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  Method of optimum controlled outlet, impingement cooling and sealing of a heat shield and a heat shield elementRelated Patent Categories: Power Plants, Combustion Products Used As Motive Fluid, Combustion Products Generator, Combustor Liner, PorousThe Patent Description & Claims data below is from USPTO Patent Application 20070245742. Brief Patent Description - Full Patent Description - Patent Application Claims
CROSS REFERENCE TO RELATED APPLICATIONS [0001] This application is the US National Stage of International Application No. PCT/EP2005/055461, filed Oct. 21, 2005 and claims the benefit thereof. The International Application claims the benefits of European application No. 04025338 EP filed Oct. 25, 2004, both of the applications are incorporated by reference herein in their entirety. FIELD OF INVENTION [0002] The present invention relates to a method for cooling a heat shield element comprising a main wall with a cold inner side and a hot outer side, wherein a coolant is introduced into an impingement region of that heat shield element and an impingement flow of said coolant is directed on a surface area of that cold inner side through a plurality of holes for both impingement cooling and flow control. [0003] Furthermore the invention relates to a heat shield element, comprising a main wall with an inner side and an outer side, having an impingement region adjacent to the inner side, said inner side having surface area which can be impinged by a coolant flow introduced through a plurality of impingement holes opposite to said surface area effecting an impingement pressure drop. BACKGROUND OF INVENTION [0004] Because of the very high temperatures attained in a combustion chamber or in channels through which hot gases flow, it is very important to provide sufficient cooling to the main walls by using as little coolant as possible. In addition hot gas ingestion has to be avoided. For this reason an arrangement of heat shield elements is used and nowadays a few cooling methods are applied for cooling the large surfaces of such arrangements. [0005] Regarding the velocity level and direction from which the coolant flow comes into contact with the area to be cooled there is the pure impingement cooling method--the coolant is blown perpendicularly to the surface and by a vigorous impact heat is transferred, the pure convection cooling method--the coolant is introduced parallel to the area and moves along it, and a combination of the above methods. [0006] Regarding the further downstream use of the coolant two types of circuitries are in use: [0007] "Open cooling": Here the coolant discharges into the hot gas (simple design but thermodynamically inefficient) [0008] "closed cooling": After cooling the air is beneficially used, it is ducted to the burners, participating in combustion. [0009] Two kinds of materials can be utilized for the construction of a heat shield element cooled by the methods that were just discussed. On and hand these are high-temperature resistant ceramics. The disadvantage of ceramic materials is their high brittleness. High-temperature Iron-, Chrome-, Nickel- or Cobalt-based metal alloys are the alternative. The thermal conductivity of metals is high, heat extraction is easily possible and the ductile metal is more forgiving the HCF- and LCF-loading. Their operating temperature is limited, however; metals must be cooled sufficiently. [0010] A possible design of the heat shield arrangement used to cool a machine component through which hot gas is passing, especially a combustion chamber of a gas turbine installation, is revealed in WO 98/13645 A1. The arrangement comprises a number of heat shield components with cooling fluid and a hot gas wall to be cooled by that fluid. This heat shield component is composed of two walls--an outer wall which is in contact with the hot gas and a parallel inner wall, so that there is a gap between those two walls. An inlet duct is constructed in such a way, that the cooling fluid is directed towards the inner wall. There it flows through a plurality of apertures, impinges against the outer wall and extends in the direction of this wall. After cooling said wall the fluid flows through an outlet duct running parallel to the inlet duct, leaves the inner room of the heat shield component and is led preferably into the burner of the gas turbine installation. [0011] In EP 1 005 620 B1 a heat shield component is described which is part of a hot gas wall to be cooled. The heat shield arrangement that consists of such heat shield components lines the walls of a hot gas space such as the combustion chamber of a gas turbine. The heat shield component comprises a hollow space; its bottom is exposed to a hot gas, which is attached to a carrier. In the hollow space there is a second hollow body element attached to the same carrier and this element has holes on its bottom. The carrier shows a plurality of inlet channels through which the cooling fluid is fed into the inner space of the hollow body element. The fluid flows through the holes at the bottom of the element, reaches the space between the hollow body element and the heat shield component and impinges the inner side of the bottom of the heat shield component. Then the warmed-up fluid is fed to an outlet channel which opens into the burner of the gas turbine. [0012] Also the EP 1 318 353 A2 discloses heat shields each comprising a liner segment and a support shell for a combustor. The support shell is spaced apart the liner segments to define chambers there between. Impingement cooling holes are arranged in the support shell for establishing impingement cooling of the liner segments from the back. Outlet opening are distributed in the liner segments to enable film cooling of the liner segments. [0013] Another arrangement is known by U.S. Pat. No. 5,396,759, disclosing a heat shield for a bulkhead of an annular combustion chamber. [0014] Different areas of the heat shield are either only impingement cooled or only film cooled or only convective cooled. Spent air is discharged such, that a continuous annular flow is achieved. [0015] Summarized, in all heat shield arrangements, especially those used in the combustion chambers of gas turbines, principally compressed air is branched off from the compressor before entering the combustion chamber and used for cooling the wall of the combustion chamber. The advantage is that at any time there is sufficient air at high pressure which can be utilized to remove the heat from the combustion chamber wall. [0016] The biggest drawback is the loss of combustion air and the burner bypass. Moreover by mixing the cold air with the hot gases in the combustion chamber the temperature level decreases. That causes a reduction of the thermodynamic efficiency and the power output of the gas turbine. SUMMARY OF INVENTION [0017] An object of the present invention is to introduce a method that reduces the overall air consumption for cooling and sealing, especially in the case of open cooling, and thus providing more air for combustion. [0018] Another object of the invention is to provide a heat shield design which utilizes that cooling method. [0019] According to the first object of the invention a flexible method is provided by a heat shield element, comprising a main wall with an inner side, which is restricted by side walls, and an outer side, which can be exposed to a hot fluid, and wherein a coolant is introduced into an impingement region of that heat shield element and an impingement flow of said coolant is directed on a surface area of that inner side through a plurality of impingement holes, causing an impingement pressure drop, wherein after impingement the coolant flow cools the main wall convectively by flowing along the inner side while the coolant converts into a discharge flow, which is drained through a number of discharge holes through said side wall from the inner side to the outer side of the side wall, causing a discharge pressure drop in series with the impingement pressure drop. The impingement pressure drop and the discharge pressure drop are tailored such as to provide a locally required mass flow, which guarantees the predetermined varying heat transfer coefficients on the cold side. [0020] This can be done by using the so-called controlled outlet flow scheme of the cooling air, i.e. one can match the holes through which the cooling air is introduced (impingement holes) with the holes through which the air is drained out of the side walls (or rims) of the heat shield element (discharge holes). Thus tailoring the in-series flow resistances allows two important things: Continue reading... 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