Ph.D. Proposal
Sungyoung Ha
(Advisor: Prof. Tim Lieuwen)
"Correlation Between the Unsteady Heat Release Rate and Combustion Noise"
Wednesday, September 11
1:30 p.m.
Food Processing Technology Building Auditorium
[North Avenue Research Area, 640 Strong St Atlanta]
Abstract:
Aircraft noise is an important noise pollutant that is becoming increasingly significant as the recent number of flights has seen unprecedented growth. Not only does it affect the quality of life around regions with heavy air traffic, but it also has large economic consequences as well. For example, during the national college entrance exams in South Korea, all commercial flights are grounded during the listening portion of the exams to ensure that regions near airports do not have a disadvantage. Following recent trends of higher bypass ratios in aircraft engines, combustion noise, or core noise, has seen a renewed interest as a significant source of aircraft noise.
To study and mitigate combustion noise, it is important to be able to distinguish different noise sources inside a combustor. Noise inside a combustor can be generally classified into either direct combustion noise or indirect combustion noise. Direct combustion noise is generated by the unsteady heat release at the flame performing work to the surrounding gas. Indirect combustion noise is generated when entropy fluctuations are accelerated or decelerated, e.g. through a nozzle, or interacts with a boundary such as a turbine blade. In addition to these acoustic sources, noise measurements can be contaminated by hydrodynamic pseudonoise which are pressure fluctuations associated with turbulence that does not propagate acoustically. In practice, it is difficult to distinguish between these noise sources as pressure measurements are a summation of fluctuations from all sources.
Typical strategies to separate combustion noise sources take advantage of the fact that direct and indirect noise have different generation mechanisms, which can lead to the two being uncorrelated. It was hypothesized that it is potentially possible to use coherence methods between chemiluminescence and pressure fluctuations instead of temperature measurements which are relatively more difficult to perform. To apply such methods, it is necessary for the chemiluminescence and direct noise to be perfectly, or at least highly coherent. However, somewhat surprisingly, there is yet no literature available that demonstrates this experimentally. In addition to there being very few works that report coherence between chemiluminescence and acoustic pressure measurements, those that do show relatively low values. This is somewhat contradictory to expectations as direct noise is known to have a linear relationship with the heat release rate. Furthermore, there has been no or minimal literature that attempts noise source separation through chemiluminescence. Despite an abundance of work literature on combustion noise, clearly there is still a large gap in literature that warrants investigation.
This work proposes to fill in such gaps in literature by focusing on the coherence between global chemiluminescence and acoustic pressure measurements both theoretically and experimentally. An analytical approach is proposed to identify the mechanisms of how coherence can be reduced and recommended practices for measurements. A canonical experiment is proposed to i) obtain quality measurements of coherence between chemiluminescence and acoustic pressure fluctuations, ii) verify the findings of the analytical study, iii) study the effect of confinement, and iv) apply findings in the context of noise source separation. The results are expected to deepen our understanding of combustion noise generation by covering aspects overlooked in previous literature and offer new perspectives on future combustion noise studies.
Committee
- Prof. Tim Lieuwen – School of Aerospace Engineering (advisor)
- Prof. Krishan Ahuja – School of Aerospace Engineering
- Prof. Jerry Seitzman – School of Aerospace Engineering