System For The Detection And Control Of Compressor Stall

Abstract

A system for the direct detection of compressor stall during acceleration in gas turbine engines and a direct control which reduces fuel flow in time to prevent the stall. One or more vanes is set at a higher angle of attack than the balance of the vanes in a compressor stage determined to be the most likely to suffer stall first. Flow separation, which is associated with the onset of compressor stall, results in a change in vane surface pressure. This pressure differential is detected by a pressure sensor through taps located in the selected vanes. Actuation of the differential pressure sensor closes a switch to energize a solenoid-operated fuel bypass valve which causes a reduction in fuel flow to eliminate the stall.

Description

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention relates generally to improvements in turbine engines, and more particularly it pertains to a new and improved compressor stall detection and control system which anticipates stall and reduces fuel flow in time to prevent complete compressor stall.

One of the most critical problems confronting developers of turbine engines has been compressor stall. Axial and centrifugal compressors both have a stall or pumping limit where flow reverses. This limit is usually encountered on starting with stationary power units and at high altitude and high speed with aircraft units. On military aircraft engines there is always a demand for the highest rate of fueling the engine during acceleration transients to obtain the highest rate of acceleration. In the prior art, the stall limit is shifted out of the operating range of the gas turbine by the use of compressor bleed, variable compressor vanes, water injection or bypass. All these methods are applied so as to reduce the aerodynamic loading on the stalled compressor stages. In prior art engines, compressor stall is also avoided by sensing several parameters. One such sensor detects the rate of change of compressor discharge pressure. Another detects changes in compressor blade characteristics sonically. However, all currently known stall detection devices do not give enough lead time on the impending stall to allow control measures to be taken.

Moreover, in the prior art, compressor stall is avoided with complex mechanisms scheduling fuel flow around the region where stall is known to occur on a "typical" compressor. Actually, characteristics of the poorest compressor must be accommodated by the control system, thereby penalizing (slowing down of acceleration time) a good compressor. Such devices have been unsatisfactory in that they are inefficient and the complex mechanisms are expensive. Those concerned with the development of the compressor art have long recognized the need for a simple and direct stall detection and control system which furnishes an early signal for acceleration control of gas turbine engines.

The above problems are overcome by the present invention which provides a system to anticipate compressor stall simply and directly by measuring a compressor parameter affected by stall and reducing fuel flow in time to prevent the stall.

SUMMARY OF THE INVENTION

The general purpose of the invention is to provide a system for the detection and control of compressor stall. After studying the problem of stall detection and control, those familiar with the state of the art have concluded that the use of some stall prone portion of the compressor on which stall can be tolerated is the only away to successfully control stall. This invention is predicated on that concept.

The main advantage of the present invention lies in its simplicity. The method of creating a lead time on stall by deliberately increasing the angle of attack of one or more vanes involves no moving parts nor a requirement for shaping a signal to a sensor. In addition, such a closed loop system lends itself to optimization of control. Since the individual compressor characteristics are sensed, each engine determines its own acceleration rate and the maximum rate of acceleration is obtained for that engine.

In actual usage, when the input of the pilot demands an increase in speed, fuel flow and speed will increase until stall is sensed by a small portion of the compressor containing stall sensors. A signal to a fuel bypass solenoid will cause a reduction in fuel flow which will eliminate the stall signal and allow fuel flow to increase momentarily until stall is again sensed and fuel is reduced again. The result is a saw tooth overlay on the acceleration fuel flow permitted to the engine. In most engines the mean fuel flow will lie above the usual acceleration schedule and result in an improvement of acceleration time.

OBJECTS OF THE INVENTION

An object of the present invention is the provision of a device to directly sense and control compressor stall, thus creating a closed loop control of this condition.

Another object is to provide a system which gives enough lead time on the impending stall to allow control measures to be taken and prevent complete compressor stall.

A further object of the invention is the provision of a closed loop control system which optimizes control of each engine to obtain the maximum rate of acceleration by sensing the individual compressor characteristics.

Still another object is to provide a stall detection system for a gas turbine engine which anticipates compressor stall directly and reduces fuel flow that is accurate and yet inexpensive.

A still further object is the provision of a simple and efficient system for the detection of compressor pre-stall conditions and the prevention of complete compressor stall.

Other objects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING

The single FIGURE is a diagrammatic view of a preferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The drawing, which illustrates a preferred embodiment of the compressor stall detection and control system, shows a vane 10 which is representative of the vanes in a compressor stage of a gas turbine engine determined to be the most likely to suffer stall first. Vanes 10 are fixedly attached to the compressor at an angle of attack denoted as .alpha.. Vane 12 is representative of the vane or plurality of vanes selected to contain the stall sensors and vane 12 is set at .DELTA..alpha. angle of attack higher than the balance of the vanes 10.

Pressure taps 14 and 16 are located on the surface of vane 12 and are connected via passages 18 and 20, respectively, to a differential pressure sensor 22. When stall is impending a change in flow will occur along the surface of vane 12. The air flow normally attached to the surface of vane 12 will separate therefrom, resulting in flow separation which is a condition associated with the onset of compressor stall. This flow separation will change the pressure along the surface of vane 12. Pressure tap 14 will indicate a decrease in pressure, while tap 16 will indicate an increase in pressure. These changes in pressure will activate differential pressure sensor 22 and cause it to close a switch 24. As seen in the drawing, closing of switch 24 will energize a solenoid 26 via a battery 28. Solenoid 26 operates a fuel bypass valve 30 which will open to bypass a portion of the fuel flow back to the engine fuel pump inlet (not shown). This will reduce fuel flow to the fuel nozzles of the turbine engine and prevent complete compressor stall by anticipating stall in the other stages of the compressor.

This invention is adaptable to modifications without deviating from the scope of the inventive concept. For example, the system is particularly adaptable to a fluidic computer fuel control where the pressure differential signal from the sensing device can be supplied directly to the fluidic computer with no transducer being necessary. For an electronic control, either the solenoid-operated fuel by pass valve 30 or a torque motor controlled valve can be operated by the differential pressure switch 24. In the preferred embodiment presented, the solenoid-operated valve 30 was selected for its simplicity.

Although the prestall device proposed by the present invention acts on fuel flow to prevent full compressor stall, the signal provided by this device can also be used to open compressor bleeds or to open a variable area exhaust nozzle to accomplish the same purpose; that is, prevent compressor stall.

In addition, the control arrangement depicted in this invention, while adequately meeting the requirements of stall prevention, is best applied in conjunction with a turbine temperature limiting device. This prevents overfueling where compressor stall may not be encountered, but turbine overtemperature may be.

The closed loop acceleration control of the present invention can also be incorporated into a new control system to permit simplification of the system or can be added to existing systems to take advantage of optimization. On existing systems it will be necessary to enrich the acceleration schedule (if the starting schedule can tolerate this enrichment), or provide a new acceleration schedule for enrichment above starting for the fuel control.

An additional advantage of the acceleration control device of the present invention is that it is as effective in manual fuel control as it is normal fuel control. On current systems, the normal fuel control only contains restraints on overfueling during acceleration. The manual fuel control is essentially a fuel valve whose opening is directly proportional to the position of the pilot's power lever. If he advances the power lever to the maximum power position, there is a tendency to overfuel the engine. In the application of the acceleration control of this invention, the constraints would be applied to the normal as well as the manual fuel control.

The control system of the present invention also allows maximum acceleration for each engine according to its particular characteristics. In designing a fuel control system to match a particular gas turbine engine, all parameters of control except starting and acceleration control are fixed values and present a fairly simple problem. Starting is generally scheduled to avoid hung and hot starts. Acceleration presents the most complex control problem. First, the engine stall conditions must be defined. Then, the fuel control schedule during acceleration must be shaped to permit the maximum fuel flow while avoiding the stall region of the compressor. The difficulty with attempting to schedule fuel flow to avoid compressor stall is that the poorest compressor must be accommodated.

This problem is avoided by the practice of the present invention.

Obviously many modifications and variations of the present invention are possible in the light of the above teachings.

Claims

What is claimed is: first;

1. A system for the detection and control of compressor stall comprising:

at least one compressor vane in a selected compressor stage fixed with respect to and set at a higher angle of attack than the balance of the vanes in the selected compressor stage, the selected compressor stage being the one most likely to suffer stall first:

a pressure sensing means located on each higher attack angle vane having first and second spaced cordwise openings for detecting a change in vane surface pressure which experiences flow separation on said higher attack angle vane surface; and

control means responsive to said pressure sensing means for preventing complete compressor stall.

2. A detection and control system as recited in claim 1, wherein said control means comprises means to open compressor bleeds and thereby prevent complete compressor stall.

3. A detection and control system as recited in claim 1, wherein said control means comprises means to reduce fuel flow and thereby prevent complete compressor stall.

4. A detection and control system as recited in claim 3, wherein said means to reduce fuel flow comprises a fluidic computer fuel control.

5. A detection and control system as recited in claim 3, wherein, said means to reduce fuel flow comprises a solenoid-operated fuel bypass valve.

6. A detection and control system as recited in claim 1, wherein said control means comprises a differential pressure sensor connected to said first and second openings.