Characterization of Nonlinear Responses of Non-Premixed Flames to Low-Frequency Acoustic Excitations
Deng Pan,Chenzhen Ji,Tong Zhu +2 more
TLDR
In this article , the authors measured the nonlinear heat release response of a methane-air non-premixed flame to low-frequency acoustic excitations using a two-microphone method.Abstract:
The response of flames’ heat release to acoustic excitation is a critical factor for understanding combustion instability. In the present work, the nonlinear heat release response of a methane–air non-premixed flame to low-frequency acoustic excitations is experimentally investigated. The flame describing function (FDF) was measured based on the overall CH* chemiluminescence intensity and the velocity fluctuations obtained by the two-microphone method. The CH* chemiluminescence and schlieren images were analyzed for revealing the mechanism of nonlinear response. The excitation frequency ranges from 10 Hz to 120 Hz. The forced relative velocity fluctuation amplitude ranges from 0.10 to 0.50. The corresponding flame Strouhal number (Stf) ranges from 0.43 to 4.67. The study has shown that the flame length responds more sensitively to changes in excitation amplitude when subjected to relatively high-frequency excitations. The normalized flame length (Lf/D) decreases from 3.79 to 2.37 with the increase in excitation amplitude at an excitation frequency of 100 Hz. The number of oscillation zones along the flame increases with increasing excitation frequency, which is consistent with the increase in the Stf. The low-pass filtering characteristic of FDF is caused by the dispersion of multiple oscillation zones, as well as the cancellation effect of the adjacent oscillation zones under relatively high-frequency excitation. The main mechanism for the local gain peak and valley is the cancellation effect of positive and negative oscillation zones with various Stf. When two adjacent oscillation regions have similar amplitudes, the overall phase-lag becomes more sensitive to changes in excitation frequency and amplitude. This sensitivity leads to nonlinear anomalous changes in the phase-lag near the frequency corresponding to the gain valley. The calculated disturbance convection time is consistent with the measured time delay in the short flame scenario. Further research is required to determine whether the identified agreement is a result of the consistent occurrence of the oscillation zone in close proximity to the flame’s center of mass, in conjunction with a precise determination of the average convective velocity.read more
References
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Journal ArticleDOI
Transfer function method of measuring in‐duct acoustic properties. I. Theory
J. Y. Chung,D. A. Blaser +1 more
TL;DR: In this paper, a broadband stationary random acoustic wave in a tube is mathematically decomposed into its incident and reflected components using a simple transfer function relation between the acoustic pressure at two locations on the tube wall.
Journal ArticleDOI
Response of a laminar premixed flame to flow oscillations: A kinematic model and thermoacoustic instability results
TL;DR: In this paper, the authors construct an analytical model to describe the dynamic response of a laminar premixed flame stabilized on the rim of a tube to velocity oscillation, and obtain a time-domain differential equation describing the relationship between the velocity perturbation and the heat release response over the entire frequency range.
Journal ArticleDOI
Experimental investigation of the nonlinear response of turbulent premixed flames to imposed inlet velocity oscillations
Ramanarayanan Balachandran,B.O. Ayoola,Clemens F. Kaminski,Ann P. Dowling,Epaminondas Mastorakos +4 more
TL;DR: In this paper, an experimental investigation of acoustically forced lean premixed turbulent bluff-body-stabilised flames in an enclosure short enough so that no coupling of the combustor downstream acoustics occurred for the frequencies studied here, which allows an unambiguous examination of the flame response to inlet velocity fluctuations.
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Nonlinear self-excited oscillations of a ducted flame
TL;DR: In this article, a theory for nonlinear oscillations of a confined flame burning in the wake of a bluff-body flameholder is developed, exploiting the fact that the main nonlinearity is in the heat release rate, which essentially saturates.
Journal ArticleDOI
A unified framework for nonlinear combustion instability analysis based on the flame describing function
TL;DR: In this article, the authors used the flame describing function (FDF) to investigate the nonlinear stability of burners by associating the FDF with a frequency-domain analysis of the burner acoustics.