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Acoustic-Flow Interaction Models for Advancing Thermoacoustic Instability prediction in Very low Emission combustors

Periodic Reporting for period 3 - AFIRMATIVE (Acoustic-Flow Interaction Models for Advancing Thermoacoustic Instability prediction in Very low Emission combustors)

Reporting period: 2021-06-01 to 2022-11-30

Gas turbines are an essential ingredient in the long-term energy and aviation mix. They can combust carbon-free and low-carbon fuels. However, low emissions combustion in gas turbines is susceptible to damaging thermoacoustic instability. This is caused by a two-way coupling between unsteady combustion and acoustic waves.

Computational methods for predicting and designing out thermoacoustic instability are needed. Methods with the prospect of being fast enough are based on coupled treatment of the acoustic waves and unsteady combustion. These exploit the amenity of the acoustic waves to analytical modelling, allowing costly simulations to be directed only at the more complex flame. Such methods show promise, but must more accurately account for the complex flow-fields in industrial combustors to realise their full potential.

This project aims to comprehensively overhaul acoustic models across the entirety of the combustor, accounting for real and important acoustic-flow interactions. These new models will offer the breakthrough prospect of extending efficient, accurate predictive capability to industrial combustors, which has a real chance of facilitating future, instability free, low emissions gas turbines.
Work so far has:
- developed the first models for the planar acoustic field in a duct whose cross-sectional area, mean flow and temperature all vary axially
- developed new ways of understanding and analysing the acoustic damping of different components
- developed a new analytical framework for predicting and understanding the noise generated by accelerated/decelerated temperature fluctuations, and has shown how the noise generated by such flows through holes can be better predicted.
- demonstrated, for the first time, how the noise generated by flow through a hole strongly depends on the exact shaping of the hole geometry, paving the way for optimization specifically targeting aeroacoustics.
- applied our coupled methods for thermoacoustic prediction to large experimental combustion rigs in Beihang University in China, yielding fresh understanding of the behaviour.

We have additionally developed the first models for the acoustic-flow interaction in simple burner geometries, the first frequency-dependent models for the aeroacoustics of heat exchanger tube rows and the first models for the noise generated by a temperature fluctuation passing over a blade. These results are all awaiting publication.
Our work on modelling of acoustic-flow interactions in duct flows, hole flows, burner flows and flows past blades has all gone beyond previous state of the art. Our computational prediction of thermoacoustic oscillations in experimental systems also continues to push the state of the art.

We expect to make further modelling developments relevant to combustor flows, and to bring these together to deliver a more sophisticated framework for computationally predicting thermoacoustic oscillations in complex systems.