Tom Shelley reports on a new non contact method for measuring the temperature and general state of ceramics and ceramic coatings in hot environments
By using pulses of ultra violet laser light to excite phosphors in ceramics or ceramic coatings, it is possible to remotely measure temperatures to an accuracy of a few degrees C.
Temperatures may be determined from the decay time of the afterglow, normally a few milliseconds, using a technique that works equally well whether the target is hot or cold.
The main target application is monitoring the temperature and state of coatings on electric power generating set gas turbine blades, but interest is also coming from others wanting to make remote measurements of various kinds.
The intention is to allow gas turbines to be safely run at higher temperatures, increasing efficiency, and reduce risk of unexpected failures.
The crucial phosphor materials are inorganic chemicals, very similar to those used to produce the colours in cathode ray tubes. Mixed in very small proportions with the ceramic being monitored, the phosphors fluoresce orange, blue or green, depending on their chemical composition and that of the host material.
The idea is the brainchild of Dr Jorg Feist, emerging during his Ph.D. studies in the Mechanical Engineering Department at Imperial College. It has since been spun out as a startup company called Southside Thermal Sciences. The company is still headquartered within the college precincts, but is now directed by Dr Feist and two colleagues, Udo Dengel, who holds a masters degree in business administration from the same institution and Dr Andrew Heyes, Dr Feist's PhD supervisor.
Dr Feist told Eureka that it would in theory be possible to determine temperature from changes in the fluorescing colour, but this is not practicable inside a glowing combustion zone. However, the 8 to 10ns pulses of ultra violet light from the laser are sufficient to excite an afterglow of around 1 to 2ms at room temperature, less at high temperatures and more at low temperatures. Temperature can then be measured to an accuracy of 3 to 6 deg C.
Alternatively, it is also possible to accurately measure temperature from the ratios of strengths of pairs of spectral emission lines, a method long established at Imperial College in their studies of gas plasmas.
Monitoring the 200 to 1,000 micron thick yttria stabilised zirconia coatings of turbine blades is crucial if they are to be run any hotter. The blades would fail even at today's temperatures, were it not for the coatings and the internal cooling, despite blades being made of single crystal high temperature alloys at typical costs of tens of thousands of pounds each.
As well as measuring temperature, it should also be possible to establish the state of the coatings, both in terms of changes in the crystalline structure of the ceramic, and their wearing away. This could be established from subtleties in the afterglow and spectra, or simply by exposure of different layers impregnated with different phosphors as the surface wore away.
An increase in working temperature of just 50 deg C improves efficiency by around 1%. Applied to a typical 500 MW generating unit, this could yield savings of up to $1 million per year savings. Reducing planned shutdown time by a single day can save around $ 50,000. Unplanned shutdowns caused by blade failures can be very expensive indeed.
Ability to monitor state of blades and coatings might also be crucial in permitting new technologies to emerge, such as coal fired gas turbines, which suffer from severe problems arising from erosion of coatings by molten coal slag. Coal slag erosion was also found to be the crucial problem in coal fired magnetohydrodynamic power generating systems, which are essentially coal fired rocket engines. Power is extracted by adding potassium to make the hot plasma electrically conductive, and then extracting it from electrodes placed at 90 deg to the direction of gas flow, in the presence of intense magnetic fields perpendicular to the current extracting electrodes and direction of gas flow. Efficiency is much higher than in conventional power generating systems but the addition of potassium makes the slag especially corrosive and potential problems caused by blade erosion end erosion of bricks in the heat regenerators have until now not been overcome.
Development is still at the research phase, and it is in research that the system is likely to find first use, such as studying temperature distributions inside turbine combustors, with a view to improving their design. Further applications are expected in improving the performance of furnaces, process plant, fuel cells, in automotive engineering development, and in monitoring machinery. The technique is in no way inhibited by partial obscuration by smoke and burning gases, provided some light can penetrate. It can be operated from a considerable distance, permitting use inside nuclear reactors and chemical plant from safe locations outside. Application to aircraft engines is likely to be much further away in the future because of the need to gain approvals based on years of rigorous testing and evaluation.
Interest is considerable, except from General Electric and Siemens who seem to think they have their own solutions to their problems. The team at STS won the Imperial College Inaugural Entrepreneurs Challenge in June 2001 and the Materials/Base Technologies category of the Wall Street Journal's European Innovation Awards at the end of 2002. They are currently looking for collaboration with industrial partners, license opportunities and further investment with a view to developing a prototype commercial system. Further research work is planned under the European Union Framework Programme 6. The technology is protected by patents both granted and applied for.
email Dr Jorg Feist
The technique is unaffected by smoke or flame, giving it much more versatility than infra red, allowing the measurement of the temperature of almost anything from a safe distance.
* The technique allows the remote measurement of the temperature of a suitable phosphor loaded coating or material.
* Partial obscuration of the light path has no effect on measurements, provided some light can get through
* Temperature measurements are accurate over a wide range.
For more technical developments see www.eurekamagazine.co.uk
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