Abstract:
A gas turbine cooled blade is constructed without an increase in the number of parts or time requirements, in which seal air is maintained at a lower temperature with the heat exchange rate being suppressed, and the heat transfer rate of the cooling medium in cooling passage is enhanced. A plurality of cooling passages (A, B, C, D, E) is provided in a blade, and the first row cooling passage (A) is covered at the blade inner and outer peripheries and communicates with the second row cooling passage (B) through communication holes ( 6 ) and with the main flow gas path through film cooling holes ( 7 ). The second row cooling passage (B) communicates with the blade inner peripheral cavity ( 10 ) to form a seal air supply passage. A plurality of ribs ( 31 ) are disposed on the inner wall of cooling passage ( 22 ) with a predetermined pitch (P). The ribs are arranged alternately and are inclined against cooling medium flow with respective higher first end contacting lower side faces of an immediately upstream rib at a position on both side portions of cooling passage ( 22 ). High heat transfer rate areas are formed on both side portions of cooling passage ( 22 ), and the average heat transfer rate in cooling passage is enhanced.

Description:
BACKGROUND OF THE INVENTION 
     1. 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. The present invention also relates to 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. 
     2. 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, the number of stages is decided by the capacity of the turbines. 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, in which the gap is formed there during manufacture, assembly, etc. 
     As a countermeasure therefor, a construction is usually employed so that a bleed air from a compressor flows 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. Thus, 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. 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 . Thus, 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 . The cooling medium is then 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 a seal tube such as used in the example of FIG. 7 is not used. 
     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 the strength of these 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  22   a ,  22   b  and  22   c  sequentially and is discharged into a gas path where a high temperature gas  25  flows. A plurality of ribs  24  are arranged inclinedly on inner walls of the cooling passages  22   a ,  22   b ,  22   c , as described later, so that the cooling medium  23  flows in each of the cooling passages as shown by 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 at an incline with a constant angle  0  relative to a flow direction of the cooling medium  23  with a rib to rib pitch P and projecting 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  23   a  as in FIG.  11 ( a ) to flow downstream from upstream there so as to move in a constant direction along the rib  24  inclined 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  23   a  approaches 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  23   a  leaves 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. As a result, 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 increased time 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 and time 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  23   a  which flows along the rib  24  in the cooling passage  22  as shown in FIG.  11 ( a ). There are formed the high heat transfer rate area  30  in the place where the swirl flows  23   a  approaches the wall surface of the cooling passage  22 , and the area of lower heat transfer rate in the place where the swirl flow  23   a  leaves 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 solve 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 at 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 the number of parts and time required. 
     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 this 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 the plurality of cooling passages is used as a seal air supply passage as well as for supplying therethrough a seal air into a cavity on a blade inner peripheral side from a blade outer peripheral side. A cooling passage of a first row at an upstream position is covered both at its blade inner peripheral side and blade outer peripheral side and communicates with a cooling passage of a second row through a communication hole bored in a partition wall between the cooling passage of the first row and the cooling passage of second row. The cooling passage of te first row also communicates with a main flow gas path through a film cooling hole bored in a blade wall passing therethrough to a blade outer surface. The cooling passage of the 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 is a smaller thermal load and smaller heat exchange rate of the seal air. Thus, an appropriate temperature for the seal air can be maintained. 
     Also, a portion of the seal air in the second row cooling passage is separated so as to flow into the first row cooling passage through 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 an increase in the 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, and the appropriate cooling of the blade leading edge portion can be done. 
     The present invention also provides a gas turbine cooled blade mentioned above, in which the seal air supply passage is formed not by the second row cooling passage but by being selected (channeled) 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 by being selected from the cooling passages downstream of the second row cooling passage. Thus, the heat exchange rate in the blade portion corresponding to that cooling passage is sufficiently small so that the temperature of the seal air can be maintained at 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. The cooling passage has 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. Each of the plurality of ribs extends from a side end of the cooling passage to a position beyond a central portion thereof to be disposed alternately right and left against the cooling medium flow direction and to be inclined in mutually opposing directions. Each rib also makes contact at its first end beyond the central portion of the cooling passage with a side face of another rib immediately upstream thereof. 
     That is, according to the present invention, each of the ribs is disposed alternately against the cooling medium flow direction and is inclined 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, the swirl flows are generated on both side portions of the cooling passage due to the cooling medium flowing against the alternately disposed and inclined ribs, and the swirl flows flow while swirling in the space formed by the ribs disposed with the predetermined pitch. Thus, the high heat transfer rate areas are formed 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. As a result, an increased and uniform high heat transfer rate area is formed in the entire cooling passage, and enhancement of the average heat transfer rate is enhanced. 
     Also, the present invention provides a gas turbine cooled blade as mentioned above, in which each of the plurality of ribs has a shape in which a height thereof reduces gradually from a higher portion at it first end beyond the central portion of the cooling passage toward a lower portion at its second end at the side end of the cooling passage. 
     That is, according to the present invention, each of the ribs has a height which reduces gradually from its higher first end to the lower second end. The higher end makes contact with the side face of the immediate upstream rib, so that there are generated the small swirl flows along the cooling medium flow at the contact position of the two ribs. These small swirl flows 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. The cooling passage has 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. A pin projects substantially perpendicularly at a predetermined position in a longitudinal direction of a rib of all or a portion of the plurality of ribs. 
     That is, according to the present invention, by the swirl flows generated by the ribs crossing the cooling medium flow, the high heat transfer rate areas are formed. In addition thereto, the pins project so that the swirl flows are further generated on the downstream side of the respective pins so as to flow along the inclined ribs. Thus, 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, in which the 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. Thus, 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, in which pin is provided in the 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. Therefore, 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 
     FIG. 1 is a perspective, partially cut away, view of a gas turbine cooled blade of a first embodiment according to the present invention. 
     FIG. 2 is a schematic cross sectional view 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 ). 
     FIG. 3 is a view showing 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. 
     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 s a view showing the heat transfer rate distribution. 
     FIG. 5 is a view showing 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. 
     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. 
     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 ). 
     FIG. 9 is a longitudinal cross sectional view of a conventional gas turbine cooled blade. 
     FIG. 10 is an enlarged view of one of the 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. 
     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. 
     FIG. 12 is a schematic cross-sectional view of an alternate embodiment of the gas turbine cooled blade of FIG. 2, wherein the FIG.  12 ( a ) is a longitudinal cross sectional view and FIG.  12 ( b ) is a cross sectional view taken along line II—II of FIG.  12 ( a ) 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A first embodiment according to the present invention will be described with reference to FIGS. 1 and 2. It is to be noted that the same parts as those in the prior art mentioned above are given the same reference numerals in the figures, and repeated description is omitted as much as possible. Characteristic points of the present embodiment will mainly be described. 
     FIG. 1 is a perspective, partially cut away, view of a gas turbine cooled blade of a first embodiment according to the present invention. 
     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 ). 
     In the present embodiment, a seal air  3  also having a blade cooling function 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. While the air cools the second row cooling passage B, a portion of the seal air  3  is separated and supplied into the first row cooling passage A, and the portion thereof is led into an inner cavity  10  as the seal air. 
     That is, as shown in FIGS. 1 and 2, a plurality of communication holes  6  are bored 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 the same as those in the prior art described above. 
     In the present embodiment constructed as described above, a cooling medium  1  having both a sealing function and a blade cooling function is supplied into the second row cooling passage B from an outer shroud  9  side. After cooling the inner surfaces of the passage, the cooling medium  1  is partially led into the inner cavity  10  as the seal air  3 . 
     The remaining part of the cooling medium  1  is supplied into the first row cooling passage A through the communication holes  6 . After cooling inner surfaces of the passage as a cooling air, the remaining portion 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. The cooling medium 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. Thus, the blade outer surfaces of the portion corresponding to the second row cooling passage B are affected by the film cooling and reduced in 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 for using a seal tube, which results in no increase in parts number and assembly time. 
     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 a 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 as shown in FIGS.  12 ( a ) and  12 ( b ). 
     In the case of such variation as this, the third row cooling passage C and subsequent cooling passages have been less thermal load than the second row cooling passage B. Consequently, the seal air is maintained at 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, and each of the ribs is 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 . Each of the ribs is also inclined with a constant angle  0  to the main flow direction of the cooling medium  23 , and there is a constant rib to rib pitch P in the main flow direction of the cooling medium  23 . 
     As shown in FIG.  3 ( a ), each of the ribs  31  is disposed at an incline in a width Wa which is smaller than an entire width W of the cooling passage  22 . Each rib has a height that gradually reduces from its higher first 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 lower second end at a downstream outer side thereof having a height f which is lower than e. 
     Each of the ribs  31  makes contact at its first end portion having the height a 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 the adjacent rib  31  at a contact portion. A plurality of ribs  31  alternatively extend at an incline 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, due to the effect of the ribs  31  disposed alternately and inclined in mutually opposing directions, the cooling medium flowing in the cooling passage  22  generates a swirl flow  23   b  which flows in a swirling motion and is inclined 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 . 
     Because the rib  31  has a shape which reduces its height from a to f, and because 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 the cooling passage. 
     In the present embodiment constructed as described 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  23   b  as shown in FIG.  4 ( d ). Because the swirl flow  23   b  is formed in the respective spaces formed between the ribs  31  on both side portions of the cooling passage  22 , the high heat transfer rate area  26  is also formed therein on both side portions of the cooling passage. 
     Also, the rib  31  has a shape in which the height is reduced from a 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 . As a result, the heat transfer rate is further enhanced. 
     It is to be noted that, as for the height e, f of the rib  31 , even in the case where a equals f, a 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 incline in mutually opposing directions. The first end portion of the rib  31  makes contact with a side surface of the upstream side rib  31 , and the rib  31  has a shape to reduce its height from a to f. Thus, the swirl flow  23   b  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 has basically the same shape of rib and the 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  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, the pin  24  is provided on each of the ribs  24 , but the pin  24  is not necessarily provided on each of the ribs  24  and 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 aside 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  23   b  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 the same function as that of the prior art shown in FIG.  11 . 
     In addition thereto, due to the 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 the inclined rib  24  so that a high heat transfer rate area  31  is formed 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 . 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 the 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 also embraces such modified forms thereof as come within the scope of the following claims.