Thermal history coating optimises combustion systems

The inherent inaccessibility and heat of combustion chambers make it an area that is not just difficult to analyse, it's sometimes impossible. But this goes for any hot environment from exhaust systems to turbochargers. Many conventional sensing methods can be ineffective. Though some expensive solutions exist, many engineers simply assume the worst and compensate for a lack of accurate data with over specified materials and over designed components.

"If you specify a high temperature alloy for an exhaust rather than a standard alloy you'll affect the cost of that part pretty significantly," says Dr Jörg Feist, managing director Sensor Coating Systems. "So, are you putting up the cost unnecessarily? It's the same with gas turbines. If you have a detailed temperature profile inside the engine then you can engineer it to run hotter, which is what everybody wants to do as it increases efficiency and reduces CO2. So why not optimise the shape, dimension and position of cooling holes in the combustion chamber and produce a design that will not cause any major hot spots?"

Sensor Coating Systems (SCS) has developed a coating and paint system that it describes as being able to capture the thermal history of parts and components.
The company was spun out of Southside Thermal Sciences (STS) in early 2012, which was itself spun out of work on phosphorescent coatings at Imperial College London in 2002.

The early STS coating uses active ions added to a host ceramic material that when excited by light, start to phosphoresce in a reversible manner, giving the materials an innate luminescence that can be measured and recorded. This led to real-time spot measurements of hot components under the general term phosphor thermometry. However, an enquiry to STS wanted to measure the temperatures across the entire surface of a combustion chamber.

"I said to the board at the time that actually, we can already do this," says Dr Feist. "Everyone was surprised, but we could. And what we have ended up with is one of the only coatings of its type in the world."

Dr Feist and his team were able to modify the coating so it would permanently change once heated. This could then be detected by non-destructive automated instrumentation.

"Essentially, phosphorescence is a process where light is used to excite a material until it starts to emit light of a different wavelength," explains Feist. "This comes from ions acting as microprobes at the atomic level within the material. These can be analysed to give important information about the state of the material itself such as temperature, pressure or crystallinity."

The technique offers significant potential in replacing thermochromic paints also known as thermal paints. These are currently used by many gas turbine manufacturers. However, they have always been far from ideal in terms of application and the information they can yield.

Thermal paints change colour irreversibly when exposed to high temperatures due to chemical changes. While they can be useful in identifying hot spots, accurately discerning temperature and temperature change needs an expert eye. Colour changes can be subtle and while compared to calibrated colour charts, the tolerance can be as much as ±40°C. Potential users of thermal paints in wider industry are often put off by the expense, toxicity and need for expert analysis.

Thermochromics are also becoming an increasing headache for those that use them due to restrictions on their supply and transportation under the Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH) legislation. The expectation is a blanket ban will soon be issued due to their toxic and carcinogenic nature. SCS hopes it can provide an alternative to fill that niche.

"Our paint coating is an oxide ceramic silicate that is non-toxic and water based," says Dr Feist. "Also our robust atmospheric plasma spray coating is a straightforward alternative. Thermochromic paint might only last a few minutes in an engine test, but we have had our coating running on heat shields at Didcot power station for over 4,500 hours, so it's very durable."

It is also much easier to analyse and does away with the need for a specialist interpreter. The historic peak temperatures are read using optical instrumentation that can be automated for full 2D thermal profiling of complex components and assemblies.

The coatings ability to survive for long durations under high thermal and mechanical loads offers fresh potential to both those familiar with the technology and those that have always seen thermal paints as too exotic.

"You can use the coating to validate the efficiency of different cooling methods or to get reliable temperature profiling data to validate a simulation," says Dr Feist. "It's likely to be used by gas turbine manufacturers as a replacement to thermochromic paints but will hopefully be used more widely.

"Also, given its relative low cost you could coat a component to see if the temperatures have exceeded a specified limit, so essentially as a warranty tool. Also, since the changes are permanent, the information can be retrieved during scheduled maintenance."

The coatings can be used to record phase changes, which cause subtle changes to the luminescence characteristic of the coating. This can be calibrated and quantitatively related to the percentage of new phase and the materials ageing characteristic can be very accurately discerned.

Similarly emission properties of the phosphorescence are primarily dependent on the dopant used, so different dopants emit light at different wavelengths and can therefore be used to quantifiably measure erosion across the surface when applied in different layers. In the same way, during corrosion the chemical composition of the coating changes, which will cause significant change to the emitted spectrum. All can be detected using the same optical equipment.

The SCS coating can offer continuous measurement of temperatures between 300°C and 1400°C to an accuracy of ±5 - 10°C. The thermal history coating is applied using an Atmospheric Plasma Spray (APS), while the thermal history paint can be sprayed using a normal air fed paint gun for applications requiring thinner coats, limited thermal exposure, rapid turnaround times or needing analysis of a large area.

Sensor Coatings Systems thermal history technology remains under development and the company is actively in collaboration with a number of international OEMs and governmental funding organisations to develop its products further. It is determined to bring data and understanding to combustion and extreme heat applications in a cost effective, and much cleaner, fashion than has been previously available. It is making it a mission to paint a much clearer picture to an area considered by many to ­­be a black art.

Justin Cunningham

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