Abstract:
A nozzle for an air cycle machine. The nozzle has a disk section having a central axis. The nozzle also includes a plurality of blades which extend a blade height H from a bladed face of the disk section. The plurality of blades are arranged radially about the disk section. The nozzle has a throat width W defined between each radially adjacent pair of the plurality of turbine blades. The nozzle includes a coating substantially encapsulating the disk section and the plurality of blades, wherein the coating contains more than 91 percent tungsten carbide by volume.

Description:
BACKGROUND 
       [0001]    The present invention relates to Air Cycle Machines (ACM), such as the type used in Environmental Control Systems in aircraft. In particular, the present invention relates to novel dimensions and coatings of turbine nozzles used in ACMs. 
         [0002]    ACMs may be used to compress air in a compressor section. The compressed air is discharged to a downstream heat exchanger and further routed to a turbine. The turbine extracts energy from the expanded air to drive the compressor. The air output from the turbine may be utilized as an air supply for a vehicle, such as the cabin of an aircraft. 
         [0003]    ACMs often have a three-wheel or four-wheel configuration. In a three-wheel ACM, a turbine drives both a compressor and a fan which rotate on a common shaft. In a four-wheel ACM, two turbine sections drive a compressor and a fan on a common shaft. 
         [0004]    Airflow must be directed into the fan section to the compressor section, away from the compressor section towards the heat exchanger, from the heat exchanger to the turbine or turbines, and from the final turbine stage out of the ACM. In at least some of these transfers, it is desirable to direct air radially with respect to the central axis of the ACM. To accomplish this, rotating nozzles may be used to generate radial in-flow and/or out-flow. 
         [0005]    Often, it is desirable for components such as nozzles to include coatings that protect the components from damage. For example, tungsten carbide coatings have been applied using detonation gun coating. 
         [0006]    Thermal spraying techniques are known in the art and are often used to apply thick coatings to change surface properties of the component. Examples of known thermal spraying techniques include detonation gun coating, in which high pressure shock waves pass through a gas stream and cause the emission of bursts of the material to be deposited. Another known method of thermal spraying is high velocity oxy fuel (HVOF), in which the fuel combusts continuously, allowing for a continuous stream of material to be deposited. 
       SUMMARY 
       [0007]    In one embodiment, a nozzle for an air cycle machine is disclosed which includes a disk section having a central axis. The nozzle also includes blades which extend from a bladed face of the disk section by a blade height H. The blades are arranged radially about the disk section. A throat width W is defined between each radially adjacent pair of the plurality of turbine blades. A coating substantially encapsulates the disk section and the plurality of blades, wherein the coating contains more than 91 percent tungsten carbide by volume. 
         [0008]    In another embodiment, a nozzle for an air cycle machine is disclosed which also includes a disk section having a central axis. The nozzle also includes blades which extend from a bladed face of the disk section by a blade height H. The blades are arranged radially about the disk section. A throat width W is defined between each radially adjacent pair of the plurality of turbine blades. The coating substantially encapsulating the disk section and the plurality of blades has a thickness between 50.8 μm and 101.6 μm. 
         [0009]    In a third embodiment, a nozzle for an air cycle machine is disclosed which also includes a disk section having a central axis. The nozzle also includes blades which extend from a bladed face of the disk section by a blade height H. The blades are arranged radially about the disk section. A throat width W is defined between each radially adjacent pair of the plurality of turbine blades. The coating substantially encapsulating the disk section and the plurality of blades comprises a metal alloy having a bond strength greater than 10,000 psi. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]      FIG. 1  is a cross-sectional view of a four-wheel Air Cycle Machine. 
           [0011]      FIG. 2  is a plan view of a turbine nozzle in the four-wheel Air Cycle Machine of  FIG. 1 . 
           [0012]      FIG. 3  is a side view of the turbine nozzle of  FIG. 2 . 
           [0013]      FIG. 4  is a plan view of a portion of the turbine nozzle of  FIG. 2 , showing the dimensions of the nozzle. 
           [0014]      FIG. 5  is a plan view of a turbine nozzle in the four-wheel Air Cycle Machine of  FIG. 1 . 
           [0015]      FIG. 6  is a side view of the turbine nozzle of  FIG. 5 . 
           [0016]      FIG. 7  is a plan view of a portion of the turbine nozzle of  FIG. 5 , showing the dimensions of the nozzle. 
           [0017]      FIG. 8  is a cross-sectional view of a three-wheel Air Cycle Machine. 
           [0018]      FIG. 9  is a plan view of a turbine nozzle in the three-wheel Air Cycle Machine of  FIG. 8 . 
           [0019]      FIG. 10  is a side view of the turbine nozzle of  FIG. 9 . 
           [0020]      FIG. 11  is a plan view of a portion of the turbine nozzle of  FIG. 9 , showing the dimensions of the nozzle. 
           [0021]      FIG. 12  is a plan view of a turbine nozzle in the three-wheel Air Cycle Machine of  FIG. 8 . 
           [0022]      FIG. 13  is a side view of the turbine nozzle of  FIG. 12 . 
           [0023]      FIG. 14  is a plan view of a portion of the turbine nozzle of  FIG. 12 , showing the dimensions of the nozzle. 
       
    
    
     DETAILED DESCRIPTION 
       [0024]      FIG. 1  is a cross-sectional view of Air Cycle Machine (ACM)  2 . ACM  2  is a four-wheel ACM, containing fan section  4 , compressor section  6 , first turbine section  8 , and second turbine section  10 , which are all connected to shaft  12 . Shaft  12  rotates about central axis  14 . 
         [0025]    Fan section  4  includes fan inlet  16  and fan outlet  18 . Fan inlet  16  is an opening in ACM  2  that receives working fluid from another source, such as a ram air scoop. Fan outlet  18  allows working fluid to escape fan section  4 . Fan blades  20  may be used to draw working fluid into fan section  4 . 
         [0026]    Compressor section  6  includes compressor inlet  22 , compressor outlet  24 , compressor nozzle  26 , and compressor blades  27 . Compressor inlet  22  is a duct defining an aperture through which working fluid to be compressed is received from another source. Compressor outlet  24  allows working fluid to be routed to other systems after it has been compressed. Compressor nozzle  26  is a nozzle section that rotates through working fluid in compressor section  6 . Compressor nozzle  26  directs working fluid from compressor inlet  22  to compressor outlet  24  via compressor blades  27 . Compressor nozzle  26  is a radial out-flow rotor. 
         [0027]    First turbine section  8  includes first stage turbine inlet  28 , first stage turbine outlet  30 , first stage turbine nozzle  32 , and first turbine blades  33 . First stage turbine inlet  28  is a duct defining an aperture through which working fluid passes prior to expansion in first turbine section  8 . First stage turbine outlet  30  is a duct defining an aperture through which working fluid (which has expanded) departs first turbine section  8 . First stage turbine nozzle  32  is a nozzle section that rotates through working fluid in first turbine section  8 . First stage turbine nozzle  32  cooperates with first stage turbine blades  37  to extract energy from working fluid passing therethrough, driving the rotation of first turbine section  8  and attached components, including shaft  12 , fan section  4 , and compressor section  6 . First stage turbine nozzle  32  is a radial in-flow rotor. 
         [0028]    Second turbine section  10  includes second stage turbine inlet  34 , second stage turbine outlet  36 , second stage turbine nozzle  38 , and second stage turbine blades  39 . Second stage turbine inlet  34  is a duct defining an aperture through which working fluid passes prior to expansion in second turbine section  10 . Second stage turbine outlet  36  is a duct defining an aperture through which working fluid (which has expanded) departs second turbine section  10 . Second stage turbine nozzle  38  is a nozzle section that cooperates with second stage turbine blades  39  to extract energy from working fluid passing therethrough, driving the rotation of second turbine section  10  and attached components, including shaft  12 , fan section  4 , and compressor section  6 . In particular, second stage turbine nozzle  38  is a radial out-flow rotor. Working fluid passes from second stage turbine inlet  34  to cavity  35 , where it is incident upon second stage turbine nozzle  38 . Working fluid then passes between nozzle blades  50  and  52  ( FIGS. 5-7 ). Turbine nozzle  38  is stationary, and the nozzle vanes guide the flow for optimum entry into the turbine rotor. The flow of causes turbine blades  39  to rotate and turb shaft  12 . 
         [0029]    Shaft  12  is a rod, such as a titanium tie-rod, used to connect other components of ACM  2 . Central axis  14  is an axis with respect to which other components may be arranged. 
         [0030]    Fan section  4  is connected to compressor section  6 . In particular, fan outlet  18  is coupled to compressor inlet  22 . Working fluid is drawn through fan inlet  16  and discharged through fan outlet  18  by fan blades  20 . Working fluid from fan outlet  18  is routed to compressor inlet  22  for compression in compressor section  6 . Similarly, compressor section  6  is coupled with first turbine section  8 . Working fluid from compressor outlet  24  is routed to first stage turbine inlet  28 . 
         [0031]    Similarly, first turbine section  8  is coupled to second turbine section  10 . Working fluid from first stage turbine outlet  30  is routed to second stage turbine inlet  34 . In this way, working fluid passes through ACM  2 : first through fan inlet  16 , then fan outlet  18 , compressor inlet  22 , compressor outlet  24 , first stage turbine inlet  28 , first stage turbine outlet  30 , second stage turbine inlet  34 , and second stage turbine outlet  38 . Additional stages may exist between those shown in  FIG. 1 . For example, often a heat exchanger (not shown) is located between compressor section  6  and first turbine section  8 . 
         [0032]    Each of fan section  4 , compressor section  6 , first turbine section  8 , and second turbine section  10  are also connected to one another via shaft  12 . Shaft  12  runs along central axis  14 , and is connected to at least compressor nozzle  26 , first stage turbine nozzle  32 , and second stage turbine nozzle  38 . Fan blades  20  may also be connected to shaft  12 . 
         [0033]    When working fluid passes through ACM  2 , it is first compressed in compressor section  6 , then expanded in first turbine section  8  and second turbine section  10 . Often, the working fluid is also heated or cooled in a heat exchanger (not shown) through which working fluid is routed as it passes between compressor section  6  and first turbine section  8 . First turbine section  8  and second turbine section  10  extract energy from the working fluid, turning shaft  12  about central axis  14 . 
         [0034]    Working fluid passing through ACM  2  may be conditioned for use in the central cabin of a vehicle powered by a gas turbine engine. By compressing, heating, and expanding the working fluid, it may be adjusted to a desired temperature, pressure, and/or relative humidity. However, due to the rapid rotation of compressor nozzle  26 , first stage turbine nozzle  32 , and second stage turbine nozzle  38  with respect to the working fluid flowpath, these parts need frequent replacement. 
         [0035]      FIG. 2  is a plan view of first stage turbine nozzle  32  arranged about central axis  14 . First stage turbine nozzle  32  includes nineteen full blades  40  arranged along a surface of disk  42 . Full blades  40  and disk  42  are made of a durable material such as steel, aluminum, or titanium. First stage turbine nozzle  32  is coated with tungsten carbide. The tungsten carbide coating on first stage turbine nozzle  32  is applied using HVOF, allowing for increased hardness and a higher percentage of tungsten carbide as opposed to other materials, such as cobalt. HVOF spraying also results in reduced variability in coating thickness as compared to traditional coating methods, such as deposition gun spraying. 
         [0036]    Disk  42  is radially symmetrical about central axis  14 . Full blades  40  are spaced equidistantly from one another about the circumferential length of disk  42 . Each of full blades  40  are also equidistant radially from central axis  14 . 
         [0037]    First stage turbine nozzle  32  is a high value component that is relatively frequently replaced. Damage to first stage turbine nozzle  32  may occur due to contact with abrasive particles. Thus, a high strength, durable coating may increase the service life of first stage turbine nozzle  32 . 
         [0038]      FIG. 3  is a side view of first stage turbine nozzle  32 . First stage turbine nozzle  32  contains full blades  40  and disk  42 , as described with respect to  FIG. 2 . 
         [0039]      FIG. 3  illustrates the thickness of first stage turbine nozzle  32 . In particular, first stage turbine nozzle  32  includes blade height H 32 . Blade height H 32  is the amount of head space between disk  42  and an adjacent component such as a shroud (not shown). Blade height H 32  as shown in  FIG. 3  is 0.686 cm (0.270 in.). In some embodiments, blade height H 32  may vary by as much as 0.01 cm (0.005 in.). However, blade height H 32  of 0.686 cm is ideal for the passage of the desired quantity of working fluid through first turbine section  8  ( FIG. 1 ). 
         [0040]      FIG. 4  is an enlarged view of a portion of first stage turbine nozzle  32 . The portion shown in  FIG. 4  shows full blades  40  arranged on disk  42 . 
         [0041]      FIG. 4  illustrates various specific dimensions of first stage turbine nozzle  32 . Nozzle passage width W 32  is the distance between each full blade  40  and the radially adjacent full blade  40 . In effect, nozzle passage width W 32  is the width of a throat through which working air may be routed. Nozzle passage width W 32  is 0.340 cm. (0.134 in.), but may deviate by as much as 0.013 cm. (0.005 in.). Flow area A 32  is the region through which working fluid may flow. Flow area A 32  converges between the vanes until it reaches the throat of first stage turbine nozzle  32 , and has a surface area of nozzle height H 32 ×nozzle passage width W 32 . Due to machining tolerances, flow area A 32  area may vary by up to 5%. Flow area A 32  is approximately 4.432 square centimeters (0.687 square in.). 
         [0042]    Nozzle passage width W 32  is optimized to ensure proper flow and energy extraction from first stage turbine nozzle  32 . Increasing or decreasing nozzle passage width W 32  would result in too much or too little flow through first stage turbine nozzle  32  Likewise, flow area A 32  is optimized to ensure an appropriate quantity of working fluid is transmitted by first stage turbine nozzle  32 . A larger flow area A 32  would result in too much working fluid passing through first stage turbine nozzle  32 , while a smaller flow area A 32  would result in too little. 
         [0043]      FIG. 5  is a plan view of second stage turbine nozzle  38  arranged about central axis  14 . Second stage turbine nozzle  38  includes seventeen full blades  50  and seventeen splitter blades  52  arranged along a surface of disk  54 . Full blades  50 , splitter blades  52 , and disk  54  are made of a durable material such as steel, aluminum, or titanium. Second stage turbine nozzle  38  is coated with tungsten carbide. The tungsten carbide coating on second stage turbine nozzle  38  is applied using High-Velocity Oxy-Fuel (HVOF) spraying, allowing for increased hardness and a higher percentage of tungsten carbide as opposed to other materials, such as cobalt. 
         [0044]    Disk  54  is radially symmetrical about central axis  14 . Full blades  50  and splitter blades  52  are interdigitated and spaced equidistantly from one another about the circumferential length of disk  54 . Thus, full blades  50  are each located between two adjacent splitter blades  52 , and splitter blades  52  are each located between two adjacent full blades  50 . Each of splitter blades  52 , and each of full blades  50 , are equidistant radially from central axis  14 . 
         [0045]    Second stage turbine nozzle  38  is a high value component that is relatively frequently replaced. Damage to second stage turbine nozzle  38  may occur due to abrasive particulates in the high velocity airflow directed by second stage turbine nozzle  38 . Thus, a highly durable coating on second stage turbine nozzle  38  may increase its service life. 
         [0046]    HVOF coating of second stage turbine nozzle causes unique physical characteristics that are not possible using traditional coating technologies, such as deposition gun coating. HVOF coating may, for example, allow for levels of tungsten carbide in excess of 91%. In addition, HVOF coating provides for surface hardness in excess of 10,000 psi. Furthermore, HVOF coating provides for reduced variability in surface coating thickness as compared to detonation gun coating. 
         [0047]      FIG. 6  is a side view of second stage turbine nozzle  38 . Second stage turbine nozzle  38  contains full blades  50 , splitter blades  52 , and disk  54 , as described with respect to  FIG. 5 . 
         [0048]      FIG. 6  illustrates the thickness of second stage turbine nozzle  38 . In particular, second stage turbine nozzle  38  includes blade height H 38 . Blade height H 38  is the amount of head space between disk  54  and an adjacent component such as a shroud (not shown). Blade height H 38  as shown in  FIG. 6  is 0.940 cm (0.370 in). In some embodiments, blade height H 38  may vary by as much as 0.01 cm (0.005 in). However, blade height H 38  of 0.940 cm is ideal for the passage of the desired quantity of working fluid through second turbine section  10  ( FIG. 1 ). 
         [0049]      FIG. 7  is an enlarged view of a portion of second stage turbine nozzle  38 . The enlarged portion shown in  FIG. 7  shows full blades  50  and splitter blades  52  arranged on disk  54 . 
         [0050]      FIG. 7  illustrates various specific dimensions of second stage turbine nozzle  38 . Nozzle passage width W 38  is the distance between each full blade  50  and adjacent splitter blade  52 . In effect, nozzle passage width W 38  is the width of a throat through which working air may be routed. Nozzle passage width W 38  is 0.222 cm (0.0875 in), but may deviate by as much as 0.013 cm (0.005 in). Flow area A 38  is the region through which working fluid may flow. Flow area A 38  is the total cross-sectional area orthogonal to the surface of disk  54  on the bladed side that is not covered by full blades  50  and splitter blades  52  and through which working fluid flows. The portion of flow area A 38  identified in  FIG. 7  is the flow area A between one full blade  50  and one splitter blade  52 . In sum, over the entire surface of second stage turbine nozzle  38 , flow area A 38  is 7.103 square centimeters (1.101 square inches). Due to minor differences in machining and/or coating, this value may be as high as 7.458 of as low as 6.748 square centimeters. 
         [0051]    Nozzle passage width W 38  is optimized to ensure proper flow and energy extraction from second stage turbine nozzle  38 . Increasing or decreasing nozzle passage width W 38  would result in too little or too much flow through second stage turbine nozzle  38 . Likewise, flow area A 38  is optimized to ensure an appropriate quantity of working fluid is transmitted by second stage turbine nozzle  38 . A larger flow area A 38  would result in too much working fluid passing through second stage turbine nozzle  38 , while a smaller flow area A 38  would result in too little. 
         [0052]      FIG. 8  is a cross-sectional view of ACM  100 . ACM  100  is a three-wheel ACM, containing fan section  102 , compressor section  104 , and turbine section  106 , all of which are connected to shaft  108 . Shaft  108  rotates about central axis  110 . 
         [0053]    Fan section  102  includes fan inlet  112  and fan outlet  114 . Fan inlet  112  is an opening in ACM  100  that receives working fluid from another source, such as a bleed valve in a gas turbine engine (not shown). Fan outlet  114  allows working fluid to escape fan section  102 . Fan blades  116  may be used to draw working fluid into fan section  102 . 
         [0054]    Compressor section  104  includes compressor inlet  118 , compressor outlet  120 , and compressor nozzle  122 . Compressor inlet  118  is a duct defining an aperture through which working fluid to be compressed is received from another source, such as fan section  102 . Compressor outlet  120  allows working fluid to be routed to other systems once it has been compressed. Compressor nozzle  122  is a nozzle section that rotates through working fluid in compressor section  104 . In particular, compressor nozzle  122  is a radial out-flow rotor. 
         [0055]    Turbine section  106  includes turbine inlet  124 , turbine outlet  126 , and turbine nozzle  128 . Turbine inlet  124  is a duct defining an aperture through which working fluid passes prior to expansion in turbine section  106 . Turbine outlet  126  is a duct defining an aperture through which working fluid which has expanded departs turbine section  106 . Turbine nozzle  128  is a nozzle section that extracts energy from working fluid passing therethrough, driving the rotation of turbine section  106  and attached components, including shaft  108 , fan section  102 , and compressor section  104 . 
         [0056]    Shaft  108  is a rod, such as a titanium tie-rod, used to connect other components of ACM  100 . Central axis  110  is an axis with respect to which other components may be arranged. 
         [0057]    Fan section  102  is connected to compressor section  104 . In particular, fan outlet  114  is coupled to compressor inlet  118  such that working fluid may be transferred from fan outlet  114  to compressor inlet  118 . Working fluid is drawn through fan inlet  112  and discharged through fan outlet  114  by fan blades  116 . Working fluid from fan outlet  114  is routed to compressor inlet  118  for compression in compressor section  104 . 
         [0058]    Similarly, compressor section  104  is coupled with first turbine section  106 . Working fluid from compressor outlet  120  is routed to turbine inlet  124 . In this way, working fluid passes through ACM  100 : first through fan inlet  112 , then fan outlet  114 , compressor inlet  118 , compressor outlet  120 , turbine inlet  124 , and turbine outlet  126 . Additional stages may exist between those shown in  FIG. 8 . For example, often a heat exchanger (not shown) is located between compressor section  104  and turbine section  106 . 
         [0059]    Each of fan section  102 , compressor section  104 , and turbine section  106  are also connected to one another via shaft  108 . Shaft  108  runs along central axis  110 , and is connected to at least compressor nozzle  122  and turbine nozzle  128 . Fan blades  116  may also be connected to shaft  20 . 
         [0060]    When working fluid passes through ACM  100 , it is first compressed in compressor section  104 , then expanded in turbine section  106 . Often, the working fluid is also heated or cooled in a heat exchanger (not shown) through which working fluid is routed as it passes between compressor section  104  and turbine section  106 . Turbine section  106  to extract energy from the working fluid, turning shaft  20  about central axis  110 . 
         [0061]    Working fluid passing through ACM  100  may be conditioned for use in the central cabin of a vehicle powered by a gas turbine engine. By compressing, heating, and expanding the working fluid, it may be adjusted to a desired temperature, pressure, and/or relative humidity. However, due to the rapid rotation of compressor nozzle  122  and turbine nozzle  128  with respect to the working fluid flowpath, these parts need frequent replacement. 
         [0062]      FIG. 9  is a plan view of turbine nozzle  128  arranged about central axis  110 . Turbine nozzle  128  includes nineteen full blades  130  arranged along a surface of disk  132 . Full blades  130  and disk  132  are made of a durable material such as steel, aluminum, or titanium. Turbine nozzle  128  is coated with tungsten carbide. The tungsten carbide coating on first stage turbine nozzle  128  is applied using HVOF, allowing for increased hardness and a higher percentage of tungsten carbide as opposed to other materials, such as cobalt. HVOF spraying also results in reduced variability in coating thickness as compared to traditional coating methods, such as deposition gun spraying, as will be described in more detail with respect to  FIGS. 15A-15B . 
         [0063]    Disk  132  is radially symmetrical about central axis  110 . Full blades  130  are spaced equidistantly from one another about the circumferential length of disk  132 . Each of full blades  130  are also equidistant radially from central axis  110 . 
         [0064]    Turbine nozzle  128  is a high value component that is relatively frequently replaced. Damage to turbine nozzle  128  may occur due to contact with abrasive particles. Thus, a high strength, durable coating may increase the service life of turbine nozzle  128 . 
         [0065]      FIG. 10  is a side view of turbine nozzle  128 . Turbine nozzle  128  contains full blades  130  and disk  132 , as described with respect to  FIG. 9 . 
         [0066]      FIG. 10  illustrates the thickness of turbine nozzle  128 . In particular, turbine nozzle  128  includes blade height H 128 . Blade height H 128  is the amount of head space between disk  132  and an adjacent component such as a shroud (not shown). In a first embodiment, blade height H 128  as shown in  FIG. 10  is 0.318 cm (0.125 in). In a second embodiment in which an increased quantity of working fluid flow is desired, blade height H as shown in  FIG. 10  may be 0.393 cm (0.155 in). In some versions of the first and second embodiments described above, blade height H 128  may vary by as much as 0.01 cm (0.005 in.). However, blade heights H  134  of 0.318 cm or 0.393 cm are ideal for ACM  100  ( FIG. 8 ) to pass a desired quantity of working fluid through turbine section  106  ( FIG. 8 ). 
         [0067]      FIG. 11  is an enlarged view of a portion of turbine nozzle  128 . The enlarged portion shown in  FIG. 11  shows full blades  130  arranged on disk  132 . 
         [0068]      FIG. 11  illustrates various specific dimensions of turbine nozzle  128 . Nozzle passage width W 128  is the distance between each full blade  130  and the radially adjacent full blade  130 . In effect, nozzle passage width W 128  is the width of a throat through which working air may be routed. Nozzle passage width W 128  is 0.241 cm (0.095 in.), but may deviate by as much as 0.013 cm. (0.005 in.). Flow area A 128  is the region through which working fluid may flow. Flow area A 128  is the total surface area of disk  132  on the bladed side through which working fluid may flow between full blades  130 . The portion of flow area A 128  identified in  FIG. 11  is the flow area A between one full blade  130  and its adjacent full blade  130 . In sum, over the entire surface of turbine nozzle  128 , flow area A 128  is 1.451 cm. squared (0.225 square inches) in the first embodiment described above, and 1.806 cm. squared (0.255 square inches) in the second embodiment described above. Due to minor differences in machining and/or coating, these values may vary by as much as 5%. 
         [0069]    Nozzle passage width W 128  is optimized to ensure proper flow and energy extraction from turbine nozzle  128 . Increasing or decreasing nozzle passage width W 128  would result in either too much or too little fluid flow through nozzle  128 A. Likewise, flow area A 128  is optimized to ensure an appropriate quantity of working fluid is transmitted by first stage turbine nozzle  128 . A larger flow area A 128  would result in too much working fluid passing through turbine nozzle  128 , while a smaller flow area A 128  would result in too little. 
         [0070]      FIG. 12  is a plan view of turbine nozzle  128 A, an alternative embodiment capable of being used in three-wheel ACM  100 . Turbine nozzle  128 A may also be arranged about central axis  110 . As with turbine nozzle  128 , described above with respect to  FIGS. 8-11 , turbine nozzle  128 A may be used in ACM  100  ( FIG. 8 ). Turbine nozzle  128 A includes twenty-three full blades  130 A arranged along a surface of disk  132 A. Full blades  130 A and disk  132 A are made of a durable material such as steel, aluminum, or titanium. Turbine nozzle  128 A is coated with tungsten carbide. The tungsten carbide coating on turbine nozzle  128 A is applied using HVOF spraying, allowing for increased hardness and a higher percentage of tungsten carbide as opposed to other materials, such as cobalt. 
         [0071]    Disk  132 A is radially symmetrical about central axis  110 . Full blades  130 A are spaced equidistantly from one another about the circumferential length of disk  132 A. Thus, full blades  130 A are located between two adjacent full blades  130 A, and each of full blades  130 A are equidistant radially from central axis  110 . 
         [0072]    Turbine nozzle  128 A is a high value component that is relatively frequently replaced. Damage to turbine nozzle  128 A may occur due to abrasive particulates in the high velocity airflow directed by turbine nozzle  128 A. Thus, a highly durable coating on second stage turbine nozzle  128 A may increase its surface life. 
         [0073]      FIG. 13  is a side view of turbine nozzle  128 A. Turbine nozzle  128 A contains full blades  130 A and disk  132 A, as described with respect to  FIG. 12 . 
         [0074]      FIG. 13  illustrates the thickness of turbine nozzle  128 A. In particular, turbine nozzle  128 A includes blade height H 128 A. Blade height H 128 A is the amount of head space between disk  132 A and an adjacent component such as a shroud (not shown). Blade height H 128 A as shown in  FIG. 13  is 0.305 cm. (0.120 in.). In some embodiments, blade height H 128 A may vary by as much as 0.01 cm. (0.005 in.). However, blade height H 128 A of 0.305 cm. is ideal for the passage of the desired quantity of working fluid through turbine section  106  ( FIG. 8 ). 
         [0075]      FIG. 14  is an enlarged view of a portion of turbine nozzle  128 A. The enlarged portion shown in  FIG. 14  shows full blades  130 A arranged on disk  132 A. 
         [0076]      FIG. 14  illustrates various specific dimensions of turbine nozzle  128 A. Nozzle passage width W 128 A is the distance between each full blade  130 A and its adjacent full blade  130 A. In effect, nozzle passage width W 128 A is the width of a throat through which working fluid may be routed. Nozzle passage width W 128 A is 0.234 cm. (0.092 in.), but may deviate by as much as 0.013 cm. (0.005 cm.). Flow area A 128 A is the cross-sectional area through which fluid may flow on the bladed side of disk  132 A between full blades  130 A. The portion of flow area A 128 A identified in  FIG. 14  is the flow area A between one full blade  130 A and the adjacent full blade  130 A. In sum, over the entire bladed surface side of disk  132 A in turbine nozzle  128 A, flow area A 128 A is 1.639 square centimeters (0.254 square inches). Due to minor differences in machining and/or coating, this value may be as high as 1.721 square centimeters or as low as 1.557 square centimeters. 
         [0077]    Nozzle passage width W 128 A is optimized to ensure proper flow and energy extraction from turbine nozzle  128 A. Increasing or decreasing nozzle passage width W 128 A would result in either too much or too little fluid flow passing across turbine nozzle  128 A. Likewise, flow area A 128 A is optimized to ensure an appropriate quantity of working fluid is transmitted by turbine nozzle  128 A. A larger flow area A 128 A would result in too much working fluid passing through turbine nozzle  128 A, while a smaller flow area A 128 A would result in too little. 
         [0078]    Each previously described turbine nozzle embodiments is coated. The coatings are applied using HVOF. Thus, each previously described turbine nozzle has a base made of any of a range of acceptable base materials, such as steel, aluminum, ceramic, or titanium. A coating is sprayed onto the base using HVOF, the coating primarily consisting of tungsten carbide. Previously, detonation gun coating was used to apply the coating. 
         [0079]    The coating applied is not pure tungsten carbide. In order to facilitate coating using detonation gun technology, the coating is often composed of 12% cobalt, plus or minus 2%. Accordingly, the surface coating has an adhesion strength of 8500 psi, plus or minus 5%. However, using HVOF, the coating may have a higher percentage of tungsten carbide. A coating applied using HVOF is often composed of 9% cobalt, plus or minus 2%. Often, the coating may contain less than 8% cobalt by volume. Accordingly, the surface coating 164 has an adhesion strength of 10,000 psi, plus or minus 5%. 
         [0080]    A coating applied using detonation gun technology typically has a minimum thickness of approximately 0.00254 cm. (0.001 in.). These prior art coatings often have a range of approximately 0.00762 cm. (0.003 in.), plus or minus 2%. A coating applied using HVOF will typically have a minimum thickness of approximately 0.00508 cm. (0.002 in.), and a range of 0.00508 cm. (0.002 in.). 
         [0081]    While the invention has been described with reference to an exemplary embodiment(s), it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. 
         [0082]    Therefore, it is intended that the invention not be limited to the particular embodiment(s) disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.