Combustion chamber and heat exchanger

Abstract

A combined combustor and recuperator is formed with the recuperator surrounding the combustor. Cold gas conduits (14, 16, 20) through the recuperator follow along involute paths toward the combustor. Hot has conduits (26) through the recuperator follow counterflow paths along corresponding involute curves outward from the combustor. The openings (18) in the combustion chamber wall through which cold gas enters the combustion chamber may be directed to impart flow direct to the cold gas to support particular desired behaviour of the cold gas in the portions of the combustion chamber concerned, e.g. supporting a stable vortex flame, enhancing mixing, providing a protective barrier layer.

Claims

1. Apparatus comprising: a combustion chamber wall enclosing a combustion chamber; and a heat exchanger integral with at least a portion of said combustion chamber wall; wherein the heat exchanger and the combustion chamber wall are formed together as one entity from a single body of material; wherein said heat exchanger transfers heat from a hot gas to a cold gas and comprises a plurality of hot gas conduits to direct hot gas along respective hot gas paths, and a plurality of cold gas conduits to direct cold gas along respective cold gas paths, wherein a portion of said combustion chamber wall is porous and said cold gas passes from at least some of said cold gas conduits into said combustion chamber through porous openings in said combustion chamber wall; and wherein: (i) at least some of said cold gas conduits directly connect to respective inlet openings in said combustion chamber wall; and (ii) at least some of said cold gas conduits directly connect to one or more cold gas plenums abutting said combustion chamber wall and plenum-inlet openings provide flow paths for said cold gas between respective ones of said one or more cold gas plenums and said combustion chamber.

2. Apparatus as claimed in claim 1, wherein said combustion chamber has a primary combustion chamber inlet and a combustion chamber outlet, and a primary fluid flow path extends between said primary combustion chamber inlet and said combustion chamber outlet.

3. Apparatus as claimed in claim 2, wherein said heat exchanger at least partially surrounds said combustion chamber in planes normal to at least a portion of said primary fluid flow path.

4. Apparatus as claimed in claim 3, wherein said heat exchanger fully surrounds said combustion chamber in said planes, said combustion chamber wall has a circular cross section in a plane normal to said primary fluid flow path and said heat exchanger has an annular cross section in said plane normal to said primary fluid flow path.

5. Apparatus as claimed in claim 2, wherein at least some of said cold gas conduits provide said cold gas to said primary combustion chamber inlet.

6. Apparatus as claimed in claim 1, wherein said plurality of hot gas conduits and said plurality of cold gas conduits share at least some conduits boundary walls.

7. Apparatus as claimed in claim 1, wherein said plurality of hot gas conduits and said plurality of cold gas conduits are disposed within said heat exchanger to provide counterflow between said hot gas and said cold gas.

8. Apparatus as claimed in claim 1, comprising one or more inner hot gas plenums proximal to said combustion chamber wall to supply said hot gas to at least some of said hot gas conduits and one or more outer hot gas plenums distal from said combustion chamber wall for collecting said hot gas from least some of said hot gas conduits.

9. Apparatus as claimed in claim 1, comprising one or more fuel conduits to supply fuel to said combustion chamber and disposed one of: proximal to one or more hot gas conduits such that heat from said hot gas heats said fuel; proximal to one or more cold gas conduits such that heat from said cold gas heats said fuel; and to absorb heat from said heat exchanger.

10. Apparatus as claimed in claim 9, wherein said combustion chamber has a primary combustion chamber inlet and a combustion chamber outlet, and a primary fluid flow path extends between said primary combustion chamber inlet and said combustion chamber outlet, and said one or more fuel conduits supply said fuel to said primary combustion chamber inlet.

11. Apparatus as claimed in claim 1, wherein respective ones of said hot gas conduits and said cold gas conduits follow involute paths that are an involute of an outer cross sectional boundary through said combustion chamber wall in a cross sectional plane containing a corresponding involute path.

12. Apparatus as claimed in claim 11, wherein said outer cross sectional boundary is circular.

13. Apparatus as claimed in claim 1, wherein said hot gas conduits and said cold gas conduits are interleaved around said combustion chamber.

14. Apparatus as claimed in claim 1, wherein said hot gas conduits have cross sectional areas normal to said hot gas paths that are greater than cross sectional areas of said cold gas conduits normal to said cold gas paths.

15. Apparatus as claimed in claim 1, wherein one or more of said cold gas conduits supply said cold gas into said combustion chamber through respective combustion chamber inlet openings, one or more of said combustion chamber inlet openings directing said cold gas to enter said combustion chamber in a mean flow direction non-normal to said combustion chamber wall at a respective one of said one or more combustion chamber inlet openings.

16. Apparatus as claimed in claim 1, wherein said apparatus is formed of consolidated powder material.

17. Apparatus as claimed in claim 1, wherein said heat exchanger is a recuperator.

18. Apparatus as claimed in claim 17, comprising: a turbine to extract energy from combustion gas from said combustion chamber and to exhaust said hot gas; and a compressor to compress said cold gas for supply to said combustion chamber, wherein said recuperator is configured to transfer heat from said hot gas leaving said turbine to said cold gas for supply to said combustion chamber.

19. Apparatus as claimed in claim 1 comprising a removable combustion chamber liner disposed between said combustion chamber wall and said combustion chamber.

20. A non-transitory storage medium storing a computer-readable data structure representing a design of an apparatus according to claim 1.

21. The apparatus as claimed in claim 1, wherein one or more of comprises wherein at least some of said cold gas conduits directly connect to one or more cold gas plenums abutting said combustion chamber wall and plenum-inlet openings provide flow paths for said cold gas between respective ones of said one or more cold gas plenums and said combustion chamber.

Description

(1) Example embodiments will now be described, by way of example only, with reference to the accompanying drawings in which:

(2) FIG. 1 schematically illustrates a cross section through a first example combined combustor and recuperator;

(3) FIG. 2 schematically illustrates a second example combined combustor and recuperator;

(4) FIG. 3 schematically illustrate a third example combined combustor and recuperator;

(5) FIG. 4 schematically illustrates a heat engine including a combined combustor and recuperator;

(6) FIG. 5 schematically illustrate a fourth example combined combustor and recuperator;

(7) FIG. 6 schematically illustrates a cross section through the combined combustor and recuperator of FIG. 5; and

(8) FIG. 7 schematically illustrates and end view (combustion chamber outlet end) of the combined combustor and recuperator of FIG. 5.

(9) FIG. 1 schematically illustrates a combined combustor and heat exchanger 2. In this example the heat exchanger is in the form of a recuperator. The combustion chamber is bounded by a combustion chamber wall 4. The combustion chamber within the combustion chamber wall 4 includes a primary flow path for the combustion gasses extending between a primary combustion chamber inlet 6 and a combustion chamber outlet 8. The combustion chamber has a substantially cylindrical form in this example embodiment, although other shapes are also possible.

(10) Fuel is passed through a fuel conduit 10 to the primary combustion chamber inlet 6. The fuel is expelled through a nozzle 12 into the combustion chamber where it is mixed with cold gas (air) which has passed through the recuperator and been heated by hot gas, which is also passed through the recuperator. Some of the cold gas is introduced through a cold gas conduit 14 directly into the primary combustion chamber inlet 6. This cold gas may pass through vanes which impart a rotating motion about the primary flow path. The fuel from the nozzle 12 mixed with this cold gas and burned (combusted). The combustion gas follows a vortex (swirling) path within a central portion of the combustion chamber toward to the combustion chamber outlet 8.

(11) Further cold gas conduits 16 within the recuperator pass cold gas directly into the combustion chamber through inlets 18 within the combustion chamber wall 4. These inlets may be directed such that they impart a mean flow direction to the cold gas entering the combustion chamber with a component of motion which rotates around the primary flow path. This rotational motion of the cold gas introduced through the conduits 18 is used to support the swirling motion imparted to the cold gas introduced through the primary combustion chamber inlet 6 and help to maintain a stable vortex within which the fuel is combusted.

(12) There are further cold gas conduits 20 within the recuperator which pass cold gas into cold gas plenums 22 which border (adjoin/abut) the combustion chamber wall 4. Openings within the combustion chamber wall 4 which connect to the cold gas plenums allow cold gas to enter the combustion chamber via the cold gas plenums 22. These outlets within the combustion chamber wall 4 which connect to the cold gas plenums 22 can have shapes which serve to direct the cold gas passing therethrough to have a mean flow direction in a particular direction. More particularly, the openings in the wall of the cold gas plenum 22 proximal to the combustion chamber outlet 8 may direct the cold gas to enter with a component of its mean flow direction parallel with and in the same direction as the primary flow path. This cold gas will have a component which is perpendicular to the primary flow path, but nevertheless the majority of its flow direction may be parallel with the combustion chamber wall 4 as illustrated in FIG. 1. This mean flow direction for the cold gas introduced proximal to the combustion chamber outlet 8 helps to form a protective relatively cool boundary layer of gas around the combustion chamber wall in this region of the combustion chamber in a manner which helps resist erosion of the combustion chamber wall 4.

(13) At a location intermediate the combustion chamber outlet 8 and the primary combustion chamber inlet 6 one or more cold gas plenums 22 have outlets which direct the cold gas to provide a degree of backflow as illustrated in FIG. 1. This backflow is such that the cold gas entering the combustion chamber has a component of its mean flow direction which is parallel to the primary flow path and in an opposite direction to the primary flow path. Such backflowing cold gas helps to mix the cold gas into the combustion vortex (flame) extending from the nozzle 12 in a manner which assists efficient and thorough combustion of the fuel.

(14) As shown in FIG. 1, the cold gas conduits 14, 16, 20 provide a cold gas path between an outer cold gas plenum 24 and the combustion chamber within the combustion chamber wall 4. The cold gas accordingly flows radially inwardly toward the combustion chamber. Thus, as the cold gas is heated as it passes along the cold gas path, the hotter portion of the cold gas will be proximal to the combustion chamber. This helps to maintain heat energy within the combustion chamber thereby improving efficiency.

(15) The recuperator includes hot gas conduits 26 which pass between an inner hot gas plenum 28 and an outer hot gas plenum 30. The hot gas, which may be exhaust gas from a turbine, enters the inner hot gas plenum 28 and flows radially outwardly through the hot gas conduits 26 from which it is collected into the outer hot gas plenum 30 before exiting the recuperator. In this way, the hot gas with the highest temperature is located within the inner hot gas plenum 28 which is closest to the combustion chamber, thereby tending to increase the amount of heat energy maintained within the combustion chamber.

(16) One or more of the hot gas conduits 32 is directed to pass proximal to the fuel conduit 10 and accordingly serves to preheat (e.g. turn into gaseous form) the fuel before it reaches the nozzle 12. In other embodiments, one or more of the cold gas conduits 14, 16, 20 may be routed to be proximal to the fuel conduit 10 to preheat the fuel, or in other embodiments sufficient heat may be conducted through the body of the combined recuperator and combustor to heat the fuel within the fuel conduit 10 to the required degree.

(17) As illustrated in FIG. 1, the hot gas conduits 26 have a diameter “y” which is larger than the diameter of the cold gas conduits “x”. The hot gas is typically less dense than the cold gas and arranging for the hot gas conduits to be greater in cross sectional area than the cold gas conduits helps to balance the mass flow of the cold gas relative to the hot gas in a manner which permits a required degree of heat transfer between the hot gas and the cold gas.

(18) The combined recuperator and combustor illustrated in FIG. 1 may be formed of consolidated powder material, such as by energy beam melting of a metal powder as part of an additive manufacturing process. Such techniques are well suited to forming a complex arrangement of conduits, plenums and openings as illustrated in FIG. 1.

(19) A feature of such additively manufactured structures is that it is possible to form such structures in a way in which the material is porous to gaseous flow. Thus, for example, some of the openings in the combustion chamber wall 4 through which cold gas flows may instead (or additionally) be provided by porous openings through a porous portion of the combustion chamber wall 4. As an example, the protective cool boundary layer of cold gas introduced proximal to the combustion chamber outlet 8 may be provided by cold gas flowing through a porous combustion chamber wall bounding the cold gas plenum 22 near the combustion chamber outlet 8 instead of passing through specific openings in the cold gas plenum 22 in that region.

(20) As previously discussed above, the cold gas flowing through the cold gas conduits 14, 16, 20 passes radially inwardly toward the combustion chamber whereas the hot gas passes radially outwardly away from the combustion chamber. This arrangement provides counterflow between the cold gas and the hot gas. The section through the combined recuperator and combustor shown in FIG. 1 illustrates the cold gas conduits 14, 16, 20 in the upper portion and the hot gas conduits in the lower portion. It will be appreciated that in practice, these conduits will be interleaved with each other around the combustion chamber wall 4 as will be illustrated in the following diagrams.

(21) The cold gas conduits 14, 16, 20 and the hot gas conduits 26 may share boundary walls along at least a portion of their lengths in order to reduce the amount of material required to build the combined recuperator and combustor as well as to improve the heat transfer efficiency. The interleaving of the cold gas conduits 14, 16, 20 and the hot gas conduits 26 facilitates such boundary wall sharing.

(22) The illustration of FIG. 1 shows that the cold gas conduits 14, 16, 20 and the hot gas conduits 26 extend outwardly from the combustion chamber wall 4 in a radial direction. In practice, the cold gas conduits 14, 16, 20 and the hot gas conduits 26 may be arranged to extend out from the combustion chamber wall 4 following involute paths that are an involute of an outer cross sectional boundary through the combustion chamber wall 4 in a plane which contains those corresponding involute paths. The use of conduits following such involute curves facilitates the conduits having a substantially constant cross sectional area along their length with a tight packing between the conduits. These features simplify gas flow and increase efficiency as well as reducing the size of the combined recuperator and combustor for a given length of conduit in which heat exchange takes place.

(23) In the examples illustrated herein the combustion chamber is of a substantially cylindrical shape and accordingly has a circular cross section. The involute paths of the conduits accordingly are an involute of a circle. However, it will be appreciated that the combustion chamber can have shapes other than that of a cylinder and in such cases the conduits can follow involute paths that are an involute of an outer cross sectional boundary of the combustion chamber wall 4 which is other than circular, e.g. elliptical. The involute paths in the examples illustrated herein lie in a plane which is perpendicular to the primary flow path through the combustion chamber. However, it is possible that these involute paths could lie in a plane which is not perpendicular to such a primary flow path and yet still meet the requirements of the involute geometry and provide closely packed and substantially constant cross sectional area conduits.

(24) In the example of FIG. 1, the combustor has a circular cross section and the recuperator is a uniform annular shape surrounding the combustor. Illustrated in FIG. 2 are conduits having shared walls and an interleaved arrangement. In the example of FIG. 2 the conduits alternate between cold gas conduits and hot gas conduits. The cold gas conduits supply cold gas moving generally radially inwardly toward the combustion chamber in which the cold gas is then mixed with the combustion gasses. The hot gas flows generally radially outwardly away from the combustion chamber and may comprise, for example, exhaust gas from a turbine from which it is desired to recover heat energy by transferring that heat energy into the cold gas which is then used within the combustion chamber.

(25) FIG. 2 also shows a removable liner 34 disposed between the combustion chamber wall and the combustion chamber. This liner 34 can be replaced, for example if the fuel is diesel and the inside of the combustion chamber suffers from a build up of coke deposits. The combustion chamber wall would still continue to condition and direct the gas flow. The liner 34 may be additively manufactured.

(26) FIG. 3 illustrates a further example embodiment of a combined recuperator and combustor. In this example embodiment, the combustor is again circular in cross section, but the recuperator is not a regular annular cross section, but instead has a scalloped section removed therefrom. This may facilitate the fitting of the combined recuperator and combustor around another part of a heat engine system in order to make a more compact whole.

(27) The examples of the recuperators shown in FIG. 1 and FIG. 2 use counter flow between the cold gas and the hot gas, i.e. the cold gas and the hot gas flow substantially parallel to each other, but in opposite directions through their respective conduits. In contrast, FIG. 3 schematically illustrates an example embodiment in which the cold gas conduits and the hot gas conduits can have a cross flow arrangement in which they cross each other e.g. in a substantially perpendicular direction. Such a cross flow heat exchanger may be preferred in some situations, such as to meet particular manifolding requirements.

(28) FIG. 4 schematically illustrates a heat engine 32 in the form of a turbomachine having a compressor 34 for compressing inlet air to generate compressed cold gas as well as a turbine 36 driven by the hot combustion gas from a combustor 38. The combustor 38 is formed integral with a recuperator 40. The compressed cold gas from the compressor 34 is supplied to the recuperator 40 where it passes through cold gas conduits, as previously described, and is heated by hot gas which passes through hot gas conduits within the recuperator 40. The hot gas within the recuperator 40 is provided by the exhaust gas from the turbine 36. The gas flow path through the turbomachine 32 of FIG. 4 is: inlet air into the compressor 34; compressed cold gas output from the compressor 34 supplied to the cold gas conduits within the recuperator 40; the compressed cold gas when it has been heated within the recuperator 40 passes into the combustion chamber of the combustor 38 where it is mixed with fuel which is burnt to generate the combustion gases; the combustion gasses flow from the combustor 38 to the turbine 36 where they serve to drive rotation of the turbine 36; and the exhaust gasses from the turbine 36 form the hot gas which is supplied to the recuperator 40 and from which residual heat energy is transferred into the compressed cold gas within the recuperator 40.

(29) FIG. 5 schematically illustrates a perspective view of a combined heat exchanger (in this example embodiment in the form of a recuperator) and combustor. FIG. 5 shows a combustor with a substantially cylindrical combustion chamber within the centre of a recuperator which completely surrounds the combustion chamber along its entire length. FIG. 6 shows a cross section through the combined recuperator and combustor of FIG. 5. The conduits through which the hot gas and the cold gas flow are interleaved with each other when passing circumferentially around the cylindrical combustion chamber. The conduits follow involute curves and have a substantially constant cross sectional area along their length facilitated by the use of such involute curves.

(30) FIG. 6, which is the cross sectional view through the combined recuperator and combustor shows the combustion chamber wall 4 having a large number of small openings within it. These openings permit cold gas from the recuperator to enter into the combustion chamber and take part within the combustion process, or protect the combustion chamber wall from excessive heat. The openings may be shaped so as to direct the cold gas entering the combustion chamber through those openings to have particular mean flow directions. Accordingly, the cold gas entering near to the primary combustion chamber inlet can be directed to swirl around the combustion chamber in a direction supporting the vortex flow of the flame extending into the combustion chamber from the primary combustion chamber inlet. The openings partway along the primary flow path within the combustion chamber can be directed to establish a degree of backflow toward the primary combustion chamber inlet in a manner which facilitates the mixing of the cold gas with the flame established at the primary combustion chamber inlet so as to improve combustion efficiency. The openings toward the combustion chamber outlet end of the combustion chamber can direct the cold gas flow toward the combustion chamber outlet in a manner whereby the cold gas forms a protective layer of cold gas close to the combustion chamber wall in a manner which helps to prevent damage/degradation of the combustion chamber wall.

(31) FIG. 7 schematically illustrates an end view of the combined recuperator and combustor looking through the combustion chamber outlet along the centre of the combustion chamber towards the combustion chamber inlet. FIG. 7 illustrates the involute curves of the interleaved cold gas conduits and hot gas conduits. The hot gas conduits are larger in cross section than the cold gas conduits.

(32) Example arrangements of the present technique are set out below in the following clauses:

(33) (1) Apparatus comprising:

(34) a combustion chamber wall enclosing a combustion chamber; and

(35) a heat exchanger integral with at least a portion of said combustion chamber wall.

(36) (2) Apparatus according to clause (1), wherein said combustion chamber has a primary combustion chamber inlet and a combustion chamber outlet, and a primary fluid flow path extends between said primary combustion chamber inlet and said combustion chamber outlet.
(3) Apparatus according to clause (2), wherein said heat exchanger at least partially surrounds said combustion chamber in planes normal to at least a portion of said primary fluid flow path.
(4) Apparatus according to clause (3), wherein said heat exchanger fully surrounds said combustion chamber in said planes.
(5) Apparatus according to any one of clauses (3) and (4), wherein said portion comprises all of said primary fluid flow path.
(6) Apparatus according to any one of clauses (4) and (5), wherein said combustion chamber wall has a circular cross section in a plane normal to said primary fluid flow path and said heat exchanger has an annular cross section in said plane normal to said primary fluid flow path.
(7) Apparatus according to any one of clauses (1) to (6), wherein said heat exchanger transfers heat from a hot gas to a cold gas and comprises:

(37) a plurality of hot gas conduits to direct hot gas along respective hot gas paths; and

(38) a plurality of cold gas conduits to direct cold gas along respective cold gas paths.

(39) (8) Apparatus according to clause (7), wherein said hot gas paths flow away from said combustion chamber and said cold gas paths flow toward said combustion chamber and are substantially parallel with said hot gas paths.

(40) (9) Apparatus according to clause (7), wherein said hot gas paths are substantially perpendicular to said cold gas paths.

(41) (10) Apparatus according to any one of clauses (7), (8) and (9), wherein said plurality of hot gas conduits and said plurality of cold gas conduits share at least some conduits boundary walls.

(42) (11) Apparatus according to clause (8), wherein said plurality of hot gas conduits and said plurality of cold gas conduits are disposed within said heat exchanger to provide counterflow between said hot gas and said cold gas.

(43) (12) Apparatus according to any one of clauses (7) to (11), wherein one or more of:

(44) at least some of said cold gas conduits directly connect to respective inlet openings in said combustion chamber wall; at least some of said cold gas conduits directly connect to one or more cold gas plenums abutting said combustion chamber wall and plenum-inlet openings provide flow paths for said cold gas between respective ones of said one or more cold gas plenums and said combustion chamber; and said combustion chamber wall is porous and said cold gas passes from at least some of said cold gas conduits into said combustion chamber through porous openings in said combustion chamber wall.
(13) Apparatus according to clause (2) and any one of clauses (7) to (12), wherein at least some of said cold gas conduits provide said cold gas to said primary combustion chamber inlet.
(14) Apparatus according to any one of clauses (7) to (13), comprising one or more inner hot gas plenums proximal to said combustion chamber wall to supply said hot gas to at least some of said hot gas conduits and one or more outer hot gas plenums distal from said combustion chamber wall for collecting said hot gas from least some of said hot gas conduits.
(15) Apparatus according to any one of clauses (7) to (14), comprising one or more fuel conduits to supply fuel to said combustion chamber and disposed one of: proximal to one or more hot gas conduits such that heat from said hot gas heats said fuel; proximal to one or more cold gas conduits such that heat from said cold gas heats said fuel; and to absorb heat from said heat exchanger.
(16) Apparatus according to clause (2) and clause (15), wherein said one or more fuel conduits supply said fuel to said primary combustion chamber inlet.
(17) Apparatus according to any one of clauses (7) to (16), wherein respective ones of said hot gas conduits and said cold gas conduits follow involute paths that are an involute of an outer cross sectional boundary through said combustion chamber wall in a cross sectional plane containing a corresponding involute path.
(18) Apparatus according to clause (17), wherein said outer cross sectional boundary is circular.
(19) Apparatus according to any one of clauses (7) to (18), wherein said hot gas conduits and said cold gas conduits are interleaved around said combustion chamber.
(20) Apparatus according to any one of clauses (7) to (19), wherein said hot gas conduits have cross sectional areas normal to said hot gas paths that are greater than cross sectional areas of said cold gas conduits normal to said cold gas paths.
(21) Apparatus according to any one of clauses (7) to (20), wherein one or more of said cold gas conduits supply said cold gas into said combustion chamber through respective combustion chamber inlet openings, one or more of said combustion chamber inlet openings directing said cold gas to enter said combustion chamber in a mean flow direction non-normal to said combustion chamber wall at a respective one of said one or more combustion chamber inlet openings.
(22) Apparatus according to clause (2) and clause (21), wherein at least one of: one or more of said combustion chamber inlet openings proximal to said primary combustion chamber inlet direct said cold gas to enter said combustion chamber with a mean flow direction rotating around said primary flow path; one or more of said combustion chamber inlet openings proximal to said combustion chamber outlet direct said cold gas to enter said combustion chamber with a mean flow direction having a component parallel with and in a same direction as said primary flow path; and one or more of said combustion chamber inlet openings proximal to an intermediate position along said primary flow path direct said cold gas to enter said combustion chamber in a mean flow direction with a component in an opposite direction from said primary flow path.
(23). Apparatus according to any one of the preceding clauses, wherein said apparatus is formed of consolidated powder material.
(24). Apparatus according to any one of the preceding clauses, wherein said heat exchanger is a recuperator.
(25). Apparatus according to clause (24), comprising:

(45) a turbine to extract energy from combustion gas from said combustion chamber and to exhaust said hot gas; and

(46) a compressor to compress said cold gas for supply to said combustion chamber, wherein said recuperator is configured to transfer heat from said hot gas leaving said turbine to said cold gas for supply to said combustion chamber.

(47) (26) Apparatus according to any one of the preceding clauses comprising a removable combustion chamber liner disposed between said combustion chamber wall and said combustion chamber.