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Author

S Chandrasekhar

Bio: S Chandrasekhar is an academic researcher. The author has contributed to research in topics: Transverse plane & Amplitude. The author has an hindex of 1, co-authored 1 publications receiving 1 citations.

Papers
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Proceedings ArticleDOI
08 Jul 2007
TL;DR: In this paper, a non-premixed diffusion flame formed by co-flowing streams of air and fuel downstream of the burner is subjected to externally imposed oscillations in pressure, velocity, and density, in the transverse direction to the mean flow.
Abstract: A non-premixed diffusion flame formed by co-flowing streams of air and fuel downstream of the burner is subjected to externally imposed oscillations in pressure, velocity, and density, in the transverse direction to the mean flow. The problem is investigated by means of numerical simulation. The flame is treated in a two-dimensional domain, and the response of its heat release rate fluctuations to the external excitation is examined. A relationship between a threshold amplitude and the frequency of the external excitation exists for which the flame blows off (at low frequency) or blows out (at high frequency). The classical response function curve dropping with frequency is captured when stable oscillations are observed. The response is linear with the imposed amplitude. The oscillatory combustion in the presence of the prevalent/imposed transverse velocity oscillations is coupled to the linear longitudinal acoustic modes of the duct by placing the burner somewhere along the length. Multiple longitudinal modes are simultaneously and spontaneously excited; the mode that grows predominantly depends upon the burner location in the duct. This shows that the combustion zone acts as a local nonlinearity in transferring energy between transverse modes and longitudinal modes, even when the acoustic oscillations are in the linear regime.

2 citations


Cited by
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Journal ArticleDOI
TL;DR: In this paper, a closed-form analytical solution for the transverse vorticoacoustic wave in a circular cylinder with headwall injection is provided. But the authors do not consider the effect of the head-wall injection on the acoustic wave.
Abstract: This work seeks to provide a closed-form analytical solution for the transverse vorticoacoustic wave in a circular cylinder with headwall injection. This particular configuration mimics the conditions leading to the onset of traveling radial and tangential waves in an idealized liquid rocket engine. Assuming a short cylindrical chamber with two injection showerhead models (a top hat, uniform flow, and a bell-shaped sinusoidal profile), regular perturbations are used to linearize the problem’s conservation equations. Flow decomposition is subsequently applied to the first-order disturbance equations, thus giving rise to a compressible, inviscid, acoustic set that is responsible for driving the unsteady motion, and to an incompressible, viscous, vortical set that is driven by virtue of coupling with the acoustic mode along solid boundaries. While the acoustic mode is readily recovered from the wave equation, the induced vortical mode is resolved using boundary-layer theory and a judicious expansion of the r...

10 citations

Journal ArticleDOI
TL;DR: 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.