Gas turbine cooled blade

BACKGROUND OF THE INVENTION:

Field of the Invention:

The present invention relates generally to a gas turbine cooled blade and more specifically to a gas turbine cooled blade having a seal air supply passage for supplying therethrough a seal air from an outer peripheral side to an inner peripheral side of a stationary blade and a gas turbine cooled blade having a structure for enhancing a heat transfer rate in a cooling passage of a moving blade or a stationary blade.

Description of the Prior Art:

Examples of the above-mentioned type gas turbine cooled stationary blade in the prior art will be described with reference to Figs. 7 and 8.

Fig. 7 is a schematic cross sectional view of one example of a prior art gas turbine cooled blade, wherein Fig. 7(a) is a longitudinal cross sectional view and Fig. 7(b) is a cross sectional view taken on line III-III of Fig. 7(a). Fig. 8 is a schematic cross sectional view of another example of a prior art gas turbine cooled blade, wherein Fig. 8(a) is a longitudinal cross sectional view and Fig. 8(b) is a cross sectional view taken on line IV-IV of Fig. 8(a).

In an actual unit of the gas turbine, number of stages is decided by the capacity of turbine, for example, in a gas turbine constructed in four stages, its second, third and fourth stage stationary blades, respectively, have moving blades disposed in front and back thereof and each of the stationary blades is structured to be surrounded by adjacent moving blades and rotor discs supporting them. Hence, it is important that a main flow high temperature gas does not flow into a gap of each portion in an interior of the stationary blade, said gap being formed there in process of manufacture, assembly, etc.

As a countermeasure therefor, such a construction is employed usually that a bleed air from compressor is flown into the interior of the stationary blade from its outer peripheral side to be supplied into a cavity portion on an inner peripheral side of the stationary blade as a seal air so that a pressure in the cavity portion is kept higher than that in a main flow high temperature gas path, thereby preventing inflow of the main flow high temperature gas.

The prior art example of Fig. 7 is of a seal air supply structure using a seal tube 4 for leading therethrough a seal air wherein the seal tube 4 is provided in a stationary blade at a position apart from an inner surface of a blade portion 5 to pass through a first row cooling passage A of a leading edge portion in the blade portion 5, thereby a blade outer peripheral side communicates with a cavity portion of a blade inner peripheral side so that a seal air 3 is supplied into the cavity portion through the seal tube 4.

Numeral 2 designates a cooling medium, which is supplied for cooling of the stationary blade to flow through the first row cooling passage A and further through a second row cooling passage B and a third row cooling passage C in the blade portion 5 and is discharged into the main flow high temperature gas from a blade trailing edge portion.

Also, another example in the prior art shown in Fig. 8 is constructed such that a sealing air 3 is supplied directly into a first row cooling passage A to be used both for a sealing air and a blade cooling air wherein such a seal tube as used in the example of Fig. 7 is not used and omitted.

In the moving blade and stationary blade of a conventional gas turbine including those blades shown in Figs. 7 and 8, there are provided cooling passages so that cooling medium is led to pass therethrough for cooling of the interior of the blade. By such cooling, gas turbine portions to be exposed to the main flow high temperature gas flowing outside thereof are cooled so that strength of said gas turbine portions is maintained so as not to be deteriorated by the high temperature.

Fig. 9 is a longitudinal cross sectional view of the conventional gas turbine cooled blade. In Fig. 9, numeral 21 designates a cooled blade (moving blade), in which a cooling passage 22 is provided passing therethrough. Numeral 23 designates a cooling medium, which flows into the blade from a base portion of the cooled blade 21 to flow through cooling passages 22a, 22b and 22c sequentially and is discharged into a gas path where a high temperature gas 25 flows. Numeral 24 designates a rib and there are provided a plurality of ribs 24 being arranged inclinedly on inner walls of the cooling passages 22a, 22b, 22c, as described later, so that the cooling medium 23 flows in each of the cooling passages like arrow 29 with a heat transfer rate therein being enhanced.

Fig. 10 is an enlarged view of one of the cooling passages of the cooled blade 21 in the prior art as described above, wherein Fig. 10(a) is a plan view thereof and Fig. 10(b) is a perspective view thereof. As shown there, in the cooling passage 22 of the cooled blade 21, the plurality of ribs 24 are provided, each extending in an entire width W of the cooling passage 22 to be disposed inclinedly with a constant angle θ relative to a flow direction of the cooling medium 23 with a rib to rib pitch P and projecting in a height e. The cooling medium 23 is led into the cooling passage 22 from outside of the cooled blade 21 to flow through the cooled blade 21 for sequential cooling therein and is discharged into the high temperature gas 25, as described in Fig. 9. At this time, the rib 24 causes turbulences in the flow of the cooling medium 23 so that the heat transfer rate of the cooling medium 23 flowing through the cooling passage 22 is enhanced.

Fig. 11 is a schematic explanatory view of a flow pattern and a cooling function thereof of the cooling medium 23 flowing in the cooling passage 22 of Fig. 10, wherein Fig. 11(a) shows a flow direction of the cooling medium 23 seen on a plan view of the cooling passage 22, Fig. 11(b) shows a flow of the cooling medium 23 seen from one side of Fig. 11(a), Fig. 11(c) shows the flow of the cooling medium 23 seen perspectively and Fig. 11(d) shows a heat transfer rate distribution in the cooling passage 22.

As shown there, in a space between each of the ribs 24, the cooling medium 23 becomes a swirl flow 23a as in Fig. 11(a) to flow toward downstream from upstream there so as to move in a constant direction along the rib 24 provided inclinedly as in Fig. 11(c). For this reason, as shown conceptually by the heat transfer rate distribution of Fig. 11(d), there is generated a high heat transfer rate area 30 on an upstream side thereof where the swirl flow 23a approaches to a wall surface of the cooling passage 22 (boundary layer there is thin). On the other hand, on a downstream side thereof where the swirl flow 23a leaves from the wall surface of the cooling passage 22 (boundary layer there is thick), the heat transfer rate tends to lower as compared with the upstream side, hence there occurs a non-uniformity of the heat transfer rate according to the place, which results in suppressing enhancement of an average heat transfer rate as a whole.

In the first prior art example shown in Fig. 7, there is provided the seal tube 4 which is disposed at the position apart from the inner surface of the blade portion 5 for exclusively leading therethrough the seal air 3. Hence, in this system, while there is an advantage that the seal air 3, making no direct contact with the inner surface of the blade portion 5, can be supplied as the seal air before it is heated by heat exchange, there is also a disadvantage of inviting an increased number of parts and an increased working man-hour in providing the seal tube 4.

Also, in the second prior art example shown in Fig. 8, while no such seal tube as the seal tube 4 is used and reduction of the parts number and working man-hour can be realized, the seal air 3 is supplied passing through the blade leading edge portion where there is a large thermal load, hence there is needed a large heat exchange rate for cooling of the blade, which results in a problem that a temperature of the seal air becomes too high.

Further, in the prior art gas turbine cooled blade shown in Figs. 9 to 11, the cooling medium flows to generate the swirl flow 23a which flows along the rib 24 in the cooling passage 22 as shown in Fig. 11(a) and there are formed the high heat transfer rate area 30 in the place where the swirl flows 23a approaches to the wall surface of the cooling passage 22 and the area of lower heat transfer rate in the place where the swirl flow 23a leaves from the wall surface of the cooling passage 22 as shown in Fig. 11(d), hence the heat transfer rate becomes non-uniform to cause a lowering of the average heat transfer rate.

SUMMARY OF THE INVENTION:

In order to dissolve the problems in the prior art as mentioned above, it is an object of the present invention to provide a gas turbine cooled blade in which a seal air is maintained to a lower temperature with its heat exchange rate being suppressed and is led into the blade for a seal air supply with no increase in number of parts and working man-hour.

It is another object of the present invention to provide a gas turbine cooled blade in which a cooling passage is made in such a structure that shapes of ribs and arrangement thereof are devised so that a high heat transfer rate area caused by a flow of cooling medium in the cooling passage is formed uniformly in a space between each of the ribs to thereby enhance an average heat transfer rate in the entire cooling passage.

In order to achieve said object, the present invention first provides a gas turbine cooled blade having therein a plurality of cooling passages extending in a turbine radial direction, a portion of said plurality of cooling passages being used as a seal air supply passage as well for supplying therethrough a seal air into a cavity on a blade inner peripheral side from a blade outer peripheral side, characterized in that a cooling passage of first row from upstream is covered both at its blade inner peripheral side and blade outer peripheral side and communicates with a cooling passage of second row from same via a communication hole bored in a partition wall between itself and said cooling passage of second row as well as communicates with a main flow gas path via a film cooling hole bored in a blade wall passing therethrough to a blade outer surface; and said cooling passage of second row communicates with the cavity on the blade inner peripheral side so as to form the seal air supply passage.

That is, according to the present invention, the seal air supplied from the blade outer peripheral side flows through the selected second row cooling passage where there are less thermal load and less heat exchange rate of the seal air, thereby an appropriate temperature as the seal air can be maintained.

Also, a portion of the seal air in the second row cooling passage is separated to flow into the first row cooling passage via the communication hole to be used as a cooling air. This cooling air first cools the blade leading edge portion which surrounds the first row cooling passage and then makes film cooling of the blade outer surface, passing through the film cooling hole. Thereby, without increase in number of parts, such as a seal tube, the seal air which is suitable to be led into the blade inner peripheral side cavity can be secured as well as the appropriate cooling of the blade leading edge portion can be done.

The present invention provides also a gas turbine cooled blade mentioned above, characterized in that said seal air supply passage is formed not by the second row cooling passage but being selected from a third and subsequent row cooling passages downstream of the second row cooling passage.

That is, according to the present invention, the cooling passage of the seal air supplied from the blade outer peripheral side to the blade inner peripheral side is formed being selected from the cooling passages of downstream of the second row cooling passage, thereby the heat exchange rate in the blade portion corresponding to that cooling passage is small sufficiently so that the temperature of the seal air can be maintained to a further lower level and the seal air which is more suitable to be led into the blade inner peripheral side cavity can be obtained.

The present invention further provides a gas turbine cooled blade having therein a cooling passage, said cooling passage having on its inner wall a plurality of ribs disposed so as to cross a cooling medium flow direction with a predetermined rib to rib pitch, characterized in that each of said plurality of ribs extends from a side end of said cooling passage to a position beyond a central portion thereof to be disposed alternately right and left against the cooling medium flow direction and inclinedly in mutually opposing directions as well as to make contact at its one end beyond the central portion of said cooling passage with a side face of another rib of immediately upstream thereof.

That is, according to the present invention, each of the ribs is disposed alternately against the cooling medium flow direction and inclinedly in mutually opposing directions while making contact with the side face of the immediate upstream rib at the position slightly biased toward the side from the central portion of the cooling passage. Thereby, there are generated the swirl flows on both side portions of the cooling passage due to the cooling medium flowing against the alternately disposed and inclined ribs and said swirl flows flow while swirling in the space formed by the ribs disposed with the predetermined pitch. Thus, there are formed the high heat transfer rate areas on both side portions of the cooling passage and there is no case of the high heat transfer rate area occurring on one side portion only as in the prior art case, which results in forming of an increased and uniform high heat transfer rate area in the entire cooling passage and enhancement of the average heat transfer rate.

Also, the present invention provides a gas turbine cooled blade as mentioned above, characterized in that each of said plurality of ribs has a shape in which a height thereof reduces gradually from a higher portion at its said one end beyond the central portion of said cooling passage toward a lower portion at its the other end at the side end of said cooling passage.

That is, according to the present invention, each of the ribs has a shape of height which reduces gradually from its one higher end to the other lower end and the higher end makes contact with the side face of the immediate upstream rib, thereby there are generated the small swirl flows along the cooling medium flow at the contact position of the two ribs, which results in assisting to enhance the heat transfer rate further, in addition to the enhancement of the average heat transfer rate by the above-mentioned invention.

The present invention further provides a gas turbine cooled blade having therein a cooling passage, said cooling passage having on its inner wall a plurality of ribs disposed so as to cross a cooling medium flow direction with a predetermined rib to rib pitch, characterized in that there is provided a pin projecting substantially perpendicularly at a predetermined position in a longitudinal direction of a rib of all or a portion of said plurality of ribs.

That is, according to the present invention, by the swirl flows generated by the ribs crossing the cooling medium flow, there are formed the high heat transfer rate areas and in addition thereto, the pins are provided projectingly so that swirl flows are further generated on the downstream side of the respective pins to flow along the inclinedly disposed ribs, thereby the high heat transfer rate areas are formed also in the area where the high heat transfer rate area had been hardly formed in the prior art, which results in forming of an increased and uniform high heat transfer rate area in the entire cooling passage and enhancement of the average heat transfer rate.

Also, the present invention provides a gas turbine cooled blade as mentioned above, characterized in that said pin is provided in plural pieces with a predetermined pin to pin pitch on the rib.

That is, according to the present invention, the pin is provided in plural pieces with the predetermined pitch between the pins on the rib on which the pin is to be provided, thereby the high heat transfer rate areas which are formed by the swirl flows generated by the pins can be further increased to form a more uniform high heat transfer rate area and the average heat transfer rate is further enhanced.

The present invention further provides a gas turbine cooled blade as mentioned above, characterized in that said pin is provided in said cooling passage so as to connect a dorsal side portion and a ventral side portion of the blade.

That is, according to the present invention, the pin is provided so as to connect the blade dorsal side and the blade ventral side, thereby the pin can be used as a reinforcing element in the cooling passage as well, in addition to the effect of the enhancement of the average heat transfer rate by the increased high heat transfer rate area.

BRIEF DESCRIPTION OF THE DRAWINGS:

<ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a perspective, partially cut away, view of a gas turbine cooled blade of a first embodiment according to the present invention.</li><li>Fig. 2 shows a schematic cross section of the gas turbine cooled blade of Fig. 1, wherein Fig. 2(a) is a longitudinal cross sectional view and Fig. 2(b) is a cross sectional view taken on line II-II of Fig. 2(a).</li><li>Fig. 3 shows a main part of a cooling passage of a gas turbine cooled blade of a second embodiment according to the present invention, wherein Fig. 3(a) is a partially enlarged plan view thereof, Fig. 3(b) is a side view thereof and Fig. 3(c) is a perspective view thereof.</li><li>Fig. 4 is a schematic explanatory view of a flow pattern and a heat transfer rate distribution of a cooling medium in the second embodiment of Fig. 3, wherein Fig. 4(a) is a plan view of the flow pattern, Fig. 4(b) is a side view thereof, Fig. 4(c) is a perspective view thereof and Fig. 4(d) is a view showing the heat transfer rate distribution.</li><li>Fig. 5 shows a gas turbine cooled blade of a third embodiment according to the present invention, wherein Fig. 5(a) is a partially enlarged plan view, Fig. 5(b) is a side view thereof and Fig. 5(c) is a perspective view thereof.</li><li>Fig. 6 is a schematic explanatory view of a flow pattern and a heat transfer rate distribution of a cooling medium in the third embodiment of Fig. 5, wherein Fig. 6(a) is a plan view of the flow pattern, Fig. 6(b) is a side view thereof, Fig. 6(c) is a perspective view and Fig. 6(d) is a view showing the heat transfer rate distribution.</li><li>Fig. 7 is a schematic cross sectional view of one example of a prior art gas turbine cooled blade, wherein Fig. 7(a) is a longitudinal cross sectional view and Fig. 7(b) is a cross sectional view taken on line III-III of Fig. 7(a).</li><li>Fig. 8 is a schematic cross sectional view of another example of a prior art gas turbine cooled blade, wherein Fig. 8(a) is a longitudinal cross sectional view and Fig. 8(b) is a cross sectional view taken on line IV-IV of Fig. 8(a).</li><li>Fig. 9 is a longitudinal cross sectional view of a conventional gas turbine cooled blade.</li><li>Fig. 10 is an enlarged view of one of cooling passages of the conventional gas turbine cooled blade of Fig. 9, wherein Fig. 10(a) is a plan view thereof and Fig. 10(b) is a perspective view thereof.</li><li>Fig. 11 is a schematic explanatory view of a flow pattern and a cooling function thereof of a cooling medium flowing in one of the cooling passages of Fig. 10, wherein Fig. 11(a) shows a flow direction of the cooling medium seen on a plan view of the cooling passage, Fig. 11(b) shows a flow of the cooling medium seen from one side of Fig. 11(a), Fig. 11(c) shows the flow of the cooling medium seen perspectively and Fig. 11(d) shows a heat transfer rate distribution in the cooling passage.</li></ul>

DESCRIPTION OF THE PREFERRED EMBODIMENTS:

A first embodiment according to the present invention will be described with reference to Figs. 1 and 2. It is to be noted that same parts as those in the prior art mentioned above are given same reference numerals in the figures with repeated description being omitted as much as possible, and characteristic points of the present embodiment will be described mainly. <ul id="ul0002" list-style="none" compact="compact"><li>Fig. 1 is a perspective, partially cut away, view of a gas turbine cooled blade of a first embodiment according to the present invention.</li><li>Fig. 2 shows a schematic cross section of the gas turbine cooled blade of Fig. 1, wherein Fig. 2(a) is a longitudinal cross sectional view and Fig. 2(b) is a cross sectional view taken on line II-II of Fig. 2(a).</li></ul>

In the present embodiment, a seal air 3 having function of blade cooling as well, like in the second example of the prior art of Fig. 8, is not led into a first row cooling passage A provided in a blade leading edge portion but is led into a second row cooling passage B where there is less thermal load, and while the air cools the second row cooling passage B, a portion of the seal air 3 is separated to be supplied into the first row cooling passage A and remaining portion thereof is led into an inner cavity 10 as the seal air.

That is, as shown in Figs. 1 and 2, there are bored a plurality of communication holes 6 in a cooling passage wall 11 which partitions the first row cooling passage A provided in the blade leading edge portion and the second row cooling passage B. There are also provided a plurality of film cooling holes 7 in walls on a dorsal side and a ventral side, respectively, of a blade portion 5 of the first row cooling passage A.

Further, an inner shroud 8 and an outer shroud 9 of the first row cooling passage A are structured to be closed in a turbine radial direction. Also, third and subsequent row cooling passages (third row cooling passage C, fourth row cooling passage D and fifth row cooling passage E) are structured same as those in the prior art described above.

In the present embodiment constructed as above, a cooling medium 1 having both of sealing function and blade cooling function is supplied into the second row cooling passage B from an outer shroud 9 side and after having cooled inner surfaces of the passage, the cooling medium 1 is partially led into the inner cavity 10 as the seal air 3.

Remaining part of the cooling medium 1 is supplied into the first row cooling passage A through the communication holes 6 and, after having cooled inner surfaces of the passage as a cooling air, is blown into a main flow high temperature gas through the film cooling holes 7 for effecting a film cooling of blade outer surfaces.

Like the cooling air in the prior art, a cooling medium 2 having passed through the third row cooling passage C enters the fourth row cooling passage D formed in a serpentine shape and the fifth row cooling passage E sequentially for cooling of blade inner surfaces and is then blown into the main flow high temperature gas from a blade trailing edge portion.

Thus, according to the present embodiment, the seal air is supplied into the second row cooling passage B where there is less thermal load and a portion of the cooling air is supplied into the first row cooling passage A through the communication holes 6 of the cooling passage wall 11 for effecting the film cooling of the blade outer surfaces, thereby the blade outer surfaces of the portion corresponding to the second row cooling passage B are applied to by the film cooling and reduced of temperature so that the thermal load of the second row cooling passage B is lowered further and temperature rise of the seal air in the second row cooling passage B is suppressed further securely. Also, because temperature rise of the seal air supplied into the inner cavity 10 via the second row cooling passage B is suppressed sufficiently, there is no need of using a seal tube, which results in no increase of parts number and working man-hour.

It is to be noted that, if a proportion of flow rates of the sealing air to be supplied into the inner cavity 10 from the second row cooling passage B and the cooling air to be supplied into the first row cooling passage A from same is to be regulated, a throttle may be provided at a cooling passage outlet on the inner cavity 10 side.

It is also to be noted that, although description has been done in the present embodiment on the seal air 3 to be supplied into the inner cavity 10 via the second row cooling passage B, the seal air 3 is not limited to that supplied from the second row cooling passage B but may be supplied from the third row cooling passage C or subsequent ones selectively.

In the case of such variation as this, the third row cooling passage C and subsequent cooling passages are in further less thermal load than the second row cooling passage B, thereby the seal air is maintained to a more preferable lower temperature so that the seal air which is suitable for the inner cavity 10 can be secured.

Next, a second embodiment according to the present invention will be described concretely with reference to Figs. 3 and 4. Fig. 3 shows a main part of a cooling passage of a gas turbine cooled blade of the second embodiment, wherein Fig. 3(a) is a partially enlarged plan view thereof, Fig. 3(b) is a side view thereof and Fig. 3(c) is a perspective view thereof. In Fig. 3, numeral 31 designates a plurality of ribs, each of said ribs being disposed on an inner wall surface of a cooling passage 22 extending alternately toward both side directions of a main flow direction of a cooling medium 23 and being inclined with a constant angle θ to said main flow direction of the cooling medium 23 and with a constant rib to rib pitch P in said main flow direction of the cooling medium 23.

As shown in Fig. 3(a), each of the ribs 31 is disposed inclinedly in a width Wa which is smaller than an entire width W of the cooling passage 22 to extend having such a height as gradually reduces from at its one higher end having a height e at a position of the width Wa which is slightly biased to a side end of the cooling passage 22 beyond a central portion thereof toward its the other lower end of downstream outer side thereof having a height f which is lower than e.

Each of the ribs 31 makes contact at its end portion of the height e with an approximately central portion of a side face of another rib 31 disposed immediately upstream thereof so as to project higher than the side face of said another rib 31 at a position of the contact portion and there are disposed alternately a plurality of ribs 31 to extend inclinedly in mutually opposing directions with the rib to rib pitch P in the main flow direction of the cooling medium 23 in the cooling passage 22.

Fig. 4 is a schematic explanatory view of a flow pattern and a heat transfer rate distribution of the cooling medium in the second embodiment of Fig. 3, wherein Fig. 4(a) is a plan view of the flow pattern, Fig. 4(b) is a side view thereof, Fig. 4(c) is a perspective view thereof and Fig. 4(d) is a view showing the heat transfer rate distribution. As shown there, by the effect of the ribs 31 disposed alternately and inclinedly in the mutually opposing directions, the cooling medium flowing in the cooling passage 22 generates a swirl flow 23b which flows swirlingly and inclinedly downstream toward a side portion of the cooling passage 22 from the central portion thereof in the respective spaces formed with the pitch P between the ribs 31.

As the rib 31 has a shape which reduces its height from e to f and there occurs a difference in the height at the portion where the ribs 31 make contact with each other, there arises a small swirl flow 27 at a corner portion of the rib 31 having the height e. As the contact portions of the ribs 31 are formed alternately on both side portions of the cooling passage 22, the small swirl flow 27 is also formed on both side portions of same.

In the present embodiment constructed as above, like in the prior art cooling structure, there is formed a high heat transfer rate area 26 on the upstream side of the swirl flow 23b as shown in Fig. 4(d), and because the swirl flow 23b is formed in the respective spaces formed between the ribs 31 on both side portions of the cooling passage 22, said high heat transfer rate area 26 is also formed therein on both side portions of same.

Also, the rib 31 changes its shape to reduce the height from e to f, hence the small swirl flow 27 occurring at the corner portion of the rib 31 in the contact portion of the ribs 31 is also generated on both side portions of the cooling passage 22 to assist generation of the high heat transfer rate area 26, which results in further enhancing the heat transfer rate.

It is to be noted that, as to the height e, f of the rib 31, even in the case where e equals f, similar effect can be obtained and the value e, f may be selected and adjusted so as to obtain a necessary high heat transfer rate.

According to the present embodiment, the ribs 31 are disposed alternately and inclinedly in the mutually opposing directions wherein the end portion of the rib 31 makes contact with side surface of the upstream side rib 31 and the rib 31 has a shape to reduce its height from e to f, thereby the swirl flow 23b is generated and the high heat transfer rate area 26 is formed uniformly on both side portions of the cooling passage 22. Moreover, the small swirl 27 is generated at the corner portion of the contact portion of the ribs 31 to assist generation of the high heat transfer rate area 26, which results in enhancing the average heat transfer rate of the entire cooled blade.

Fig. 5 shows a gas turbine cooled blade of a third embodiment according to the present invention, wherein Fig. 5(a) is a partially enlarged plan view, Fig. 5(b) is a side view thereof and Fig. 5(c) is a perspective view thereof. As shown there, the present third embodiment is made basically on a same shape of rib and same arrangement thereof as in the prior art, shown in Fig. 10, with an improvement being added to enhance a heat transfer rate in a low heat transfer rate area.

In Fig. 5(a), there is provided a pin 28 on the rib 24 at a position of an approximately central portion C of an entire width W of the cooling passage 22. The pin 28 has a shape of diameter d and height h, as shown in Fig. 5(b). In the figure, there is provided the pin 24 on each of the ribs 24 but the pin 24 is not necessarily provided on each of the ribs 24 but may be provided on every two, three or more ribs 24.

Fig. 6 is a schematic explanatory view of a flow pattern and a heat transfer rate distribution of the cooling medium in the third embodiment of Fig. 5, wherein Fig. 6(a) is a plan view of the flow pattern, Fig. 6(b) is a side view thereof, Fig. 6(c) is a perspective view and Fig. 6(d) is a view showing the heat transfer rate distribution. As shown there, the swirl flow 23b is generated by the rib 24 and the high heat transfer rate area 30 is thereby formed, as shown in Figs. 6(a) and (d). This high heat transfer rate area 30 has a same function as that of the prior art shown in Fig. 11.

In addition thereto, by existence of the pin 28, there is generated a swirl flow 32 on a downstream side of the pin 28. The swirl flow 32 flows along inclination of the rib 24 so that there is formed a high heat transfer rate area 31 on the opposing side of the high heat transfer rate area 30, as shown in Fig. 6(d). Thus, by selecting the diameter d and the height h of the pin 28 arbitrarily, the heat transfer rate at the high heat transfer rate area 31 can be adjusted.

If the entire width W of the cooling passage 22 is large, the pin 28 may be provided in plural pieces along a longitudinal direction of the rib 24 and in this case, the high heat transfer rate area can be enlarged.

In the present embodiment where the pin 28 is provided projectingly, it will be preferable if the pin 28 is provided so as to connect a dorsal side portion and a ventral side portion of the blade, because the pin 28 may function in this case not only for acceleration of cooling but also as a reinforcing element of the blade which is a hollow blade having a thin wall structure.

According to said third embodiment, like in the second embodiment, the high heat transfer rate area is enlarged, thereby the average heat transfer rate can be enhanced.

It is understood that the invention is not limited to the particular construction and arrangement herein illustrated and described but embraces such modified forms thereof as come within the scope of the following claims.