Abstract:
The airflow distribution of bleed air extracted from a plurality of turbine engines ( 11, 13, 15, 17 ) is equalized by an airflow sharing system having electronic airflow sensors ( 49, 51, 53, 55 ) and closed-loop control algorithm to equalize the pressure-drop characteristics of multiple bleed air branches to flow-share equally. The pressure-drop characteristic of each airflow branch is controlled to the same setpoint characteristic by negative feedback. The closed-loop control can be implemented with an electronic circuit or as a computational process in a digital controller.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims the benefit of U.S. Provisional Patent Application No. 60/251,910 (Attorney Docket No. H0001054) Filed Dec. 7, 2000 and entitled AIRFLOW SHARING 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention relates to airflow distribution of bleed air extracted from turbine engines, and more particularly to a system for equalizing the airflow distribution among a plurality of engines.  
           [0003]    It is a common practice to bleed air from one or more gas turbine aircraft engines to provide pneumatic and thermal power to different aircraft systems, for example, for cabin pressurization and temperature control, equipment environmental control, thrust reversing systems, anti-icing equipment, and pneumatically powered equipment. It is desirable to distribute the burden of supplying air for these auxiliary functions among the several engines of the aircraft. U.S. Pat. No. 4,765,131 entitled AIRCRAFT ENGINE BLEED AIR FLOW BALANCING TECHNIQUE recognizes these problems, measures the pressure drop across a heat exchanger and employs limited authority negative feedback to control a pressure regulating valve for each of two duct systems. Without accurate allocation of the airflow burden among the several engines, the engine having the greatest burden experiences disproportionate diminished fuel economy, elevated operating temperature, and increased wear and maintenance requirements. These problems are discussed in greater detail in the U.S. Pat. No. 4,765,131 patent.  
           [0004]    In an application entitled BLEED AIR FLOW REGULATORS WITH FLOW BALANCE, U.S. Pat. No. 5,155,991 recognizes the shortcomings of the U.S. Pat. No. 4,765,131 patented system due to errors in sensing the pressure drop across the heat exchanger and measures not only the pressure differential across a heat exchanger (precooler), but also another downstream pressure differential and precooler outlet temperature and pressure values in an attempt to refine the equalization of the flow of bleed air from each of several gas turbine engines. The U.S. Pat. No. 5,155,991 control scheme for each flow branch requires the flow information from its own branch and the flow information from the other branch. The cross-feeding of flow information is the key to equalize the flows in the two branches. The system is suitable only for a two-branch flow system. A control scheme for a four-engine system is an extremely challenging control problem, with cross-feeding of all four flow information to all four control channels, creating enormous interaction problem. There remains a need for comparatively simple and economical, yet highly accurate and reliable technique for allocating bleed airflow among several aircraft engines, and this need is particularly acute for applications involving more than two engines.  
           [0005]    It is desirable to equally share the air supply responsibilities among the several engines of an aircraft, and to do so in an economical and accurate manner.  
         SUMMARY OF THE INVENTION  
         [0006]    The present invention provides solutions to the above problems by measuring individual flow rates and a common downstream manifold pressure, and uses electronic/computer means to implement negative-feedback control for creating identical, e.g., linear, pressure-versus-flow characteristics for all flow branches. Flow branches with identical pressure-drop characteristics will allow the supply sources to naturally and accurately flow-share.  
           [0007]    The invention comprises, in one form thereof, a technique for allocating an aircraft air supply demand among several independently operable aircraft engines, each supplying air to a common manifold by way of a corresponding air duct having an inlet pressure control valve for regulating the duct airflow. The rate of airflow through each individual duct may be measured and that measurement used in determining the desired air pressure set point. Additionally, a common manifold air pressure is measured. A combination of the determined and measured pressures is used to generate a corrective control command to the corresponding pressure control valve for each duct independently of the others. Airflow as used herein means the air flow rate, e.g. measured in pounds of air per minute, while pressure drop is measured, for example in pounds per square inch.  
           [0008]    An advantage of the present invention is that all ducts can be independently set by controlling individual pressure regulating valves to create identical pressure-drop characteristics downstream of the valves as a function of mass flow rate regardless of differences in downstream pressure drops due to the individual duct length, cross-section area, shape and internal contamination, which is indeterminate in time. Ducts with identical pressure-drop characteristics will flow-share naturally. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0009]    A more complete understanding of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings in which:  
         [0010]    [0010]FIG. 1 illustrates multiple-engine bleed air sources, pressure regulated and flowing into a common manifold before being distributed to a number of different destinations;  
         [0011]    [0011]FIG. 2 shows modifications to FIG. 1 to achieve airflow sharing in accordance with the invention;  
         [0012]    [0012]FIG. 3 shows, in accordance with the invention, four airflow branches with the same pressure-drop characteristics;  
         [0013]    [0013]FIG. 4 shows a closed-loop control algorithm for implementing the present invention;  
         [0014]    [0014]FIG. 5 is a schematic illustration of the fluid flow controlling process according to the present invention; and  
         [0015]    [0015]FIG. 6 is a graphical illustration of the fluid flow controlling process according to the present invention. 
     
    
       [0016]    Corresponding reference characters indicate corresponding parts throughout the several drawing views.  
       DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0017]    Referring now to the drawings and particularly to FIG. 1, there is shown an aircraft pneumatic system for supplying pressurized air from a plurality of aircraft engines  11 ,  13 ,  15  and  17  to multiple air utilization devices including upstream equipment  19   a ,  19   b ,  19   c  and  19   d  and downstream equipment  21 . There may be no upstream equipment and whatever upstream equipment is present maintains the airflows separate from one another. There is no mixing of the flows upstream of the heat exchangers such as  29 . Airflow from engine  11 , for example, passes through an air duct or conduit portion  23 , into the upstream equipment associated with that engine (if any), then by duct portion  25  to the inlet of a pressure regulating valve  27 . From the outlet of valve  27 , the airflow continues through additional airflow equipment  29 , which may, for example, comprise a heat exchanger for reducing the air temperature to a preferred level. The airflow exits the equipment  29  by way of conduit portion  31  and enters a common manifold  33  where the engine # 1  airflow is mixed with the airflow from the other engines. The common airflow is supplied to the downstream utilization equipment  21  and thereafter exits the aircraft.  
         [0018]    In FIG. 1, there are a plurality of air temperature reducing heat exchangers  29 ,  35 ,  37  and  39 , each having an air inlet and an air outlet. A like plurality of air passageways similar to  25  lead from an engine to the inlet of a corresponding heat exchanger and each passageway includes a controllable pressure reduction valve  27 ,  41 ,  43 , and  45  for controlling the airflow from the associated engine to the associated heat exchanger. The multiple-engine bleed air sources  11 ,  13 ,  15  and  17  are pressure regulated by  27 ,  41 ,  43  and  45  respectively and flow into a common manifold  33  before being distributed to a different number of destinations as indicated generally at  21 . The common manifold  33  merges the airflows exiting the plurality of heat exchangers into a common airflow path and a plurality of air passageways similar to  31  each lead from a heat exchanger to the manifold. The prior art technique, as exemplified by the aforementioned U.S. Pat. No. 4,765,131, utilizes the pressure differential across a heat exchanger for controlling a corresponding valve. The bleed air extracted from turbine engines of an airplane must be equally distributed among the engines, otherwise the engine with the higher airflow will run hotter and will have increased wear and reduced life. Small differences in pressure regulation are inevitable and airflow ducts downstream are different in length and have different pressure-drop characteristics. The result is that airflow distribution will not be equal.  
         [0019]    [0019]FIG. 2 is similar to FIG. 1 in showing a plurality of engines or other air flow supplies feeding air to optional upstream equipment  19   a ,  19   b ,  19   c  and  19   d  by way of conduits such as  23 . The individual airflow paths include pressure regulators such as  27  as before, and heat exchangers or other airflow equipment such as  47  the outlets of which merge in a manifold  33 . Heat exchangers are typical pressure-drop elements of a typical system, but are not elements that are necessary to make this flow-sharing scheme work. Furthermore, the flow-sharing scheme is applicable to systems other than pneumatic systems on aircraft. Each conduit also includes an airflow sensing device  49 ,  51 ,  53  or  55  for measuring the airflow therethrough. A pressure sensor  57  is positioned within the manifold  33  for determining a common fluid passageway outlet pressure. A plurality of control arrangements  59 ,  61 ,  63 , and  65  are each associated with and control a corresponding valve  27 ,  41 ,  43  and  45  respectively. Each control arrangement specifies a set point pressure as a function of the flow rate and controls the flow rate through each heat exchanger independently of the flow rates through the other heat exchangers. This equalizes the pressure-drop characteristics of airflow paths from the outlet of each valve to the manifold causing those paths to equally share the flow. As before, air utilization equipment  21  receives air by way of the common airflow path through the manifold  33 . Using the individual flow sensor signal and the common pressure sensor signal, the unequal pressure-drop characteristics of the four airflow branches are transformed by the closed-loop control algorithm of FIGS. 4 and 5 into four airflow branches with the same pressure-drop characteristics as shown in FIG. 3. The algorithm of FIGS. 4 and 5 is independently replicated for each of the illustrative four ducts by the control arrangements  59 ,  61 ,  63  and  65  in FIG. 2 and as functionally illustrated at  87 ,  89 ,  91  and  93  in FIGS. 3 and 6.  
         [0020]    Referring primarily to FIGS. 4 and 5, the method of allocating the aircraft air supply demand among the several independently operable aircraft engines  11 ,  13 ,  15  and  17  should now be clear. A plurality of pressure control set points (pressure setpoint function  69 ) are predetermined at  67  for each flow branch. These provide the desired air pressure drop for each flow branch. Preferably, the pressure drop across the control valve is a linear function of duct flow rate, and in a preferred form, the linear function has a negative slope (a strictly decreasing function) with air pressure decreasing as duct flow rate increases. The actual setpoint value to which the common manifold pressure is compared is calculated at  67  with the individual duct flow rates measured at  81 . The common manifold air pressure is measured as indicated at  71  by sensor  57 . The selected setpoint is combined with or compared to the common manifold pressure as indicated at  73 , and the result is utilized to control the corresponding valve as indicated at  75 . The result of adjusting the pressure control valve provides a pneumatic feedback as indicated at  77  thereby closing the feedback loop. Comparison  73  may, if desired, include a threshold function below which no adjustment occurs to avoid continuous minute changes or “hunting.” That is, the flow rate specifies the abscissa value along the linear function  69  thereby determining a specific ordinate value of pressure.  
         [0021]    The control transfer function  83  of FIG. 4 facilitates closed-loop control. Closed-loop control forces the pressure parameter under control (the pressure downstream of the corresponding control valve) to follow the setpoint closely. Since the setpoint is typically not set to a fixed pressure number, but rather is a linearly drooping pressure as a function of flow rate, the controlled pressure as measured at  57  in the common manifold must follow the linear function also. The pressure vs. flow relationship (setpoint function  69 ) may be a continuous linear function, two or more linear segments, a finite set of discrete values, or other strictly decreasing relationship. The control transfer function and the linear variable setpoint function are computational modules in software if computer control is used, or electronic circuit functions if analog control is used, and supply control signals to torque motor driver  85  which functions to adjust the corresponding valve. The control transfer function  83  is typically a proportional plus integral function that result in reducing the error from the summing junction  73  to zero in the steady state.  
         [0022]    Uncontrolled, i.e., natural pressure drop is proportional to the square of the mass flow rate. The constant of proportionality is determined by the air density and mechanical design of the duct, however, a linear setpoint function is preferred for the control algorithm so that closed-loop control action will result in a controlled pressure that is a linear function of the flow rate, instead of the quadratic relationship that obeys the law of physics. Having two (or more) identical linear droop characteristics in the two (or multiple) airflow ducts results in balanced flow distribution.  
         [0023]    It is important that, at the maximum operating flow rate, the pressure drop determined by the variable setpoint functions be slightly greater than the pressure drop that would be naturally encountered by the duct. This should include the anticipated build-up of contamination although the effect of contamination would typically be only a small fraction of the total pressure drop due to the length, cross-section area and shape. The technique of the present invention works well with unequal engine supply pressures, as long as the supply pressure is high enough to produce the flow rate required in a duct. This is accomplished by controlling the “equivalent flow impedance” of the pressure regulating valve.  
         [0024]    [0024]FIG. 6 shows the four monotonically decrasing pressure-drop characteristics  87 ,  89 ,  91  and  93  of the four flow branches as shown in FIG. 3, as a result of closed-loop control as shown in FIG. 2. FIG. 6 also includes an illustrative common sensed manifold pressure line  95 . This line  95  represents the only steady-state solution for the common manifold pressure that the four control loops must continuously seek. An excess flow rate as illustrated by the exaggerated flow rate at line  97  would be impossible in the steady state since that would have created a lower pressure in flow branch  87  below the single common manifold pressure. Likewise, a low flow rate as illustrated by the exaggerated flow rate at line  99  would be impossible in the steady state since that would have created a high pressure in flow branch  89  above the single common manifold pressure.