1. Field of the Invention
The present invention relates to a gas turbine blade, in particular, having an improved cooling passages formed inside the blade.
2. Prior Art
In the latest gas turbine plant, a technique of making a gas turbine high temperature has been remarkably developed, and a gas turbine inlet combustion gas temperature has been transferred to 1500.degree. C. or more via a former range of 1000.degree. C. to 1300.degree. C.
In the case where the inlet combustion gas temperature of the gas turbine is made 1500.degree. C. or more, an allowable thermal stress of a gas turbine blade, which is representative of a gas turbine stationary blade or a gas turbine movable (rotating) blade, has already reached the limit although a heat-resisting material has been developed. In an operation having many times of start up and shut down, or in a continuous operation over a long time, there is the possibility that accidents such as a crack and breakdown happen in the heat-resisting material. For this reason, in the case where the gas turbine inlet combustion gas temperature is made high, an air is used as a technique for keeping the gas turbine blade within an allowable temperature by cooling an interior of the gas turbine blade.
However, in the case of cooling the gas turbine blade with the use of the air, the air supply source is an air compressor connected directly to the gas turbine. For this reason, several ten percents (%) of high pressure air supplied from the air compressor to the gas turbine are used for cooling the gas turbine blade. In the relationship between heat input and heat output, the gas turbine plant, which uses much cooling air, has a plant heat efficiency lower than a gas turbine plant which uses a small amount of cooling air. Therefore, it is important to reduce the cooling air so as to improve the plant heat efficiency.
In order to improve the plant heat efficiency, recently, in the gas turbine plant, an air supplied into the gas turbine blade is circulated, and then, is again recovered, so-called, an open loop system is reconsidered.
Moreover, in the gas turbine plant, the following technique has been studied. That is, a steam is used as a cooling medium in order to make high the gas turbine inlet combustion gas temperature and to secure a high power. In that case, the steam supplied into the gas turbine blade is circulated.
As described above, in the recent gas turbine plant, even in the case where the air or steam is used as a cooling medium, the cooling medium supplied into the gas turbine blade is again recovered, and then, the recovered cooling medium is supplied for heat utilization to other equipments, whereby it is expected that the plant heat efficiency is further improved.
In the case of supplying a cooling medium into the gas turbine blade, the cooling medium is circulated to the gas turbine blade to be cooled, and thereafter, is supplied for heat utilization to other equipments. Therefore, a plant heat efficiency can be further improved unlike the conventional case where the cooling medium after cooling the blade joins together with a gas turbine driving gas (main stream). Further, the cooling medium cools the inside of the blade, and thereafter, is recovered, so that there is no disturbance of a stream line of the gas turbine driving gas. Therefore, a blade efficiency can be improved.
Even promising cooling medium recovery type gas turbine plant described above has some problems in the case of supplying the cooling medium into the blade and circulating it. One of these problems is to improve a heat transfer coefficient and to reduce a pressure loss.
Ordinarily, a leading edge or trailing edge of the gas turbine blade is requested having a thin wall thickness to improve a flow performance in spite of receiving a high thermal load of the gas turbine driving gas. Further, the leading edge or trailing edge of the gas turbine blade is required having a streamline shape having a larger curvature. For this reason, a cooling passage section area and the ratio of cooling surface area to an outer surface area inevitably become small as compared with the middle of the blade. In the case of the aforesaid cooling medium recovery type gas turbine, it is disadvantageous to plant efficiency to provide film cooing or ejection holes in a blade wall. For this reason, the following problem arises. That is, a cooling efficiency as a design value is not obtained by convection cooling of merely circulating the cooling medium. Further, a pressure loss of the cooling medium becomes great, and a velocity of flow lowers, resulting in local superheat. Therefore, effective cooling method is required for a blade leading edge and trailing edge.
Recently, in order to improve a heat transfer coefficient of the cooling medium, there has been frequently proposed a technique of providing a rod-like rib in a cooling passage of the gas turbine blade.
However, in the case of providing a rib which functions as a heat transfer accelerating element in the cooling passage of the blade, a pressure loss increases unless the heat transfer accelerating element is located on a proper position. As a result, a flow rate of cooling medium excessively increases, and for this reason, a heat transfer coefficient as a design value can not be obtained. Therefore, proper arrangement of ribs or new ribs are required in order to effectively cool the gas turbine blade.