This invention relates to combustion apparatus and, more specifically, to means for effectively cooling combustion chambers. For convenience of illustration and discussion, the invention will be described in connection with a jet engine of the gas turbine type. However, it will be appreciated that the structure is suitable for any high temperature application which requires effective film cooling of combustion apparatus.
Aircraft engines presently in operational use and those under development for future applications are designed to operate at extremely high temperatures. In order to prolong the life of combustors associated with such engines, new alloys have been developed which are highly compatible with the high temperature environment. However, it has also been found that by cooling the combustor under operating conditions the thermal fatigue life characteristic of the combustor are enhanced.
Generally, it is accepted practice in the art to cool combustion chambers by providing a moving film of cooling air between the inner surface of the liner and the hot gases of combustion. The film of cooling air forms a protective barrier between the liner and the hot gases and also provides for convective cooling of the liner.
Generally, in prior art devices the protective film is introduced into the combustion chamber from a plenum of cooling air surrounding the exterior of the combustor. This has been accomplished by providing for the introduction of cooling air through a series of apertures in an upstream portion of the liner into an annular lipped pocket. The streams of cooling fluid entering through the apertures are permitted to mix and coalesce within the pocket to form a uniform annular boundary layer of cooling air which is directed by the lip along the inner surface of the combustor liner.
It is well known that the lip associated with the aforementioned cooling arrangement is subject to thermal stresses which cause warpage and buckling of the lip under operating conditions. One of the approaches utilized in the past to overcome warpage and buckling of the lip has been to include in the downstream portion of the lip a series of circumferentially spaced dimples which provide localized stiffening to resist the buckling tendency induced by the thermal stresses. While the inclusion of dimples in this manner served well to overcome lip distortion, the dimples were found to create wakes in the film of cooling air discharged along the inner surface of the liner. The wakes were found to destroy the uniformity of the cooling air barrier and permit hot gas of combustion to directly contact the inner liner of the combustor thereby reducing its operating life.
Some attempts have been made to eliminate the wakes caused by dimples disposed in combustor lips. United States Patent 3,826,082 discloses an arrangement wherein the lateral walls of the dimple converge in the downstream direction. Unlike previous dimples which diverged in the downstream direction, the arrangement taught in the referenced patent sought to direct cooling air into the area immediately downstream of the dimple hence filling the area with cooling air rather than hot gases of combustion. Incorporation of dimples with converging lateral walls into combustors has proved to be at least partially successful in reducing the deleterious effects on the liner which result from dimple wakes. However, while converging lateral walls, as taught by the above-referenced patent, serve to form an exit slot characterized by increasing width in the aft direction, such walls do not insure that the cross-sectional flow area of the exit slot is also increasing. Rather, since the height of the dimples increases in the aft direction, the cross-sectional area flow area of the exit slot decreases in the same direction. Consequently, cooling air flowing through the exit slot of decreasing flow area must accelerate and converge as it flows through successive downstream cross sections of the exit slot. As the cooling air exits the slot in the aforedescribed converging manner, wakes are formed in the boundary layer of cooling film. Said another way, wakes are formed in the boundary layer of cooling film as a direct result of the flow of cooling air out of the exit slot in an accelerated convergent manner caused by the decreasing cross-sectional flow area of the exit slot. Such convergent flow will be present even though the pair of lateral walls associated with each dimple converge toward each other and form an exit slot with increasing width in the aft direction. The present invention is directed toward providing dimple construction in a lip associated with a cooling slot disposed in a combustor liner wherein each dimple is characterized by a constant height portion so as to provide, in cooperation with convergent lateral walls, an exit slot of increasing cross-sectional flow area in the aft direction. An exit slot having an increasing cross-sectional flow area will cause cooling air to exit the slot in a divergent manner so as to flow into and fill wake areas immediately aft of each dimple.