Bleed air control by mixing

Abstract: A gas turbine engine bleed air system (10) in which air flow from a high pressure (HP) and a low pressure (LP) compressor stage of a gas turbine engine are mixed together, by controlling the pressure at the outlet of a proportional valve, (PRV) the pressure is controlled by modulating the flow area of the proportional valve (HPSOV) through which the high pressure air flows in order to achieve a scheduled pressure value, where the scheduled pressure value is determined as a function of a temperature error at a location downstream of the proportional valve (PRV) in the bleed air system.

Description: Technical Field This invention relates to the control of bleed air flow from an aircraft's gas turbine engine, and more particularly to controlling the set point of the bleed air temperature. Background Art As known, both commercial and military aircraft use bleed air from the aircraft's gas turbine engine to supply pneumatic and thermal energy to on-board systems which provide aircraft air conditioning, cabin pressurization, thermal anti-ice (TAI), and engine thrust reversal. The bleed air sources are the engine's high pressure (HP) and low pressure (LP) compressor stages; the HP stage providing bleed air at a pressure and temperature typically twice that of the LP stage. Prior art bleed air control systems, such as that illustrated in U.S. Patent 4,482,114 to Gupta et al., select bleed air alternately from the HP stage and the LP stage, but not from both simultaneously. This type of system uses bistable valves to control flow in the bleed conduits from the HP and LP stages. An open/close (i.e., bistable) valve in the HP bleed conduit is gated on or off to provide full or zero HP bleed flow to a bleed air manifold, and a check valve in the LP conduit provides LP flow to the manifold in the absence of HP flow. Improved efficiency prior art bleed systems provide a two tier HP flow arrangement in which two HP open/close valves are used. A low flow bistable valve is placed in parallel with a high flow valve. The HP high flow valve opens only when the low flow valve cannot supply the user system requirements. These systems, however, neither modulate the bleed air flow nor mix the HP and LP flows to achieve set point control of flow pressure or temperature. If the temperature of the LP bleed air is too low for system requirements (e.g., inlet air for the TAI system), the bleed system control opens the HP bleed valve. This results in a step increase in flow pressure and temperature which exceeds the bleed air requirements of the user system. The excess energy is wasted, while engine thrust is reduced by loss of the bleed air. A prior art system which does provide mixing of the HP and LP air flows is disclosed in U.S. Patent 4,285,466 to Linscheid et al.. This system uses a plurality of nozzle ejectors, each of which passes a different mass of HP compressed air into an air-mixing chamber where the HP air is mixed with LP air. Each nozzle is bistable and provides step control of HP air flow through it. As a result only a discrete number of mixed air temperatures are available since there are only a discrete number of nozzle states. Disclosure of the Invention An object of the present invention is to provide a gas turbine engine bleed air system which mixes high pressure and low pressure bleed air from a gas turbine engine in a manner to control the temperature of the mixed bleed air flow to a selected value. According to the present invention the high pressure (HP) flow passes through a proportional valve to mix with the low pressure (LP) flow, the proportional valve modulating the HP flow area to achieve a selected mixed air pressure which corresponds to a desired mixed air temperature. The present invention may be used for providing mixed bleed air to a thermal anti-ice (TAI) system. Closed loop temperature and pressure control are used to modulate the flow area of the proportional valve to control pressure at the valve outlet, in order to regulate temperature at the user equipment inlet. This control can be performed without additional bleed air sensors, since the stations from which feedback information is obtained already contain sensors for other bleed system control functions. Using a proportional valve to control the flow of the HP air allows the bleed air system to meet the demands of the systems it supplies, while extracting less energy from the engine. These and other objects, features and advantages of the present invention will become more apparent in light of the following detailed description of a best mode embodiment thereof as illustrated in the accompanying drawings. Brief Description of the Drawings Fig. 1 is a schematic diagram of the bleed air system of the present invention; and Fig. 2 is an illustration of the control logic used in the embodiment of Fig. 1; Fig. 3 is a functional illustration used in the description of the embodiment of Fig. 1; and Fig. 4 is another functional illustration used in the description of the embodiment of Fig. 1. Best Mode for Carrying Out the Invention In Fig. 1, the bleed air system 10 of the present invention receives the engine's low pressure (LP) and high pressure (HP) bleed air through conduits 12,13 respectively, and the engine fan air through conduit 14. Conduits 12,13 route bleed air to a source selector 15, which comprises a high pressure shut-off valve (HPSOV) 16 and a check valve 17. The HPSOV 16 regulates the flow of HP air entering a mixing conduit 18 in response to an electrical signal on a line 19 from an electronic controller 20. The HPSOV 16 is a proportional valve which allows the electronic controller 20 to modulate the area of the HPSOV through which HP air flows. The check valve 17 prevents HP air from back flowing to the engine LP compressor stage. Operation of the source selector 15, and more particularly the control of the source selector by the electronic controller 20, is described in detail hereinafter. The mixing conduit 18 is connected to a pressure regulating valve (PRV) 24 which regulates the pressure in a conduit 25 to provide a pressure regulated flow of warm air to a precooler 26. The precooler 26 receives cool fan air on conduit 28 from a fan air valve (FAV) 30 which regulates the quantity of engine fan air passing from conduit 14 through the precooler 26. The precooler uses the fan air to remove heat from the pressure regulated flow of warm air from the PRV 24. The HPSOV 16 operates in two modes: a bistable valve emulator mode, and a proportional valve mode. In the bistable mode the HPSOV operates in either the full open or full closed state similar to a bistable valve. However, under certain engine operating conditions, the HPSOV is operated in the proportional mode to modulate the HP airflow and reduce the amount of bleed air energy extracted from the engine. A pressure sensor 40 measures static air pressure in the mixing conduit 18 via a non-flowing conduit 42, and provides an electrical feedback signal indicative of the static mixed air pressure on a line 44 to the electronic controller 20. A temperature sensor 46 (e.g., a thermistor) measures the temperature of the mixed air exiting the precooler, and provides an electrical feedback signal indicative of the sensed mixed air temperature on a line 48 to the electronic controller 20. The sensors 40,46 provide feedback information to individual inner and outer closed loop control functions within the controller 20. Fig. 2 illustrates the control logic of the electronic controller for controlling the HPSOV in the proportional mode. The control logic comprises compensation for two loops: the outer closed loop on temperature 58 and the inner closed loop on pressure 59. The inner (pressure) loop is faster than the outer (temperature) loop (i.e., the inner loop has a higher bandwidth than the outer loop). It is to be noted that the control logic for the PRV in the emulator mode is neither illustrated or discussed since it has no bearing on the practice of the present invention. The controller receives the sensed pressure and the sensed temperature feedback signals on lines 44,48 respectively. The temperature signal is input to a lead/lag filter 60 which provides a filtered temperature signal on a line 62 to a summing junction 64. The summing junction provides the difference between a reference temperature signal on a line 66 and the filtered temperature signal on the line 62 as a temperature error signal on a line 68. The temperature error signal 68 is presented to a lead/lag filter 70 which provides a filtered temperature error signal on a line 72 to an integrator 74 with gain. The integrator 74 provides the time integrated value of the filtered temperature error signal as a pressure reference signal on a line 76 to the inner loop 59 (i.e., the signal on the line 76 is in the same units as the sensed pressure on the line 44). Summing junction 77 takes the difference between the pressure reference signal on line 76 and the sensed pressure signal on the line 44 to provide a pressure error signal on a line 82. The pressure error signal is multiplied by a non-linear gain 84 and the resultant product is presented to a lead/lag filter 86. The filtered signal from the lead/lag filter 86 is presented on a line 88 to an integrator 90 with a gain which provides an output signal on a line 92, and to a proportional amplifier 94 having gain K p which provides a proportional signal on a line 96 into a summing junction 98. The summing junction 98 provides a proportional plus integral signal on a line 102 to a limiter 104 which limits the magnitude of a current signal on the line 19 to the HPSOV. The inner pressure control 59 maintains the air pressure in the mixing conduit 18 substantially equal to the reference pressure signal on the line 76 (set as a function of integrated temperature error) by modulating the flow area of the HPSOV 16. Testing has illustrated that closed loop temperature control alone is not sufficient to control the temperature of the mixed air while operating the HPSOV in the proportional valve mode. However, adequate HPSOV proportional valve mode control is achieved using an outer closed loop on temperature with an inner closed loop on pressure. To control the HPSOV as a proportional valve, the summing junction 64 calculates the error between the filtered temperature signal on the line 62 and the reference temperature signal on the line 66, and provides the error signal on the line 68 into the lead/lag 70. The integrator 74 provides the reference pressure on the line 76 for the inner loop which sets the command signal to the HPSOV on the line 19. In the proportional valve mode, the HPSOV 16 is opened a small amount to provide some HP air for mixing with the LP air in the mixing conduit 18 (Fig. 1). The amount that the HPSOV is open is controlled by the control logic of Fig. 2. This small flow requirement is established by the fact that the check valve 17 will remain open only when the HPSOV outlet pressure is not substantially greater than the LP air pressure in the conduit 12. Fig. 3 illustrates a plot 108 of the mixing conduit static pressure 109 versus HPSOV flow area 110. An operating line 111 defines mixing conduit static pressure versus the HPSOV flow area relationship. When the HPSOV flow area is zero (i.e., the HPSOV is closed) the mixing conduit static pressure is equal to the LP conduit air pressure minus the pressure losses in the check valve and the LP line. As the HPSOV flow area increases along the operating line 111, it reaches a flow area value A c 112 where the LP flow has been reduced to zero (i.e., the check valve 17 closes). As the flow area increases beyond A c , the LP check valve remains closed and the mixing conduit air pressure increases in a substantially proportional manner until the flow area reaches a value of A o 114 which represents the maximum HPSOV flow area. At the maximum flow area A o , the mixing conduit static pressure is equal to the HP conduit air pressure, 115. To facilitate mixing, the pressure of the air exiting the HPSOV must be approximately equal to the pressure of the LP bleed air in order to keep the check valve 17 (Fig. 1) open. Therefore, the HPSOV has a limited operating region 116 (bounded in dashed lines) where mixing occurs. If the HPSOV flow area increases beyond the limited operating region 116 the check valve will close terminating the mixing. Therefore, to sustain mixing the HPSOV flow area has to take on values within the limited operating region 116. It should be understood the specific limited operating region has to be determined based on the characteristics (e.g., friction) of the components within the system. Fig. 4 illustrates a plot 118 of the mixing conduit air temperature 120 versus HPSOV flow area 122. An operating line 124 defines the mixing conduit air temperature versus the HPSOV flow area relationship. When the HPSOV flow area is zero (i.e., the HPSOV is closed) the mixing conduit air temperature is approximately equal to the LP conduit air temperature. As the HPSOV flow area increases along the operating line 124, it reaches a flow area value A c 126 where the mixing conduit air temperature reaches the value of the HP conduit air temperature. As the HPSOV flow area increases beyond A c 126, the mixing conduit air temperature remains constant at the HP conduit air temperature since the HP conduit is supplying all the bleed air entering the mixing conduit 18. While HPSOV operates in the proportional valve mode under the control of the logic in Fig. 2, the air temperature in the mixing conduit 18 can only take on values in a region 128, along the operating line 124. The particular value is determined by the reference temperature on the line 66 of the outer closed loop on temperature 58 (Fig. 2). While the best mode embodiment of the present invention uses an electronic controller, it should be understood the control logic illustrated in Fig. 2 may also be performed mechanically. Furthermore, within the electronic controller 20 the control laws of the present invention can be performed in either hardware or software. It should also be noted the present invention is not limited to mixing of only two bleed air flows. A plurality of bleed air flows may be mixed together by using proportional valves to control each bleed air flow, with the exception of the lowest pressure bleed air flow. As described hereinbefore the inner pressure loop is faster than the outer temperature loop. As an example, the inner loop may have a bandwidth of 10 radians/second, whereas the slower outer loop bandwidth may be approximately 0.5 radians/second. However, it should be understood the bandwidth of each of these two loops has to be determined in view of the dynamics of the particular bleed air system to be controlled, just as in any control system design, and as such these bandwidth numbers are only examples. In addition it should be understood that if the bleed air system operates in only the proportional mode, the temperature sensor 46 may be placed in the mixing conduit 18 similar to the positioning of the pressure sensor 40, since the precooler is not operational during the proportional mode. Also, while the exemplary embodiment discussed hereinbefore senses static pressure, it is obvious total pressure may be sensed as an alternative embodiment. Notwithstanding the foregoing detailed description, simply stated the present invention mixes high pressure and low pressure bleed air flows from a gas turbine engine to achieve a desired mixed air temperature, by modulating the flow area of the proportional valve through which the high pressure air flows, the pressure at the outlet of the proportional valve is controlled in order to achieve a scheduled pressure value, where the controller determines the scheduled pressure value. Although the present invention has been shown and described with respect to a best mode embodiment thereof, it should be understood by those skilled in the art that various other changes, omissions and additions to the form and detail thereof, may be made therein without departing from the scope of the claims.

Claims: Apparatus for mixing together the high pressure and low pressure compressed air from a gas turbine engine to create a temperature regulated mixed air flow, comprising: a check valve (17) disposed to receive the low pressure compressed air; a proportional valve (16), disposed to receive the high pressure compressed air, having a variable flow area responsive to a valve area command signal (19) and through which the high pressure compressed air is discharged; a mixing conduit (18), having a plurality of inlets, a first of said plurality of inlets for receiving compressed air from said check valve (17), a second of said plurality of inlets for receiving compressed air from said proportional valve (16), such that the air from each of said plurality of inlets is mixed together within said mixing conduit (18) and discharged as a temperature regulated flow of bleed air; a temperature sensor (46) for sensing the air temperature of said temperature regulated flow of bleed air, and for providing a temperature signal (48) indicative thereof; characterized by: a pressure sensor (40) for sensing the air pressure of said temperature regulated flow of bleed air, and for providing a pressure signal (44) indicative thereof; and a controller (20) for computing said valve area command signal (19) as a function of said temperature signal value (48) and said pressure signal value (44), such that said valve area command signal takes on a value that drives the temperature of the temperature regulated flow of bleed air towards a desired mixed air temperature value. The apparatus of claim 1 wherein said controller further comprises control logic (58,59) which computes a reference pressure signal value (76) as a function of the difference between said sensed temperature signal (48) value and the desired mixed air temperature signal value (66), and computes said valve area command signal value (19) in response to the difference between said pressure reference signal value (76) and said sensed pressure signal value (44). The apparatus of claim 2 wherein said control logic (58,59,) further comprises: a first compensator (70,74) for providing said reference pressure signal value, having an integrator (74) which provides an outer control loop (58) on temperature with said sensed temperature signal value (48) as a temperature feedback signal and said desired mixed air temperature value (66) as the loop reference signal; and a second compensator (84,86,90,94,98), having a proportional plus integral path (94,90) which provides an inner control loop (59) on pressure with said sensed pressure signal value (44) as a pressure feedback signal and said pressure reference signal value (76) as the loop reference signal for providing said valve area command signal value. The apparatus of claim 2 wherein said control logic (58,59); further comprises: a first compensator (70,74) for providing said reference pressure signal value, having a lead/lag filter (70) which provides an outer control loop (58) on temperature with said sensed temperature signal value (48) as a temperature feedback signal and said desired mixed air temperature value 66 as the loop reference signal; and a second compensator (84,86,90,94,98), having a proportional plus integral path (94,90) which provides an inner control loop (59) on pressure with said sensed pressure signal value (44) as a pressure feedback signal and said pressure reference signal value (76) as the loop reference signal for providing said valve area command signal value. Apparatus for mixing together the high pressure and low pressure compressed air from a gas turbine engine to create a temperature regulated mixed air flow, comprising: a check valve (17) disposed to receive the low pressure compressed air; a proportional valve (16), disposed to receive the high pressure compressed air, capable of operating alternatively in a bi-static mode and a proportional mode, having a variable flow area responsive to a valve area command signal (19) and through which the high pressure compressed air is discharged; a mixing conduit (18), having a plurality of inlets, a first of said plurality of inlets for receiving compressed air from said check valve (17), a second of said plurality of inlets for receiving compressed air from said proportional valve (16), such that the air from each of said plurality of inlets is mixed together within said mixing conduit (18) and discharged as a temperature regulated flow of bleed air; a pressure sensor (40) for sensing the air pressure of said temperature regulated flow of bleed air, and for providing a pressure signal 44 indicative thereof; a conduit; characterized by: a pressure regulating valve (24) having a variable flow area, disposed to receive said temperature regulated flow of bleed air, and provide a pressure and temperature regulated flow of bleed air 25 into said conduit; a temperature sensor (46) for sensing the air temperature of said pressure and temperature regulated flow of bleed air in said conduit, and for providing a temperature signal 48 indicative thereof; and a controller (20) having control logic (58,59) which computes a reference pressure signal value (76) as a function of the difference between said sensed temperature signal (48) value and a desired mixed air temperature signal value (66), and computes said valve area command signal value (19) in response to the difference between said pressure reference signal value (76) and said sensed pressure signal value (44). A method of providing a temperature regulated flow of bleed air by mixing together a low pressure bleed air flow and a high pressure bleed air flow from a gas turbine, comprising the steps of: modulating, in response to a command signal value, the flow area of a proportional valve through which the high pressure bleed air flows; combining the modulated high pressure bleed air flow and the low pressure bleed air in a mixing conduit; sensing the temperature of the mixed bleed air flow in the mixing conduit; sensing the pressure of the mixed bleed air flow in the mixing conduit; and computing said command signal value as a function of the sensed temperature and sensed pressure signal values.

Inventors: BRUUN EUGENE ROBERT, KOS JOSEPH MICHAEL, BRUUN, EUGENE ROBERT, KOS, JOSEPH MICHAEL
Applicants: UNITED TECHNOLOGIES CORP, UNITED TECHNOLOGIES CORPORATION
Application year: 1992
Main classification: F02C7
IPC codes: B64D13, F02C7, F02C6