Linear and nonlinear entropy-wave response of technically-premixed jet-flames-array and swirled flame to acoustic forcing

Abstract

Broadband combustion noise is a major part of the total noise radiated by modern jet engines. It comprises two components: direct noise originating from the unsteady heat release of the flame, and indirect noise resulting from the acceleration of entropy fluctuations in the turbine stages. Not only do these entropy waves contribute to the noise pollution of aeroengines, they can also have a crucial role in the feedback loop leading to thermoacoustic instabilities, which induce vibrations and thermal loads that are highly detrimental for combustors and turbine components. Thus, there is a critical need to understand and model the complex mechanisms associated with the generation and the advection of these waves. This study presents quantitative measurements of the production of entropy waves in a technically premixed turbulent combustor, subject to acoustic forcing. Entropy transfer function (ETF) relating acoustic input, obtained with microphones, to entropy wave output, obtained with OH-LIF thermometry at a distance of four flame heights from the burner outlet, were measured between 40 Hz and 90 Hz. These ETF were obtained using two burners of same length with technical premixing of air and natural gas, operated at the same thermal power: a matrix burner producing an array of turbulent jet flames, and a burner producing a single swirled turbulent flame. It is found that the ETF of the matrix burner exhibits a low-pass behavior, with a gain ranging from approximately 0.7 at 40 Hz down to 0.25 at 90 Hz, while the gain of the swirled flame ETF was not exceeding 0.2. It is also demonstrated that entropy wave production with the matrix burner is highly nonlinear, with a dramatic drop of the ETF gain occurring beyond a certain level of acoustic forcing amplitude. These measurements can be used to derive predictive nonlinear models of thermoacoustic instabilities involving entropy wave feedback.