Double Wall Combustion Chamber

Abstract

A combustion chamber for a gas turbine power plant of the step-liner type including a plurality of annular double wall step-liner portions. The portions are of various sizes and are arranged concentrically and in order of increasing size from the upstream end toward the downstream end of the chamber. Each double wall liner portion includes an alternating smooth wall member and a serpentinous wall member. In one step, the smooth wall member is the radially inner wall and an overlapping portion of the serpentinous wall member is the radially outer wall. In the adjacent larger step, the serpentinous wall member is the radially inner wall and an overlapping portion of a larger diameter smooth wall member is the radially outer wall. The effect of this construction is to provide a combustion chamber capable of withstanding higher burning temperatures.

Description

BACKGROUND OF THE INVENTION

One serious problem in gas turbine power plants is reliable combustion chambers or combustors, capable of withstanding high temperatures for extended periods of time.

Historically, a substantial step forward was taken when the step-liner type combustion chamber having corrugated spacer members between adjacent liners was disclosed by E. F. Miller, U.S. Pat. No. 2,610,467, issued on Sept. 16, 1952, and assigned to the present assignee. In this now well known construction, cooling air is admitted from the plenum chamber through axially extending spaces in the corrugated members, to provide a flow of relatively cool air to insulate the inner surface of the combustion chamber wall from the hot combustion gases. Other improvements on this construction are disclosed in the following patents: S. Way, U.S. Pat. No. 2,448,561 patented on Sept. 7, 1948; W. L. Christensen, U.S. Pat. No. 2,537,033, patented Jan. 9, 1951; R. A. Sforzine, U.S. Pat. No. 2,549,858, patented Apr. 24, 1951; and E. A. DeZubay et al., U.S. Pat. No. 2,573,694, patented Nov. 6, 1951, all of the preceding patents being assigned to the present assignee.

Of course, the higher the temperature of the combustion chamber walls during normal operation, the more subject the chambers are to thermal stress and strain. This requires constant maintenance programs whereby combustion chambers must be periodically inspected to ensure their operating reliabilities. More recently, because of the advent of larger gas turbine power plants, it has become desirable to operate the plants at higher and higher temperatures. Furthermore, because of the economy involved, it is more desirable to burn heavy residual fuels, which are high in contaminants, rather than the purer fuels, such as No. 2 distillate fuel. However, residual fuels radiate substantially more heat to the combustor walls, so that combustor life and reliability is substantially reduced.

One such solution to enable combustion chambers to operate at higher temperatures is the refractory or ceramic combustion chamber. But as of now, the high velocities and violent pulsations in the combustion chambers have prevented the use of refractories in combustors in commercial applications.

Another such solution is to introduce more cooling air to the combustor walls. However, this decreases the overall efficiency of the turbine and has an adverse effect on the temperature distribution pattern of the gases when it is introduced to the turbine blades since there is a large temperature differential between the blade tips where the cooler air is and the blade centers causing serious thermal stress and strain.

It would be desirable, then, to design a combustion chamber which would operate at higher temperatures for extended periods of time, or in the alternative operate at cooler temperatures than present combustors operating at normal conditions, so that it would be subjected to fewer thermal stress and strains than present combustors. This in turn would require less periodic inspections since its reliability would be substantially increased. It would further be desirable to design a combustor that would be economical to construct and easy to assemble.

SUMMARY OF THE INVENTION

The following disclosure relates to a combustion chamber structure for a gas turbine and more particularly to an improved apparatus of this type.

A gas turbine power plant has a compressor section, in which is disposed at least one combustor or combustion chamber, and a turbine section. The combustion chamber is of the step-liner type and is comprised of a plurality of double wall portions. The double wall portions are of stepped configuration, each of the portions being of greater diameter than the preceding portion from the upstream to the downstream end of the combustor. Each liner portion includes an alternating smooth wall member and a serpentinous or corrugated wall member.

In one step or portion, the axially extending smooth wall forms the radially inner wall and a partially telescoping or overlapping serpentinous wall member forms the radially outer wall. One part of the serpentinous member overlaps approximately half of the smaller diameter smooth member and approximately one-half of the serpentinous member extends in a downstream axial direction. A second larger diameter smooth wall member, in turn overlaps the extending half of the adjacent serpentinous member, to cooperatively form a second double wall portion.

The adjacent ends of the alternating smooth members of the wall portions are approximately aligned in a radial direction relative to the axial centerline of the combustor. The alternating serpentinous members are also radially aligned at their adjacent ends.

Each serpentinous member partially forms an annular array of convoluted axially extending passageways to provide fluid communication between the compressor section and the chamber within the combustor.

Compressor air used to cool the combustor walls enters the convoluted passageways from the compressor, and the air flow through these passageways cools the outer surface of the radially inner wall of the smooth member of the double wall stepped liner, by convection. After flowing through the convoluted passageways, thereby cooling a portion of the radially inner double wall step-liner, the air flows along the inner surface of the larger diameter upstream smooth wall member to provide a cool film of air to thereby insulate the smooth wall from the hot combustion gases.

The double wall construction enables the cooling air to provide multiple functions: (a) to cool each double wall portion of the step-liners by convective cooling and (b) to cool each double wall portion by providing a cool film or air to insulate the inner wall surface from the hot combustion gases. A greater cooling effect is achieved when compared to present temperatures of combustion chambers, or, in the alternative where higher combustion chamber temperatures must be handled, the construction permits a more efficient utilization of available cooling air to enable operation at the higher temperatures. One U.S. Patent disclosing dual/cooling in a combustion chamber is No. 3,545,202.

Further advantages of this combustion chamber construction is that approximately the same man-hours are required for constructing this combustor relative to prior combustors, and there is substantially, no increase in the cost associated therewith.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is an axial sectional view of a portion of the upper half of a gas turbine power plant provided with combustion apparatus incorporating the invention;

FIG. 2 is an enlarged sectional view of the combustion apparatus illustrated in FIG. 1;

FIG. 3 is a sectional view taken along line III--III in FIG. 2; and

FIG. 4 is a view in perspective of a portion of the combustion apparatus illustrated in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings in detail and especially to FIG. 1, there is shown a portion of a gas turbine power plant 10 having combustion apparatus generally designated 11. However, the combustion apparatus may be employed with any suitable type of gas turbine power plant. The power plant 10 includes an axial flow air compressor 12 for directing air to the combustion apparatus 11 and a gas turbine 14 connected to the combustion apparatus 10 and receiving hot products of combustion therefrom for motivating the power plant.

Only the upper half of the power plant and combustion apparatus has been illustrated, since the lower half may be substantially identical and symmetrical about the centerline or axis of rotation RR' of the power plant.

The air compressor 12 includes, as well known in the art, a multi-stage bladed rotor structure 15 cooperatively associated with a stator structure having an equal number of multi-stage stationary blades 16 for compressing the air directed therethrough to a suitable pressure value for combustion in the combustion apparatus 11. The outlet of the compressor 12 is directed through an annular diffusion member 17 forming an intake for the plenum chamber 18, partially defined by a housing structure 19. The housing 19 includes a shell member of circular cross-section, and as shown of cylindrical shape, parallel with the axis of rotation RR' of the power plant 10, a forward dome-shaped wall member 21 connected to the external casing of the compressor 12 and a rearward annular wall member 22 connected to the outer casing of the turbine 14.

The turbine 14, as mentioned above, is of the axial flow type and includes a plurality of expansion stages formed by a plurality of rows of stationary blades 24 cooperatively associated with an equal plurality of rotating blades 25 mounted on the turbine rotor 26. The turbine rotor 26 is drivingly connected to the compressor rotor 15 by a tubular connecting shaft member 27, and a tubular liner or fairing member 28 is suitably supported in encompassing stationary relation with the connecting shaft portion 27 to provide a smooth air flow surface for the air entering the plenum chamber 18 from the compressor diffuser 17.

Disposed within the housing 19 are a plurality of tubular elongated combustion chambers or combustors 30 of the telescopic step-liner type. The combustion chambers 30 are disposed in an annular mutually spaced array concentric with the centerline of the power plant and are equally spaced from each other in the housing 19. The chambers 30 are arranged in such a manner that their axes are substantially parallel to the outer casing 19 and with the centerline RR' of the power plant 10. It is pointed out that this invention is applicable to other types of combustors such as the single annular basket type, shown and described in the Miller patent, previously cited or the can-annular type having composite features of the canister and annular types.

Since the combustors 30 may be substantially identical, only one will be described. As shown in FIG. 1, each combustor 30 is comprised of three sections: an upstream primary portion 32, an intermediate secondary portion 33 and a downstream transition portion 34.

The forward wall 21 of the combustion apparatus 11 is provided with a central opening 36 through which a fuel injector 37 extends. The fuel injector 37 is supplied with fuel by a suitable conduit 38 connected to any suitable fuel supply (not shown) and may be of the well known atomizing type formed in a manner to provide a substantially conical spray of fuel within the primary portion 32 of the combustion chamber 30. A suitable electrical igniter 39 is provided for igniting the fuel and air mixture in the combustor 30.

In the primary portion 32 of the combustor 30 are a plurality of liner portions 42 of circular cross-section and in the example shown, the liner portions are cylindrical. The portions 32 are of stepped construction, each of the portions having a circular section of greater circumference or diameter than the preceding portion from the upstream to the downstream end of the combustor to permit telescopic insertion of the portions. Some portions 42 have an annular array of apertures 44 for admitting primary air from within the plenum chamber 18 into the primary portion 32 of the combustor to support combustion of fuel injected therein by the fuel injector 37. The combustor further includes the intermediate portion 33 which is provided with a plurality of annular rows of apertures 46 for admitting secondary air from the plenum chamber 18 into the combustor 30 during operation, to cool the hot gaseous products and make it adaptable to the turbine blades 24 and 25. The transition portion 34 is provided with a forward portion 48 of cylindrical shape disposed in encompassing and slightly overlapping relation with the intermediate portion 33 as shown by the locking spring structure 47 (FIG. 2). The transition portion 34 is also provided with a rearward tubular portion 49 that progressively changes in contour from circular cross-section at the jointure with the cylindrical portion 48 to arcuate cross-section at its outlet end portion 50. The arcuate extent of the outlet 50 is such that, jointly with the outlets of the other combustors 30, a complete annulus is provided for admitting the hot products of combustion from the combustors to the blades 24 and 25 of the turbine 14, thereby to provide full peripheral admission of the motivating gases to the turbine 14.

In accordance with the invention, the liner portions 42 of the combustor 30 are substantially comprised of a plurality of double walled portions 52 (for example five). Each of the portions 52 have a circular cross-section of greater diameter than the preceding portion, from the upstream to the downstream end of the combustor relative to the flow of air therethrough. The double wall construction of the liner portions 42, as shown, is confined to the primary portion of the combustor 30 although it is not limited thereto.

Another way of viewing the double wall portions 52 is that each of the portions includes alternating smooth and corrugated or serpentinous wall members 54 and 56, respectively, which overlap each other approximately to the center of each respective member. The resulting plurality of the double wall portions form the primary portion of the combustor 30.

As best seen in FIGS. 2 and 4, each double wall portion 52 includes an alternating smooth wall member 54 and a serpentinous or corrugated wall member 56. The smooth wall member 54 shown is an elongated tubular or cylindrical member (FIGS. 2 and 3) and each serpentinous member 56 is in the form of circumferentially corrugated rings comprised of generally trapezoidal shaped elements 57 (FIG. 3).

Referring to one of the first upstream step-liners 42a, the double wall portion 52 is comprised of an axially extending smooth wall member 54, which forms the radially inner wall of the double wall 52, and a larger diameter telescoping serpentinous wall member 56 which overlaps approximately one-half of the smooth wall member 54. Approximately one-half of the serpentinous member 56 overlaps the smaller diameter smooth member 54 and the other half of the serpintinous member 56 extends in a downstream axial direction. Therefore, the double wall portion of the step liner 42a comprises a radially inner wall formed of a smooth wall member 54 and a radially outer wall formed of a serpentinous member 56.

An adjacent second step liner 42b is larger in diameter than the step liner 42a. The inner wall is comprised of the half of the serpentinous member 56 which extends beyond the step 42a in an axial direction. A second larger diameter smooth wall member 54 overlaps the axially extending serpentinous wall member 56. The second larger diameter smooth wall member 54 overlaps approximately one-half of the serpentinous member 56 and approximately one-half of the smooth member extends in a downstream axial direction. Therefore, the second double wall portion 52 is comprised of a radially inner wall of a serpentinous member 56 and a radially outer wall of a smooth member 54 to cooperatively form the second step liner portion 42 b.

The adjacent downstream end 61 of the first corrugated member 56 and the adjacent upstream end 62 of the second corrugated member 56 are substantially aligned in a radial direction relative to the axial centerline of the combustor 30 and do not overlap in the preferred embodiment. Furthermore, the adjacent downstream end 63 of the first smooth member 54 and the adjacent upstream end 64 of the second smooth member 54 are also substantially radially aligned at their adjacent ends and do not overlap.

The subsequent larger diameter downstream liner portions 42 are substantially similar to the first two liner portions 42a and 42b previously described. Each liner portion includes an alternating smooth wall member 54 an alternating serpentinous wall member 56 with one part of the serpentinous member overlapping approximately half of the smaller diameter smooth member and the other half extending in a downstream axial direction. In the alternate step, the larger diameter smooth wall member overlaps approximately one-half of the upstream adjacent serpentinous member to cooperatively form another double wall portion 52. In the subsequent step portions 42 the adjacent ends of the alternating smooth members are substantially radially aligned and the adjacent ends of the serpentinous members are also substantially radially aligned.

As best seen in FIGS. 3 and 4, the trapezoidal shaped elements 57 of the serpentinous member 56 have inwardly depressed portions 66, which contact the smaller diameter smooth wall member 54, and outwardly depressed portions 67, which are in spaced relation with the inner wall member or smooth member 54. In the second step-liner portion 42b (FIG. 2) the outwardly projecting portions 67 contact the outer or smooth wall member 54 and the inwardly depressed portions 66 are in spaced relation therefrom and form the inner wall of the double wall portion 52.

As best seen in FIG. 4, the serpentinous members 56 are securely fastened to the smaller diameter smooth wall member 54 by any suitable means 69, preferably by spot welding at a plurality of points of contact. The larger diameter smooth wall members 54 are in turn securely fastened by any suitable means 70, preferably by spot welding at a plurality of points of contact to the smaller diameter serpentinous member 56 along the outwardly depressed portions 67, which are substantially flattened to receive a more rigid spot weld. Only one annular row of spot welds are used to allow for thermal expansion of the members 54 and 56 in an axial direction.

The outwardly depressed portions 67 in the serpentinous member 56 cooperatively with the smooth wall portion 54 in the first step-liner portion 42a, form a plurality of axial passageways 72 to provide for entry of cooling air indicated by arrow E in FIGS. 2 and 4 from the plenum chamber 18 (FIG. 1) into the combustion chamber 30.

Referring to the second step-liner portion 42b, a second plurality of axial passageways 74 are jointly defined by the outer smooth wall member 54 and the inwardly depressed portions 66 of the serpentinous member 56. These passageways 74 also provide for entry of cooling air indicated by arrows G from the plenum chamber 18 into the combustion chamber 30.

As previously described, a plurality of apertures 44 are provided within the primary portion 32 of the combustor 30. As best shown in FIGS. 2 and 4, the apertures 44 are partially formed in both the smooth and serpentinous wall members 54 and 56, respectively. In the preferred embodiment, each aperture 44 is wholly formed in the smooth member 54 as indicated by 44a and partially formed in the serpentinous members 56 as indicated in 44b. Furthermore, each aperture 44 is in registry with one of the passageways 72, so that a portion of the air from the plenum chamber entering the combustor through aperture 44 is also directed through axial passageway 72 for cooling purposes.

In operation, air is compressed in the compressor 12 (FIG. 1) and flows into the plenum chamber 18 within the casing 19. A portion of the pressurized air enters through the primary air apertures 44 where it is mixed with fuel to form a combustible mixture. The hot products of combustion then move to the intermediate portion 33 of the combustor 11 where secondary air enters the air inlet apertures 46 to cool the gaseous products. The air then is directed through the transition portion 34 through the outlet 50 to turn the turbine rotor structure 26, the shaft 27 and compressor rotor structure 15.

As previously mentioned under "Background Of The Invention," one of the most serious problems with the combustors is to design them to withstand high temperatures for extended periods of time. The cooling of the combustor is as follows. The compressor air from the plenum chamber 18 (FIG. 1) enters the convoluted axial passageways 72 in a first step-liner portion 42a because of the pressure drop between the plenum chamber and the combustor. As the air flows through the passageways 72 the air convectively cools the inner smooth wall member 54 (or the downstream half) and also convectively cools the outer serpentinous wall member 52 (or the upstream half) as indicated by the arrows E in FIG. 2. The air then continues to flow into the second step-liner portion 42b, where it film cools the serpentinous inner wall member 56 (or the downstream half) and the air continues to flow to the adjacent downstream portion to provide an insulating film cooling blanket for the smooth radially inner wall member 54 (or the downstream half). This multiple cooling technique is similar in subsequent step-liner portions 42 as indicated by cooling air represented by arrows F entering convoluted axially extending passageways 72 and convectively cooling the radially inner smooth wall 54, the outer serpentinous wall member 56 and then continuing to provide film cooling for the serpentinous member and the subsequent downstream smooth wall member 54 of the double wall portions 52.

Additional cooling air represented by arrows G (best seen in FIGS. 2 and 4) enters the axially extending passageways 74 in the second step 42b to provide convective cooling for the larger diameter smooth wall member 54 and the inner serpentinous member 56 and film cooling for the downstream smooth wall member 54. This multiple cooling function is similar in subsequent step-liner portions.

In large commercial gas turbines, typical tests have indicated that combustors burning No. 2 distillate fuel had an average temperature in the primary portion of approximately 1,600.degree.F. Laboratory tests have indicated using the double wall combustor disclosed herein, that the average temperature in the same primary portion were approximately 1,000.degree.F. under similar testing conditions. This means that the double wall combustor during actual operating conditions runs at approximately 600.degree.F. cooler than present combustors which represents a decrease of about 361/2 percent when compared to present combustors.

Furthermore, this reduction in temperature of 600.degree.F. is estimated to extend the useful operating life of the combustors of approximately five times when compared to conventional combustors, while the increase in cost involved in building the double wall combustor relative to the conventional combustor is almost negligible.

As previously mentioned, it is more economical to switch to heavy fuels (those with high hydrocarbon contents and impurities) rather than burn conventional No. 2 distillate fuel. However, since heavy residual fuels radiate substantially more heat than No. 2 distillate fuel, combustor walls become hotter and combustor life is correspondingly substantially reduced. As an example, it is not uncommon for combustor wall portions to reach 1,900.degree.F. while burning heavy residual fuels. Using the double wall combustor construction, the wall temperature of the combustors can be kept within present operating temperatures and have useful lives equal to that of conventional combustors.

Another advantage to the double wall combustor, is that no additional cooling air is needed to cool the walls and the temperature distribution pattern of the gases introduced to the turbine blades are not effected thereby. Alternately, to maintain the same temperature of the combustor walls, the amount of compressor air could be substantially reduced thereby increasing the overall efficiency of the turbine because of the reduction in back work.

An additional advantage of the double wall combustor is that since the combustor walls are cooler, the pressurized air within the plenum chamber is also kept cooler resulting in a cooler outer casing structure.

What is disclosed then, is a double wall combustor that provides multiple cooling functions: (a) convectively cools each double wall portion, and (b) film cools each double wall portion. The double wall combustor runs substantially cooler when compared to present temperatures of combustors or in the alternative can operate at substantially higher temperatures without having a corresponding decrease in combustor life. When operated at normal conditions, the combustor has an increase of useful life of five times that of present combustors, with substantially no increase in construction cost associated therewith. Finally, the temperature distribution pattern of the hot combustion gases are not affected by the double wall construction.

Since numerous changes may be made in the above-described construction, and different embodiments of the invention may be made without departing from the spirit and scope thereof, it is intended that all the matter contained in the foregoing description or shown in the accompanying drawings, shall be interpreted as illustrative and not in a limiting sense.

Claims

We claim as our invention:

1. In a combustion chamber arranged for admission of fuel to the upstream end thereof and discharge of hot gaseous products from the downstream end thereof, an elongated tubular wall structure of a step-liner construction incrementally increasing in cross sectional area in downstream direction and having a double wall, said double wall comprising, serpentinous wall members and smooth wall members, said smooth wall members being disposed adjacent each other in such a manner that upstream smooth wall members are circumferentially smaller than adjacent downstream smooth wall members and a serpentinous wall member overlaps adjacent smooth wall members to form said tubular double wall structure, said serpentinous and smooth wall members being so disposed that an upstream portion of each of said serpentinous members enwraps and fastens to a downstream portion of a smooth wall member and telescopes within and fastens to an upstream portion of an adjacent smooth wall member, whereby said combustion chamber has a double wall throughout said elongated tubular step liner structure.

2. The structure recited in claim 1, wherein the adjacent smooth members have ends which are substantially radially aligned.

3. The structure recited in claim 1, wherein

the serpentinous members have ends which are substantially radially aligned.

4. The structure recited in claim 1, wherein the serpentinous wall members partially define an array of first passageways, generally extending in the axial direction, to permit cooling fluid to convectively cool the enwrapped smooth wall member and gaseous products within the combustion chamber and the adjacent downstream smooth wall members.

5. The structure recited in claim 4, wherein the cooling fluid also convectively cools the portion of the serpentinous wall members enwrapping the downstream portion of the smooth wall members.

6. The structure recited in claim 4, wherein the cooling fluid further film cools the adjacent serpentinous wall members telescoping in the upstream portion of the smooth wall members.

7. The structure recited in claim 4, wherein the serpentinous wall members partially define an array of second passageways generally extending in the axial direction to permit cooling fluid to convectively cool the serpentinous wall members and the enwrapped smooth wall member, and furthermore to allow cooling fluid to provide a cooling film of fluid adjacent the downstream portion of the smooth wall members.

8. The structure recited in claim 1 wherein there is no substantial overlap between adjacent smooth wall members.

9. The structure recited in claim 1 wherein there is no substantial overlap between adjacent serpentinous wall members.

10. The structure recited in claim 1, having a primary combustion zone adjacent the downstream end thereof.

11. The structure recited in claim 10, and further comprising an array of primary air inlet apertures disposed in the primary combustion zone.

12. The structure recited in claim 1 and further comprising an array of apertures disposed adjacent the upstream end of the combustion chamber for admitting air thereto, said apparatus being formed jointly in the smooth and serpentinous member.

13. The structure recited in claim 12, wherein each of the apertures is wholly formed in a smooth wall member and partially formed in adjacent serpentinous wall members.

14. The structure recited in claim 1, wherein the serpentinous wall members are tubular and are comprised of elements having a substantially trapezoidal shaped cross-section.

15. The structure recited in claim 14, wherein the trapezoidal shaped elements jointly define inwardly and outwardly depressed portions the outer surfaces of which contacts the adjacent smooth wall members.

16. In a gas turbine power plant comprising combustion apparatus having an outer casing structure, said casing structure at least partially defining a pressurized plenum chamber,

a plurality of combustion chambers disposed in an annular array within said casing structure, the centers of said chambers being equidistant from each other in an annular direction,

each of said combustion chambers having a tubular double wall structure of step-liner construction, incrementally increasing in cross sectional area in downstream direction,

said double wall structure comprising serpentine wall members and smooth wall members,

said smooth wall members being disposed adjacent each other in such a manner that upstream smooth wall members are circumferentially smaller than adjacent downstream smooth wall members and a serpentinous wall member overlaps adjacent smooth wall members to form said tubular double wall structure, said serpentinous and smooth wall members being so disposed that an upstream portion of each of said serpentinous members enwraps and fastens to a downstream portion of a smooth wall member and telescopes within and fastens to an upstream portion of an adjacent smooth wall member, whereby said combustion chamber has a double wall throughout said tubular step liner structure.

17. The structure recited in claim 16, wherein the adjacent smooth members have ends which are substantially radially aligned.

18. The structure recited in claim 16, wherein

the serpentinous members have ends which are substantially radially aligned.

19. The structure recited in claim 16, wherein the serpentinous wall members partially defines a first array of passageways extending generally in an axial direction to provide fluid communication between the plenum chamber and the combustion chamber,

said first passageways permitting cooling fluid to convectively cool the enwrapped smooth wall member and the enwrapping serpentinous member,

and said passageways being arranged to permit said cooling fluid to provide a fluid film between hot combustion gases within the combustion chamber and the adjacent serpentinous wall members telescoped within the smooth wall member and to produce a fluid film between the hot combustion gases and the enwrapped smooth wall members.

20. The structure recited in claim 19, wherein the serpentinous wall members partially defines a second array of passageways extending generally in an axial direction,

said passageways permitting cooling fluid to convectively cool the serpentinous wall members and telescoped with the smooth wall members,

and furthermore, to allow the cooling fluid to provide a film of cooling fluid adjacent the enwrapped smooth wall members.