Exhaust gas diffuser   

BACKGROUND OF THE INVENTION

The subject matter disclosed herein relates to turbines and, in particular, to diffusers for use with gas turbines and steam turbines.

Typical gas turbines include a diffuser cone, or diffuser, coupled to the last stage bucket of the rotor. The diffuser serves, generally, to increase static pressure of the exhaust gas by decreasing the kinetic energy of the exhaust gas. Generally, this may be achieved by increasing the cross-sectional area of the diffuser in the direction of exhaust gas flow.

Of course, diffusers are not perfectly efficient. One source of loss and turbulence generation in exhaust gas diffusers is the result of flow interaction with struts and manways. The struts are structural member which transfer the rotor load from a center to an outer casing (outer wall) and then the outer casing transfers the load to a foundation. Aerodynamically, struts present blockages between diffuser inner and outer wall of the diffuser. The inner wall typically surrounds a portion of the rotor shaft and possible other elements.

The loss due to flow interaction with struts may be amplified with high turbine exit Mach numbers, which can be driven by high-flow turbine operating conditions and flow distributions that concentrate a large amount of flow near the hub flow path. Struts are typically located near the diffuser inlet, which by design is the region of highest diffusion gradient. Therefore, any loss incurred in this region can have significant impact on the performance and acoustics behavior of the total diffuser system.

According to one aspect of the invention, a center body for an exhaust gas diffuser is disclosed. The center body includes an outer wall and an inner wall spaced apart from the outer wall, the inner wall forming a continuous curve between an entry location and an end of the center body.

According to another aspect of the invention, an exhaust gas diffuser is disclosed. The diffuser includes a center body having an inlet adapted for coupling to a gas turbine, the center body having an outer wall, an inner wall and an end downstream from the inlet, the inner wall forming a continuous curve from an entry location to the end. The center body includes a second portion coupled to the end of the of center body.

Another aspect of the present invention is directed to a gas turbine that includes a turbine casing that surrounds a portion of the gas turbine. The turbine also includes an exhaust gas diffuser coupled to the turbine casing that includes a center body having an inlet adapted for coupling to a gas turbine, the center body having an outer wall, an inner wall and an end downstream from the inlet, the inner wall forming a continuous curve from an entry location to the end.

These and other advantages and features will become more apparent from the following description taken in conjunction with the drawings.

The subject matter, which is regarded as the invention, is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other features, and advantages of the invention are apparent from the following detailed description taken in conjunction with the accompanying drawings in which:

FIG. 1 is cut-away side-view of a prior art diffuser;

FIG. 2 is cut-away side-view of a diffuser according to one embodiment;

FIG. 3 shows the turbulence kinetic energy that may exists in the diffusers shown in FIG. 1;

FIG. 4 shows the turbulence kinetic energy that may exists in the diffusers shown in FIG. 2;

FIG. 5 is a graph that show the variation of turbulence intensity across the span of the radius at center body end

FIG. 6 is cut-away side-view of another diffuser according to one embodiment;

FIG. 7 is cut-away side-view of yet another diffuser according to one embodiment;

FIG. 8 shows the turbulence kinetic energy in a diffuser as shown in FIG. 2; and

FIG. 9 shows the turbulence kinetic energy in a diffuser as shown in FIG. 7.

The detailed description explains embodiments of the invention, together with advantages and features, by way of example with reference to the drawings.

DETAILED DESCRIPTION

In one embodiment of the present invention, flow-path area is controlled in the strut passage through shaping of the diffuser inner barrel (inner wall). This flow path shaping is achieved through having maximum opening at the minimum area section in diffuser that is at maximum strut thickness with controlled variation upstream and downstream of strut generates inner wall flow path shape and is often constrained by flow separation from walls. The center body end is the function of center body radius and the diffuser inner wall angle, Similarly casing is shaped retaining the end radius of outer wall and is function of radial swirl. In particular, and contrary to the prior art in which the inner wall includes one or more “flat” sections, the inner wall may, in one embodiment, may be formed such that the inner wall is a curved from the inlet to an end of a center body section of the diffuser. The result is reduced strut blockage and, therefore, a reduction of loss through the strut. In another embodiment, the outer wall may be shaped upstream of the struts, therefore lowering the Mach number across the struts, without pushing outer wall diameters, which may be constrained due to engine shipping limits.

FIG. 1 shows a cut-away side view of a prior art diffuser 100. The diffuser 100 includes a center body portion 102 and a second portion 104. The center body portion 102 and the second portion 104 may be formed separately and joined together in use. The center body portion 102 includes a rotor chamber 106. The rotor chamber 106 surrounds a portion of gas turbine rotor (not shown) while in operation.

The center body portion 102 has an inner wall 108 formed of at least a first plane 110 and a second plane 112. The first plane 110 extends from the hub 114 to the second plane 112. The second plane 112 extends from the first plane 110 to the center body end 118. The second portion 104 may include a cylindrical channel formed to surround a portion of a rotor or other elements.

The center body portion 102 may also include one or more struts 120 formed between the inner wall 108 and the outer wall 122. The strut 120 serves to hold the inner wall 108 and the outer wall 122 in a fix relationship to one another. The number of struts 120 is variable and is commonly either four, five or ten.

The diffuser 100 has a diffuser inlet 124 that is typically coupled to an output of gas turbine (not shown) and a diffuser outlet 126 which may be coupled to a silencer. While in general, the design shown in FIG. 1 may work for its intended purpose, such a design may have one or more shortcomings. First, as described above, a primary source of loss and turbulence generation in exhaust gas diffusers is flow interaction with struts 120. This loss may be amplified with high turbine exit Mach numbers (e.g., high diffuser inlet 124 entry speeds), which can be driven by high-flow operating conditions and flow distributions that concentrate a large amount of flow near the hub 114 flow path. Struts 120 are typically located near the diffuser inlet 124, which by design is the region of highest diffusion gradient. Therefore, any loss incurred in this region can have significant impact on the performance of the total diffuser system.

Additionally high turbulence generation has detrimental effect to acoustic behavior of exhaust gas diffuser, silencer design, and it may induce or be an additional source of vibration to a first tube bundle of an HRSG coupled to the diffuser, which may lead to HRSG failure.

FIG. 2 shows an example of an example of a diffuser 200 according to one embodiment. The diffuser 200 includes a center body 202 and a second portion 104. In operation, exhaust gas from the gas turbine flows through the diffuser in the direction shown by arrow A. In this description, an object is “downsteam” of another object or location if displaced from it in the direction of arrow A and is “upstream” if it is displaced from it in a direction opposite of arrow A.

The center body 202 includes an inner wall 204 and an outer wall 206. Struts 120 hold the inner wall 204 in a fixed relationship with the outer wall 206. The inner wall 204 forms an inner chamber 208 through which a portion of a rotor may pass. The center body 202 also includes an inlet 211.

According to one embodiment, the inner wall 204 curves from an entry location 210 to a center body end 212. In one embodiment, the curve is a continuous curve. In one embodiment, the curve is a spline. In one embodiment, the inner wall is curved from a slight distance downstream of the entry location 210 to the center body end 212. In one embodiment, the inner wall 204 forms a continuous curve from the center body end 212 to a region upstream of one or more of the struts 120. The entry location 210 may be at any location between the inlet 211 and the struts 120 in one embodiment.

Shown by way of contrast and not part of the embodiment shown in FIG. 2, are first plane 110 and a second plane 112 (shown in dashes) from FIG. 1. As evident in FIG. 2, forming a curved inner wall 204 increases the cross-sectional area of the center body 202. This may, in one embodiment, reduce turbulence caused by the struts 120 without expanding the outer diameter of the outer wall 206 from what is conventionally used. In another embodiment, utilizing a curved inner wall 204 may allow for a smaller diameter outer wall 206.

FIGS. 3 and 4 show, respectively, the turbulence kinetic energy (k) in center bodies 102 and 202 shown in FIGS. 1 and 2. K is defined as the fluctuations in velocity and has dimensions of (length2/time2). In FIGS. 3 and 4 the region having the lowest k values (on the order of 0.0 to 100 m2/s2) is identified by reference numeral 302. As can be seen, FIG. 3 has much less of the low k value than FIG. 4 does. Accordingly, the turbulence kinetic energy of center body 202 is lower than center body 102. Lower k values mean lower losses.

FIG. 5 shows the variation of turbulence intensity (x axis) across the span of the length of a center body (y axis). The turbulence intensity is defined, in this example, as: I=((k2/3)1/2)/(average velocity at center body inlet). In FIG. 5, curve 402 represents I for center body 102 and curve 404 represents I for center body 202. For both curves, 402 and 404, the center body includes 10 struts 120.

Furthermore, experimental data has shown that the recover factor (RF) of the embodiment shown in FIG. 2 is greater than that of FIG. 1. Indeed, experimental data has shown upwards of a 10 percent increase in RF as between the embodiment of FIG. 2 and that shown in FIG. 1.

FIG. 6 shows another embodiment of a diffuser 502. In this embodiment, the diffuser 502 includes a modified outer wall 504. The outer wall 504, in this embodiment, is formed such that it includes a curved portion 506. The curved portion 506 extends from an inlet 508 of the diffuser 502 to a location downstream of the inlet 508. In one embodiment, the diffuser 502 has a radius r and the curved portion 506 does not extend more than a distance r from the center line 510 of the diffuser. In one embodiment, the inner wall 512 may be formed as shown in either FIG. 1 or FIG. 2.

FIG. 7 shows a cut-away side view of yet another embodiment of the present invention. The diffuser 600 shown in FIG. 7 includes a center body 602 and a second portion 604. The second portion 604 may be formed separately from the center body 602 and joined thereto along an entry side wall 605 of the second portion 604. Of course, the center body 602 and second portion 604 could be formed as a unitary member.

The second portion 604 includes and inner wall 612. In one embodiment, the entry side wall 605 is connected to the inner wall 612 by curve 606. This is in contrast to the prior art and as exemplified by dotted lines 608 and 610.

Advantageously, utilization of curve 606 reduces turbulence kinetic energy as shown in FIG. 8 and FIG. 9. FIG. 8 shows turbulence kinetic energy in a diffuser as shown in FIG. 2. FIG. 8 includes regions 702, 704 and 706 that are regions of increased turbulence. FIG. 9 shows turbulence kinetic energy for a diffuser with a second portion 604 as shown in FIG. 7. The diffuser shown in FIG. 9 does not include regions 702, 704 and 706. This indicates that there is less turbulence in the configuration where a second portion 602 as shown in FIG. 7 is utilized.

It shall be understood that the various embodiments of the present invention have been shown separately for sake of simplicity in explanation. It shall be further understood that any embodiment disclosed herein may be combined with any other embodiment herein. For example, the curved inner wall of FIG. 2 may be implemented with a diffuser having a curved outer wall as shown in FIG. 6. In addition, either or both of the embodiments shown in FIGS. 2 and 6 may include a second portion as shown in FIG. 7.

While the invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the invention is not limited to such disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Additionally, while various embodiments of the invention have been described, it is to be understood that aspects of the invention may include only some of the described embodiments. Accordingly, the invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims.