Cooled gas turbine blade

Abstract: A gas turbine cooling blade for maintaining a cooling effect and preventing sticking of deposits to the belly surface of the blade. This blade is provided with a relatively big cooling hole 5 formed on the belly part of a hollow stator blade 1 at an acute angle with the blade surface for blowing off cooling air and a relatively small cooling hole 6 disposed on the downstream thereof at an acuter angle with the blade surface so as to bring the jet of cooling air along therewith for blowing off cooling air.

Description: BACKGROUND OF THE INVENTION The present invention relates to a cooled gas turbine blade as defined by the preamble portion of claim 1. Fig. 4 is a sectional view showing the cooling structure of the conventional gas turbine hollow stator blade. A hollow stator blade 11 is formed integrally with inside and outside shrouds(not shown in the Figure) by means of precision molding. Within the hollow stator blade 11 an insert 13 having a plurality of cooling holes 12 is installed and cooling air is flown thereinto from the outside shroud. As shown by the arrows in the Figure, the cooling air is flown out of the hole of the insert 13, brought into collision with the inner wall of the hollow stator blade 11, an impingement cooling is carried out, and then it is flown into a hollow chamber A formed between the insert 13 and the hollow stator blade 11. Then, the stator blade is cooled while the cooling air is flown toward the rear edge of the blade, a part of the cooling air is flown out of a film cooling hole 14 along a blade profile and thereby a blade surface is film-cooled. The blade rear edge including a pinfin 16 is convection-cooled by the cooling air flown out of a slit 15 thereon. Further, on a blade front edge exposed most to high-temperature gas a blade front edge part film cooling hole 18 called a shower head is provided. When the gas turbine cooling blade of such a conventional type is used for burning heavy oil, etc., as described below, deposits 17 get stuck to a blade belly part where a flow speed is relatively slow clogging the film cooling hole 14. These deposits are oxides made of such corrosive components as S(sulfur), Na(sodium) and the like included in fuels and Ca(calcium), Fe(iron), Si(silicon) and others included in intake air. They get solidified and stuck to the cooled blade surface when they are brought into contact therewith though they are melted on an area of high-temperature gas at the front stage of the gas turbine, and they tend to stick more to the blade belly part where the flow speed is relatively slow. In the case where the gas turbine cooling blade having the cooling structure described above is used for the gas turbine operated by burning, for example, crude oil and heavy oil other than such standard fuels as kerosene, gas oil, naphtha and the like, as many ashes and residual carbons are contained in heavy oil, deposits get accumulated on the belly side of the turbine blade and thereby the cooling performance of the air-cooling blade is greatly reduced within a short period of time. Consequently, high-temperature corrosion is generated. A prior art cooled gas turbine blade is described in EP-0 501 813 A1. In this blade a pair of cooling holes are provided to extend through the side wall of the blade in inclined relation to the external side wall. The holes merge and intersect with one another at the internal side wall of the blade wall and at the external side wall of the blade wall the cooling holes are displaced with respect to each other in the downstream direction. SUMMARY OF THE INVENTION The object of the present invention is to provide a cooled gas turbine blade in which deposits are reliably prevented from stickung on the blade surface while the cooling of the blade surface is guaranteed. According to the present invention there is provided a cooled gas turbine blade, comprising a first cooling hole for blowing off cooling air, said first cooling hole being arranged in an outer wall on a belly part of the blade at an acute angle within a range from 45° to 90° to the outer surface of the blade, a second cooling hole for blowing off cooling air, said second cooling hole being arranged in said outer wall downstream of the first cooling hole (5) at an acute angle within a range from 25° to 40° to said outer surface of the blade, characterized in that the second cooling hole is relatively small compared to said first cooling hole. Produced deposits get easily solidified and stuck to the downstream side surface of a film cooling hole as they are brought into contact with a film layer formed on the boundary layer of the blade surface. Thus, according to the present invention, the relatively big cooling hole buried on the blade surface is provided on the upstream of the relatively small cooling hole for blowing off a jet of cooling air specialized in carrying out a cooling operation along the blade surface and by means of the jet of cooling air from the relatively big cooling hole penetrating the boundary layer formed on the blade surface, produced deposits just before sticking are blown off, and thus sticking thereof is prevented. Also, from the relatively small downstream side cooling hole arranged on the downstream a jet of cooling air is blown off along the blade surface so as to supplement the cooling effect of the jet of cooling air from the relatively big upstream side cooling hole. By working of both of these holes sticking of deposits is prevented, and thus the film-cooling can be sufficiently performed. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a sectional view showing one embodiment according to the present invention. Fig. 2 is an enlarged view showing a part of the above embodiment wherein cooling holes are provided. Fig. 3 is a view showing the example of arranging relatively large and small cooling holes. Fig. 4 is a sectional view showing the cooling structure of the hollow stator blade of the conventional gas turbine. DESCRIPTION OF THE PREFERRED EMBODIMENT One embodiment according to the present invention will be described in detail with reference to the accompanying drawings. As shown in Figs. 1 and 2, the cooling stator blade 1 of a gas turbine is provided with an insert 2 having a plurality of cooling holes 2' for impingement cooling on the inside and a hole 3 for film-cooling for the object of reinforcing a cooling operation while a blade front edge part film cooling hole 4(shower head) is provided on the front edge part of the blade. On the belly part of the blade, a relatively big cooling hole 5 buried at an acute angle with the blade surface and inclined toward the blade rear edge and a relatively small cooling hole 6 buried at an acuter angle with the blade surface on the downstream (blade rear edge side) thereof, inclined toward the blade rear edge and so arranged as to bring the direction of the blown-off jet of cooling air along the blade surface are provided in combination. As similar to the one shown in Fig. 4, a hollow chamber A is formed between the insert 2 and the cooling stator blade 1, cooling air is flown from an outside shroud(not shown in the Figure) into the insert 2 and it is blown off from a slit on the blade rear edge. According to this embodiment, a large amount of air to film-cool the blade surface is jetted off from the relatively big cooling hole 5 formed on the blade belly part and thereby deposits just before sticking to the belly surface of the blade can be blown off. From the relatively small cooling hole 6 disposed on the downstream of the big cooling hole 5 cooling air is jetted off along the blade surface in order to supplement the cooling effect of the air spurted out of the hole 5. By the air blown off from both of these holes 5 and 6, film cooling effect can be maintained, deposits apt to accumulate on the belly surface of the blade can be blown off, and thus their sticking can be prevented. Further, the relatively big cooling hole 5 must be formed having an ejection angle α within the range of ≥45° to ≤90° so that the ejected air penetrates a boundary layer formed along the blade surface. In this way, deposits just before sticking to the blade surface can be blown off by the air entering the boundary layer with the low flow speed, and thus it is made hard for deposits to stick to the blade surface. On the other hand, the relatively small cooling hole 6 provided on the downstream of the relatively big cooling hole 5 (better if provived immediately thereafter) must be formed having an ejection angle β within the range of ≥20° to ≤40°, preferably 30° so as to make film efficiency highest. Thus, a film cooling film is formed along the blade surface. Further, blown air pressure adjustment is carried out for the insert 2 provided within the blade and a blowing rate (see below) is set around 1.0 where film efficiency is considered to be the highest. Blowing rate= pv(blowing) p'v'(main flow) Herein, p,v are density and speed of blown air while p',v' are density and speed of main flow fluid. In this way, an air film can be formed on the downstream side blade surface of the relatively small cooling hole 6 without penetrating the boundary layer to be formed on the blade surface. The gas turbine cooling blade according to the present invention is not only useful for the gas turbine operated by burning crude oil and heavy oil but also for the ones operated by burning by-product gas produced at chemical plants, by-product liquid fuels and blast furnace gas or for other types including a gasfied coal gas turbine, etc., which produce many deposits. Further, it is useful in maintaining the film cooling effect without sticking of deposits to the belly side of the blade by means of small and large diameter cooling holes buried thereon having different angles to the blade surface as described in the claims. Thus, the gas turbine cooling blade according to the present invention is capable of solving such problems as a reduction in the cooling performance of the cooling blade of the gas turbine operated by burning heavy oil, etc., within a short period of time, generation of high-temperature corrosion due to this and which can be extremely effective in improvement and maintenance of the reliability of the gas turbine.

Claims: A cooled gas turbine blade (1), comprising a first cooling hole (5) for blowing off cooling air, said first cooling hole (5) being arranged in an outer wall on a belly part of the blade (1) at an acute angle within a range from 45° to 90° to the outer surface of the blade (1), a second cooling hole (6) for blowing off cooling air, said second cooling hole (6) being arranged in said outer wall downstream of the first cooling hole (5) at an acute angle within a range from 25° to 40° to said outer surface of the blade (1), characterized in that the second cooling hole (6) is relatively small compared to said first cooling hole (5).

Inventors: TERAZAKI MASAO, TSUKAGOSHI KEIZO, TERAZAKI, MASAO, TSUKAGOSHI, KEIZO, TERAZAKI, MASAO, C/O TAKASAGO MACHINERY WORKS OF, TSUKAGOSHI, KEIZO, C/O TAKASAGO MACHINERY WORKS OF
Applicants: MITSUBISHI HEAVY IND LTD, MITSUBISHI JUKOGYO KABUSHIKI KAISHA
Application year: 1995
Main classification: F01D5
IPC codes: F01D9, F01D25, F01D5