Sensor Specifications
Sensor Specifications
PIWG conducts turbine instrumentation research and development in areas that are of interest to all PIWG Members and Partners. Based on survey input from PIWG’s membership, a listing of top technology focus areas was developed.
PIWG collectively focuses its energies and resources to develop state-of-the-art instrumentation and sensor technologies. To facilitate progress PIWG has formed technical sub-teams in the focus areas composed of technical specialists from member organizations chosen for their expertise in their particular disciplines. These sub-teams meet on their own, either through teleconferencing or in person, to address technologies in their related fields.
The technology areas listed below focus on development efforts required to meet anticipated instrumentation needs and offer potential technological solutions.
Technology Areas
Hot Section Dynamic Strain Measurements
Dynamic strain measurements on gas turbine engine turbine components, especially airfoils are needed to substantiate the design life and to monitor or investigate issues such as HCF and other problems identified in the field. Thus resistive strain gages are installed on various engine structures in particular on gas turbine engine airfoils. These strain gages are then used in test to evaluate vibratory modes of interest; looking at both amplitude and frequency. The strain gages are often positioned to detect more than one vibratory mode (engine orders) using stress ratio information. Strain gages have seen wide application on fan, compressor, and turbine airfoils to make sure design intent is achieved, excessive strain levels are avoided, and margin of operational safety is assured.
Frequencies and amplitudes are both measurements of interest from the strain gages. Dependent on mode shapes and blade design the strain gages can be applied to both the pressure and suction side of the rotating and stationary airfoils. Dynamic strain measurements are then used in Goodman diagram analyses of total strain on a particular component to estimate fatigue life.
Up to about 700F (370C), foil backed commercially available strain gages are applied with epoxies that provide a minimal adhesive line and conform to the airfoil surface contour. The strain gages alloy selection is made depending on strain field, temperature, expected strain level, fatigue life, and wiring restrictions. The application epoxies are selected based primarily on temperature capability. For strain measurements greater than 700F (370C) found in gas turbine engine hot section application both flame sprayed aluminium oxide and high purity ceramic cements are used. The most widely used technique is wire wound strain gages, which are installed with flame sprayed high purity aluminium oxide.
Alternative technologies for dynamic strain measurements in gas turbine engines has included thin film deposited strain gages and more recently fiber Bragg gratings as well as Fabry Perot interferometers. Thin film dynamic strain gages are fabricated directly on the hardware using sputtered vacuum chambers and have dielectric as well as metallization material limitations. Thin film resistive strain gages have found reasonable acceptance within the OEM community for compressor airfoil test and evaluation.
Current Sensor Needs
Most importantly, the strain gage installations and associated leadwires must be able to survive the high temperatures found in gas turbine engines, handle the CTE mismatch, adhere under high centrifugal loading, and survive severe erosive and oxidizing gas path conditions. The overall thickness of the strain gage installation must be minimized. For epoxy installations .010” to .012” is expected. In contrast the flamespray installation is generally slightly thicker to .013”-.015”. Ultimately the thickness must be minimized to reduce cooling air disruption on film cooled airfoils and to reduce the installation effects. The strain gage installation increases the structures mass and stiffness modifying the structural frequency and damping behaviour of the component being tested. With the improvement in engine blade materials and coatings, the engine cycle temperatures and the airfoil surface temperature have increased to 2000F or more, which has a detrimental effect on strain gage life. For example in some applications, the flame sprayed turbine airfoil strain gage lives have become marginally acceptable, especially in the high pressure turbine on the pressure side of the airfoil where strain gages are located radially outward from the blade platform.
Nickel based alloy resistive strain gages with modified constituents suffer severe oxidation if not properly overcoated from the hot corrosive flowpath. In contrast platinum-tungsten gages have very high gage factors and its resistance changes as temperature increases, but also exhibits better oxidation resistance. Of primary concern for all development test in the flowpath is the possible delamination of the strain gages from the airfoils during engine operation. For example flame spray free filament strain gage installations, can delaminate when the surface temperatures exceed 1800F – 2000F.
Improvements in hot section strain gage technology will come from advances in both material and application technologies. Advancements in ceramic materials have demonstrated semiconducting materials that have potential but require development to yield a reliable instrument grade sensors having reliability and calibrations repeatability. Goals for engine airfoils application, especially rotating blades, would be dynamic strain gages survive for 20 hours at surface temperatures up to 2100F – 2200F. The base installation materials need to provide insulation to ground for the strain gage of >1 Megohm @ 2000F with an overall thickness of less than 0.015 inch.
Strategic Advisory Board (SAB) Members Addressing This Need
Aerodyn Engineering, Inc.
Cleveland Electric Laboratories Company, Inc.
Surface Temperature Mapping
Measurement of surface temperature over 80% of blade airfoil surface for all blades in the turbine is necessary for test monitoring and to verify turbine durability at the combustion temperatures necessary for high thermal efficiency in aircraft and power generation turbine engine applications.
Current Sensor Needs
Conventional techniques using embedded thermocouples create unacceptable stress concentrations, particularly where the wall thickness of the blade is small, and have limited life and airfoil coverage. Thus, the need is for less intrusive techniques (for example thin film thermocouples) or non-intrusive techniques (for example pyrometry and thermographic phosphors) of surface temperature mapping. Also, ceramic thermal barrier coatings are generally translucent to the operating wavelengths (0.4 to 1.8 microns) of current pyrometers used in turbine development. Consequently a surface temperature cannot be determined from the radiant power measured. Also, there is a need to measure the surface temperatures non-metal components (for example ceramic metal composites).
Strategic Advisory Board (SAB) Members Addressing This Need
High-Temperature Dynamic Pressure
Accurate, high response pressure measurement is required to adequately characterize transient gas path conditions in gas turbine test articles. Specific applications include operability, stall boundary definition, combustion dynamics, and quantifying blade passing forcing functions. High response pressure measurement necessitates that the sensor be mounted as close to the gas path as possible thus minimizing (ideally eliminating) response degradation resulting from installation pneumatic volume. A consequence of this mounting constraint is that the sensor will be exposed to gas path temperatures.
Current Sensor Needs
The table below outlines the general high temperature dynamic pressure transducer requirements for the near future, taking into account realistic expectations for technology development. The primary limiter for this type of device is currently temperature capability. In the very near term we would like to have available for purchase devices that can operate at up to 1100 degrees F. Within the next five years, on the order of 1500 degrees should be reasonably expected. And, the current long-term need is for sensors that can operate in excess of 2000 degrees F. For gas turbine engine validation and development minimum sensor life expectancies in the hundreds of hours range is acceptable and for production engines tens of thousands of hours is required. Currently piezoresistive transducers are favored over competing technologies because of their ability to measure both the absolute pressure level and the dynamic pressure variation with a single device. However, depending on capabilities, other technologies will be considered. Cooling of the sensors tends to add complication to the measurement and can be impractical. Varying operating temperature must not compromise measurement accuracy and influence on the sensor’s pressure signal must be inherently negligible or compensated out. In general the sensors should be tolerant of contaminants such as water, fuel, dirt, sand, and bugs. Consideration should also be given to the ruggedness and temperature capability of the transducer’s ancillary components, such as cabling, electronic modules, and connectors.
| INPUT | |
| Range (psi) | 25 – 750 |
| Excitation (VDC) | 5 – 10 |
| Input Impedance (Ohms) | 350 – 2500 |
| OUTPUT | |
| Full Scale Output (mV) | 100 |
| Isolation From Case (Meda-Ohms) | 20 |
| Output Impedance (Ohms) | 350 – 2500 |
| Combined Uncertainties @ Ambient Temperature | ≤ 1% FS |
| Temp. Effect on Zero | +/- 2% |
| Temp. Effect on Sensitivity | 0 to 2% FS |
| Natural Frequency | ≥ 100kHz |
| Acceleration Sensitivity | ≤ 0.001% FS/g |
| ENVIRONMENTAL | |
| Diaphragm Operating temperature Range (deg. F) | 0 – 1400 |
| Compensated Temp. Range (deg. F) | 0 – 1400 |
| Lead Wite Operating Range (deg. F) | 0 – 1400 |
| PHYSICAL | |
| Diameter (in.) | 0.062 – 0.19 |
| Length (in.) | < 1.25 |
| Temperature Compensation | internal |
| Protection Screen | optional |
Strategic Advisory Board (SAB) Members Addressing This Need
Blade Tip Deflection
The basic principle of operation of NSMS (also commonly referred to as “blade tip timing”) is that the blade tip times of arrival at probes mounted in the case are compared with a fast, very stable system clock referenced to an accurate 1/rev signal. Blade vibration results in either a retarding or advancing of the blade passage time relative to the expected nominal blade passage time. From the advancement or retarding of the blade tip passage time an NSMS/blade tip timing system is able to infer the blade tip deflection. When this blade tip deflection is then used as an input to a finite element analysis of the blade, it is then possible to infer the blade stresses. NSMS/blade tip timing has the unique capability to measure the stresses of all of the blades in an instrumented stage. NSMS/blade tip timing systems have evolved through several generations (see Table 1) with the first systems being capable only of blade flutter monitoring. As part of an Air Force funded effort to develop a generation 4 NSMS system to address high cycle fatigue (HCF) needs a NSMS/blade tip timing sensor specification was tabulated which lists sensor requirements for both the cold and hot sections of turbine engines (see Table 2).
Current Sensor Needs
NSMS/Blade tip timing systems commonly use optical probes. An advantage of optical probes is that they are capable of providing a high degree of spatial resolution. A disadvantage of optical probes is that they require frequent cleaning of the probe tip. For blade health monitoring and flight test applications non-optical probes are preferred in order to avoid the cleaning requirements inherent in optical probes. The sensor specifications given in Table 2 provide a benchmark against which the performance of non-optical NSMS/blade tip timing probes may be compared.
Strategic Advisory Board (SAB) Members Addressing This Need
Planar Gas-Path Temperature
In the hot section, knowledge of gas temperature is necessary to estimate heat transfer to surfaces, combustion stoichiometry, and burner pattern factor. In the compressor, the efficiency of each stage can be related to the temperature rise across each stage.
Current Sensor Needs
The most common method of measuring gas temperature is the thermocouple probe. Its use at high temperature is limited by the melting point of the thermocouple and by chemical attack by the constituents of the gas stream. Hence higher melting point, more chemically inert thermocouple materials are desirable. The various optical and acoustic methods are not subject to the above limitations and should also be further developed.
Strategic Advisory Board (SAB) Members Addressing This Need
Blade Tip Clearance
Measurement of rotating blade tip to shroud clearances for fan, compressor, and turbine components are necessary to set operating clearances for best performance and efficiency of the rotating component. Diagnostic rotating clearance measurement systems are needed for both steady state correlations of clearances with engine operating conditions and also dynamic measurements. Clearance measurement systems with high enough bandpass frequency can be used to measure rotor dynamics and vibration. Past sensors have been optical, capacitance, eddy current, microwave, and contact.
Current Sensor Needs
Important to be compact and non-intrusive in size, 0.25 inch diameter for compressors and turbines. Need the capability to operate at high-pressure turbine temperatures. Required measurement range of 0 to 0.200 inches for compressors and turbines, and measurement range of 0 to 0.400 inches for large by-pass fans. Repeatable to within 0.0005 inches is needed. For use to record dynamic data, must measure individual blade clearance and have a frequency response of 1 Mega Hertz (Full Power Bandwidth). Tip Clearance System will need real time measurement and display.
Strategic Advisory Board (SAB) Members Addressing This Need
Particulate Emissions
FAA and ICAO has requested that the SAE E31 Committee provide recommended measurement techniques and procedures for measurements of turbine exhaust particulate matter (PM) including both non-volatile (“soot”) and volatile (condensed volatile gases) components. SAE E31 published (SAE AIR-5892) their opinion of the state-of-the-art of most mature potential non-volatile measurement techniques, and a similar report is in the works for addressing volatile PM. Measurement techniques are required for quantification of particle mass, particle size, number and particle chemical constituents at the nozzle exit plane.
Current Sensor Needs
Measurements of size (5 nm to 2.5 um), number, mass and chemical constituents are required for nonvolatile and volatile particles in the exhaust of aircraft turbine engines for stationary source compliance (test facilities) and health concerns around airports, test facilities and so on. NASA, DoD, FAA, universities and other agencies and private companies are working to develop measurement techniques, instrumentation and sampling methodologies for exit plane characterization of turbine PM. Sampling turbine exhaust at the harsh conditions of the nozzle exit plane offers challenges to transport unaltered sampled gas to the PM analyzers. Technologies for characterizing nonvolatile particles are more mature than for volatile particles. Volatile particles don’t exist at the exit plane conditions, but form due to the condensation of volatile gases as the exhaust mixes and cools with bypass and ambient air. Characterization of volatile particles at the exit plane will require the development of mixing chamber to control the process of volatile particle formation and subsequent measurement analysis, or a measure of exit plane precursor gases and a methodology to accurately predict downstream volatile particle products.
Upon approval by the EPA, DoD will use modern PM measurement technologies to quantify and report PM emissions form military engines. For civil aviation, the SAE E31 Committee will evaluate and recommend techniques and procedures for characterization of PM in response to FAA/EPA regulations if/as imposed.
Strategic Advisory Board (SAB) Members Addressing This Need
Heat Flux
The measurement of heat flux (watt/m2) is of interest in directly determining the cooling requirement of hot section blades and vanes. In addition if the surface and gas temperatures are known, the measurement of heat flux provides a value for the convective heat transfer coefficient that can be compared with the value provided by CFD codes.
Current Sensor Needs
It is difficult to measure heat flux without having the measurement disturb the measure and, so if a physical sensor is used to measure heat flux, a thin film design is desirable. It must be constructed of materials capable of withstanding the harsh environment in the hot section. Non-intrusive optical methods, such as the use of temperature sensitive paints, should also be pursued since in many cases they do affect measurement at all.
Gas Species Emissions
Gas turbine engine exhaust gas emissions include CO2 and H2O as well as CO, NOx, unburned hydrocarbon fuel (THC) and smoke. Gas samples are extracted from many points in the exhaust plume at the nozzle exit by a probe rake system, transported through many feet of heated tubing, and analyzed by gas analyzers. Profiles of engine performance in terms of combustion efficiency and fuel/air ratio indicate zones of incomplete combustion. Bulk averages of the multi-point samples are used to define the pollution emission indexes, pounds of pollutant generated per 1000 pounds of fuel consumed.
As aircraft and stationary source gas turbine engines become more efficient emission measurements systems must be capable of analyzing very low levels of CO, NOx, and THC. Gas turbine engine emission measurements are costly in terms of infrastructure and engine run time required, especially at high power conditions. Therefore, new cost-effective methods are required that facilitate emissions measurements during the engine development through certification cycle.
With endorsement from the regulatory agencies, new gas analysis techniques are being evaluated. Among the new techniques are FTIR multi-gas analyzers and MEMS chemical gas sensors that may expand the role of emission measurements from ground testing to on-board diagnostics and active engine control functions. Development of new emission measurement processes is on-going in universities and industry with funding from agencies such as SBIR, DoE, and OSD T&E/S&T. Oversight, evaluation and documentation of advancements is within the charter of the SAE E-31 Committee for Aircraft Engine Exhaust Measurements.
Strategic Advisory Board (SAB) Members Addressing This Need
High-Temperature Static Strain
Static strain (DC or mean strain) is usually well understood when assessing gas turbine engine component structural integrity, where calculated load induced strain is combined with laboratory low temperature engine bench static strain measurements and testing information. The largest unknown static strain occurs with assembly and interaction with adjacent engine components, followed by operation induced changes to the baseline FEA analyses. While the inability to measure high temperature static strain is a shortcoming, it is one of the easier parameters to accurately calculate in the engine environment, since nearly all of the static strain encountered in the engine environment is the result of centrifugal loading and/or coefficient of thermal expansion mismatches.
Stress and strain are related by Young’s modulus for the component being instrumented.
In its simplest form: Stress = Strain x Young’s modulus
The basis for needing strain measurements and engine component structural integrity is found in the Goodman-Soderberg Diagram which documents maximum static (mean) strain which is combined with maximum dynamic (alternating) strain. Total strain combines both static and dynamic loading (fatigue), which often leads to a reduction in component life and durability.
Goodman and Soderberg Failure Criteria
The current accepted sensor used for measuring steady state static (mean) strain is the resistance strain gage, which changes electrical resistance with applied strain. The strain gage has been the basis for measuring component life and performance since it was invented in 1938 by Dr. Arthur Ruge and Edwin Simmons. Various alloys are employed today for strain gages including nickel based, tungsten, palladium, and others. Unfortunately, all of these alloys change resistance with temperature, which causes as “apparent change” in strain. Nickel with its many alloys is the primary strain gage material today. Nickel based wire wound strain gages are used up to about 750 degrees F, but are limited by alloy stability . Foil strain gages are used to 600F but above that experience backing failure and epoxy degradation in test / operation. Due to material characteristics, above 750F the gage factor of strain gage alloys such as MCrAlY or NiCoCrAlY becomes non-linear due to both the temperature and rate of temperature change. A large body of work has been done over the past 40 years to try to develop a simple reliable strain gage alloy that is repeatable for static strain measurements up to 2000 F. Unfortunately irreproducible alloy behavior and high apparent strain due to temperature has precluded any large successes.
Current Sensor Needs
The engine community would like to have a single quarter arm bridge resistance strain gage that can easily installed on engine hot section components and operate in a Wheatstone bridge completion circuit data system.
Ongoing strain gages development work within the OEM community, small business and academia SAB members, and agencies such as NASA GRC are conducting development efforts to identify and qualify new strain gage materials which will allow static strain measurements to be conducted at temperatures over 750F. Reliable repeatable strain gage behavior with minimal temperature induced apparent strain is the goal.
Optics and optical sensing methods provide a new approach to the acquisition of static strain measurements using approaches such as Fiber Bragg Grating (FBG) and Fabry Perot Interferometer (FPI) technologies. This technique holds promise but requires higher temperature fiber materials, packaging, the ability to install the optical sensor into the engine environment, routing fibers out of the engine and data signal processing /acquisition. Ultimately sensor durability reliability and repeatability will be the differentiator.
Strategic Advisory Board (SAB) Members Addressing This Need
Thermal Barrier Coating Health
High performance modern engines, both aero and land-based, operate at ever increasing temperatures to take advantage of associated increases in thermodynamic efficiency. The engine operating temperatures are substantially higher than that sustainable by the existing base metal capability. The successful operation of engine components in the hot gas path, such as the stationary vanes and rotating blades, relies on both insulating ceramic thermal barrier coatings (TBCs) and hot part cooling.
Thermal barrier coating integrity is essential to the protection of hot parts. Both rotating and stationary parts need to be monitored. Prominent TBC failure modes that health monitoring must address include:
- TBC loss caused by erosion of TBCs or foreign object damage
- TBC spalation, debonding, or delamination caused by sintering when exposed to over temperatures or application defects
- Cracks cause by strain accumulation
Current Sensor Needs
Sensing methods are needed to monitor the thermal barrier coating integrity of both stationary and rotating parts in the high temperature turbine areas. Coverage is needed for all critical part surfaces. Temperature mapping of the surface is often used as an indication of thermal barrier coating health. Techniques currently applied to monitor TBC health include embedded sensors, radiation pyrometry or imaging, and thermographic phosphors.
- Embedded sensors such as thermocouples are to measure critical properties at points on the surface. Limitations include the fact that they can create unacceptable stress concentrations on the part, only discrete points on the surface are measured, and life of the embedded sensor is limited.
- Radiation pyrometry has been used robustly as a nonntrusive method to monitor line of sight temperature indications. Short wavelength infrared (SWIR) pyrometers are a standard techniques used in testing. SWIR pyrometers can provide accurate surface temperature measurements for fully oxidized metal parts because their radiative properties provide high emittance, low reflectance, and no transmittance. Ceramics thermal barrier coatings tend to have low and variable emittance, significant reflectance, and some transmittance at short wavelengths, which will cause increased uncertainties in measurements of TBC coated parts.
- Long wavelength infrared (LWIR) pyrometry operates at wavelengths around 10 microns where ceramics TBC are opaque and have high emittance and low transmittance. The LIWR measurements, however, have interferences from the radiance of combustion products like H2O and CO2.
- IR imaging can be used to map the surface of hot coated parts. The IR mapping provides a visual image of the coating health and a qualitative temperature map. Imaging requires, however, line of sight access or access through optical fiber guides. IR imaging measurements have the same accuracy limitations as SWIR and LWIR pyrometry.
- Thermographic phosphors and other temperature indicating techniques have been used to measure temperature distributions as an indication of TBC health. Temperature indicating materials include temperature indicating paints and thermographic phosphors that use both thermal and laser excitation. The phosphors investigated are rare earth doped ceramics that can be either in separate layers or mixed with the TBC during preparation.
Each of the methods under development has many technical and application challenges to meet test measurement and on-line TBC monitoring needs, including:
- The monitoring system should be capable of providing real-time data, and the sensors should have life capability in excess of normal outage times at operating conditions.
- The monitor will need to be an early indication of TBC coating condition changes.
- The monitor will need to monitor rotating and stationary parts globally so that deterioration of particular areas can be detected.
- The monitor will need to be sensitive to subtle changes in TBC conditions.
- Health monitoring resolution, accuracy, and temperature ranges requirements vary with engine type and specific test or monitoring objectives.
Strategic Advisory Board (SAB) Members Addressing This Need
Bearing Health
The purpose of this sensor technology is to actively monitor and trend all useful ball and roller ball bearing health data in a gas turbine engine environment. Measuring the key inputs for a high speed, highly loaded bearing operating in a turbine engine environment can be a challenging task. Sensors and any associated routing leads must be small and unobtrusive enough to fit within the space available while not affecting the performance or life of the bearing that is being characterized, or any surrounding structures. The data generated should be sufficient to evaluate all aspects of bearing health in real time and provide early evidence of bearing degradation, allowing catastrophic bearing failures to be avoided.
Current Sensor Needs
Useful data for bearing health evaluation include ball pass (frequency), cage speeds and ratios to shaft speed. Additionally, the temperatures and vibration levels of bearing components can assist in the trending of bearing health. The sensors must be suitable for integration into existing and future rigs and engines, and must be able to withstand the thermal, mechanical, and chemical environment that a gas turbine engine bearing experiences. The sensors must also be designed such that sensor failures will not affect the normal operation of the bearing or degrade bearing life.
Current tools available for bearing monitoring include, but are not limited to: proximity probes, accelerometers, thermocouples, and load measurement devices. These technologies are not always practical for turbine engine testing. Future technologies could address miniaturizing of existing technologies and data transmission and storage techniques required to transmit this data with minimal additional instrumentation routing within the engine. A more ambitious approach would be to build fully instrumented bearings capable of gathering useful data without any additional instrumentation routing or space claim from the existing engine. The ideal solution would be a low cost technology capable of being implemented in production engines and capable of operating safely for at least 40 hours for development, and preferably capable of gathering and transmitting data for 5,000 hours or more for in-service health monitoring.
