Exploring Lean Frontiers

As an undergraduate, for a long time, I simply thought supervisors wait around all day to supervise us — and sometimes even lecture. Obviously, I knew they must do research of some sort, after all (not to state the obvious), it is an academic institution; but what does doing research really mean? If anything, something about it sounded rather tame. However, as a result of my 4th year project, I’ve recently started to catch a glimpse of how exciting research can be, and how some fairly novel concepts are being explored in random room CUED — in my case, only 25 meters away from the 2nd year heat transfer lab.

Supervised by Dr James Dawson and post-doc researcher Nicholas Worth, my project looks at combustion instabilities in a model gas turbine combustor. Combustion instabilities are the thermo-acoustic self-excitations that occur when the heat release from the flames synchronises with the pressure waves in the system, causing large-scale oscillations. The heat release is the “thermo” part and the pressure wares are the “acoustic” part, hence “thermo-acoustic”. This synchronisation only happens under certain conditions, however the oscillations generated can be enough to cause significant damage to the engine. BUT, before you get too worried, this is a development issue for lean-burn combustors; not the rich-burn type found on your typical Boeings and Airbus’. Albeit only a development issue for these lean (premixed) combustion must be addressed and understood for its challenges if any commercially viable engine based on this design is ever to be manufactured.

Although substantial advances have already been made in this field, it is still not possible to theoretically or numerically predict thermo-acoustic instabilities with adequate accuracy. Much of the work so far has been done for single flames, with these results scaled up to multiple flames. The success has been debatable. Modelling multiple flames in this way seems to be a good approximation for when the flames are widely spaced apart, but a fairly poor one when the flames are close together. This suggests that the fundamental physics are not being captured well, particularly under conditions of merging. Experimental investigations of two flames by Drs J. Dawson and N. Worth showed that flame merging has significant effects on the flame structure — this is one of the important parameters governing the presence of thermo-acoustic instabilities. Furthermore, these instabilities are either axial or circumferential in nature, however it is the circumferential modes that tend to be the more prominent problem. Following on from these findings, a lab scale model combustor was designed and built in CUED — in the same workshop used for the 1st year structural design project — where tangential instabilities have been found, and are currently being assessed.

On a side note, it is worth noting that no one has ever built a lab-scale annular set of flames before and obtained self-excited instabilities (for various reasons). But, if you think about it, at first though it all sounds very simple… If you want to understand the behaviour self-excited instabilities in a real gas turbine combustor: (i) just build a model of it; (ii) non-dimensionalise everything like your life depends on it (because it kind of does); and (iii) plot some graphs? Yes, this would be fine to do (-ish), except that there are many drawbacks to this method, some of which include time, money and effort. Moreover, the understanding gained, although probably useful, is not necessarily that insightful. This is because you are essentially assessing the output of a black box, and not what is happening inside the black box. For example: if an unstable system were to exhibit vortex shedding, it would not be completely clear whether this shedding is a by-product or driver of the resonance, or what coupling exists between the two? This is not to say that understanding the output is unimportant (it is); more that analysing the output is only part of the bigger picture. Looking at this output is the core of my 4th year project.

A lot of progress has been made so far with the annular combustor. As mentioned before, the instability modes have been found, and further to this; they have been recorded using pressure measurements and flame images. A schematic and photographs of the model combustor are shown to the right. The pressure data obtained from several pressure transducers mounted flush with the burner’s inside walls. The flame images were taken using a high-speed camera (HSC), fitted with an intensifier (IRO) to image the different UV wavelengths of the flame. These images were then post-processed using MATLAB. This is not as easy as plotting scatter graphs with lines of best fit — the of my bulk data analysis skills prior to this project. For example, it took 300 lines of code to convert ≈14,400 raw image files into the 18 phase-averaged mean-normalised images, some of which are shown above. Due to space limited, only the case of a near-perfect circumferential travelling wave is displayed. The wave travels at a fairly constant speed of ≈2000 mph! Other permutations also exhibited travelling waves of a similar form, but often coupled with standing waves. All this data and more is still being collected and processed. And since so much of it is new, the results found could be groundbreaking…

So there you have it, there’s more happening in CUED than just example papers, lectures and the standard credit laboratory sessions. Cutting edge research is taking place, probably being done by your supervisor; and you have to turn the corner to see it!

Editor: Tafara Makuni.
Edited: July 2021.

Author: Tafara Makuni.
Published: Lent Edition 2012.

Artist from London.

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