In gas turbine engines, air is compressed at an initial stage, then is heated in combustion chambers, and the hot gas so produced drives a turbine that does work, including rotating the air compressor.
To achieve a good overall efficiency in a gas turbine engine, one consideration is the reduction of losses of fluid pressure, such as due to friction and turbulence, between the compressor and the intakes of the combustion chambers. In a common gas turbine engine design, compressed air flows from the compressor, through an annular diffuser, into a plenum in which are positioned transitions and other components, and then from the plenum into the intakes of combustion chambers.
Generally, a diffuser converts a high velocity, low pressure fluid flow into a low velocity, higher pressure fluid flow. That is, a diffuser's efficiency is measured in terms of conversion of dynamic head (i.e., velocity) to static pressure. In a gas turbine engine, the annular diffusers that convey compressed air from the compressor into the plenum typically comprise an annular diverging passage. This diverging from intake end to aft end acts to decelerate the fluid flow from the compressor, and to raise the static pressure by converting its kinetic energy into pressure energy. Among other effects, this approach provides for the fluid to enter the combustion chambers at a velocity providing for sustained combustion.
However, when fluid enters the plenum from the diffuser during operation, it may have dual roles—to pass to the combustion chamber with minimal pressure loss, and to provide cooling to the transitions disposed in the plenum. Design optimization for conversion of dynamic pressure to static pressure may not provide for overall optimization when other functional objectives, such as efficient cooling of the transitions, are taken into account. That is, optimizing the annular diffuser to provide the most efficient conversion without considering alternatives that would more efficiently cool the transitions may not provide the most efficient overall gas turbine engine.
One general approach to improve fluid flow efficiency in the plenum, and thereby improve overall efficiency, is to modify the end of the diffuser so as to redirect fluid flow toward a more radially outward direction. For example, a curved diffuser may be employed wherein the aft end has a bend that directs the fluid flow radially outward, instead of axially aft. Conceptually this may provide 1) a more direct, flow-efficient route to the combustion chamber intakes, and 2) less turbulence/frictional losses in the parts of the plenum where the mid-sections and aft ends of the transitions are located. Another, different approach, is the use of radially offset splitters within the diffuser, such as described in U.S. Pat. No. 5,737,915, U.S. Pat. No. 5,335,501, or in U.S. Pat. No. 5,134,855, or a diffuser with radially extending struts (dividers) and/or annular flow separators, as disclosed in U.S. Pat. No. 6,554,569 (see FIG. 8 and accompanying text).
However, radial diversion of a substantial portion of compressed fluid from an annular diffuser, without more, may not effectively provide a desired resolution when the fluid flow from the compressor is desired to be used to cool the transitions. Particularly, when there is a relatively long expanse of transition disposed aft of a radially split annular diffuser, given the tendency of fluid flow from such diffuser to deviate radially and forward, toward the combustion chambers' intakes, such an approach does not provide a balanced cooling fluid flow across the length of such transition. This may necessitate implementation of transition cooling approaches that are relatively costly on a capital and/or an operational basis.
Further as to such approaches, generally it is known that cooling transitions with fluid flow from the compressor may be implemented by direct convection cooling (i.e., directing the fluid flow across the outside surface of the transitions, see for one example U.S. Pat. No. 4,903,477), by open fluid cooling (in which a portion of the compressed fluid passes through channels in the transition and then enters the flow of combusted gases within the transition, see for one example U.S. Pat. No. 3,652,181), by channel cooling (i.e., conveying fluid from outside the transition, through channels in the transition walls, and into the transition), by impingement cooling (where fluid is directed at the transition exterior walls through apertures positioned on plates or other structures close to these walls, see U.S. Pat. No. 4,719,748 for one example), and by combinations of these approaches.