Abstract:
A cooling system for a gas turbine engine includes a fuel deoxygenator for increasing the cooling capacity of the fuel. The fuel deoxygenator removes dissolved gases from the fuel to prevent the formation of insoluble deposits. The prevention of insoluble deposits increases the usable cooling capacity of the fuel. The increased cooling capacity of the deoxygenated fuel provides a greater heat sink for cooling air used to protect engine components. The improved cooling capacity of the cooling air provides for increased engine operating temperatures that improves overall engine efficiency.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     This invention generally relates to a cooling system for a gas turbine engine, and specifically to a cooling system including a fuel deoxygenator to increase the usable heat absorption capability of fuel used for cooling cooling air.  
         [0002]     A gas turbine engine typically includes a compressor, a combustor and a turbine. Air entering the compressor is compressed and directed toward the combustor. In the combustor, fuel is combined with the high-pressure air and ignited. Combustion gases produced in the combustor drive the turbine. High engine temperatures provide better fuel burn rates and engine efficiencies that extend the range of an aircraft. The high engine and combustion gas temperatures are greater than can normally be accommodated by metal parts of the engine. Typically, a portion of air from the compressor is bled off and directed over parts of the engine to form a cooling boundary layer that insulates exposed surfaces from the hot combustion gases.  
         [0003]     Cooling the bleed air from the compressor allows the engine to be operated at increased combustion gas temperatures while maintaining the same temperature in engine components. It is known, to use fuel as a cooling medium to cool air from the compressor. The usable cooling capacity of fuel is limited by coke formation caused by oxidative reactions with dissolved oxygen within the fuel. These reactions cause the formation of insoluble materials referred to as “coke” or “coking”. Coke deposits can cause degradation of fuel delivery performance. Therefore, the usable cooling capacity of the fuel is limited by the amount of dissolved oxygen within the fuel. Further, the usable cooling capacity of the fuel limits the amount of heat that can be transferred from the engine cooling air, and that in turn limits sustainable engine operating temperatures.  
         [0004]     Accordingly, it is desirable to develop an engine cooling system with increased fuel cooling capacity for absorbing greater amounts of heat from cooling air.  
       SUMMARY OF INVENTION  
       [0005]     This invention is an engine cooling system including a fuel deoxygenator for removing dissolved oxygen from fuel to increase the usable cooling capacity of fuel used for cooling engine cooling air.  
         [0006]     A gas turbine engine includes a compressor compressing air to a high pressure. The high-pressure air is mixed with fuel in a combustor and ignited to produce hot combustion gases. The hot combustion gases drive a turbine. The turbine is cooled by air bled off from the compressor. The cooling air from the compressor is cooled within a fuel/air heat exchanger. Removing substantially all the dissolved oxygen in the fuel deoxygenator increases the usable cooling capacity of the fuel by increasing the temperature at which coke deposits are formed.  
         [0007]     The increased cooling capacity of the fuel enables an increase in the amount of heat that can be absorbed from the cooling air, which in turn enables increased engine operating temperatures. As appreciated, higher engine temperatures result in greater engine efficiencies, that in turn result in favorable performance improvements.  
         [0008]     Accordingly, the engine cooling system of this invention increases the usable cooling capacity of fuel used to cool engine cooling air.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]     The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawings that accompany the detailed description can be briefly described as follows:  
         [0010]      FIG. 1  is a schematic view of a gas turbine engine and cooling air system according to this invention;  
         [0011]      FIG. 2  is a schematic view of a fuel deoxygenator according to this invention;  
         [0012]      FIG. 3  is a schematic view of another deoxygenator according to this invention; and  
         [0013]      FIG. 4  is a cross-sectional view of a permeable membrane and porous substrate of the fuel deoxygenator. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0014]     Referring to  FIG. 1 , a gas turbine engine assembly  10  includes a compressor  12  a combustor  14  and a turbine  16 . Airflow  26  entering the compressor  12  is compressed to a high pressure and directed towards the combustor  14 . In the combustor  14 , fuel  22  is mixed with the high-pressure air and ignited. Resulting hot combustion gases  15  are exhausted to drive the turbine  16 .  
         [0015]     Hot combustion gases  15  exhausted to drive the turbine  16  are typically at temperatures that can potentially damage metal components of the engine  10 . An air passage  19  leading from the compressor  12  supplies high-pressure air  18  to the turbine  16 . High-pressure air  18  creates a boundary layer that insulates metal components from the hot combustion gases  15  flowing over the turbine  16 .  
         [0016]     The air  18  within the air passage  19  cooling the turbine  16  must be at a temperature that provides the desired cooling benefits to the turbine  16 . The greater the temperature of the air flowing over the turbine  16 , the more flow required. More flow from the compressor  12  decreases overall engine efficiencies. For this reason the air  18  within the air passage  19  is first routed through a fuel/air heat exchanger  20 . Air  18  within the air passage  19  is placed in thermal contact to reject heat to fuel  22  in the fuel/air heat exchanger  20 .  
         [0017]     Usable cooling capacity of the fuel  22  is increased by removing dissolved oxygen. The presence of dissolved oxygen within the fuel causes most aircraft fuels to break down at temperatures greater than about 250° F. The breakdown of fuel results in the formation of insoluble coke deposits on components within fuel passages  23  and the combustor  14 . The formation of undesirable coke deposits causes degradation of engine efficiencies and/or requires additional maintenance. The fuel system includes a fuel deoxygenator  24  for removing dissolved oxygen from the fuel  22 .  
         [0018]     Referring to  FIG. 2 , a schematic view of a fuel deoxygenator  24 ′ according to this invention is shown and includes a plurality of tubes  34  disposed within a housing  36 . The fuel  22  is flowed around the tubes  34  from an inlet  38  to an outlet  40 . Tubes  34  include a composite permeable membrane  30  that absorbs oxygen molecules dissolved within the fuel  22 . A strip gas  32  flowing through the tubes  34  creates a partial pressure differential across the composite permeable membrane  30  that draws dissolved oxygen from the fuel  22  into the tubes  34  and out with the strip gas  32 . Oxygen is then removed from the strip gas  32  and exhausted from the system. The strip gas  32  is then recycled through the fuel deoxygenator  24 ′. Deoxygenated fuel exits through the outlet  40  and into the fuel/air heat exchanger  20  for absorbing heat from cooling air  18 .  
         [0019]     Referring to  FIG. 3 , another embodiment of a fuel deoxygenator  24 ″ is shown and includes a series of fuel plates  42  stacked one on top of the other. The composite permeable membrane  30  is included on each of the fuel plates  42  to define a portion of fuel passages  46 . Fuel enters through an inlet  48  and exists through an outlet  50 . An opening  49  is open to a vacuum source  56 . Fuel  22  passes within the fuel passages  46  defined by the stacked fuel plates  42 . The fuel plates  42  are disposed within the housing  44  that defines the inlet  48  and the outlet  50 . The use of the fuel plates  42  allows for the adaptation of the fuel deoxygenator  24 ″ to various applications by the addition or subtraction of fuel plates  42 . Although embodiments of fuel deoxygenators are shown and described, a worker skilled in the art with the benefit of this application would understand that other configurations of fuel deoxygenators are within the contemplation of this invention.  
         [0020]     Referring to  FIG. 4 , the composite permeable membrane  30  is shown in cross-section and preferably includes a permeable layer  52  disposed over a porous backing  51 . The porous backing  51  supplies the required support structure for the permeable layer  52  while still allowing maximum oxygen diffusion from fuel. The permeable layer  52  is coated on to the porous backing  51  and a mechanical bond between the two is formed. The permeable layer  52  is preferably a 0.5-20 μm thick coating of Teflon AF 2400 over a 0.005-in thick porous backing  51  of polyvinylidene fluoride (PVDF) with a 0.25 μm pores size. Other supports of different material, thickness and pore size can be used that provide the requisite strength and openness. Preferably the permeable layer  52  is Dupont Teflon AF amorphous fluoropolymer however other materials known to workers skilled in the art are within the contemplation of this invention, such as Solvay Hyflon AD perfluorinated glassy polymer and Asahi Glass CYTOP polyperfluorobutenyl vinyl ether. Each composite permeable membrane  30  is supported on a porous substrate  54 . The porous substrate  54  is in communication with the vacuum source  56  to create an oxygen partial pressure differential across the composite permeable membrane  30 .  
         [0021]     In operation a partial pressure differential is created by the vacuum source  56  between a non-fuel side  55  of the permeable membrane  30  and a fuel side  57 . Oxygen indicated at arrows  58  diffuses from fuel  22  across the composite permeable membrane  30  and into the porous substrate  54 . From the porous substrate  54  the oxygen  58  is pulled and vented out of the fuel system.  
         [0022]     The efficiency of a gas turbine engine is related to the temperatures that the engine can achieve. Higher temperatures enable better fuel burn capabilities that in turn result in longer range for the aircraft. Increased temperatures are enabled by cooled cooling air  18  from the compressor  12  that is routed through the fuel/air heat exchanger  20 . As appreciated, air bled from the compressor  12  reduces the efficiency of the engine  10 . The reduction of air bled from the compressor  12  facilitated by the increased heat sink capacity of deoxygenated fuel increases overall engine operating efficiency.  
         [0023]     The foregoing description is exemplary and not just a material specification. The invention has been described in an illustrative manner, and should be understood that the terminology used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations of the present invention are possible in light of the above teachings. The preferred embodiments of this invention have been disclosed, however, one of ordinary skill in the art would recognize that certain modifications are within the scope of this invention. It is understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically described. For that reason the following claims should be studied to determine the true scope and content of this invention.