Abstract:
An apparatus and method are provided for permitting free rotation of turbine nozzle vanes, while minimizing or eliminating leakage on the vane side-walls. The device incorporates a plurality of nozzle vanes arranged around the circumference of a radial or mixed inflow turbine. Each of the nozzles is individually attached to a rod positioned perpendicular to the flow direction. The nozzle rods in the circumferential array protrude through a movable annular back-wall, and are rotated by a linkage or gear. Between discrete movements of the vane mechanism, a bellows device is used to provide pressure-actuated force in the movable back-wall, thereby clamping the nozzle between the two surfaces. Prior to rotating the nozzles, pressure within the bellows interior is decreased via venting to cause a retraction of the back wall.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit under 35 U.S.C. § 119(e) of U.S. provisional application Ser. No. 60/561,567, filed Apr. 13, 2004. The aforementioned provisional application is incorporated herein by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to the field of turbomachinery and, more specifically, to an apparatus and method for the control of gas entering a radial inflow or mixed flow turbine. 
     BACKGROUND OF THE INVENTION 
     A radial turbine is a practical device for converting gas pressure and temperature to shaft power. The majority of radial turbines incorporate fixed geometry nozzle vanes, or airfoils, to optimally guide the gas entering the rotor. In such cases the principal flow parameters, such as pressure, mass flow rate, and temperature, remain in fixed proportion, and cannot be individually controlled. The variable area nozzle provides an additional degree of control freedom, permitting independent control over inlet temperature, pressure, and flow through the turbine stage. Many examples of variable area nozzles have been deployed on such products as hover craft and turbochargers. 
     Most practical variable area nozzles incorporate a mechanism to rotate the nozzle vane. This is particularly critical for gas turbines where the inlet gas temperature is typically in the range from about 1000 degrees F. to 2500 degrees F. Changing the stagger angle alters the throat between nozzle vanes and changes the flow angle entering the rotor. To permit free rotation of the common variable position nozzle, clearance between the nozzle back-wall and the shroud line, or static turbine structure, is required. This clearance represents a performance loss as a percentage of the air entering the rotor will flow beneath or over top of the nozzle vane with a largely radial trajectory. This flow vector is not optimal for work extraction within the rotor, and generally contributes to inefficiency within the turbine stage. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved variable position turbine nozzle apparatus and method, which permit free rotation of turbine nozzle vanes, while minimizing or elimination leakage above and below the vane. The present development incorporates a plurality of nozzle vanes arranged around the circumference of a radial inflow turbine. The nozzles are individually attached to a rod positioned perpendicular to the flow direction. The array of circumferential nozzle rods protrudes through a movable annular back-wall. In a preferred embodiment, a linkage or gear connects the plurality of rods to allow simultaneous rotation of all nozzles for opening and closing the passage between nozzle vanes, although other means for positioning the vanes are also contemplated. Clearance may be provided between the movable back-wall, the rotatable nozzle vane, and the fixed turbine shroud, such that, by retracting the back-wall in the axial direction, the nozzle is free to rotate without friction or interference with the shroud or back-wall. In the full forward position, the movable back-wall closes the gaps on either side of the nozzle. One or more bellows devices may be used to provide pressure-actuated force in the movable back-wall, thereby clamping the nozzle between the two surfaces. With these general features, the complete apparatus may be capable of discrete steps with free angular movement, followed by periods of clamped fixed geometry operation. The apparatus is well-suited for applications that tolerate momentary nozzle vane leakage, followed by extended operation at fixed geometry conditions. When the nozzle is fixed by the clamping pressure of the bellows, the aerodynamic performance of the vane is equal to the fixed geometry, non-leaking nozzle. 
     The loads on the turbine structural components may also be balanced in a turbomachine employing the variable position nozzle vanes in accordance with this teaching. Evaluation of the natural loads within the turbomachine shows that the turbine shroud is typically forced towards the rotor, as the pressure behind the shroud is greater than that within the rotor passage. This sustained pressure and high temperature leads to creep effects that may ultimately cause the rotor to rub catastrophically on the shroud. By optimally sizing the bellows, the force acting through the nozzle (the gap clamping force) may be designed to balance the pressure forces acting to push the shroud into the rotor. 
    
    
     
       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 preferred embodiments and are not to be construed as limiting the invention. 
         FIG. 1  depicts a turbo-alternator in accordance with an exemplary embodiment of the invention. 
         FIGS. 2 and 3  are enlarged views illustrating the bellows, nozzle pivot rod, and the annular sliding back-wall elements of the embodiment depicted in  FIG. 1 . 
         FIG. 4  depicts an exemplary mechanism for rotation of the nozzle vanes array. 
         FIG. 5  depicts an alternative method for affecting the bellows movement. 
         FIG. 6  is a block diagram illustrating the relationship between the external variables, the input signals, the output devices, and the programmable controller. 
         FIG. 7  is a flow diagram illustrating a method of control for the variable position nozzle according to an exemplary embodiment. 
         FIG. 8  is a flow diagram illustrating an exemplary operating mode for maintaining a turbine inlet temperature and power set points given changes in the external variables. 
         FIG. 9  is a pictorial depiction of the nozzle vanes, vane rotating mechanism, and the turbine rotor according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  depicts a turbomachine  20 , which represents one possible embodiment of the invention. In the depicted embodiment, the device  20  is a turbo-alternator, although it will be recognized by those skilled in the art that the present invention may be adapted to other types of generators and turbomachines. The turbomachine  20  includes a gas turbine section  23  and an alternator or generator section  24 , combined on a common shaft/bearing system  22 . A practical use for this preferred embodiment device is to convert the energy from expanding gas to electrical power. High-temperature, high-pressure gas enters through an inlet flange  27 , flows circumferentially around a turbine scroll  1 , and enters a nozzle section  2  of the turbine stage  23 . Accelerating through the nozzle  2  with a tangential vector, the gas enters a rotating turbine rotor  3 . The velocity vector, the mass flow rate, as well as intensive state properties such as temperature and pressure, influence the potential energy converted to shaft power. The lower pressure and temperature gas exits the turbine section  23  at a conduit  9  through a flange  25 . Electrical power converted by the alternator  26  may be connected to the load with conventional wiring. 
       FIG. 2  provides a close-up detail, depicting the nozzle  2 , which is a circumferential array of vanes surrounding the rotor  3 . The vanes are angled to direct the flow into the rotor  3  with a high degree of tangency. The number of vanes in the nozzle typically varies from 11 to 19 for small turbo machines, although other numbers are also contemplated. Each nozzle is attached, e.g., integrally attached, to a rod  6  that is driven by a linkage device, represented by element  11 . The linkage device  11  may be any mechanical linkage for transforming linear to rotational motion or for otherwise effecting rotational movement of the nozzles, and may include pivoting rods, a gear arrangement, or the like. For example, in an alternative embodiment (not shown) the linkage device may include a solar gear engaging a plurality of planetary gears, each engaging one of the rods  6 . The linkage device  11  may be located on either side of a bellows device  5 . The bellows  5  may be an annular design or, alternatively, a series of two or more discrete cylindrical bellows units  5  situated symmetrically around the nozzle  2 . The bellows  5  is secured, e.g., welded to the annular element  40  that extends axially to the back wall  4  of the nozzle passage  2 . The annular back-wall  4  is a ring designed with ample radial clearance so as to permit axial movement of the back wall  4  along a cylindrical surface  29 . The relative pressure within the bellows  5  affects the axial movement of the annular back-wall  4 . 
       FIG. 3  shows an isolated view of the bellows  5 , nozzle pivot rod  6 , and the annular sliding back-wall element  4 , with the mechanism for capturing and rotating the rods  6  removed for ease of exposition. The bellows piston force, Fb, acts through the sliding annular element  4  pushing the nozzle vanes  2  into the shroud  7 . When properly sized, the bellow force Fb, is nominally approximately equal to the force, Fs, resulting from the pressure of the turbine inlet fluid multiplied by the projected area of the shroud  7 . 
     In  FIG. 4 , a push-rod  17  actuates the linkage  11  to cause the rotation of the array of nozzle vanes  2 . A linear stepper motor  15  provides the force to push the rod  17 , move linkage  11 , and rotate the array of nozzle vanes  2 . Many practical mechanisms for interconnecting an electronic actuator, such as the linear stepper motor  15 , and a slow-moving rotary device are known by those skilled in the art of mechanical design. 
     Conduits  12  and  19 , and valves  13  and  18 , are used in conjunction to provide pressurization or venting of the internal space  36  (see  FIG. 5 ) defined within the bellows  5 . The pressure used for the process is preferably obtained from the turbine system supply pressure  16 , since it is naturally above that in the nozzle  2  passages. Due to the accelerating flow of gas through the nozzle passages, the static pressure is substantially depressed on the flow side of back-wall element  4 , relative to that of the supply pressure  16 . However, as an alternative, the supply  16  may be maintained by an auxiliary source not in common with the turbine supply. When supplied with pressure from source  16 , the bellows  5  exerts a piston load acting through the movable turbine back-wall  4 , thereby clamping the nozzle  2  between the back-wall  4  and the stationary shroud  7 . When operating conditions warrant a nozzle position change, air pressure is vented through the valve  18  and conduit  19 , thus causing the bellows  5  and the nozzle back-wall  4  to retract. This movement frees the nozzle vane from its clamping force and allows unrestricted rotation via the push rod  6  and associated linkage or gear mechanism  11 . Once the stepper motor  15  has completed the programmed movement, the solenoid valve  18  is closed and the solenoid valve  13  opens to re-pressurize the bellows  5  and push the movable back-wall  4  to its clamped position. 
     An alternative method for affecting the bellows  5  movement is shown in  FIG. 5 . An elevated pressure in a cavity  30  communicates with the inner volume  36  of the bellows  5  via a small port  33 . This port operates on the principle of a hydraulic or pneumatic snubber. When valve  18  is opened and gas is permitted to escape through conduit  19 , the pressure within the bellows  5  drops. The hysteresis in the circuit from the high pressure cavity  30 , the interior of the bellows  5 , and the low pressure sink (possibly ambient) is controlled by the relative flow resistance of the snubber port  33 , and that through valve  18  and conduit  19 . For a relatively small snubber port  33 , the bellows interior pressure will drop when valve  18  is opened, permitting free movement of nozzle vanes  2 . The linear contraction of the bellows  5  may be limited an internal stop member  10 , such as a pin or the like, providing a passive stop as the pressure within the cavity  36  drops. This pin  10  may be sized to limit the clearance gap to a value that is optimal or suitable for permitting free movement of the nozzle vane  2  in the passage. Upon closing valve  18 , the bellows  5  reinflates, gradually applying the clamping force to the nozzle vanes. The snubber port  33  may be employed to passively manage gradual reinflation of the bellows, thus permitting the use of a simple on/off single point solenoid control to the vent bellows. 
     In a practical application, the need to change the turbine nozzle position is dictated by external variables, such as a demand for power, changing input from the fuel source, a natural variation in the supply pressure, and so forth.  FIG. 6  illustrates the relationship between external variables  43 , input signals  44 , output devices  46 , and a controller  45 , such as a digital microprocessor or microcontroller, or other programmable controller. In a preferred embodiment, the primary input signal may be a temperature signal from a turbine inlet temperature-sensing device  14  (see  FIG. 4 ). In applications where the gas temperature may be too high to obtain long-term reliable temperature measurement, the temperature-sensing device  14  may be located at a cooler location, such as the turbine exit  9  (see  FIG. 4 ). By employing an algorithm that includes measurement of the upstream and downstream pressures, the turbine inlet temperature may alternatively be determined or calculated by inference based on the outlet temperature. 
       FIG. 7  illustrates a method of controlling the variable position nozzle under conditions where solar is the only heat source. A change in either external variable, i.e., solar insolation and/or supply pressure, will cause a change in the turbine inlet temperature. Dropping temperature causes a reduction in overall efficiency, and excess flow consumption. Likewise, a rising temperature could have catastrophic results as a gas turbine generally operates near the property limits of the materials. A simplified feedback loop is shown in  FIG. 7 . An indication of the turbine inlet temperature (e.g., either by direct measurement or by inference as detailed above) is read into the microprocessor and compared against some predefined or preselected control set point. As used herein, the term “set point” is intended to encompass not only a discrete value, but also, an acceptable range of values. To correct the error signal when the inlet temperature sensed is outside of the preselected conditions, the variable position nozzle will perform the unclamp-rotate-reclamp sequence described above. 
       FIG. 8  describes a typical operating mode set to maintain a turbine inlet temperature set point and a power set point given changes in the external variables (e.g., system supply pressure, thermal input, and operator demand). In embodiments wherein solar energy is employed as a heat source, maintaining a power set point additionally requires control over a supplemental (hybrid) fuel source, since under solar-only operation, the power is a function of the solar availability. In  FIG. 8  the temperature response loop is represented as the inner loop in the diagram and preferably operated with a faster time constant that the outer loop. In this example the inner loop works to maintain a turbine inlet temperature set point as described above by asserting control over the heat source, such as a fuel control valve. In the event the inlet temperature exceeds the preselected set point, the heat input is decreased. In the event that the temperature is less than the temperature set point, the heat input is increased. 
     The depicted outer loop, operating with a slower time constant works to control the flow through the turbomachine to achieve the power set point. Comparing the power measurement from the input device to the set point, the microprocessor commands the actuating system to rotate the vane mechanism towards the closing (more tangential) position when the sensed power value is below the preselected set point and towards the more open, or radial position when the measured power is above the preselected set point. This nested loop arrangement is stable and properly constrained as long as the nozzle vanes operate within their physical limits of rotation, between maximum open and maximum closed positions. 
       FIG. 9  shows an isometric view of the nozzle vanes  2 , the vane rotating mechanism  11 , and the turbine rotor  3 . The turbine is shown as having a bladeless rotor for ease of exposition. 
     In certain embodiments, the variable position nozzle vane system may permit free movement of the vanes in a high temperature environment prone to large thermal strain. In yet other embodiments, the variable position nozzle vane system may permit relaxation of machining tolerances in the nozzle area, which may result in manufacturing cost savings. In further embodiments, a positively sealed nozzle vane may be provided, with top and bottom surface maintained in a pneumatically clamped position to prevent gas leakage, which may raise the efficiency of the variable geometry turbine stage. In yet other aspects, the bellows diameter may be optimally sized so as to provide an axial thrust acting through the nozzle that serves to counteract the natural forces on the shroud that tend to close (creep) the shroud into the turbine rotor. 
     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 disclosure herein, whereby it is to be distinctly understood that the foregoing descriptive matter is to be interpreted merely as illustrative of the invention and not as a limitation.