materials. The strictest limits regards materials most exposed to mechanical stresses, i.e. those of the blades of the first series of turbines. The overall rate of mixture enrichment is thus established in such a manner that the maximum gas temperature does not exceed the technological limits of the materials they are made of. The gas temperature thus determines - through its average value - the efficiency, while - through its maximum value - it has influence on the technological aspects of resistance of the materials and thus the duration of the components and operating reliability. The most favourable temperature distribution, which minimises thermal stresses and maximises energy efficiency, is the perfectly homogeneous one wherein the maximum and average values coincide. Such ideal distribution is not attainable, and all combustors have a more or less marked non-homogeneity of the gas temperature at the outlet. The difference between the maximum and average value, compared to the average temperature increase of the main fluid through the combustor, is expressed by a pattern factor PF parameter given by: PF=(T_mx-T_med)/(T_med-T_air)

The medium temperature, T_mβd, of the hot gases depends on the quantity of the oxidising air, while the maximum value, T_mx depends on distribution thereof, i.e. how it is introduced into the flame pipe. Lastly, T_air indicates the temperature of the compressed air at the inlet of the combustion section.

It is thus important to reduce the PF as much as possible, such reduction being obtained by means of a suitable repartition of the air through the side holes of the basket. An amount of air generally equivalent to that required to complete the combustion of the fuel is introduced in the first section of the flame pipe of the diffusion combustors, both through the burner and the holes of the primary air. Another portion of the air is used for cooling the walls of the flame pipe. The residual portion of the air, required to reach the value of the mixture rate capable of lowering the temperature below the technological limit, is introduced through a series of holes arranged on the perimeter of the basket downstream of the holes of the primary air. The jets coming from such holes, known as dilution holes, also serve the purpose of homogenising the hot gas flow temperature as much as possible. Usually the combustion chambers of the cannular combustors, either multiple- can or single can type, are cylindrical. Temperature homogenisation, which is attained by means of lateral air jets which enter transversely with respect to the hot gas flow, must be carried out both in radial and azimuthal directions.

Radial homogenisation consists in reducing the temperature gradient between the centre and periphery and for such purpose it is required that the jets penetrate in depth as much as possible in such a manner to reach the hottest layers of the hot gases, diluting them even by means of formation of vortices inside the flow of the gases which also extend the coldest peripheral layers of the hot gases. The penetration of a single jet depends on its momentum, which in turn depends on the flow rate and velocity thereof.

The flow rate of the jet in turn depends on the area of the hole. Given a certain value of the amount of combustion air available for dilution, it is preferable that the holes be as few as possible in such a manner to increase the flow rate through the single hole and thus the penetrating force of the single jet. However, it is also necessary to avoid strong azimuthal non-homogeneities. Hence, the limit case of a single jet, or also of two opposite jets, is not used in that it would create strong azimuthal non-homogeneities. The compromise solution between the penetrating force and homogeneous distribution of the dilution jets usually occurs with four coplanar holes, i.e. located on the same transverse section of the basket, and orthogonal to each other. The penetration of the lateral air jets may also be increased, considering the same mass flow rate, by increasing the momentum thereof, by increasing the passage speed in the dilution holes. The latter in turn depends on the pressure drop through the basket, which could be increased through homogeneous reduction of all the openings present in the basket in such a manner to preserve the partitioning of the oxidizing air between the various passages. However, this solution is not advisable in that the increase of the pressure drop through the basket leads to the machine losing power and efficiency.

The abovementioned problem does not occur in a combustor having an annular configuration, such as for example the combustors disclosed in US6260359 or in US2004182085. In this type of combustor, the combustion chamber is still made up of a single volume, but in this case it is torus-shaped and arranged in such a manner to extend the main axis of the gas turbine. Therefore, the flow of the main fluids, air and hot gases, always develops, on average, in an axially symmetrical way around the main axis of the machine, also in the combustion section, thus attaining an ideal azimuthal homogeneity of the turbine inlet temperature.

Thus, the technical problem faced with the present invention concerns the high non-homogeneity of the temperature of the hot gases entering the turbine, which is typical of single can combustors with bifurcated collector. Such non-homogeneity leads to the material - in some areas - being subjected to excessive thermal stress, which currently require setting limits to the operating conditions in order to reduce such stresses, these limits negatively affecting the gas turbine power level and efficiency.

A single can combustion chamber oriented perpendicularly to the main axis of the turbine machine, but with a different shape of the collector, is also present in a configuration of a combustor similar to that described above, and shown for example in US7000400. As a matter of fact, in this case, instead of being bifurcated, the collector is spiral-shaped with single volute having a decreasing transverse section which distributes the hot gases along the entire perimeter of the turbine inlet ring.

In order to improve the mixing of the dilution air and thus the homogeneity of the temperature of the hot gases, the abovementioned document suggests a differentiation of the diameter between the dilution holes in such a manner that the flow entering from those having larger diameter penetrates further into the flow of the hot gases. However, in the gas turbine according to patent US7000400 dealing with a collector of the spiral type, i.e. made up of a single volute which is wound, with decreasing section, around the entire turbine inlet ring, there is only one flow of hot gases, and there are no privileged directions along which the flow of the dilution air can be increased.

Objects and Summary of the Invention

The main object of the present invention is to improve the temperature homogeneity in the turbine hot gas inlet section, for single can combustors with bifurcated collector. A particular object of the present invention is to provide a gas turbine with a combustion section made up of a single can combustor combined with a hot gas bifurcated collector, wherein the two streams into which the hot gas flow is divided in the collector have a greater temperature homogeneity with respect to that obtainable in prior art bifurcated collectors. The objects are attained by means of the combustion section of a gas turbine according to the present invention whose essential characteristics are set forth in claim 1. Further important characteristics are outlined in the dependent claims. Brief description of the drawings

Further features and advantages of the combustion section for gas turbines with single can combustor and hot gas bifurcated collector will be apparent from the following description of an embodiment thereof given as a non-limiting example with reference to the attached drawings wherein: figure 1 is a perspective view of the assembly of a gas turbine with single can combustor and hot gas bifurcated collector according to the present invention; figure 2 is a longitudinal schematic view of the gas turbine according to the invention; figure 3 is a schematic perspective cross-section view of the combustor of the gas turbine according to the invention.

Detailed description of the Invention

Referring to figures 1 and 2, a gas turbine made up of, as known, a compressor 2, a combustion section 3 and a turbine or expander 4 is generally indicated with 1.

The rotors of the compressor and the turbine are keyed on a single shaft, whose axis represents the main axis of the machine indicated with X, while the combustion section extends laterally to the machine axis X.

The combustion section 3 comprises a substantially cylindrical-shaped combustor 5 arranged according to an axis Y perpendicular to the machine axis X and a collector section 6, or insert, extended from an end of the combustor and arranged coaxially to the machine axis X.

As shown in figure 2, the combustor 5 comprises a combustion chamber 7 comprising a cylindrical-shaped basket 8, laterally delimited by a cylindrical jacket 9 and closed at an end by a conical cap 10, with a burner 11 mounted at the centre thereof. The basket 8 is accommodated in an external containment housing, called air box and indicated with 12, made up of an external cylinder 13 and a sealing cover 14.

The jacket 9, delimiting the combustion chamber 7, and the external cylinder 13 delimiting the air box 12, are coaxial and the oxidizing air feeding the flame generated by the burner raises in the annular volume 22 (ascent ring) comprised between external cylinder 13 and jacket 9.

As shown particularly in figure 3, arrays of primary holes 15 are formed on the lateral wall of the jacket 9, immediately downstream of the burner 11. The air required to complete the combustion passes through hole 15, while a series of dilution holes 16a, 16b is formed in proximity to the combustion chamber outlet.

The collector or insert section 6 defines two substantially torus-shaped chambers thereinto coaxial to the machine axis X. The external chamber or plenum 17 is intended to channel the compressed air coming from the compressor 2 towards the ascent ring 22, while the internal chamber, indicated with 18, is intended for channelling the hot gases coming from the combustion chamber 7 towards the turbine 4. In particular, the collector section 6 comprises an external wall 19 having a substantially spheroid-shaped development which encloses the external chamber 17 of the compressed air and an internal wall 20 having a mainly torus development which encloses the internal chamber 18 which thus forms the hot gas bifurcated collector.

The external wall 19 has a cylindrical connection portion 19a axially and sealingly connected to the external cylinder 13 of the combustor 5. Similarly, the internal wall 20 has a connection portion 20a, coaxial to the connection portion 19a, joined to the open end of the basket 8 delimiting the combustion chamber 7. The external chamber 17 also communicates with the outlet section of the compressor 2, whereby the compressed air enters the external chamber 17 and therein changes direction to raise in the ring 22 up to the burner. The internal chamber 18 is instead in communication with the mouth of the turbine 4. After passing through the cylindrical connection portion 20a, the hot gases generated in the combustion chamber 7 are divided in two streams at a bifurcation 18a of the internal chamber 18. The two streams pass through the two branches of the internal chamber 18, converging into the turbine 4 through an annular outlet section 21 , thus completing the reorientation of the gases from a perpendicular direction to a direction parallel to the machine axis X.

According to the invention, the area of passage through the various holes belonging to the array of dilution holes 16a, 16b is differentiated in such a manner to increase the air flow through the holes located substantially at the symmetry plane X-Y of the combustor coplanar to the machine axis X (main dilution flow), with respect to the air flow passing through the other holes aligned on a transverse axis Z perpendicular to the abovementioned plane (secondary dilution flow). This solution allows modifying the distribution of the temperature in the transverse sections of the flame pipe comprised between the dilution plane and the bifurcation 18a of the hot gas collector 18, in such a manner that the area of maximum temperature is extended in the transverse direction Z with respect to the machine axis. Such extension, whose amount depends on the ratio between the main dilution flow and the secondary dilution flow, may be carried up to form a two-lobe configuration of the maximum temperature area. This distribution allows reducing the temperature of the hot gases in the central part of the jet and attain better azimuthal homogeneity, along the perimeter of the inlet ring, of the hot gases entering the turbine. In the particular, yet very frequent, case wherein the series of dilution holes consists of four angularly equally spaced holes and aligned to the main axes of the machine, the above described modification provides for a larger area of the pair of holes oriented along the machine axis X, indicated with 16a in figure 3, with respect to the area of the pair of holes oriented in the direction Z transverse to said axis, indicated with 16b.

Though the four-hole configuration is the generally preferred one, configurations with more than four holes may also be envisaged, wherein the holes closest to the longitudinal symmetry plane X-Y of the machine are enlarged with respect to those located close to the transversal plane Z-Y.

In a particularly preferred embodiment of the invention the ratio between the sections of the main dilution holes 16a and the sections of the secondary dilution holes 16b is comprised between 2 and 4.

Obviously it may be advantageous to use, under specific circumstances, dilution holes 16a having a different section relative to each other. In particular it may be advantageous to widen the turbine side dilution hole to a greater extent or, at least, to widen only this one with respect to the compressor side dilution hole.

The efficiency of the proposed configuration was verified numerically through the CFD method, by simulating the distribution of the temperatures of the hot gases both with homogeneous dilution holes, and with differentiated holes, adopting suitable values of the ratio between the flow rate of the main dilution flow and the flow rate of the secondary dilution flow. A value of the abovementioned ratio, which, according to the test CFD simulation, was particularly useful, is 3:1.

The innovative configuration allowed improving the penetration of the two jets corresponding to the two main dilution jets. The hot gas temperature distribution in the transverse section of the flame pipe upstream of the bifurcation is passed from a generally concentric conformation, with only one central maximum value, to a two-lobe conformation with two maximum values aligned along the direction Z transversal to the machine axis. These two lobes correspond to two hot gas jets which move in the two branches of the bifurcated collector and generate two relative maximum zones of the inlet gas temperature to the turbine that are located at the opposite sides of the outlet ring in the transversal direction Z. However, the level of these two maximum values is lower with respect to the single maximum which is generated in the configuration according to the known art, thus improving the shape factor of the hot gas temperature distribution. The lower difference between the medium temperature and the maximum temperature of the hot gases attained in this way allows the average temperature of the hot gas jet to be increased by increasing the flow rate of the fuel, considering the same flow rate of oxidising air (usually fixed for a given machine), and thus the power and the efficiency of the machine is also increased. Otherwise, it helps reducing the maximum temperature and thus the thermal stress of the material, improving the reliability of the machine or reducing the needs of inspection and periodic replacement of hot parts.

Variations and/or modifications may be brought to the combustion section of a gas turbine with single can combustor having a bifurcated collector according to the present invention without departing from the scope of the invention as defined by the following claims.

Claims

1. A gas turbine (1) comprising a single can combustor (5) and an hot gases bifurcated collector (18, 20), wherein the combustor (5) comprises a combustion chamber (7), whose axis (Y) is substantially orthogonal to the main machine axis (X), said combustion chamber (7) being delimited by a wall (9), primary holes (15) being formed on said wall (9) for supplying combustion air coming from a compressor (2), dilution air holes (16a, 16b) being further formed on said wall near the outlet section of the combustion chamber, through which the combustion chamber communicates with said hot gases bifurcated collector (18, 20), the mean lying plane of said collector being substantially orthogonal to the main machine axis (X), characterized in that said dilution air holes are formed in a neighborhood of the direction of the main axis (X) and in a neighborhood of the direction (Z) orthogonal to said main axis (X) and the dilution air holes (16a) formed in the neighborhood of the direction of the main axis (X) have a section larger than the section of the dilution air holes (16b) formed in the neighborhood of the direction (Z) orthogonal to said main axis (X).

2. The gas turbine according to claim 1 , wherein the ratio between the sections of the dilution air holes (16a) formed in the neighborhood of the direction of the main axis (X) and of the dilution air holes (16b) formed in the neighborhood of the direction (Z) orthogonal to said main axis (X) is comprised between 2 and 4.

3. The gas turbine according to claims 1 or 2, wherein a first pair of dilution air holes (16a) formed at diametrically opposed sides in the direction of the main axis (X) and a second pair of dilution air holes (16b) formed at diametrically opposed sides in the direction (Z) orthogonal to said main axis (X) are provided, said first pair of dilution air holes (16a) having a larger section than said second pair of dilution air holes (16b).

4. The gas turbine according to any one of the previous claims, wherein the dilution air holes (16a) with larger section have different sectional area.

5. The gas turbine according to claim 4, wherein the dilution air holes (16a) formed in the neighborhood of the main axis (X) and facing the turbine section have a larger section than the dilution air holes (16a) formed in the same direction and facing the compressor section.