Method and apparatus for cooling a wall within a gas turbine engine

According to the present invention, a method and an apparatus for cooling a wall within a gas turbine engine is provided. The apparatus includes and the method provides for a cooling circuit disposed within a wall having utility within a gas turbine engine. The cooling circuit includes a forward end, an aft end. The pedestals extend between first and second portions of the wall. The characteristics and array of the pedestals within the cooling circuit are chosen to provide a heat transfer cooling profile within the cooling circuit that substantially offsets the profile of the thermal load applied to the wall portion containing the cooling circuit. At least one inlet aperture provides a cooling airflow path into the forward portion of the cooling circuit from the cavity. A plurality of exit apertures provide a cooling airflow path out of the aft portion of the cooling circuit and into the core gas path outside the wall.

BACKGROUND OF THE INVENTION
 1. Technical Field
 This invention relates to gas turbine engines in general, and to cooling
 passages disposed within a wall inside of a gas turbine engine in
 particular.
 2. Background Information
 A typical gas turbine engine includes a fan, compressor, combustor, and
 turbine disposed along a common longitudinal axis. The fan and compressor
 sections work the air drawn into the engine, increasing the pressure and
 temperature of the air. Fuel is added to the worked air and the mixture is
 burned within the combustor. The combustion products and any unburned air,
 hereinafter collectively referred to as core gas, subsequently powers the
 turbine and exits the engine producing thrust. The turbine comprises a
 plurality of stages each having a rotor assembly and a stationary vane
 assembly. The core gas passing through the turbine causes the turbine
 rotors to rotate, thereby enabling the rotors to do work elsewhere in the
 engine. The stationary vane assemblies located forward and/or aft of the
 rotor assemblies guide the core gas flow entering and/or exiting the rotor
 assemblies. Liners, which include blade outer air seals, maintain the core
 gas within the core gas path that extends through the engine.
 The extremely high temperature of the core gas flow passing through the
 combustor, turbine, and nozzle necessitates cooling in those sections.
 Combustor and turbine components are cooled by air bled off a compressor
 stage at a temperature lower and a pressure greater than that of the core
 gas. The nozzle (and augmentor in some applications) is sometimes cooled
 using air bled off of the fan rather than off of a compressor stage. There
 is a trade-off using compressor (or fan) worked air for cooling purposes.
 On the one hand, the lower temperature of the bled compressor air provides
 beneficial cooling that increases the durability of the engine. On the
 other hand, air bled off of the compressor does not do as much work as it
 might otherwise within the core gas path and consequently decreases the
 efficiency of the engine. This is particularly true when excessive bled
 air is used for cooling purposes because of inefficient cooling.
 One cause of inefficient cooling can be found in cooling air that exits the
 wall with unspent cooling potential. A person of skill in the art will
 recognize that cooling air past through a conventional cooling aperture
 typically contains cooling potential that is subsequently wasted within
 the core gas flow. The present invention provides convective cooling means
 that can be tailored to remove an increased amount of cooling potential
 from the cooling air prior to its exit thereby favorably affecting the
 cooling effectiveness of the wall.
 Another cause of inefficient cooling can be found in poor film
 characteristics in those applications utilizing a cooling air film to cool
 a wall. In many cases, it is desirable to establish film cooling along a
 wall surface. A film of cooling air traveling along the surface of the
 wall increases the uniformity of the cooling and insulates the wall from
 the passing hot core gas. A person of skill in the art will recognize,
 however, that film cooling is difficult to establish and maintain in the
 turbulent environment of a gas turbine. In most cases, air for film
 cooling is bled out of cooling apertures extending through the wall. The
 term "bled" reflects the small difference in pressure motivating the
 cooling air out of the internal cavity of the airfoil. One of the problems
 associated with using apertures to establish a cooling air film is the
 film's sensitivity to pressure difference across the apertures. Too great
 a pressure difference across an aperture will cause the air to jet out
 into the passing core gas rather than aid in the formation of a film of
 cooling air. Too small a pressure difference will result in negligible
 cooling airflow through the aperture, or worse, an in-flow of hot core
 gas. Both cases adversely affect film cooling effectiveness. Another
 problem associated with using apertures to establish film cooling is that
 cooling air is dispensed from discrete points, rather than along a
 continuous line. The gaps between the apertures and areas immediately
 downstream of those gaps are exposed to less cooling air than are the
 apertures and the spaces immediately downstream of the apertures, and are
 therefore more susceptible to thermal degradation.
 Hence, what is needed is an apparatus and a method for cooling a wall that
 can be tailored to provide a heat transfer profile that matches a thermal
 load profile, one that effectively removes cooling potential from cooling
 air, and one that facilitates film cooling.
 DISCLOSURE OF THE INVENTION
 It is, therefore, an object of the present invention to provide an
 apparatus and method for cooling a wall having a selectively adjustable
 heat transfer profile that can be adjusted to substantially match a
 thermal load profile.
 According to the present invention, a cooling circuit is disposed within a
 wall inside a gas turbine engine. The cooling circuit includes a forward
 end, an aft end, a first wall portion, a second wall portion, and a
 plurality of pedestals. The first and second wall portions extend
 lengthwise between the forward and aft ends of the cooling circuit, and
 are separated a distance from one another. The pedestals extend between
 the first and second wall portions. The characteristics and array of the
 pedestals within the cooling circuit are chosen to provide a heat transfer
 cooling profile within the cooling circuit that substantially offsets the
 profile of the thermal load applied to the wall portion containing the
 cooling circuit. At least one inlet aperture extends through the first
 wall portion to provide a cooling airflow path into the forward portion of
 the cooling circuit from the cavity. A plurality of exit apertures extend
 through the second wall portion to provide a cooling airflow path out of
 the aft portion of the cooling circuit and into the core gas path outside
 the wall.
 The present cooling circuits are designed to accommodate non-uniform
 thermal profiles. The temperature of cooling air traveling through a
 passage, for example, increases exponentially as a function of the
 distance traveled within the passage. The exit of a cooling aperture is
 consequently exposed to higher temperature, and therefore less effective,
 cooling air than is the inlet. In addition, the wall portion containing
 the passage is often externally cooled by a film of cooling air. The film
 of cooling air increases in temperature and degrades as it travels aft,
 both of which result in a decrease in cooling and consequent higher wall
 temperature traveling in the aft direction. To ensure adequate cooling
 across such a non-uniform thermal profile (typically present in a
 conventional cooling passage) it is necessary to base the cooling scheme
 on the cooling requirements of the wall where the thermal load is the
 greatest, which is typically just upstream of the exit of the cooling
 passage. As a result, the wall adjacent the inlet of the cooling passage
 (i.e., where the cooling air within the passage and the film cooling along
 the outer surface of the wall are the most effective) is often overcooled.
 The present invention cooling circuit advantageously avoids undesirable
 overcooling by providing a method and an apparatus capable of creating a
 heat transfer cooling profile that substantially offsets the profile of
 the thermal load applied to the wall portion along the length of the
 cooling circuit.
 Another advantage of the present cooling circuits is a decrease in thermal
 stress within the component wall. Thermal stress often results from
 temperature gradients within the wall; the steeper the gradient, the more
 likely it will induce undesirable stress within the wall. The ability of
 the present cooling circuit to produce a heat transfer profile that
 substantially offsets the local thermal load profile of the wall decreases
 the possibility that thermal stress will grow within the wall.
 Another advantage of the present cooling circuit is that it decreases the
 possibility of hot core gas inflow. Each cooling circuit is an independent
 compartment designed to internally provide a plurality of incremental
 pressure drops between the inlet aperture(s) and the exit apertures. The
 pressure drops allow for a low pressure drop across the inlet aperture and
 that increases the likelihood that there will always be a positive flow of
 cooling air into the cooling circuit. The positive flow of cooling air
 through the circuit, in turn, decreases the chance that hot core gas will
 undesirably flow into the cooling circuit.
 These and other objects, features and advantages of the present invention
 will become apparent in light of the detailed description of the best mode
 embodiment thereof, as illustrated in the accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION
 Referring to FIGS. 1 and 2, a gas turbine engine 10 includes a fan 12, a
 compressor 14, a combustor 16, a turbine 18, and a nozzle 20. Within and
 aft of the combustor 16, most components exposed to core gas are cooled
 because of the extreme temperature of the core gas. The initial rotor
 stages 22 and stator vane stages 24 within the turbine 18, for example,
 are cooled using cooling air bled off a compressor stage 14 at a pressure
 higher and temperature lower than the core gas passing through the turbine
 18. The cooling air is passed through one or more cooling circuits 26
 (FIG. 2) disposed within a wall to transfer thermal energy from the wall
 to the cooling air. Each cooling circuit 26 can be disposed in any wall
 that requires cooling, and in most cases the wall is exposed to core gas
 flow on one side and cooling air on the other side. For purposes of giving
 a detailed example, the present cooling circuit 26 will be described
 herein as being disposed within a wall 28 of an airfoil 29 portion of a
 stator vane or a rotor blade. The present invention cooling circuit 26 is
 not limited to those applications, however, and can be used in other walls
 (e.g., liners, blade seals, etc.) exposed to high temperature gas.
 Referring to FIGS. 2-5 and 5A, each cooling circuit 26 includes a forward
 end 30, an aft end 32, a first wall portion 34, a second wall portion 36,
 a first side 38, a second side 40, a plurality of first pedestals 42, and
 a plurality of alternately disposed T-shaped second pedestals 43 and third
 pedestals 45. The third pedestals are shaped to nest between adjacent
 T-shaped second pedestals 43. The first wall portion 34 has a cooling-air
 side surface 44 and a circuit-side surface 46. The second wall portion 36
 has a core-gas side surface 48 and a circuit-side surface 50. The first
 wall portion 34 and the second wall portion 36 extend lengthwise 52
 between the forward end 30 and the aft end 32 of the cooling circuit 26,
 and widthwise 54 between the first side 38 and second side 40. The
 plurality of first pedestals 42 extend between the circuit-side surfaces
 46,50 of the wall portions 34,36. At least one inlet aperture 56 extends
 through the first wall portion 34, providing a cooling airflow path into
 the forward end 30 of the cooling circuit 26 from the cavity 58 of the
 airfoil 29. A plurality of exit apertures 60 extend through the second
 wall portion 36 to provide a cooling airflow path out of the aft end 32 of
 the cooling circuit 26 and into the core gas path outside the wall 28. The
 exit apertures 60 are formed between the T-shaped second pedestals 43 and
 nested third pedestals 45, the first wall portion 34, and the second wall
 portion 36.
 The size, number, and position of the first pedestals 42 within the cooling
 circuit 26 are chosen to provide a heat transfer cooling profile within
 the cooling circuit 26 that substantially offsets the profile of the
 thermal load applied to the portion of the wall containing the cooling
 circuit 26; i.e., the cooling circuit may be selectively "tuned" to offset
 the thermal load. For example, if a portion of wall is subjected to a
 thermal load that increases in the direction extending forward to aft (as
 is described above), the size and distribution of the first pedestals 42
 within the present cooling circuit 26 are chosen to progressively increase
 the heat transfer rate within the cooling circuit 26, thereby providing
 greater heat transfer where it is needed to offset the thermal load.
 Decreasing the circuit cross-sectional area at a lengthwise position (or
 successive positions if the thermal load progressively increases), is one
 way to progressively increase the heat transfer within the cooling circuit
 26. For clarity sake, the "circuit cross-sectional area" shall be defined
 as the area within a plane extending across the width 54 of the circuit
 through which cooling air may pass. The decrease in the circuit
 cross-sectional area will cause the cooling air to increase in velocity
 and the increased velocity will positively affect convective cooling in
 that region. Hence, the increase in heat transfer rate. If, for example,
 all of the first pedestals 42 have the same cross-sectional geometry,
 increasing the number of first pedestals 42 at a particular lengthwise
 position within the circuit 26 will decrease the circuit cross-sectional
 area. The circuit cross-sectional area can also be decreased by increasing
 the width or changing the geometry of the first pedestals 42 to decrease
 the distance between adjacent first pedestals 42. The heat transfer rate
 can also adjusted by utilizing impingement cooling or tortuous paths that
 promote convective cooling. FIG. 5 shows a distribution of first pedestals
 42 that includes first pedestals 42 disposed downstream of and aligned
 with gaps 62 between upstream first pedestals 42. Cooling air traveling
 through the upstream gaps 62 is directed toward the downstream pedestals
 61 elongated in a widthwise direction. The positioning of the second
 pedestals 43 encourages impingement cooling.
 The amount by which the convective cooling is increased at any particular
 lengthwise position within the cooling circuit 26 depends upon the thermal
 load for that position, for that particular application. It is also useful
 to size the inlet aperture 56 of the cooling circuit 26 to produce a
 minimal pressure difference across the aperture 56, thereby preserving
 cooling potential for downstream use within the cooling circuit 26. A
 cooling circuit heat transfer profile that closely offsets the wall's
 thermal local thermal load profile will increase the uniformity of the
 temperature profile across the length of the cooling circuit, ideally
 creating a constant temperature within the wall portion 36.
 Although this invention has been shown and described with respect to the
 detailed embodiments thereof, it will be understood by those skilled in
 the art that various changes in form and detail thereof may be made
 without departing from the spirit and scope of the claimed invention.