DE69623098T2 - AVOIDING THE PUMPING OF A COMPRESSOR - Google Patents

Description

Technical area

The present invention relates to methods for diagnosing and preventing stall in rotary compressors, for example aircraft jet engines.

Background of the invention

For a dynamic rotary compressor operating under normal, steady-state flow conditions, the flow through the compressor is essentially uniform around the annulus, i.e. it is axisymmetric, and the flow rate averaged over the annulus is uniform. In general, if the compressor is operating against the mass flow too close to the maximum pressure rise on the constant speed compressor pressure rise line diagram, disturbances acting on the compressor can cause it to enter a region of the performance diagram where fluid dynamic instabilities known as rotary stall and/or surge develop. This region is bounded on the compressor performance diagram by the surge/stall line. The instabilities degrade compressor performance and can lead to permanent damage, so they should be avoided.

Rotating stall can be considered a two-dimensional phenomenon that creates a localized region of reduced or reversed flow through the compressor that rotates around the annulus of the flow path. This region is referred to as the "stall cell" and typically extends axially through the compressor. Rotating stall creates reduced output (measured in annulus-averaged pressure rise and mass flow) from the Compressor. In addition, the stall cell loads and unloads the compressor blades as it rotates around the annulus and can induce blade fatigue failure. Surging is a one-dimensional phenomenon defined by oscillations in the annulus-averaged flow through the compressor. In severe surge conditions, flow reversal through the compressor can occur. Both types of instabilities should be avoided, especially in aircraft applications.

In practical applications, the closer the operating point is to the maximum pressure rise, the less the compression system can tolerate a given level of disturbance without entering a rotating stall and/or surge condition. The initiation of a rotating stall results in a sudden increase (within 1 to 3 rotor revolutions) from a state of efficient, axisymmetric operation with high pressure rise to a state of inefficient, non-axisymmetric operation with reduced pressure rise. It requires lowering the operating line on the compressor performance diagram to a point well below the point at which the stall occurred in order to return the compressor to axisymmetric operation (i.e., to eliminate the rotating stall region). In practical applications, the compressor may need to be shut down and restarted to eliminate (or recover from) stall due to stall hysteresis. Initiating surge produces a similar performance and operational degradation, but surge occurs for different reasons.

Because of these potential instabilities, compressors are typically operated with a "stall margin". The stall margin is a measure of the relationship between the maximum pressure rise, i.e. the pressure rise at stall, and the pressure ratio on the operating curve for the compressor for the instantaneous flow rate. In theory, the larger the stall margin, the greater the disturbance that the compressor system can tolerate. can be achieved before stall and/or surge occurs. Therefore, the goal in compressor design is to incorporate sufficient stall margin to avoid operation in a condition where an anticipated disturbance is likely to induce stall and/or surge. In gas turbine engines used to power aircraft, stall margins of 15 to 30% are common. Since operating the compressor below the maximum pressure rise results in a reduction in operating efficiency and performance, there is a trade-off between stall margin and performance. The stall margin can be reduced by engine operating conditions, such as aircraft angle of attack and yaw, and acceleration (conditions that momentarily change the rise in actual pressure), and can decrease over time due to component wear, such as increased clearances between the compressor blade tips and the compressor boundary wall.

Description of the invention

An object of the present invention is to avoid compressor stall, particularly in aircraft jet engines.

A method for controlling the acceleration operation of a gas turbine engine is known from European patent application EP-A2-0401152. This system allows the engine to accelerate with an adequate stall margin by controlling the flow of fuel to the combustor in response to certain engine operating parameters during acceleration.

According to a first aspect, the present invention provides a controller for a rotary compressor according to claim 1.

According to a second aspect, the invention provides a method for preventing stall in a rotary compressor according to claim 10.

In a preferred embodiment of the present invention, compressor flow is measured with one or more pressure sensors to produce a signal that is passed through a bandpass filter having a lower roll-off between 0.01 and 1 of N2 (the compressor rotation frequency) and an upper roll-off between 1 and 10 of N2. The output of the filter is smoothed and compared to a "design value" for compressor flow discontinuities and an error is generated which is integrated. One or more compressor bleed valves are opened when the integral exceeds a predetermined threshold.

In a first embodiment of the invention, compressor bleed air valves are opened for a fixed duration when the threshold is exceeded.

In a second embodiment of the present invention, when the threshold is exceeded and the bleed air valves are open, the design value is temporarily changed (e.g. decreased) until the bleed air valves close.

A feature of the invention, which derives from the ability of the invention to detect very early signs of rotating stall, is that a stall control utilizing the present invention can be used to improve the operation of a compressor (pumping) system having a compressor that is susceptible to rotating stall under certain circumstances. A feature of the invention is that it can be used in gas turbine engines and refrigeration systems, for example in some air conditioning systems or refrigeration systems.

The above and other objects, features and advantages of the present invention will become more apparent from the following description and drawings.

Short description of the drawings

Figure 1 is a functional diagram of a gas turbine engine using a static pressure sensor and a signal processor to control the opening and closing of compressor bleed valves to avoid stall using the time varying output of the pressure sensor, in accordance with the present invention.

Fig. 2 shows various compressor stage static compressor flow pressure sensors, bleed valve positions and signal processing steps to control the opening and closing of the bleed valves according to the present invention.

Fig. 3 shows transfer functions or operations used in any of the steps shown in Fig. 2.

Figure 4 is a three-dimensional plot of the magnitude of the fluctuations in N2 and the duration of the fluctuations and shows the pressure fluctuations in N2 used for machine diagnostics according to the present invention.

Fig. 5 is a three-dimensional plot of the magnitude of the pressure fluctuations and N2 and the duration of the fluctuations and shows the pressure fluctuations that typically occur at lower frequencies (below N2) and on the State-of-the-art stall detection devices typically respond.

Best mode for carrying out the invention Active stall prevention

Fig. 1 shows a bypass gas turbine turbofan engine 10 which uses a static pressure sensor 12 to provide a signal PR1 indicative of characteristics of the compressor flow 14 prevailing at a location in the compressor stage, for example between the eighth and ninth compressor stages. The signal PR1 is provided to a signal processor (SP) 16, which is assumed to include a central processing unit and associated memories programmed to cyclically perform calculation steps using the signal PR1 and the control/transfer functions 20, 22, 24 and 26 in Fig. 2 to generate a signal ACON.

The signal processor also receives a compressor speed (N2) signal which represents the compressor speed or frequency (ie rotor frequency). The signal ACON controls the opening of the compressor bleed valves 18 using the following control rule which will be explained in more detail using the software functional block diagram of Fig. 2 and Fig. 3:

In this equation 1, α₁₁ = an instantaneous level of discontinuity in the flow properties as manifested in the pressure signal PR1, and αk is a stored or "construction" value for the current level of discontinuity.

In Fig. 1, a so-called "FADEC" or "Full Authority Digital Electronic Control" 28 controls the fuel flow to the engine burners 2 as a function of a power lever advance (PLA) setting of a power control element 4 located in the cockpit. The fuel control may be considered to include a signal processor for controlling the fuel flow based on a variety of engine operating parameters and, although a separate signal processor 16 has been shown for carrying out the specific operations associated with the present invention, it is conceivable that a FADEC may be programmed to carry out these operations and generate the ACON signal for controlling the bleed air valves 18.

Referring to Fig. 2, it can be observed that a compressor has a plurality of stages, that bleed valves 18 are selectively located at certain stages, and that the static pressure sensor 12 is located before these stages (upstream in the compressor flow), although in some applications the sensor or sensors 12 may be located after (downstream of) the bleed valves 18. It should be assumed that the signal processor 16 is programmed to perform steps which achieve the functions of blocks 20, 22, 24 and 26. The pressure signal PR1 generated by the sensor 12 will have a time varying characteristic and will produce a compressor flow 14 signature including an indication of flow discontinuities along with flow and sensor noise. The pressure signal PR1 is narrowly filtered in block 20, with the bandpass frequency ranging from 0.1N2 to N2 with 2 pole roll-offs at the upper and lower frequencies. One effect is the smoothing of the signal PR1. The output of the filter function 20, the signal PR2, is used in an absolute value 22 function to generate the absolute value signal PR2 for the spectrum of information passed through the filter function 20. To remove unwanted noise in the signal PR2, the output from block 22 is passed to a low pass filter with a roll-off at 1 Hz and generates the signal PR3 which is effectively a measure of the unsteady flow condition associated with an imminent compressor stall, in other words the remaining stall boundary region. The next block 26 begins the operations shown in Fig. 3. At operation 30, the precursor value PR3 is subtracted from a stored value Amax (block 32), which is the maximum or design value for the precursor value and, if exceeded, manifests unstable compressor flow in the value of the Error 1 signal. Assuming the output from the scaling block 30 is zero, the output, Error 2, from a second summer 30 would be Error 1. The value for Error 2 is integrated at operation 38. The output of the integration step is limited at operation 40, and the output Aint (from the limiter 40) is scaled at operation 42, producing the bleed air control output signal Acon. At logic operation 44, the bleed air valves 18 are controlled to fully open when Acon exceeds a stored threshold; otherwise, the bleed air valves 18 remain fully closed. The block 44 should be able to perform one of the following operations once the bleed valves are open. It can provide a signal to decrease the value of Amax slightly, e.g. by 10%, while the bleed valves are open, and return Amax to its full value when the valves close again (the open signal ends). Alternatively, as shown by the broken block, a timer function 44 can be used to open the bleed valves for a fixed interval when the Acon signal is generated. The output from the operation 40 is subtracted from the output from the integrator operation 38 at the summing device 46, and the error from the summing device 46 is added to of operation 34 and applied to the summer 36 which reduces error 2 to prevent the integrator operation from winding up above the value of Aint over time. It can be seen that the bleed valves 18 open rapidly when the precursor signal (signal PR1) indicates a flow condition near the stall boundary; that is, the time-varying characteristic flow values normally encountered at early stages of a rotating stall are within the bandwidth of the filter 20 and last long enough for Acon to exceed the threshold.

Referring to Figure 4, at the rotational frequency of the rotor disk, the pressure fluctuations (the TP signal) occur hundreds of rotor revolutions before an actual stall/or surge. In Figure 4, the X-axis shows the time in seconds before stall, with the stall and/or surge occurring at approximately zero (0) seconds. The Z-axis shows the magnitude of the pressure disturbance in pounds per square inch squared (psi²) or amplitude squared. The Y-axis indicates the machine order, the frequency of pressure fluctuation (the value of TP divided by N2 (the rotation frequency of the rotor disk), where the value one (1) is the rotation frequency of the rotor disk and one-half (0.5) is half the rotation frequency of the rotor disk. This shows that the pre-stall pressure disturbance at N2 can be detected a few seconds before stall and/or surge. The value of N2 in this example is about one hundred (100) revolutions per second. Accordingly, monitoring the pressure fluctuations at N2 detects the pre-stall condition a few hundred rotor revolutions before an actual stall.

Such early detection provides sufficient warning to take corrective action to prevent or minimize stall and/or surge. The state of the art has focused on monitoring pressure fluctuations at 30-70% of the rotor disk rotation frequency or 0.3 to 0.7 of the engine thrust order shown on the Y axis. As can be seen in Fig. 5, the prior art techniques allowed warning of impending stall and/or surge only a few rotor revolutions before the actual stall. Fig. 5 is analogous to Fig. 4, but is scaled to show rotating stall disturbances at approximately one-half (0.5) or fifty percent (50%) of the rotor disk rotation frequency.

The preferred embodiment described here used a discontinuous pressure quantity as a form of measurement. Other discontinuous pressure parameters can be monitored to predict the onset of stall and/or surge. For example, gas density, velocity, temperature, or any other discontinuous flow quantity can be monitored to determine the onset of stall and/or surge. Velocity can be measured using hot wire anemometers or a static Pitot tube. Temperature can be measured using a thin wire thermocouple.

The test or diagnostic equipment described and shown in Fig. 2 is an example of test equipment that can be used to monitor the amplitude of the pressure fluctuations in accordance with the invention. Other equipment can be used to monitor the pressure fluctuations in the compressor. For example, the data acquisition system can be either a digital data acquisition system, digital tape, FM analog tape, or any other type of system capable of recording pressure disturbances (sensor output) with sufficient frequency bandwidth to resolve the disturbances to the rotation frequency of the rotor disk. For example, software packages that can be used in analyzing the pressure and rotor speed data are the MATLAB® program from The Math Works, Inc. of Natick, Massachusetts and the SNAP-MASTER® setup program from Hem Data Corporation of Southfield, MI. DAQBOOK® data acquisition hardware from Iotech of Cleveland, Ohio was used to generate the output values from these programs.

Claims (13)

1. Control system for a rotary compressor with a compressor bleed air valve (18), characterized by: a first device (12) for providing a first time-varying signal (PR1) which manifests the compressor flow; second means (20) for providing a second signal (PR2) manifesting the magnitude of the first signal between a first frequency less than the compressor speed and a second frequency greater than or equal to the compressor speed; and signal processing means (24, 26) for providing a first processor signal from the second signal manifesting a difference between the magnitude of the second signal and a stored value (32) for the second signal, for integrating the difference to produce a control signal and for producing a bleed air signal (ACON) to open the compressor bleed air valve (18) when the control signal exceeds a threshold value. 2. Control according to claim 1, further characterized in that the first device comprises a static pressure sensor (12) arranged in a compressor stage. 3. Control according to claim 1, further characterized in that the signal processing device (24, 26) has means for generating the bleed air signal (ACON) for a selected time interval. 4. The controller of claim 1, further characterized in that the second device (20) has two-pole slip at the first and second frequency. 5. Control according to claim 1, characterized in that the first frequency is 0.1 of the compressor speed. 6. The controller of claim 1, further characterized in that the first frequency is between 0.01 and 1 times the rotation frequency and the second frequency is between 1 and 10 times the rotation frequency. 7. Control according to claim 6, further characterized in that the signal processing device (24, 26) has means (44a) for generating the bleed air signal (ACON) for a fixed time interval. 8. Control according to claim 1, further characterized in that the signal processing device (24, 26) comprises means (44) for changing the magnitude of the stored value (32) to a temporary value when the bleed air signal (ACON) is generated. 9. The controller of claim 8, further characterized in that the temporary value is less than the magnitude of the stored value (32) when the bleed air signal is generated. 10. Method for preventing flow separation in rotary compressors, characterized by: Recording the size (PR3) of the time-varying characteristic value of the compressor flow in a bandwidth around the rotation frequency of the compressor; Generating an integral value (38) by integrating the difference between the size (PR3) and a design value (32) for the size; and Increasing the compressor mass flow when the integral value (38) exceeds a threshold value. 11. The method of claim 10, further characterized by increasing the mass flows for a fixed period of time. 12. The method of claim 10, further characterized by decreasing the design value (32) while generating the integral value (38). 13. The method of claim 10, further characterized in that the bandwidth has a lower frequency roll-off at 0.01 to 1 times the rotation frequency and an upper frequency roll-off at 1 to 10 times the rotation frequency.