High precision temperature measurements for reacting flows

The effective and fast design of low emission gas turbines depends critically on the ability of engineers to make accurate and precise predictions of gas temperatures within the combustion chamber. This project aims to produce instantaneous temperature measurements of the highest accuracy and precision ever in model and industrial scale combustors. These precision measurements aim not only provide the basis for validation of models by industrial and academic users, but also to create a path for development of a lower cost, high precision thermometry technique for deployment in realistic combustors.
The two key factors governing the design of continuous flow combustors are maintaining low emissions - particularly nitric oxides - and keeping the system away from thermoacoustic instabilities. The spatial and statistical distribution of burned gas temperatures is the single most important factor governing the formation of nitric oxide (NO): a local change of 50 K can lead to a change of 70% in thermal NO formation rates at typical combustion temperatures. Validation of emission prediction models is hemmed by the lack of availability of statistical and spatial information on temperatures. Thermoacoustic instabilities are created by a feedback effect in which acoustic waves generated by the unsteady acceleration of the flow during combustion in a confined environment lead to further unsteadiness in heat release. Two factors associated with the flame are important: the response of the flame to acoustic perturbation, and the generation of temperature non-uniformities (called entropy spots): the former leads directly to density fluctuations and acoustic waves, and the latter couple the boundary conditions to reflect as pressure waves. The identification of the origin of combustion instabilities is complex, as several factors can contribute fluctuations, yet usually only pressure information is available, sometimes aided by relative total heat release fluctuations via chemiluminescence. Nevertheless, statistical measurements of temperatures in either model or industrial scale gas turbine flames are relatively uncommon, because of difficulties with physical probes or optical methods relying on calibration of signal amplitudes. The proposed measurements do not rely on amplitudes, but on the measurement of signal frequency, which can be made significantly more precisely (down to errors of 0.2%) than comparable techniques. Furthermore, the present measurements will enable the direct simultaneous measurements of NO and temperature with a single laser, thus creating a unique statistical database for model validation. Finally, the technique will enable for the measurement of temperature fluctuations through a nozzle at very high precision, which has not been done previously. The high precision measurements will have a direct impact on assessing the quality of model predictions for NO and instabilities, and when translated into design codes, into the design of cleaner and more stable power and propulsion systems.

Gas turbine power generation is currently responsible for 40% of power in the UK, as well as the primary propulsion mode for aeroengine combustion for the foreseeable future, with the UK industry as a leading equipment supplier. The effective and fast design of low emission combustion devices depends critically on the ability of engineers to make accurate and precise predictions of gas temperatures within the combustion chamber. This project will produce instantaneous temperature measurements of the highest accuracy and precision ever in model and industrial scale combustors. These precision measurements will not only provide the basis for validation of models by industrial and academic users, but also to create a path for development of a lower cost, high precision thermometry technique for deployment in realistic combustors.
The two key factors governing the design of continuous flow combustors are maintaining low emissions - particularly nitric oxides - and keeping the system away from thermoacoustic instabilities. Validation of emission prediction models is hemmed by the lack of availability of statistical and spatial information on temperatures. In this project, we will be able to obtain unprecedented precision in temperature measurements, as well as obtain simultaneous measurements of NO and temperature in turbulent premixed and stratified flames. This unique statistical database, which will build on a base of a very well characterized turbulent flame including all major species, will help untangle the relationship between temperature statistics and the resulting non-linearity in NO production in premixed flames. Models produced by researchers in academia and used in the design of gas turbines will be able to take advantage of this database for validation, ultimately leading to a speed up in the design and development of combustion chambers for gas turbines as well as furnaces and other devices.
Thermoacoustic instabilities are created by a feedback effect in which acoustic waves generated by the unsteady acceleration of the flow during combustion in a confined environment lead to further unsteadiness in heat release. The prediction of thermoacoustic instabilities is a more intricate problem than the prediction of NO emissions: here many factors at play, including injector and flame aerodynamics, injector mixing characteristics, temperature non-uniformities and, in an annular combustor, the interactions between acoustics and the flames distributed around the annulus. In this proposal we isolate two leading factors, the flame response and temperature non-uniformities, to identify the relative importance as the boundary conditions are varied, in a representative single-sector combustor. The study will for the first time characterize the simultaneous temperature and pressure response of the system through the combustor and converging nozzle and be able to identify the evolution of these variables. Models for the system will be able to incorporate the actual rather than postulated spatial and time resolved variation of temperature, allowing the separation between modes, and a greater understanding of flame behaviour and flame-system interaction.
In spite the significant advances in computational models and resources, testing of gas turbine combustor remains a necessary and expensive part of the development of new systems. A single day of tests costs upwards of hundreds of thousand pounds, and many additional man-hours in interpreting and extrapolating the results obtained. The proposed experiments will provide a significant leap forward in creating the highest precision thermometry technique, and producing detailed and statistically quantified NO and temperature measurements throughout a model combustor. Successful implementation will allow industrial designers to move forward more confidently and rapidly, reaping the benefits of lower emissions, lower instability power systems.