• Combustion Generated Noise

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Combustion Generated Noise,
Organised by the Combustion Physics Group
Institute of Physics, London
27 September, 2004

Meeting Synopsis

This one-day series of presentations comprised a range of industrial presentations, where practical issues and current practice for controlling and mitigating combustion noise were outlined, together with ongoing research programmes aiming to provide improved understanding and prediction of the various thermo-acoustic processes involved. One objective of the day was to provide a forum for cross-fertilisation of ideas of combustion noise problems across different industries and applications. The day was co-sponsored by The British Section of the Combustion Institute, and the British Flame.

Proceedings commenced with an overview of current industrial gas turbine operator practice, from Stuart James (E.ON UK). Observations of the influence of operating parameters, such as ambient conditions and fuel composition, upon combustion dynamics were discussed, followed by a management-based system for monitoring and mitigation, with case study from the ‘Connays Quay’ plant. A series of complementary academic talks followed, presented by Alex Riley (Cambridge), James Dawson (Cardiff, now Cambridge) and Chris Lawn (Queen Mary and Westfield), where phenomenological understanding of oscillation/flame interactions was elucidated via fundamental studies in simplified experimental facilities, derivation of suitable flame transfer functions and reduced ‘thin-flame’ modelling. Suggested methods for mitigation, such as variation in geometrical combustor shape and active control were indicated. Andy McIntosh (Leeds), in the spirit of the day, explained the extraordinary performance of natures own ‘pulse combustor’, the Bombardier Beetle, and drew analogies with a potential re-ignition system for gas turbines. This tiny creature essentially blasts its enemies with explosive jets at a rate of 2-3/ms, with exceptionally high efficiency discharge. Early CFD studies had focussed on the shape of its heart-shaped combustor, whilst more recent studies are concentrating upon the possible existence of a pressure release valve, facilitating release of slightly superheated jets.

The afternoon session began with Greg Sims (QinetiQ, formerly DERA) describing a broad range of combustion noise technologies developed at QinetiQ, for both aero and stationary gas turbine powerplant. Greg described developments in flame transfer function measurements, various mitigation methods, including passive acoustic damping, active control methods and plant monitoring and optimisation. Linear thermoacoustic modelling and vortex dynamic issues were also discussed. Mark Jermy described work undertaken at Cranfield to understand better the interaction between combustion generated acoustic waves and fuel-air mixing in liquid fuel systems, with the resultant modulation of equivalence ratio. The influence of fuel injector type on combustion noise generated was emphasised. In appropriately scaled experiments, and using a range of advanced laser diagnostic systems (LIF, LSD, Mie), a transfer function was derived describing the interaction between AFR, SMD and acoustic perturbation, facilitating prediction of injectors likely to invoke strong combustion instabilities. Rama Balachandran (Cambridge) discussed the response of turbulent premixed flames to imposed acoustic oscillations, through experimental studies on ethylene-air flames. A variety of diagnostic techniques to measure velocity fluctuation and heat release were developed and employed to quantify heat-release/velocity transfer functions. The origin of non-linear response of the flame to high amplitude forcing was highlighted. Finally, Chris Carlson (Ricardo) provided an overview of previous developments and current state-of-art in the automotive sector. In the only audible demonstration of the day, he showed how combustion generated noise from diesel engines have considerably reduced over the last few years. Chris introduced the basics of combustion noise in the internal combustion engine, design concepts for noise improvement, and facilities used for directly measured combustion noise. He emphasised that customer satisfaction under idling conditions, when driving noise is minimised, is a priority for manufacturers.

The meeting closed with a discussion which raised importance of improved flame transfer functions, the likely importance of CFD modelling, and raised the question of the significance of the interaction of other 3-dimensional instabilities known to prevail in many combustors, most notably the PVC.