Monitoring of foreign object ingestion in engines

Abstract: The invention described in this specification relates to apparatus for monitoring the intake of foreign bodies and objects into an engine, particularly a gas turbine engine whereby sensors are located at or near the engine intake for detecting any electrostatic charge induced on the sensors by the passage of foreign bodies therepast. The specification particularly discloses an arrangement for a gas turbine engine in which sensors are provided in the exhaust duct and in the inlet duct and that signals from sensors in the inlet duct and exhaust duct are compared and contrasted.

Description: This invention relates to an apparatus for monitoring the intake of foreign bodies into a gas turbine engine. In our European patent applications published as EP-A-256845 and EP-A-256846 both claiming priority from our earlier British Patent Applications Nos. 8620239 and 8702553 now abandoned we describe and claim methods of measuring electrostatic charges in the exhaust gas stream of a gas turbine engine with a view to determining the on-set of faults and degeneration generally of the engine performance. Such methods are well know and may be achieved by providing a plurality of sensors in the exhaust duct of the engine to give a corresponding plurality of signals. The signals can be analysed or recorded and subsequently analysed, for example, by measuring the time displacement and amplitude variations of the signals sensors to give an indication of the general area of the engine in which the fault or problem lies and the type of fault which has occurred. Hitherto, it has been believed that charged particles are created in the engine as the result of a distress event occurring in the engine and that under normal operating conditions the degree of electrostatic activity within the engine will be at a minimum level corresponding to normal wear of the engine. Indeed, US-A-4607337 discloses an engine diagnostic system in which electrostatic probes are positioned in the combustion chamber and at the exhaust of a gas turbine engine. The system processes signals from the probes to determine when an event occurs within the engine. We have found that debris ingested into the intake of an engine, particularly a jet engine carries an electrostatic charge. This may be monitored on an intake sensor as a single event. Such debris may comprise stones, rivets and other particulate material of a substantial nature in addition to sand, salt, grit and small particles of dirt. All the debris will be sensed to a greater or lesser extent and which will then pass down either through the by-pass duct of a jet engine or through the inner cor of the engine to be expelled from the exhaust system. Thus, the present invention resides in the realisation that debris entering the engine in addition to debris created by wear within the engine, carries an electrostatic charge. According to the invention there is provided an apparatus for monitoring the intake of foreign bodies into a gas turbine engine, the apparatus being characterised by: at least one sensor located at or near the intake of an engine, said at least one sensor being effective to sense the passage of electrostatic charge associates with said foreign bodies adjacent said at least one sensor and for producing a signal indicative of the passage of said electrostatic charge; and signal conditioning means coupled to said at least one sensor and effective for conditioning said signal produced by said at least one sensor. It will be appreciated that some of the debris sensed in the exhaust gas stream of an engine will have resulted from material that has been ingested through the air intake of the engine and passed right through the engine. In jet engines the introduction of foreign objects into the air intake can have catastrophic consequences and it has been known that, for example, metal objects have produced serious structural faults in engines particularly during take-off and landing. Other objects such as small stones and dust, can pass through the engine without significant effect and yet can product electrostatic signals in the exhaust gas stream. The ability to monitor the intake of the engine enables more information to be obtained relating to the running of the engine. The above and further features of the invention are set forth with particularity in the appended claims and together with advantages thereof will become clearer from consideration of the following description of an exemplary embodiment of the invention given with reference to the accompanying drawings. In the drawings: Figure 1 is a general layout of a foreign object ingestion detection device in operative combination with an exhaust detention device as applied to gas turbine jet engines; Figure 2 is a diagram of a unit for determining the charge of particles passing an intake sensor or probe system; Figure 3 is a chart showing a typical charge signal from a small stone sensed in the apparatus of Figure 2; Figure 4 is a diagram of a simulated engine for sensing the ingestion of foreign objects; and Figure 5 is a signal trace of an object sensed in the apparatus of Figure 4. Turning first to Figure 1, a jet engine indicated generally has an intake 11 and a hot exhaust 12 with a by-pass 13 between intake 11 and exhaust 12. The intake of the engine comprises a duct having a substantially circular internal surface having a circumferential intake sensor 14 comprising four segmental sensor elements, each of which has been mounted onto the internal surface of the intake duct 11. The sensors may be arcuate sensors or spot sensors; arcuate sensors may be spaced about ducting within the engine, while spot sensors may be disposed in a staggered ring around an engine ducting or at other convenient places therein. Two parameters determine the performance of each sensor, these are the length of the sensor within the ducting and the overall surface area. The effect of length on the sensor in the direction of gas flow on signal shape and duration has shown that signal amplitude is related to the sensor surface area and that charged debris sets up a charge field which is long compared to the change when considered in the axial length of the sensor. In consequence, the engine signal duration is not discernable and the axial length of the sensor does not, therefore, appear to effect the frequency response of a sensor within a reasonable range of length. In practical terms it is preferred that the minimum length of any sensor is constrained to approximately 10mm usually by the problem of providing adequate lead-in connectors. The maximum length of a sensor is limited by the available space and by the considerations of capacitance. The surface area of a sensor should be maximised and the capacitance should be minimised. In practice a compromise has to be achieved as capacitance is proportional to the surface area for any particular dielectric material. In maximising the surface area, therefore, capacitance could increase to an unacceptable level. As a compromise, a nominal value of 50mm is a preferred maximum length for a sensor for use with the present invention. It will be appreciated, therefore, that the selection of different dielectric materials will enable improved sensor construction. Each sensor may comprise an insulating layer, a bonding layer for bonding said insulating layer to a support surface and a conducting layer carried by said insulating layer characterised in that each of the layers is applied by spraying or coating. The insulating layer maybe a ceramic layer and the spraying or coating may be effected by plasma spraying or flame spraying. Each of the sensors comprises an insulating layer of epoxy resin of the order of 0.5mm thick together with a charge collecting layer applied thereover of silver loaded epoxy resin of approximately 0.05mm in thickness. Each of the charge collecting layers is provided with an electrical connector comprising a central cylindrical stud having a flanged head adapted to engage the conductive layer of each sensor with the stud passing through the wall of the intake duct via an insulating grommet for connection to an intake signal conditioning unit indicated generally at 15. In addition to the foregoing, sensors may also be included in the engine by-pass 13 to detect debris which is ingested into the engine and passes via the by-pass out of the engine as well as that which passes through the high temperature sections of the engine. Therefore a similar peripheral sensor system 17 is provided within by-pass 13 and is coupled to by pass signal conditioning means 18 which is connected at a signal processing station indicated generally at 20. For a high temperature sensor the bonding coat may have a thickness within the range of 0.5 to 1.5mm, the ceramic layer may have a thickness of 0.5 to 1.6mm and the conductor coating or layer may have a thickness of 0.01 to 0.05mm. In a typical high temperature sensor the bonding layer may comprise a nickel chromium alloy containing 6% of aluminium. The ceramic layer may be selected from magnesium zirconate or a composition containing alumina, titanium oxide, silica and iron oxide. The conducting layer may be selected from, a stainless steel containing 17% by weight of chromium and 12% of nickel together with up to 3.5% of molybdenum and 1.5% of silicon, or may be a nickel layer of 99%+ purity. The exhaust duct 12 carried four high temperature segmental sensors 21 circumferentially disposed about the internal surface of the exhaust duct and electrically connected to signal conditioning means 22 which in turn is also connected to a signal processor 20. The sensors 21 in the exhaust duct of the engine are similar to the sensors disclosed in our European applications EP-A-256845 and EP-A-256846. Each sensor may be formed directly on the engine casing or in the alternative, may be formed on a support plate adapted to be secured to the engine casing. The engine casing may be recessed to accommodate the sensor so that the sensor surface follows substantially the internal surface of the engine casing thus providing the minimum interruption or disturbance to the gas flow. The geometrical size and type of both the low temperature sensors in the intake and by-pass and the high temperature sensors in the exhaust will be much the same. The material, however, will depend on the local operating environment such, for example, as temperature and humidity. At each location it is desirable that the thermal expansion of the material should be matched as closely as possible to ensure that cracking and loss of material by degradation does not occur. At the intake, ambient conditions will be a relatively low temperature with a wide range of relative humidities. The sensor materials, therefore, should not be susceptible to water absorption. The ceramic materials tend to be porous and will not, therefore, be entirely satisfactory for an intake environment. A low temperature sensor may comprise an epoxy based insulator with an epoxy conductor layer thereon. The insulating layer may be a pure epoxy resin and conducting layer may be a silver loaded epoxy resin. This may be covered with an insulating layer to effect waterproofing and to protect from impact. The advantages are that such materials have a minimum distortion due to thermal effects and, therefore, enjoy reduced possibility of damage from interacting within the environmental conditions. They are relatively easy to apply to irregularly shaped areas such as intake ducts, and a sensor so formed will provide minimum obstruction to flow passed the sensor. The signal processor 20 further has input date 24 from the engine and flight data 24 provided from aircraft flight data from other on board systems. The signal processor 20 provides a plurality of outputs 25 to an event recorder 26 which records events for subsequent analysis at the completion of the flight regime for the engine. An alarm detector 27 may also be coupled to processor 20 for any significant incidence to be drawn to the attention of the flight crew. During the development of the present invention a laboratory experiment was conducted using the apparatus illustrated in Figure 2. A centrifugal suction rig 40 is disposed with an outwardly directed inlet 41 and a substantially horizontal lower outlet 42. The inlet 41 is provided with an upstanding cylindrical tube 41a constituting an inlet duct. The tube 41a is provided towards its lower end 43 with an annular ring of circumferential segments 44 disposed on the internal surface and formed by an insulating layer of epoxy resin and a conductive layer of silver loaded epoxy resin of thickness dimensions substantially as described herein above. The inlet tube 41a is of substantially 150mm internal diameter and has a length of approximately 400mm. The upper end 45 is juxtaposed an inclinable plate 46, the angle of inclination of which can be adjusted. Articles of debris 47 are located on the upper surface of inclined place 46 and released one at a time or in groups into the inlet end 45 of inlet tube 41a. The fan is capable of producing an air flow within the inlet tube 41a of 12 metres per second. Release of an article of debris 47 into the entrance of the tube results in the article of debris passing substantially centrally of the sensor ring 44 with a velocity approaching 5 metres per second. The sensor ring 44 is coupled to debris detection unit 50 and then via an oscilloscope 51 to a chart recorder 52. Various size small stones ranging from a nominal 2 metres diameter up to a nominal 20 metre diameter have been tested individually. To avoid any unintentional precharging of the debris, the metal plate is connected directly to the probe tube, the debris detection unit, the oscilloscope and the chart recorder. During the experiment it was noted that each piece of debris passing the sensor produced an induced charge therein; a typical charge signal is shown in Figure 3 which is the charge signal for a stone of a nominal diameter of 6mm. Figure 4 shows a test bench arrangement to demonstrate the present invention. The bench comprises an air intake 61 debouching into a debris separator 62. The debris separator is connected with combustor 63 which exhausts into a polouche 64 and debouches into an exhaust 65. The inlet 61 is provided with an inlet mouth 66 in juxtaposition with a turntable assembly 67. A debris accelerator comprises a conduit 68, a branch of which 69 communicates with polouche 64 to provide a by-pass working section 70. The intake 61 is provided with three sensors 71, 72 and 73 each of different construction to enable relative tests on the efficiency of different sensors and materials to be evaluated. The sensors are provided with signal conditioning means juxtaposed each sensor and the condition signals fed by means of conductors 74 passing through cable duct 75 to a control room 76 where the signals are monitored and recorded. In operation the combustor 63 is ignited by the supply of fuel within the combustion ducts 63. Air is drawn into the system by means of inlet 66 and additional air is drawn through debris accelerator tube 68 and by-pass 69 to reproduce the general conditions within a jet engine. The turntable assembly is adjustable and enables selected components or solid items to be ingested into the engine and duct to be presented to inlet 66 so that its passage passed sensors 71, 72 and 73 may be monitored by means of the signals induced in each of the sensors. In a particular experiment, sensor 71 comprised an insulated layer of flexible epoxy 25mm wide, a conducting layer of silver loaded epoxy 13mm wide and a top insulayer of flexible epoxy 25mm wide. Sensors 72 and 73 each comprised an insulating layer of flexible polymer material, a conductive layer of silver loaded epoxy and a top insulating layer of a flexible polymer material. The overall dimensions of all three sensors were identical. The lead-out connection to the signal conditioning was effected using a cable which is bonded to the conductive layer employing a conductive epoxy. Individual signal conditioning units were operatively connected with each sensor on the intake working section. Debris was injected into the intake working section using the turntable assembly to simulate as realistically as possible ingestion of the article into a jet engine intake, such as will be caused by the effect of thrust or reverse thrust of a jet engine. Debris was collected in the debris separator to ensure that the test engine was not damaged. The accompanying Figure 5 shows a typical signal from this test facility caused by a bolt passing through the intake section. Having thus described the present invention by example it will be apparent to those possessed of the appropriate skills that modifications and variations may be made thereto without departing from the scope of the invention as defined in the claims appended hereto.

Claims: An apparatus for monitoring the intake of foreign bodies into a gas turbine engine, the apparatus being characterised by: at least one sensor (14) located at or near the intake (11) of an engine (10), said at least one sensor (14) being effective to sense the passage of electrostatic charge associated with said foreign bodies adjacent said at least one sensor and for producing a signal indicative of the passage of said electrostatic charge; and signal conditioning means (15, 18, 22) coupled to said at least one sensor and effective for conditioning said signal produced by said at least one sensor. An Apparatus as claimed in Claim 1, characterised in that at least one sensor (14) comprises a plurality of electrically discrete sensor elements disposed in a ring about the intake (11) of the engine, said signal conditioning means comprising a plurality of signal conditioning circuits, each said sensor element being coupled to a respective one of said signal conditioning circuits for conditioning signals thereof. An apparatus as claimed in Claim 1 or 2, characterised in that the engine comprises a by-pass passage (13) and further including additional sensors (17) disposed in the by-pass passage whereby electrostatic activity in the by-pass passage can be monitored. An apparatus as claimed in any preceding claim, characterised in that said signal conditioning means (15) are effective for producing conditioned outputs from said at least one sensor (14) and in that the apparatus further comprises signal processing means (20) responsive to the conditioned outputs and effective to identify from said outputs events signifying the ingestion into the engine of a foreign body. An apparatus as claimed in Claim 4 as dependent on Claim 3, characterised in that said signal conditioning means (18) are further effective for producing conditioned outputs from said additional sensors (17), said signal processing means (20) being responsive thereto to identify events occurring within the engine. An apparatus as claimed in Claim 4 as dependent on Claim 3 or Claim 5, wherein the engine includes an exhaust duct, the apparatus being characterised by further sensors (21) at the exhaust duct. An apparatus as claimed in Claim 6, characterised in that said conditioning means (22) are effective for producing conditioned outputs from said exhaust duct sensors 21, said signal processing means being responsive thereto to distinguish between events occurring within the engine as a result of normal engine running and events occurring as a result of debris being ingested into the engine. An apparatus as claimed in any preceding claim, characterised in that said signal processing means (20) is further arranged to process signals originating in the at least one sensor (14) for measuring a rate of change in said sensed electrostatic charge. An apparatus as claimed in any preceding claim, characterised in that signal processing means (20) is further arranged to process signals originating in the at least one sensor (14) for measuring the magnitude of said sensed electrostatic charge.

Inventors: FISHER CELIA ELIZABETH, FORFITT ROY, FISHER, CELIA ELIZABETH, FORFITT, ROY
Applicants: STEWART HUGHES LTD, STEWART HUGHES LIMITED
Application year: 1988
Main classification: G01N27
IPC codes: G01N27, F02C7