Combustion Liner Cooling Slot Stabilizing Dimple

Abstract

A combustor liner film cooling slot, of the variety including an annular space extending circumferentially of the liner which space is defined between overlapping portions of telescoping liner segments, a cooling fluid plenum substantially circumscribing the liner, means for transferring cooling fluid from the plenum to the space, means for exhausting fluid from the space onto the liner, and an overhanging lip extending substantially axially downstream of the space for facilitating attachment of the fluid to the liner in a protective film barrier, is provided with a plurality of circumferentially spaced stabilizing dimples disposed within the overhanging lip. Each dimple includes a radially depressed portion which has a larger circumferential width at its upstream extremity than its downstream extremity. The depression is disposed toward the associated cooler liner segment and is provided with a geometry which enhances stress relief of the lip as well as cooling fluid attachment as a protective film barrier upon the heated side of the liner downstream from the lip.

Description

BACKGROUND OF THE INVENTION

This invention relates to combustion chambers and, more particularly, to means for effecitively cooling combustion chambers. The present invention will be discussed in the environment of combustion chambers for use in gas turbine engine; however, the concepts of this invention are broadly applicable to any situation wherein similar combustion chambers are utilized.

The invention herein described was made in the course of or under a contract, or a subcontract thereunder, with the United States Department of the Air Force.

Development of high temperature operating cycles within gas turbine engines has put increased emphasis upon the development of combustion chambers capable of withstanding extremely high temperatures. Improvements in liner alloys and other combustion chamber materials has aided in this quest. To further enhance combustion chamber dependability, efficient and reliable means for cooling combustion chambers are necessary.

To date, the most efficient combustion chamber cooling techniques have involved, in part, the formation of a protective film boundary of cool air or other cooling fluid between the hot gases of combustion and the liner portions forming and defining the combustion zone. Typically, a combustion chamber liner defining a combustion zone also partially defines a cool fluid plenum usually circumscribing the combustion zone. Means are commonly provided for transferring a portion of the cool fluid from the plenum into the combustion zone to form the protective film barrier from the hot combustion products in the chamber.

In order to accomplish effective film propagation, means must be provided for directing the fluid in a film upon the liner's inner or heated surface. This means must effectively perform "attachment" (that is, the disposition of the fluid in a low-turbulence boundary layer immediately adjacent the liner to be protected) without aspirating or entraining hot gases from the combustion zone. Such entrainment would negate the effectiveness of the film cooling by creating a turbulent interchange whereby hot gases of combustion would directly impinge on the liner.

Effective attachment requires substantially uniform and low cool fluid velocity from the source of the fluid circumferentially about the liner in order to avoid turbulence as well as localized hot streaks or hot spots which arise in the absence of adequate film protection. At the same time, the means for transferring the cool fluid from the surrounding plenum to the liner has, in the past, commonly taken the form of a plurality of spaced apertures circumferentially disposed about the liner. (These apertures are used since large mechanical and thermally induced stresses upon the liner require substantial mechanical strength and, thus, prevent a more homogeneous transfer system such as a continuous circumferential slot or the like.)

As a result, it has become common to utilize a relatively long, axially extending, overhanging lip to direct the fluid upon the heated side of the liner. The lip defines an annular space extending circumferentially of the liner within which space the fluid has an opportunity to coalesce in velocity by the mechanism of frictional engagement of the fluid with the lip and associated liner segment. The coalescing changes the flow from a plurality of individual jets, corresponding to the individual apertures, into a flow having substantially uniform circumferential velocity.

While the extended overhanging lip has performed well with respect to avoiding aspiration and providing attachment, there has been an unfortunate structural repercussion in this design in that, under the thermal stresses associated with combustion, the lip has become warped and structurally incapable of continuing its proper function. The reason for this undesirable situation is that thermal gradients imposed upon the liner skin can operate upon residual material stresses to cause localized closure of the cooling slot (that is, a bending and buckling of the overhanging lip into engagement with the associated liner to block flow near the point of engagement) with a resultant interruption of cooling flow. This can cuase heat damage to the unprotected liner portions and resultant accelerated detrimental effects on combustor life.

Current practice to prevent such slot closure is to stabilize the overhanging lip by the addition of closely spaced, conical dimples. The dimple provides mechanical support to prevent lip closure as well as enhancing the flexibility of the lip. The flexible dimples further provide a means for absorbing thermal stresses without buckling. Such a configuration provides a significant life increase over the non-dimpled lip. However, long term usage is hampered by stress levels in the conical dimple which may result in fatigue cracking since the shape of the dimple concentrates thermal stress at the apex of the cone. In addition, this form of dimple creates an aerodynamic disturbance which reduces the film cooling effectiveness downstream of the dimple.

The present invention provides an improvement which results in significant reductions in both the local stress level and aerodynamic disturbance which, in turn, enhance life and cooling effectiveness. These advantages are accomplished without compromise to the desirable lip support and flexibility characteristics.

BRIEF SUMMARY OF THE INVENTION

It is, therefore, a principal object of the present invention to provide a combustor liner film cooling slot having a reinforced overhanding lip with improved life and film cooling characteristics.

In order to accomplish this and other objects, which will become apparent from the detailed description hereinafter, the present invention, in one embodiment thereof, provides a combustion chamber liner which comprises a number of telescoping, partially overlapping segments. A cooling fluid plenum substantially circumscribes the liner, and means are provided for transferring cooling fluid from the plenum to an annular space defined between adjacent overlapping liner segments. The radially inner of the overlapping segments forms an overhanging lip extending substantially axially downstream of the space which facilitates attachment of the fluid to the liner in a protective film barrier. This lip is provided with a plurality of circumferentially spaced stabilizing dimples, each dimple including a radially depressed portion which has an upstream, substantially linear extremity (or junction with the lip surface) and a downstream arcuate extremity, which occurs at the downstream edge of the lip. Each dimple has a larger width in the circumferential direction at its upstream extremity than at its downstream extremity. Furthermore, each dimple departs from the lip surface to a first depth near its upstream extremity and to a second depth near its downstream extremity, the second depth being greater than the first. Each dimple also includes lateral extremities at the junction of the depressed portion with the lip surface, the lateral extremities of any one dimple converging in the downstream direction. As a result, the adjacent of the lateral extremities of adjacent dimples diverge in the downstream direction. Finally, the depression extends in the direction of the associated liner segment.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be made clearer by reference to the following description taken together with the appended drawings wherein:

FIG. 1 is a simplified cross-sectional view of a combustion chamber of a gas turbine engine employing cooling slots;

FIG. 2 depicts a cross-sectional view of a reinforcing dimple according to the prior art;

FIG. 3 illustrates a pictorial view of the prior art dimple of FIG. 2 taken along line 3--3;

FIG. 4 shows a cross-sectional view of a cooling slot according to the present invention;

FIG. 5 is a pictorial view taken along line 4--4 of FIG. 3 and showing details of the reinforcing dimple of the present invention;

FIG. 6 is a graphical representation of stress levels in prior art and dimples according to the present invention; and

FIG. 7 is a graphical representation of comparative aerodynamic efficiencies.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

FIG. 1 depicts a combustion chamber, designated generally 30, and illustrating the relationship of the present invention to substantially typical combustion chambers of the gas turbine engine variety. An outer liner 32 combines with axially segmented liner 34 to define an outer plenum 36. An inner liner 38 combines with an inner portion of the segmented combustor liner 40 for the purpose of defining a radially inner cooling fluid plenum 42. The combustion zone itself is designated 44 and is defined by liners 34 and 40 as well as by an upstream dome 46 which cooperates with a fuel nozzle 48 through fuel for combustion is directed into the combustion zone. An air/fuel inlet 50 is defined between axial extensions 52 and 54 of liners 34 and 40, respectively.

In general operation, the combustion chamber described is substantially similar to those in present use. A flow of atmospheric air is pressurized by means of a compressor (not shown) upstream of the combustion zone 44 with the compressor discharge directed partially into plena 52 and 54 as well as into the fuel/air inlet 50. A quantity of fuel is mixed with the portion of air entering fuel/air inlet 50 and is ignited within combustion zone 44. The rapid expansion of the burning gases and the configuration of liners 34 and 40 results in the gases being forced from combustion zone 44 through an outlet 56 and into engagement with a turbine 58. Rotary portions of the turbine are driven by this exiting fluid and a portion of the energy thereof serves to drive the upstream compressor through an interconnecting shaft. The remaining energy of the gas stream provides energy for driving thrust toward the left in FIG. 1.

The cooling of liners 34 and 40 is the subject of the present invention, and will be described with respect to the remaining figures. FIG. 2 illustrates a cross-sectional view of a cooling slot according to the prior art. FIG. 3 is a view of this same slot taken along line 3--3 of FIG. 2 and further particularizes the cooling slot of the prior art. Referring back to FIG. 1, liner 34' may be seen to be divided substantially into a number of axially adjacent segments. A typical segment 60' can be seen to be in telescopic cooperation with a typical segment 62' downstream thereof by means of a junction designated generally by the numeral 64'. At this junction is disposed a cooling slot configuration which, in substance, comprises a cooling film promoter for passing cool fluid from plenum 36' in a protective film barrier upon liner 34', the latter liner partially defining a hot gas passage (the combustion zone 44'). The segment 60' can be seen to partially define the plenum 36' as well as the combustion zone 44'. Segment 62' likewise partially defines the plenum and the hot gas passage, and these liner segments cooperate to form an annular space 66' extending circumferentially of the liner between them. The space 66' includes a substantially closed upstream end 70' and a downstream-facing open end or exit 68'. Thus, the space is substantially isolated from the combustion zone except for communication through exit 68'.

As can be seen in FIG. 1, the annular space 66' is disposed between a portion of downstream segment 62' and an xial extension of the upstream segment 60' of liner 34'. It can further be seen that the space 66' is connected to the surrounding plenum 36' by means of a plurality of circumferentially spaced apertures 74' through a portion of the liner. It is the function of these apertures to deliver cool fluid from the plenum to the space 66' and therefrom to the heated portions of combustion liner 34' for the formation of a protecive film barrier. A prerequisite to the formation of such a film barrier is that the cool fluid which is intended to form the barrier be free of large velocity gradients in the circumferential direction, which gradients could lead to aspiration of hot gases thus adversely affecting the benefits of the barrier.

It would, therefore, be beneficial to the barrier formation to provide a more uniform means for delivering cool fluid to the space than the plurality of apertures 74'. However, the cool fluid quantity must be metered, and these apertures are a most effective means for accomplishing metering. In addition, the structural forces imposed upon the liner require that it be formed in a particularly strong fashion. A more uniform delivery system, such as a continuous annular slot between the plenum 36' and space 66' would substantially weaken the liner in comparison to apertures 74. Hence, the apertures are a preferred delivery mechanism.

But, yet, the apertures do serve to transfer the fluid to the space 66' in a plurality of spaced jets with substantial circumferential velocity gradients therebetween. There must, then, be provided means for allowing a coalescence of these velocities prior to the direction of the fluid onto the combustor liner for the purpose of forming a barrier. To this end, the overhanging lip 72' extends a substantial distance downstream of the junction 76' between liner segments 60' and 62'. Thus, the space 66' is given a substantial axial length. By reason of this length, the fluid passing from plenum 36' in a plurality of jets through apertures 74' is given a predetermined period of residence within space 66' as it passes downstream therein. During this time, the viscous forces within the fluid and frictional engagement thereof with the surfaces of the lip 72' and downstream liner segment 62' cause the fluid to develop a more uniform circumferential velocity. In order to achieve the desired amount of velocity equalization, the space 66' must be substantial in length and, hence, the overhanging lip 72' must likewise extend axially downstream for a substantial distance. In this way, the overhanging lip 72' serves to facilitate the attachment of the cool fluid to the liner in a protective film barrier.

But, at the same time, the axial length of the lip is such that residual stresses within the structure defining the lip combine with thermal stresses resulting from the heat of combustion within chamber 44' to cause structural deformation of the lip that can result in local closing of the exit 68', through which the cool fluid is normally exhausted from the space onto the liner. Should this occur, localized hot streaks would develop wherein cooling fluid film protection would fail. This, in turn, could result in local damage to or destruction of the combustion chamber liner.

In order to strengthen the overhanging lip 72' against such stresses, as well as relieve the residual stresses therein, there have been provided in the past a plurality of reinforcing dimples within the material of the lip itself. Such dimples, as stated, are illustrated in FIGS. 2 and 3. In these figures, the dimples are designated 80, and can be seen to be substantially conical in the cross section, originating in a point 82 at their upstream ends and terminating in a substantially circular cross section 84 at their downstream ends. The dimples are formed as depressions in the lip material itself, with the depression extending in the direction from the lip surface toward the associated downstream liner segment 62' (and, hence, away from the centerline of the combustion zone). The lateral extremities 86 and 88 of an individual dimple comprise the intersections between the conical cross section of the depression and the plane of lip 72. These lateral extremities diverge in the downsream direction with respect to any individual dimple; and, contemporaneously, the adjacent lateral extremities of adjacent dimples converge in the downstream direction. These prior art dimples have been found to improve operational reliability of the overhanging lips associated therewith in that the lip is strengthened against deformation and local exit closure. However, these dimples provide stress concentrations and result in fatigue cracking when subjected to long-term use. In addition, the dimple configurations create an aerodynamic disturbance which reduces film cooling effectiveness downstream of the dimple.

The present invention comprehends the usefulness of dimples as reinforcing means for overhanging lips, but overcomes the principal objections thereto in the prior art. FIGS. 4 and 5 depict reinforcing dimples according to the present invention and their cooperation with the overhanging lip and associated cooling slot structure. As may be seen from FIG. 5, each dimple, designated 90, includes a substantially radially depressed portion 92, this portion having a substantially larger circumferential width 94 proximate its upstream extremity than circumferential width 96 proximate its downstream extremity. In addition, it can be seen that the dimple extends to and terminates in the downstream edge 72a of lip 72.

Referring to FIG. 4, it is therein illustrated that the depressed portion 92 of the dimple extends radially from the surface of lip 72 in the direction outwardly of the center of the combustion zone and toward the associated downstream liner segment 62. The depressed portion departs from the surface of the lip to a first depth near its upstream extremity and to a second and greater depth near its downstream extremity. In other words, the depressed portion of the dimple converges toward the downstream liner segment 62 in the downstream direction.

In the particular embodiment depicted in FIGS. 4 and 5, the upstream extremity 98 of each dimple is substantially linear in the circumferential direction with respect to the lip, while the downstream extremity is substantially arcuate describing an arc 100 in the cross section. Furthermore, the dimple of this embodiment includes lateral extremities 102 and 104 which are substantially linear and which converge in the downstream direction corresponding to the narrowing of the depressed portion 92 from greater width upstream at 94 to smaller downstream width 96. As a result of this latter characteristic, the lateral extremities 102 and 104 of individual dimples converge in the downstream direction while the adjacent extremities of adjacent dimples diverge in the downstream direction.

It has been found experimentally that the reinforcing dimple of the present invention thus described serves to increase the life of the overhanging lip and, hence, the reliability of the associated combustor liner over extended periods of application. One facet of this improvement rests in a substantial increase in overall fatigue strength due to reduction in the localized stresses in the lip structure proximate the dimple. This improvement is dramatically exemplified in FIG. 6 which displays a graphical representation of stress versus position over various stations through dimples according to the prior art and present invention. It can be appreciated from this Figure that local stresses are generally maintained at levels approximately one-half of the magnitude of the prior art dimple by means of utilization of the present invention. Experiments have illustrated that this stress reduction results in a life gain of on a ratio of 15 to 1 over the prior art cooling slot.

An additional benefit of the present invention relates to the efficiency with which the cooling fluid operates to remove heat from the combustion chamber liner. This can be expressed in terms of cooling film effectiveness, .eta., which is depicted in graphical form in FIG. 7 and plotted versus axial distance downstream of the dimple along the liner surface. The figure illustrates a comparison in efficiency between the prior art dimple and the dimple according to the present invention, both directly downstream of individual dimples and in the areas between dimples downstream thereof. As the figure discloses, a marked improvement in film barrier effectiveness is achieved by utilization of the dimples according to the present invention. This improvement can be traced to aerodynamic improvements of the present dimple configuration which cause the film flow streamlines to converge and, thus, minimize disturbance or wake effects downstream of the dimples.

It has been illustrated that the present invention results not only in significant life improvements but also in cooling improvements and the efficiency thereof with respect to gas turbine engines to which the present invention is applied. Numerous variations of the particular embodiment disclosed can be made without departing from the spirit of the invention. For example, neither the lateral nor the upstream extremities of the dimples need be linear so long as the interrelationship therebetween is maintained. Similarly, the downstream extremity of the dimples need not necessarily be arcuate. Other variations will occur to those skilled in the art which also do not depart from the spirit of the invention. It is intended that all such variations be within the scope of the appended claims.

Claims

What is considered to be new and desired to be secured by Letters Patent of the United States is:

1. In a combustor liner film cooling slot of the variety including an annular space extending circumferentially of the liner and defined between overlapping portions of telescoping liner segments, a cooling fluid plenum substantially circumscribing the liner, means for transferring cooling fluid from the plenum to the space, means for exhausting fluid from the space onto the liner, and an overhanging lip extending substantially axially downstream of the space for facilitating attachment of the fluid to the liner in a protective film barrier, the improvement comprising:

a stabilizing dimple disposed within said lip, the dimple including a substantially radially depressed portion, the depressed portion having a first circumferential width proximate its upstream extremity and a second circumferential width proximate its downstream extremity, the first width being greater than the second.

2. The improvement of claim 1 wherein said lip terminates in a downstream edge, and said dimple extends to said edge.

3. The improvement of claim 1 wherein said depressed portion departs from the surface of said lip to a first depth proximate its upstream extremity and to a second depth proximate its downstream extremity, said second depth being greater than the first.

4. The improvement of claim 1 wherein said upstream extremity of said depressed portion is substantially linear and said downstream extremity thereof is substantially arcuate.

5. The improvement of claim 1 wherein said depresssed portion further includes lateral xtremities, and said lateral extremities substantially converge in the downstream direction.

6. The improvement of claim 5 further including a plurality of said dimples so characterized, said dimples spaced circumferentially about said lip.

7. The improvement of claim 6 wherein adjacent of the lateral extremities of adjacent dimples substantially diverge in the downstream direction.

8. The improvement of claim 7 wherein said depressed portion of each of said dimples departs from the surface of said lip to a first depth proximate its upstream extremity and to a second depth proximate its downstream extremity, said second depth being greater than said first depth.

9. The improvement of claim 8 wherein said depressed portion extends radially toward said liner.