Patent Publication Number: US-8984895-B2

Title: Metallic ceramic spool for a gas turbine engine

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims the benefits, under 35 U.S.C. §119(e), of U.S. Provisional Application Ser. No. 61/363,113 entitled “Metallic Ceramic Spool for a Gas Turbine Engine” filed on Jul. 9, 2010, which is incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates generally to gas turbine engines and in particular to a gas turbine spool design combining metallic and ceramic components. 
     BACKGROUND 
     There is a growing requirement for alternate fuels for vehicle propulsion and power generation. These include fuels such as natural gas, bio-diesel, ethanol, butanol, hydrogen and the like. Means of utilizing fuels needs to be accomplished more efficiently and with substantially lower carbon dioxide emissions and other air pollutants such as NOxs. 
     The gas turbine or Brayton cycle power plant has demonstrated many attractive features which make it a candidate for advanced vehicular propulsion as well as power generation. Gas turbine engines have the advantage of being highly fuel flexible and fuel tolerant. Additionally, these engines burn fuel at a lower temperature than comparable reciprocating engines so produce substantially less NOx per mass of fuel burned. 
     A multi-spool intercooled, recuperated gas turbine system is particularly suited for use as a power plant for a vehicle, especially a truck, bus or other overland vehicle. However, it has broader applications and may be used in many different environments and applications, including as a stationary electric power module for distributed power generation. 
     The thermal efficiency of gas turbine engines has been steadily improving as the use of new materials and new design tools are being brought to bear on engine design. One of the important advances has been the use of ceramics in various gas turbine engine components which has allowed the use of higher temperature operation and reduced component weight. The use of both metallic and ceramic components in an engine which may have wide variations in operating temperatures, means that special attention be given to the interfaces of the these different materials to preserve the intended component clearances. Control of clearances generally leads to fewer parasitic performance losses. Fewer parasitic performance losses incrementally improves engine efficiency. 
     There therefore remains a need for innovative designs for gas turbine compressor/turbine spools fabricated from a combination of metallic and ceramic materials that maintain a desired control of clearances between various compressor and turbine components. 
     SUMMARY 
     These and other needs are addressed by the various embodiments and configurations of the present invention which are directed generally to a gas turbine spool assembly design combining metallic and ceramic components in a way that controls clearances between critical components over a substantial range of engine operating temperatures and pressures. 
     In a first embodiment, a ceramic turbine rotor rotates just inside a ceramic shroud and separated by a small clearance gap. The ceramic rotor is connected to a metallic volute. In order to accommodate the differential rates of thermal expansion between the ceramic rotor and metallic volute, an active clearance control system is used to maintain the desired axial clearance between ceramic rotor and the ceramic shroud over the range of engine operating temperatures. This clearance control means is comprised of an impingement-cooled conical arm, a shroud carrier and a sliding seal system that allows the metallic volute to expand and move independently of the ceramic shroud thus allowing the clearance gap between ceramic rotor and ceramic shroud to remain substantially constant. 
     With proper design of the impingement cooling air flow and conical arm, the clearance control system can automatically maintain an approximately constant width of clearance gap between the rotor blades and the shroud over most or all of the operating conditions of the engine, from idle to full power. This in turn minimizes leakage of gas flow between the rotor blades and shroud. This clearance control system thus allows metallic and ceramic components to be used without compromising overall engine efficiency. As can be appreciated, the active clearance control system described herein can be designed to 1) maintain an approximately constant width of clearance gap between the rotor blades and the shroud over most or all of the operating conditions of the engine; 2) a slightly decreasing width of clearance gap between the rotor blades and the shroud over most or all of the operating conditions of the engine; 3) a slightly increasing width of clearance gap between the rotor blades and the shroud over most or all of the operating conditions of the engine; or 4) a prescribed width of clearance gap between the rotor blades and the shroud over most or all of the operating conditions of the engine. 
     In a second embodiment, a ceramic turbine rotor rotates just inside a ceramic shroud which is part of a single piece ceramic volute/shroud assembly. As temperature increases, the ceramic volute expands at approximately the same rate as ceramic shroud and tends to increase the axial clearance gap between the ceramic rotor and ceramic shroud, but only by a small amount compared to a metallic volute attached to the shroud in the same way. A compliant metallic bellows connecting the outer case of the turbo-compressor spool assembly and the ceramic shroud does not allow the case to pull shroud away from the rotor. 
     In one embodiment, a gas turbine engine comprising at least one turbo-compressor spool assembly, wherein the at least one turbo-compressor spool assembly comprises a compressor in mechanical communication with a turbine, a volute directing an inlet gas towards an inlet of a rotor of the turbine and a shroud adjacent to the rotor of the turbine, the shroud directing an outlet gas towards an outlet of the at least one turbo-compressor spool assembly and a clearance control device to substantially maintain, during the at least one turbo-compressor spool assembly operation, an operational clearance between the rotor and shroud at a level no greater than about 110% of a non-operational clearance between the rotor and shroud when the at least one turbo-compressor spool assembly is non-operational. 
     In another embodiment, a method, comprising providing an engine comprising at least one turbo-compressor spool assembly, wherein the at least one turbo-compressor spool assembly comprises a compressor in mechanical communication with a turbine, a volute adjacent to a rotor of the turbine directing an inlet gas towards an inlet of the turbine rotor, and a shroud adjacent to the turbine rotor, the shroud directing an outlet gas towards an outlet of the at least one turbo-compressor spool assembly and substantially maintaining, during the at least one turbo-compressor spool assembly operation, an operational clearance between the rotor and shroud at a level no greater than about 110% of a non-operational clearance between the rotor and shroud when the at least one turbo-compressor spool assembly is non-operational. 
     In another embodiment, a gas turbine engine, comprising at least one turbo-compressor spool assembly, wherein the at least one turbo-compressor spool assembly comprises a compressor in mechanical communication with a turbine, a volute directing an input gas to a rotor of the turbine, and a shroud adjacent to the turbine rotor, the shroud directing an outlet gas towards an outlet of the at least one turbo-compressor spool assembly, wherein the volute and shroud each comprise a ceramic material to maintain, during the at least one turbo-compressor spool assembly operation, at least an operational clearance between the rotor and shroud of no more than about 110% of a non-operational clearance between the rotor and shroud when the at least one turbo-compressor spool assembly is non-operational. 
     The present invention is illustrated for a gas turbine engine with an output shaft power in the range from about 200 to about 375 kW. The diameter of the ceramic turbine rotor is about 95 mm and the desired clearance gap between the ceramic rotor and shroud is about 0.38 mm. The diameter of the ceramic turbine rotor commonly ranges from about 75 to about 125 mm, more commonly from about 85 to about 115 mm, and even more commonly is about 95-mm and the desired clearance gap between the ceramic rotor and shroud commonly ranges from about 0.25 to about 0.50 mm, more commonly ranges from about 0.30 to about 0.45 mm, and even more commonly is about 0.38 mm. Without impingement cooling, the axial motion of the shroud with respect to the rotor at operating temperature is in the range of about 0.7 to about 1 mm which will substantially increase the clearance gap between the ceramic rotor and shroud. The clearance gap increases from the desired 0.38 mm to as much as about 1 mm, or a potential three-fold (about 300%) increase in gap width which, in turn, would result in an approximately three-fold increase in leakage mass flow rate. The present disclosure, by contrast, can maintain the axial motion of the shroud at operating temperature to a level commonly of less than about 0.06 mm, more commonly of no more than about 0.05 mm, more commonly of no more than about 0.04 mm, more commonly of no more than about 0.03 mm, and even more commonly of no more than about 0.02 mm. Stated differently, the axial motion of the shroud at operating temperature is maintained at a level of commonly no more than about 16%, more commonly no more than about 13%, more commonly no more than about 10.5%, more commonly no more than about 8.0%, and even more commonly no more than about 5%. 
     As can be appreciated, the impingement-cooling-driven clearance control method of the present invention can be applied to any spool of any size gas turbine engine. 
     These and other advantages will be apparent from the disclosure of the invention(s) contained herein. 
     The above-described embodiments and configurations are neither complete nor exhaustive. As will be appreciated, other embodiments of the invention are possible utilizing, alone or in combination, one or more of the features set forth above or described in detail below. 
     The following definitions are used herein: 
     Ceramic refers to an inorganic, nonmetallic solid prepared by the action of heat and subsequent cooling. Ceramic materials may have a crystalline or partly crystalline structure, or may be amorphous (e.g., a glass). Some properties of several ceramics used in gas turbines are shown in Table 1. 
     An engine is a prime mover and refers to any device that uses energy to develop mechanical power, such as motion in some other machine. Examples are diesel engines, gas turbine engines, microturbines, Stirling engines and spark ignition engines 
     A gasifier is that portion of a gas turbine engine that produce the energy in the form of pressurized hot gasses that can then be expanded across the free power turbine to produce energy. 
     A gas turbine engine as used herein may also be referred to as a turbine engine or microturbine engine. A microturbine is commonly a sub category under the class of prime movers called gas turbines and is typically a gas turbine with an output power in the approximate range of about a few kilowatts to about 700 kilowatts. A turbine or gas turbine engine is commonly used to describe engines with output power in the range above about 700 kilowatts. As can be appreciated, a gas turbine engine can be a microturbine since the engines may be similar in architecture but differing in output power level. The power level at which a microturbine becomes a turbine engine is arbitrary and the distinction has no meaning as used herein. 
     A recuperator as used herein is a gas-to-gas heat exchanger dedicated to returning exhaust heat energy from a process back into the pre-combustion process to increase process efficiency. In a gas turbine thermodynamic cycle, heat energy is transferred from the turbine discharge to the combustor inlet gas stream, thereby reducing heating required by fuel to achieve a requisite firing temperature. 
     A regenerator is a heat exchanger that transfers heat by submerging a matrix alternately in the hot and then the cold gas streams wherein the flow on the hot side of the heat exchanger is typically exhaust gas and the flow on cold side of the heat exchanger is typically gas entering the combustion chamber. 
     Spool means a group of turbo machinery components on a common shaft. 
     A turbine is any machine in which mechanical work is extracted from a moving fluid by expanding the fluid from a higher pressure to a lower pressure. 
     Turbine Inlet Temperature (TIT) as used herein refers to the gas temperature at the outlet of the combustor which is closely connected to the inlet of the high pressure turbine and these are generally taken to be the same temperature. 
     A turbo-compressor spool assembly as used herein refers to an assembly typically comprised of an outer case, a radial compressor, a radial turbine wherein the radial compressor and radial turbine are attached to a common shaft. The assembly also includes inlet ducting for the compressor, a compressor rotor, a diffuser for the compressor outlet, a volute for incoming flow to the turbine, a turbine rotor and an outlet diffuser for the turbine. The shaft connecting the compressor and turbine includes a bearing system. An example of a turbo-compressor spool assembly is shown in  FIG. 5  herein. 
     A volute is a scroll transition duct which looks like a tuba or a snail shell. Volutes may be used to channel flow gases from one component of a gas turbine to the next. Gases flow through the helical body of the scroll and are redirected into the next component. A key advantage of the scroll is that the device inherently provides a constant flow angle at the inlet and outlet. To date, this type of transition duct has only been successfully used on small engines or turbochargers where the geometrical fabrication issues are less involved. 
     As used herein, “at least one”, “one or more”, and “and/or” are open-ended expressions that are both conjunctive and disjunctive in operation. For example, each of the expressions “at least one of A, B and C”, “at least one of A, B, or C”, “one or more of A, B, and C”, “one or more of A, B, or C” and “A, B, and/or C” means A alone, B alone, C alone, A and B together, A and C together, B and C together, or A, B and C together. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention. In the drawings, like reference numerals refer to like or analogous components throughout the several views 
         FIG. 1  is a schematic of an intercooled, recuperated gas turbine engine cycle with reheat. This is prior art. 
         FIG. 2  is a stress-temperature map showing ceramic failure regimes. 
         FIG. 3  is a schematic of a spool with a metallic compressor rotor and a ceramic turbine rotor. This is prior art. 
         FIG. 4  is a schematic of a gas turbine compressor/turbine spool with ceramic and metallic components that has an axial clearance problem. 
         FIG. 5  is a schematic of a gas turbine compressor/turbine spool with ceramic and metallic components and active sealing. 
         FIGS. 6   a - b  are schematics of a metallic conical arm for controlling clearances. 
         FIGS. 7   a - d  are schematics of a metallic volute and ceramic shroud components. 
         FIG. 8  is a schematic of the details of the interface and sealing system between a ceramic shroud and a metallic shroud carrier. 
         FIG. 9  is schematic of a gas turbine compressor/turbine spool with a one piece ceramic volute and shroud. 
         FIGS. 10   a - b  are schematics of a ceramic volute and shroud. 
     
    
    
     DETAILED DESCRIPTION 
     Gas Turbine Engine Architecture 
       FIG. 1  is a schematic of an intercooled, recuperated gas turbine engine cycle with reheat. This configuration of gas turbine components is known. Gas is ingested through optional valve  101  into a low pressure compressor (LPC)  102 . The outlet of the low pressure compressor  102  passes through an intercooler (IC)  103 , which removes a portion of heat from the gas stream at approximately constant pressure. The gas then enters a high pressure compressor (HPC)  104 . The outlet of high pressure compressor  104  passes through a recuperator (RECUP)  105  where some heat from the exhaust gas is transferred, at approximately constant pressure, to the gas flow from the high pressure compressor  104 . The further heated gas from recuperator  105  is then directed to a combustor (COMB)  106  where a fuel is burned, adding heat energy to the gas flow at approximately constant pressure. The gas emerging from the combustor  106  then enters a high pressure turbine (HPT)  107  where work is done by the turbine to operate the high pressure compressor. The gas from the high pressure turbine  107  then enters a reheat combustor (REHEAT)  108  where additional fuel is burned, adding heat energy to the gas flow, again at approximately constant pressure. The gas from the reheater  108  then drives a low pressure turbine (LPT)  109  where work is done by the turbine to operate the low pressure compressor. The gas from the low pressure turbine  109  then drives a free power turbine (FPT)  110  where energy is extracted and converted to rotary mechanical energy of a shaft. The shaft of the free power turbine  110 , in turn, drives a transmission (TRANS)  111  which drives an electrical generator (GEN) or mechanical drive shaft  112 . As can be appreciated, an alternate version of this engine architecture can omit the reheat combustor  108  or relocate reheat combustor  108  between low pressure turbine  109  and free power turbine  110 . 
     The low pressure compressor  102  is coupled to the low pressure turbine  109  by shafts  131  and  132  which may be coupled by a gear box  121 . Alternately, the low pressure compressor  102  may be coupled to the low pressure turbine  109  by a single shaft. The components including low pressure compressor  102 , shafts  131  and  132 , gear box  121  and low pressure turbine  109  comprise the low pressure spool of the gas turbine engine. 
     The high pressure compressor  104  is coupled to the high pressure turbine  107  by shafts  133  and  134  which may be coupled by a gear box  122 . Alternately, the high pressure compressor  104  may be coupled to the high pressure turbine  107  by a single shaft. The components including high pressure compressor  104 , shafts  133  and  134 , gear box  122  and high pressure turbine  107  comprise the high pressure spool of the gas turbine engine. 
     The various components described above may be made from a variety of materials depending on the mechanical and thermal stresses they are expected to encounter, especially in a vehicle engine application where components may be subjected to a range of mechanical and thermal stresses as the engine load varies from idle to full power. For example, the low pressure spool components may be made from metals, typically steel alloys, titanium and the like. The high pressure spool components may be made from a combination of metals and ceramics. For example, the turbine rotors may be made from silicon nitride while turbine shroud and volutes may be made from ceramics such as silicon carbide. The compressor and turbine housings or cases are generally made of steel to contain a potentially fragmenting ceramic volute, rotor or shroud. 
     The combustor and reheater may be made from metals but they may also be made from ceramics. For example, a ceramic thermal oxidizer (also known as a thermal reactor) may function as a high-temperature combustor or as a reheater. 
     Metals, for example, offer strength and ductility for lower temperature components. Ceramics offer light weight for high rpm components and excellent thermal performance for higher temperature components. Higher temperature operation especially in the combustors and high pressure turbine rotors can lead to higher overall thermal engine efficiencies and lower engine fuel consumption. Thus, in the quest for better engine performance, ceramics will be used more and more and in combination with metal components. One of the impediments to achieving efficiency gains by the use of both metals and ceramics is the parasitic flow losses that can result when these materials are used together over a variable range of temperatures. These losses occur because of the differential thermal expansion rates of ceramics and metals. 
     Ceramic Materials 
       FIG. 2  is a stress-temperature map illustrating ceramic failure regimes. This graphic shows that if flexure stress and temperature experienced by a ceramic component are high then the component operates in the fast fracture regime and the ceramic component lifetime would be expected to be unpredictable and typically short. This graphic also shows that if flexure stress and temperature experienced by a ceramic component are low then the component operates in the no failure regime and the ceramic component lifetime would be expected to be predictable and typically long. If the flexure stress is high but the temperature is low then the component operates in a region characterized by Weibull strength variability. If the flexure stress is low but the temperature is high then the component operates in a region characterized by slow crack growth and the ceramic component lifetime would be expected to be somewhat unpredictable and variable. 
     Some gas turbine engines, especially microturbines, have used ceramic components in prototype situations. These have been used for relatively high temperatures and have operated in the slow crack growth region. These engines have experienced failure of the ceramic components. One of the design goals used in the present invention is to maintain ceramic component operation well inside the no failure regime so that incidences of component failure are minimized and component lifetime is maximized. A number of turbochargers have used ceramic components, most notably ceramic rotors, operating in the no failure region. 
     The following table shows some important properties of ceramics that are typically used for gas turbine components. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                   
                   
                 Silicon 
                 Silicon 
                   
               
               
                   
                 Alumina 
                 Cordierite 
                 Carbide 
                 Nitride 
                 Mullite 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Density 
                 3,700-3,970 
                 2,600 
                 3,210 
                 3,310 
                 2,800 
               
               
                 (kg/m3) 
               
               
                 Specific 
                 670 
                 1,465 
                 628 
                 712 
                 963 
               
               
                 Heat 
               
               
                 (J/kg/K) 
               
               
                 Thermal 
                 24 
                 3 
                 41 
                 27 
                 3.5 
               
               
                 Conductivity 
               
               
                 (W/m/K) 
               
               
                 Coefficient 
                 8.39 
                 1.7 
                 5.12 
                 3.14 
                 5.3 
               
               
                 Thermal 
               
               
                 Expansion 
               
               
                 (μm/m/K) 
               
               
                 Thermal 
                 200-250 
                 500 
                 350-500 
                 750 
                 300 
               
               
                 Shock 
               
               
                 Resistance 
               
               
                 (ΔT (K)) 
               
               
                 Maximum 
                 3,925 
                 1,645 
                 1,675 
                 1,775 
                 1,975 
               
               
                 Use 
               
               
                 Temperature 
               
               
                 (K) 
               
               
                   
               
            
           
         
       
     
       FIG. 3  is a schematic of compressor-turbine spool with a metallic compressor rotor and a ceramic turbine rotor. This is prior art. This figure illustrates a compressor/turbine spool typical of the present invention. A metallic compressor rotor  302  and a ceramic turbine rotor  303  are shown attached to the opposite ends of a metal shaft  301 . The ceramic rotor shown here is a representation of a 95-mm diameter rotor fabricated from silicon nitride that was designed for use in turbocharger applications. 
     Design with Axial Clearance Problem 
       FIG. 4  is a schematic of a gas turbine compressor/turbine spool assembly with ceramic and metallic components. This configuration does not have active rotor/shroud clearance control but does have an unacceptable axial clearance growth problem when the assembly is heated to operational temperatures. A ceramic turbine rotor  403  is shown attached to a metallic shaft  405  which is attached to a metallic compressor rotor (not shown, see  FIG. 3 ). Ceramic rotor  403  is separated by a small clearance gap (see  FIG. 8  for detail) from a ceramic shroud  402 . Ceramic shroud  402  is attached to a metallic volute  401 . The ceramic shroud  402  is also attached to a compliant metallic bellows  406  which is, in turn, attached to an outer metal case  404 . The metallic volute  401  can be fabricated from a high temperature alloy such as Hastelloy-X. The ceramic rotor  403  can be fabricated from silicon nitride, for example, and is capable of operating safely at turbine inlet temperatures in the approximate range of 1,400 K. Ceramic shroud  402  can be fabricated from silicon carbide, for example, and has a coefficient of thermal expansion similar to that of silicon nitride. The use of a rotor and shroud fabricated from the same or similar ceramics is designed to substantially maintain rotor/shroud radial clearance over a wide range of engine operating temperatures. In the design of  FIG. 4 , the metallic volute  401 , which is exposed to turbine inlet temperatures is less likely to catastrophically fail than a ceramic volute such as described below in  FIG. 9 . However, there will be differential axial and radial expansion between the metallic volute  401  and ceramic shroud  402  which can result in growth of an axial clearance gap between ceramic rotor  403  and ceramic shroud  402 . This, in turn, can lead to parasitic flow losses with the growth of an axial clearance gap between the rotor blade tips and the shroud as the shroud moves axially away from rotor  403  with increasing temperature of the assembly. 
     In this configuration, when the assembly is heated, ceramic rotor  403  and ceramic shroud  402  have approximately the same coefficient of thermal expansion and so they expand radially approximately by the same amount thus retaining the approximate initial radial clearance between rotor  403  and shroud  402 . However, as the assembly is heated, case  404 , the compliant bellows  406  and volute  401  all have coefficients of thermal expansion typical of metals and therefore expand much faster with increasing temperature than the ceramic rotor  403  and ceramic shroud  402 . The metallic volute  401  is fixed in position with respect to case  404  as it is held within a circumferential groove in case  404 . Nevertheless, the right side of the volute expands and pushes shroud  402  to the right. Case  404  and bellows  406  also expand to the right but the compliance of the bellows does not allow the case  404  to strongly pull shroud  402  to the right. The expansion of the metallic volute  401  does, however, cause the axial clearance between rotor and shroud to increase and increases the axial clearance gap beyond that which is desired. 
     Therefore, a preferable design would be a metallic volute interfaced with a ceramic shroud with a means of controlling the axial expansion of the shroud over the range of anticipated operating temperatures from idle through full power operation. Such a design should be capable of providing a means of limiting parasitic flow leakage from the high pressure side of the rotor  403  around the outside of the shroud  402 . 
     Present Invention 
     Metallic Volute Ceramic Rotor/Shroud Embodiment 
       FIG. 5  is schematic of a gas turbine compressor/turbine spool assembly with ceramic and metallic components and with an active clearance control system. In this embodiment, a ceramic turbine rotor  501  and a metallic compressor rotor  502  are shown on a metal spool shaft  503 . The ceramic rotor  501  rotates just inside ceramic shroud  505 , driven by gas entering via metallic volute  504 . This configuration differs from that of  FIG. 4  as the compliant bellows attachment means is replaced by an active clearance control means. This clearance control means is comprised of an impingement-cooled conical arm  507  and several moveable parts broadly shown as  506  which are moved by conical arm  507  during operation of the engine. The function of the clearance control means is to maintain a desired axial clearance between ceramic rotor  501  and the ceramic shroud  505  over the range of engine operating temperatures. Ceramic shroud  505  is connected by a metallic shroud carrier (item  703  of  FIG. 7 ) which in turn is connected to metal housing  508 . As the operating temperature varies over the power range of the engine, the metal case  508  to which the ceramic shroud carrier is attached moves axially with respect to the ceramic rotor. However, ceramic shroud  505  slides within the shroud carrier thus allowing the clearance gap between ceramic rotor  501  and ceramic shroud to remain substantially constant as described in more detail in  FIG. 8 . The way in which all these parts function with varying temperature is described fully in  FIG. 8 . As will also be apparent from  FIG. 8 , metallic volute  504  is not attached to ceramic shroud  501  but rather the two components can slide axially relative to one another. The impingement cooling of conical arm  507  is provided by a cooler air flow bled from the output of the high pressure compressor (commonly the bleed gas flow is in a temperature range of about 400 K to about 800 K, more commonly of about 450 K to about 700 K, more commonly of about 475 K to about 600 K, and even more commonly of about 500 K to about 530 K) and directed via a small channel to the region to the right of the flexing section of conical arm  507 . The temperature of the bleed air or gas from the high pressure compressor output is commonly between about 35% to 50% of the output temperature of the high pressure turbine gas outlet. 
     As in the configuration described in  FIG. 4 , metallic volute  504  can be fabricated from a high temperature alloy such as Hastelloy-X, ceramic rotor  501  can be fabricated from silicon nitride, for example, and ceramic shroud  505  can be fabricated from silicon carbide, for example. 
       FIG. 6  is a schematic of a metallic conical arm for controlling clearances.  FIG. 6   a  shows an isometric view of the conical arm  601 .  FIG. 6   b  shows a cut away view of the conical arm and shows a cylindrical pusher section  603  and a conical flexing section  602 . The cylindrical pusher section  603  is also referred to as an armature. When there is no impingement cooling, the temperature of the conical flexing section  602  ranges from about 800 to about 1,080 K. When there is impingement cooling, the temperature of the conical flexing section  602  is lower than in the absence of such cooling. When there is impingement cooling, commonly the temperature of the conical flexing section  602  is less than about 800 K, more commonly ranges from about 450 K to about 750 K, and even more commonly ranges from about 575 K to about 725 K. This cooling of the conical arm causes it to push the sealing mechanism and ceramic shroud to the left (as viewed in  FIG. 5 ), thereby maintaining the desired clearance between the ceramic rotor and ceramic shroud. The above temperature ranges are typical for a specific engine configuration and are given to illustrate the principle of operation of the conical arm. 
       FIG. 7  is a schematic of a metallic volute and ceramic shroud components.  FIG. 7   a  shows a metallic volute  701  which is typically a cast component.  FIG. 7   b  shows an isometric cutaway view of the metallic volute showing circumferential rings and grooves  702  that serve as a labyrinth seal as described more fully in  FIG. 8 .  FIG. 7   c  shows a ceramic shroud  703  with pins  704  that position and hold the shroud with respect to the shroud carrier. A two piece (clamshell) metallic shroud carrier  705  is shown in  FIG. 7   d . This shroud carrier adapts the shroud  703  to a metal case (shown below in  FIG. 8 ). For example, if the shroud carrier  703  is fabricated from Hastelloy-X and the shroud is fabricated from silicon carbide ceramic, the coefficient of thermal expansion of the metallic shroud carrier, which in turn is attached to the metal case (see  FIG. 5 ), is larger than the coefficient of thermal expansion of the ceramic shroud, commonly being approximately 3 times that of the ceramic shroud. The coefficient of thermal expansion of the metallic shroud carrier may be the same or different than the coefficient of thermal expansion of the metallic volute. This differential expansion will lead to axial movement of the shroud relative to the ceramic rotor since the shroud carrier moves with the metal case. If the axial clearance between the rotor and shroud is not controlled, then parasitic flow leakage will occur around the rotor blade tips and inside of the shroud. This parasitic leakage can cause an overall engine efficiency in the range of about ½% to about 2%. It can also lead to increased erosion of the rotor blade tips and upstream edge of the shroud. The present disclosure can substantially minimize parasitic leakage and provide a higher overall engine efficiency. 
       FIG. 8  is a schematic of the details of the active clearance control for maintaining a desired clearance  809  between ceramic rotor  801  and ceramic shroud  802 . This figure shows a ceramic rotor  801  separated from a ceramic shroud  802  by a small clearance gap  809  which allows ceramic rotor  801  to rotate freely relative to ceramic shroud  802 . This figure also shows the sealing system between the metallic volute  803  and ceramic shroud  802 . The metallic volute  803  is attached to a metallic labyrinth seal cylinder  808 . The sealing system allows the ceramic shroud  802  to slide axially relative to the metallic volute  803 . The labyrinth seal is provided by the circumferential rings shown on the outside of the labyrinth seal cylinder  808 . A metallic conical arm  804  is shown inserted into a metallic push plate  805  which in turn is in contact with metallic shroud carrier  806 . Metallic conical arm  804  is referred to as an armature and is the cylindrical pusher section shown as item  603  of  FIG. 6 . The shroud carrier  806  is a two piece component described previously in  FIG. 7   d . A metallic labyrinth seal sleeve  807  holds the various components in place and its inside diameter forms a sealing surface for the labyrinth seal teeth on labyrinth seal cylinder  808 . 
     As noted in  FIG. 4 , the use of a rotor and shroud fabricated from the same or similar ceramics is designed to substantially maintain rotor/shroud radial clearance over a wide range of engine operating temperatures. 
     The coefficient of thermal expansion of the metallic components are substantially greater than that of the ceramic components. For example, thermal expansion of a Hastelloy-X shroud carrier is 3 times that of a silicon carbide shroud. 
     Ceramic shroud  802  is connected by a metallic shroud carrier  806  which is ultimately connected to the metallic turbine case or housing (item  508  in  FIG. 5 ). As the operating temperature of the gas turbine engine varies, the ceramic shroud  802  moves axially with respect to ceramic rotor  809 . In the absence of an active clearance control system, the axial clearance gap  809  would increase as the operating temperature of the turbine increases. As this clearance gap increases, more of the flow through the turbine bypasses the turbine blades by flowing through gap  809  causing a decrease in turbine efficiency. 
     When the conical arm  804  (shown in full in  FIG. 6 ) is cooled by impingement cooling, the cylindrical pusher section of conical arm  804  is forced to the left (as viewed in  FIG. 5  and  FIG. 8 ), pushing on pusher plate  805  which then moves shroud carrier  806  and shroud  802  to the left, in a direction that decreases clearance gap  809 . By controlling the amount of impingement cooling of the conical arm, the tendency of the gap to increase by the expansion of the metal turbine housing (item  508  in  FIG. 5 ) is balanced by the action of the conical arm which tends to decrease clearance gap  809 . With proper design of the impingement cooling air flow and conical arm, the clearance control system can automatically maintain an approximately constant width of clearance gap  809  over most or all of the operating conditions of the engine (from idle to full power). This in turn maintains the desired optimum clearance between ceramic rotor  801  and ceramic shroud  802  and thereby minimizes leakage of gas flow between the rotor blades and shroud. This clearance control system thus allows metallic and ceramic components to be used without compromising overall engine efficiency. 
     The configuration shown in  FIGS. 4 ,  5  and  9  are all based on a gas turbine engine design in which the full power mass flow rate is approximately 1.25 kg/s; the two-stage compression ratio is about 15, the high pressure turbine inlet temperature is about 1,400 K and the full shaft power of the free power turbine is about 375 kW. The diameter of the ceramic turbine rotor is about 95-mm and the desired clearance gap between the ceramic rotor and shroud is about 0.38 mm. Without impingement cooling, the axial motion of the shroud with respect to the rotor at operating temperature is in the range of 0.7 to 1 mm which will substantially increase the clearance gap between the ceramic rotor and shroud. This illustrates the importance of the impingement-cooling-driven clearance control system of  FIG. 8 . Without this system, the clearance gap between the ceramic rotor and shroud increases from the desired 0.38 mm to as much as 1 mm, or a potential three-fold increase in gap width which, in turn, would result in an approximately three-fold increase in leakage mass flow rate. 
     As can be appreciated, the impingement-cooling-driven clearance control method described in  FIG. 8  can be applied to any spool of any size gas turbine engine. 
     Ceramic Volute, Rotor and Shroud Embodiment 
       FIG. 9  is schematic of a gas turbine compressor/turbine spool assembly with ceramic and metallic components. A ceramic turbine rotor  903  is shown separated by a small clearance gap from a ceramic shroud  902  which is integral with a ceramic volute  901 . The volute, shroud and rotor are housed inside a metal case  904 . The ceramic shroud  902  is also attached to a compliant metallic bellows  906  which is attached to an outer metal case  905 . For example the ceramic rotor  903  can be fabricated from silicon carbide and is capable of operating safely at turbine inlet temperatures in the approximate range commonly of from about 850 to about 1,800 K, more commonly of from about 950 to about 1,650 K and even more commonly of about 1,400 K. Ceramic shroud  902  and volute  901  can be fabricated from silicon carbide, for example, which has a coefficient of thermal expansion similar to that of silicon nitride used for rotor  903 . 
     In this embodiment, when the assembly is heated during engine operation, the ceramic rotor  903  and ceramic shroud  902  have approximately the same coefficient of thermal expansion and so they expand radially approximately by the same amount thus retaining the approximate initial radial clearance between rotor  903  and shroud  902 . The right side of ceramic volute  901  expands at approximately the same rate as ceramic shroud  902  and tends to push shroud  902  to the right but only by a small amount. As the assembly is heated, case  905  and bellows  906  have coefficients of thermal expansion typical of metals. Case  905  and compliant metallic bellows  906  also expand to the right but the compliance of the bellows does not allow the case  905  to pull shroud  902  to the right. The expansion of the ceramic volute  901  is relatively small and does not cause the axial clearance gap between rotor and shroud to increase beyond that which is desired. 
     The use of a rotor and volute/shroud fabricated from the same or similar ceramics adequately thus controls radial and axial shroud clearances between the rotor  903  and shroud  902  and maintains high rotor efficiency by controlling the clearance and minimizing parasitic flow leakages between the rotor blade tips and the shroud. 
     The advantages of this design approach are:
         similar coefficient of thermal expansion of ceramic volute/shroud and rotor gives excellent shroud clearance control   maintains good form stability—will keep its shape at high temperatures   has good thermal shock properties   allows complicated shapes can be easily cast   is cost effective compared to high temperature turbine metals       

     The temperature of the flow exiting the combustor into the volute that directs the flow to the high pressure turbine may be in substantially the same range as the turbine inlet temperature. The temperature of the flow exiting the high pressure turbine into the shroud that directs the flow towards the low pressure turbine may be in the range of from about 1,000 to about 1,400 K, more commonly from about 1,000 to about 1,300 K, and even more commonly of approximately 1,200 K. Stated differently, the inlet temperature of the high pressure turbine is commonly higher than, more commonly about 5% higher than, more commonly about 10% higher than, more commonly about 15% higher than, and even more commonly about 20% higher than the high pressure turbine gas outlet temperature. A one-piece volute and shroud may be exposed to a temperature differential in the range of about 100 K to about 300 K and more commonly about 160 K to about 200 K. 
     The disadvantages of this design approach are:
         the amount of stress that can be sustained at high temperature in the volute is unpredictable (especially if the materials operate in the slow crack growth or fast fracture regions as shown in  FIG. 3 )   the potential for catastrophic failure of the volute is significant since ceramics generally don&#39;t yield, they behave elastically until they fracture and break abruptly       

     This design of a single piece or two piece ceramic volute and shroud for use with a ceramic turbine rotor is preferred if the ceramic material used can be operated well within the no failure region as shown in  FIG. 3 . 
       FIG. 10  is a schematic of an example of a two piece ceramic volute and shroud such as described in  FIG. 9 .  FIG. 10   a  is an isometric view showing the volute  1001  and the shroud  1002 . The volute/shroud can be made in one piece or multiple pieces. A typical material for such a volute/shroud is silicon carbide.  FIG. 10   b  shows a side cutaway view again illustrating the volute  1003  and the shroud  1004 . Arrows indicate flow direction. 
     The invention has been described with reference to the preferred embodiments. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof. 
     A number of variations and modifications of the inventions can be used. As will be appreciated, it would be possible to provide for some features of the inventions without providing others. 
     The present invention, in various embodiments, includes components, methods, processes, systems and/or apparatus substantially as depicted and described herein, including various embodiments, sub-combinations, and subsets thereof. Those of skill in the art will understand how to make and use the present invention after understanding the present disclosure. The present invention, in various embodiments, includes providing devices and processes in the absence of items not depicted and/or described herein or in various embodiments hereof, including in the absence of such items as may have been used in previous devices or processes, for example for improving performance, achieving ease and\or reducing cost of implementation. 
     The foregoing discussion of the invention has been presented for purposes of illustration and description. The foregoing is not intended to limit the invention to the form or forms disclosed herein. In the foregoing Detailed Description for example, various features of the invention are grouped together in one or more embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed invention requires more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive aspects lie in less than all features of a single foregoing disclosed embodiment. Thus, the following claims are hereby incorporated into this Detailed Description, with each claim standing on its own as a separate preferred embodiment of the invention. 
     Moreover though the description of the invention has included description of one or more embodiments and certain variations and modifications, other variations and modifications are within the scope of the invention, e.g., as may be within the skill and knowledge of those in the art, after understanding the present disclosure. It is intended to obtain rights which include alternative embodiments to the extent permitted, including alternate, interchangeable and/or equivalent structures, functions, ranges or steps to those claimed, whether or not such alternate, interchangeable and/or equivalent structures, functions, ranges or steps are disclosed herein, and without intending to publicly dedicate any patentable subject matter.