BACKGROUND OF THE INVENTION Conventional combustors in combustion turbine systems use compressed air to cool components subject to high temperatures. For example, a conventional combustor that uses compressed air for cooling is disclosed in U.S. Patent number 5,394,688 to Parker, et al. (1995), which is incorporated herein by reference in its entirety. As shown in Figure 3, the conventional Parker combustor has two adjustable, counter- rotating primary mixing passages 90 and 92, a dual fuel nozzle 18, a secondary mixing passage 68, an outer liner 40, an intermediate liner 42, and a primary liner 44.

The primary liner 44 encloses a primary combustion zone 36, in which a lean mixture of fuel and air-is burned.

The primary liner 44 is encircled by a cylindrical intermediate liner 42, which is encircled by a cylindrical outer liner 40. The liners 40, 42, and 44 are concentrically arranged so as to form an annular cooling air passage 70 between the primary liner 44 and the intermediate liner 42, and the annular secondary mixing passage 68 between the intermediate liner 42 and the outer liner 40.

The primary liner 44 typically is cooled because it is directly exposed to hot combustion gas in the primary combustion zone 36. To cool the primary liner 44, a cooling

portion 66 of a compressed air stream 8 from a compressor 2 flows through the cooling air passage 70. To enhance cooling of the liner 44 in the conventional combustor, an approximately cylindrical baffle 80 is located at the outlet of the passage 70 and extends between the inner liner 44 and the middle liner 42, as shown in Figure 4. The baffle 80 is attached at its downstream end 108 to a downstream end of the middle liner 42 via welds 104. The front end 106 of the baffle 80 is spring loaded to bear against an outer surface of the primary liner 44. The spring section is formed by a series of saw cuts at the front end 106, which, unfortunately, causes leakage of air that is not useful for cooling.

Although Figures 3 and 4 show the front end 106 of the baffle 80 extending upstream only about one-third the length of the primary liner 44, the Parker patent teaches that the baffle 80 may encircle the entire large diameter portion of the primary liner 44. As further shown in Figure 4, a number of holes 100 are distributed around the circumference of the baffle 80 so as to divide the cooling air 66 into a number of jets 102 that impinge on the outer surface of the primary liner 44.

Conventionally, cooling air 66 is supplied by the compressor 2. Diverting compressed air from the compressor 2 for cooling the combustor 4 has two drawbacks. First, using such air diminishes turbine efficiency because the cooling air is not available to support combustion. Second, using such air increases flame temperature that causes increased formation of oxides of nitrogen, which are atmospheric pollutants.

Therefore, there exists a need for a combustor that minimizes the amount of cooling air needed to cool the combustor. Specifically, because the primary liner 44 is in direct contact with hot gas in the primary combustion zone 36, there exists a need to minimize the quantity of air required to cool the primary liner 44.

SUMMARY OF THE INVENTION A combustor that minimizes the amount of compressed air needed for cooling is provided. The combustor has two concentric primary mixing passages, an outer secondary mixing passage, a fuel nozzle, and an intermediate liner that centrally encloses a primary liner so as to form an annular cooling air passage between the liners. A cooling air stream that flows through the cooling air passage removes heat from the primary liner. The primary liner has numerous effusion cooling holes through which a portion of the cooling air stream passes. The primary liner also has cooling fins disposed on its outer surface, over which another portion of the cooling air stream flows. A baffle assembly, located within the cooling air passage, directs yet another portion of the cooling air stream to impinge against the primary liner outer surface. The baffle assembly comprises a conical member and a cylindrical platefin member.

The present invention also encompasses a method for cooling the primary liner of a combustor. The method comprises the steps of passing a portion of a cooling air through the liner effusion holes, passing another portion of the cooling air across the cooling fins, and directing yet another portion of the cooling air to impinge against the primary liner.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic diagram of a gas turbine system having an exemplary combustor that employs the present invention.

Figure 2 is a longitudinal cross-section through the combustion section of the gas turbine system shown in Figure 1.

Figure 3 is a longitudinal cross-section through a conventional prior art combustor.

Figure 4 is a detailed view of a portion of the prior art combustor of Figure 3.

Figure 5 is a longitudinal cross section through the exemplary combustor shown in Figure 2 illustrating the present invention.

Figure 6 is a detailed view of the portion of the exemplary combustor of Figure 5 illustrating the present invention.

Figure 7 is a partial cross sectional view of the exemplary combustor illustrating the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, wherein like reference numerals designate corresponding structure throughout the views, there is shown in Figure 1 a schematic diagram of a gas turbine system 1 of the type having a combustor 4 employing the present invention. The gas turbine system 1 has a compressor 2 that is driven by a turbine 6 via a shaft 26.

Ambient air 12 is drawn into the compressor 2 and compressed.

Compressed air 8 produced by the compressor 2 is directed to a combustion system that includes one or more combustors 4, each of which include a fuel nozzle 18 that introduces into the combustor gaseous fuel 16, oil fuel 14, or both gaseous fuel 16 and oil fuel 14. Within the combustors 4, the fuel is burned in compressed air 8, thereby producing a hot compressed gas 20 for driving the turbine 6. Figure 2 shows the gas turbine system 1 combustion section 30 of the type employing the present invention. The hot gas 20 exiting from the combustion section 30 is directed by a duct 5 to the turbine section 6.

Now referring to Figure 5, the combustor 4 employing the present invention is shown. The combustor 4 includes a fuel nozzle 18, two concentric mixing passages 90 and 92, an outer liner 40, an intermediate liner 42, and a primary liner 44. An inner surface 130 of the primary liner 44 forms a primary combustion zone 36. The primary liner 44 includes three portions: a truncated conical portion 124, a cylindrical portion 126, and a tapered portion 128. The conical portion 124 yields to the-cylindrical portion 126, which ends in the tapered portion 128. The intermediate liner 42 is

substantially cylindrical and concentrically surrounds the primary liner 44. A cooling air passage 70 is formed in the space between the intermediate liner 42 and the primary liner 44.

In accordance with the present invention, the liner conical portion 124 has numerous small-diameter effusion holes 144 that enable an effusion portion 66' of the cooling air stream 66 to flow from the cooling air passage 70 into the primary combustion zone 36. An outer surface 132 of the cylindrical portion 126 has numerous cooling fins 135, which are axially oriented and angularly spaced equidistant apart.

The cooling fins 135 are formed by conventional methods, for example by casting or by machining. The cooling fins 135 are in contact with a platefin portion 66'' of the cooling air stream 66. The quantity, dimensions, and spacing of the effusion cooling holes 144 and the cooling fins 135 will vary according to combustor size, materials, and operating conditions, as will be understood by those skilled in the art.

Also in accordance with the present invention, a baffle assembly 134 is contained within the cooling air passage 70. An annular impingement air passage 152 is formed between the intermediate liner 42 and the baffle assembly 134.

The baffle assembly 134 includes a cylindrical platefin member 136, a conical member 138, and an equal number of inner blocks 140 and outer blocks 142. The platefin member 136 concentrically surrounds the cooling fins 135. In the preferred embodiment, the platefin member 136 is radially set apart from the cooling fins 135 to accommodate differential thermal expansion. Radial clearance is preferably zero when the platefin member 136 and the cylindrical section 126 are at normal operating temperature. Because the cylindrical section 126 is hotter than the platefin member 136, the cylindrical section 126 has larger thermal expansion and therefore closes the radial gap.

Preferably, the inner blocks 140 are welded to the liner plate fin section 136 around its outer circumference.

The outer blocks 142 are welded to an inner surface of the intermediate liner 42. A contact surface 143 of each one of the outer blocks 142 receives a contact surface 141 of each one of the inner blocks 140. The inner block surface 141 slides on the outer block surface 143 as the baffle assembly 134 thermally expands and contracts relative to the intermediate liner 42. Preferably, the contact surfaces 141, 143 are coated with a wear resistant coating, as will be understood by those skilled in the art. Alternately, the blocks 140, 142 may be formed from a wear resistant alloy.

Preferably, the blocks 140, 142 are spaced approximately equidistant apart, as shown in Figure 7. The arc length and spacing of the blocks 140, 142 will vary according to the combustor diameter, block height, and operating conditions such as cooling air flow rate and operating temperature, as will be understood by those skilled in the art.

The conical member 138 substantially surrounds the liner tapered portion 128. A downstream end of the conical member 138 is attached to the intermediate liner 42. The upstream end of the conical member 138 is attached to a downstream end of the platefin member 136. Fillet welds 150 and 151 are preferably used for attaching the conical member 138 to the intermediate liner 42 and to the platefin member 136, respectively, although spot welds may also be used for weld 151. The downstream end of the conical member 138 is isolated from the hot combustion gas to reduce weld stress in the weld 150 to a safe value. The conical member 138 has numerous impingement cooling holes 146 for directing an impingement portion 66''' of the cooling air stream 66 onto the liner tapered portion 128. The baffle assembly 134 is not formed by saw cuts, thus, eliminating air leakage air of earlier designs.

The present invention also encompasses a method of cooling the combustor 4 by removing heat from the primary liner 44 by the cooling air stream 6. According to the practice of the present invention, the cooling air stream is divided into three portions: effusion air 66', platefin air

66'', and impingement air 66''' . First, the effusion air stream 66' cools the liner conical portion 124 by flowing through the effusion cooling holes 144.

Second, the platefin air stream 66'' cools the liner 44, especially the cylindrical portion 126, by flowing over the liner outer surface 132 and the cooling fins 135. The cooling fins 135 enhance heat transfer from the primary liner 44 to the platefin air stream 66'' by conducting heat from the liner 44 and by providing a relatively large surface area (compared with a straight cylinder) to enhance convective heat transfer to the platefin air stream 66''.

Third, the impingement air stream 66''' cools the liner tapered portion 128 by direct impingement. The impingement air 66''' flows outside of the platefin member 136 through the impingement air passage 152. The impingement air 66''' is directed through the conical member 138 onto the surface of the tapered section 128 by the impingement cooling holes 146. Thus, all the cooling air flow 66 is used for cooling -- there is no "wasted" air due to leakage.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification, as indicating the scope of the invention.