Andrew L. Heyes

Simultaneous temperature, mixture fraction and velocity imaging in turbulent flows using thermographic phosphor tracer particles.

This paper presents an optical diagnostic technique based on seeded thermographic phosphor particles, which allows the simultaneous two-dimensional measurement of gas temperature, velocity and mixture fraction in turbulent flows. The particle Mie scattering signal is recorded to determine the velocity using a conventional PIV approach and the phosphorescence emission is detected to determine the tracer temperature using a two-color method. Theoretical models presented in this work show that the temperature of small tracer particles matches the gas temperature. In addition, by seeding phosphorescent particles to one stream and non-luminescent particles to the other stream, the mixture fraction can also be determined using the phosphorescence emission intensity after conditioning for temperature. The experimental technique is described in detail and a suitable phosphor is identified based on spectroscopic investigations. The joint diagnostics are demonstrated by simultaneously measuring temperature, velocity and mixture fraction in a turbulent jet heated up to 700 K. Correlated single shots are presented with a precision of 2 to 5% and an accuracy of 2%.

The characterization of Y2O2S:Sm powder as a thermographic phosphor for high temperature applications

Thermographic phosphors may be used to measure surface temperatures in hostile and high temperature environments and have applications in gas turbine combustors and high temperature regions of the turbine. Most phosphors are excited by UV light and exhibit a temperature sensitive exponential decay in emission once excitation has ceased. This can be characterized using a photomultiplier enabling temperatures to be measured at discrete points on the surface. However, one phosphor, YAG:Dy, is known to exhibit temperature sensitivity in the relative intensity of specific lines in its emission spectrum. Emission intensity can be recorded as an image using a CCD camera and hence this type of response can lead to the measurement of surface temperature distributions. In the paper the energy level characteristics of Dy leading to the intensity ratio response are discussed. Another lanthanide, Sm, is shown to exhibit similar characteristics and has been experimentally investigated using Y2O2S:Sm powder. Y2O2:Sm has been shown to exhibit intensity ratio sensitivity over a temperature range from room temperature to 1100 K and to be suitable for temperature measurement by this means with an uncertainty of approximately ±1%. It has also been shown to exhibit lifetime decay sensitivity over the temperature range from 900 to 1425 K and to be suitable for temperature measurement by this means with an uncertainty of approximately ±1%. In both cases the upper temperature limit is a function of the instrumentation used and the dynamic response may extend further. Decay time constants for Y2O2S:Sm are very short (3 µs at 1400 K), compared to those for YAG:Dy, making it suitable for use on moving surfaces by either response mode. This material, or other Sm doped phosphors, may therefore be useful for surface temperature measurement on rotating turbine blades.

Eu-doped Y 2 O 3 phosphor films produced by electrostatic-assisted chemical vapor deposition

Europium-doped yttrium oxide (Y 2 O 3 :Eu) thermographic phosphor films were deposited on Ni-alloy substrates using a novel and cost-effective electrostatic-assisted chemical vapor deposition (EACVD) technique. The thermoluminescence properties were studied under irradiation by an ultraviolet laser. It was found that crystallized Y 2 O 3 : Eu films could be deposited at a temperature as low as 550 °C. Annealing of the as-deposited films at higher temperatures (>1000 °C) improved the luminescence properties due to further crystallization processes. The correlation of the lifetime decay and temperature change of the films showed that the EACVD-deposited films are suitable for use in phosphor thermometry for high-temperature applications.

Oxygen quenching of phosphorescence from thermographic phosphors

Thermographic phosphor thermometry is a technique for surface temperature measurement which may be employed in the hot sections of a gas turbine allowing temperature detection up to around 1400 °C with uncertainties better than for other remote standard techniques such as pyrometry. The phosphors have been regarded as pressure insensitive and indeed have been used to provide reference temperature data for the correction of pressure-sensitive paint data. The authors wish to employ the technique in gas turbine combustors where oxygen partial pressure varies widely due to its consumption in the combustion reaction. An experiment was therefore conducted to confirm the pressure/oxygen insensitivity of two high-temperature phosphors. However, this revealed a response to oxygen partial pressure that implies an uncertainty in temperature measurements within the primary zone of a combustor of typically 1%. There does not appear to have been any previous report of such a response in the literature and this note therefore serves as a caution to those employing the thermographic phosphor thermometry technique where oxygen partial pressure varies significantly.

Rapid technique for wind-tunnel model manufacture

Introduction I N wind-tunnel-based experimental aerodynamic projects a substantial proportion of project resources in terms of both time and cost is consumed by the design and manufacture of wind-tunnel models. This can limit the scope of experiments and cause long lead times. Thus, methods that can expedite the model-building process are highly beneficial. The manufacture of wind-tunnel models has traditionally been a highly skilled and time-consuming process.1 By their nature aerodynamic bodies can have complex three-dimensional curvature and often the experiment requires internal details such as the provision for surface-pressure tappings. Thus, wind-tunnel models are often constructed from a number of parts, which require accurate fitting and hand finishing to obtain the necessary high surface finish. Ideally, the wind-tunnel experiment is an integral part of the design process, which may call for a prescribed range of model configurations to be tested, or alternatively for the iterative optimization of a particular feature of the model. In the former case the range of configurations that can be tested is limited by the cost of each model and in the latter case by the speed at which a model feature can be changed. A number of studies2−5 have demonstrated the advantages of rapid prototyping techniques for building wind-tunnel models in speed and cost relative to traditional techniques. Dimensional tolerances and surface finish may be inferior, which in turn increases experimental uncertainty, but nevertheless the method is quite adequate for preliminary studies. In this Note we outline the use of rapid prototyping for producing wind-tunnel models with internal features. A most advantageous aspect of the rapid-prototyping method is the ability to produce internal forms, that would be difficult to design and manufacture by traditional techniques. Rapid-prototyping technology is reviewed briefly in context and two case studies from the work of the authors are presented.

Design Overview of a Three Kilowatt Recuperated Ceramic Turboshaft Engine

A three kilowatt turboshaft engine with a ceramic recuperator and turbine has been designed for small unmanned air vehicle (UAV) propulsion and portable power generation. Compared with internal combustion (IC) engines, gas turbines offer superior reliability, engine life, noise and. vibration characteristics, and compatibility with military fuels. However, the efficiency of miniature gas turbines must be improved substantially without severely compromising weight and cost, if they are to compete effectively with small IC engines for long-endurance UAV propulsion. This paper presents a design overview and supporting analytical results for an engine that could meet this goal. The system architecture was chosen to accommodate the limitations of mature, cost-effective ceramic materials: silicon nitride for the turbine rotors and toughened mullite for the heat exchanger and turbine stators. An engine with a cycle pressure ratio below 2:1, a multistage turbine, and a highly effective recuperator is shown to have numerous advantages in this context. A key benefit is a very low water vapor-induced surface recession rate for silicon nitride, due to an extremely low partial pressure of water in the combustion products. Others include reduced sensitivity to internal flaws, creep, and foreign object damage; an output shaft speed low enough for grease-lubricated bearings; and the potential viability of a novel premixed heat-recirculating combustor.

Emission lifetimes of europium-doped pyrochlores for phosphor thermometry

The luminescent lifetime of La2Zr2O7 and La2Hf2O7 has been determined as a function of temperature. We have shown that the luminescence of both materials can be used to determine the temperature of a surface up to 1073K. The results are qualitatively explained via multiphonon emission. Phonon energies and the number of phonons needed to cross the energy gap are estimated. The results are useful in the design of phosphors for noncontact thermometry in high-temperature applications.

Optical Nondestructive Condition Monitoring of Thermal Barrier Coatings

This paper describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and online condition monitoring in gas turbines. Increases in turbine inlet temperature in the pursuit of higher efficiency will make it necessary to improve or upgrade current thermal protection systems in gas turbines. As these become critical to safe operation, it will also be necessary to devise techniques for online condition monitoring and NDE. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving NDE for TBCs. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia (YSZ)) with a rare-earth activator to provide the coating with luminescence when excited with UV light. This paper describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented along with the results of comparative thermal cycle testing of TBSCs, produced using a production standard air plasma spray system. The latter show the durability of TBSCs to be similar to that of standard YSZ TBCs and indicate that the addition of the rare-earth dopant is not detrimental to the coating. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Experimental data are presented, indicating that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers covered by as much as 500 μm of standard TBC material. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.56 mm thickness.

Photo-Stimulated Phosphorescence for Thermal Condition Monitoring and Nondestructive Evaluation in Thermal Barrier Coatings

This article describes recent developments of the thermal barrier sensor concept for nondestructive evaluation (NDE) of thermal barrier coatings (TBCs) and on-line condition monitoring in gas turbines. New and enhanced instrumentation to measure surface temperature distributions and heat flux and to monitor TBC health are regarded as a priority by the industry. The authors have proposed thermal barrier sensor coatings (TBSCs) as a possible means of achieving such measurements. TBSCs are made by doping the ceramic material (currently yttria-stabilized zirconia) with a rare earth activator to provide the coating with luminescence when excited with ultraviolet (UV) light. The article describes the physics of the thermoluminescent response of such coatings and shows how this can be used to measure temperature. Calibration data are presented from a coating produced using a production standard spray system. Also discussed is the manufacture of functionally structured coatings with discreet doped layers. The temperature at the bond coat interface is important with respect to the life of the coating since it influences the growth rate of the thermally grown oxide layer, which in turn destabilizes the coating system as it becomes thicker. Preliminary experimental data are presented that indicate that dual-layered TBSCs can be used to detect luminescence from, and thereby the temperature within, subsurface layers. A theoretical analysis of the data has allowed some preliminary calculations of the transmission properties of the overcoat to be made, and these suggest that it might be possible to observe phosphorescence and measure temperature through an overcoat layer of up to approximately 1.33 mm thickness.

Producing Hydrogen and Power Using Chemical Looping Combustion and Water-Gas Shift

A cycle capable of generating both hydrogen and power with ‘inherent’ carbon capture is proposed and evaluated. The cycle uses chemical looping combustion (CLC) to perform the primary energy release from a hydrocarbon, producing an exhaust of CO. This CO is mixed with steam and converted to H2 and CO2 using the water-gas shift reaction (WGSR). Chemical looping uses two reactions with a re-circulating oxygen carrier to oxidise hydrocarbons. The resulting oxidation and reduction stages are preformed in separate reactors — the oxidiser and reducer respectively, and this partitioning facilitates CO2 capture. In addition, by careful selection of the oxygen carrier, the equilibrium temperature of both redox reactions can be reduced to values below the current industry standard metallurgical limit for gas turbines. This means that the irreversibility associated with the combustion process can be reduced significantly, leading to a system of enhanced overall efficiency. The choice of oxygen carrier also affects the ratio of CO vs. CO2 in the reducer’s flue gas, with some metal oxide reduction reactions generating almost pure CO. This last feature is desirable if the maximum H2 production is to be achieved using the WGSR reaction. Process flow diagrams of one possible embodiment using a zinc based oxygen carrier are presented. To generate power, the chemical looping system is operated as part of a gas turbine cycle, combined with a bottoming steam cycle to maximise efficiency. The WGSR supplies heat to the bottoming steam cycle, as well as helping to raise the steam necessary to complete the reaction. A mass and energy balance of the chemical looping system, the WGSR reactor, steam bottoming cycle and balance of plant, is presented and discussed. The results of this analysis show that the overall efficiency of the complete cycle is dependant on the operating pressure in the oxidiser, and under optimum conditions, exceeds 75%.Copyright