Aditya Saurabh

Route to chaos for combustion instability in ducted laminar premixed flames

Complex thermoacoustic oscillations are observed experimentally in a simple laboratory combustor that burns lean premixed fuel-air mixture, as a result of nonlinear interaction between the acoustic field and the combustion processes. The application of nonlinear time series analysis, particularly techniques based on phase space reconstruction from acquired pressure data, reveals rich dynamical behavior and the existence of several complex states. A route to chaos for thermoacoustic instability is established experimentally for the first time. We show that, as the location of the heat source is gradually varied, self-excited periodic thermoacoustic oscillations undergo transition to chaos via the Ruelle-Takens scenario.

Chaos in an imperfectly premixed model combustor

This article reports nonlinear bifurcations observed in a laboratory scale, turbulent combustor operating under imperfectly premixed mode with global equivalence ratio as the control parameter. The results indicate that the dynamics of thermoacoustic instability correspond to quasi-periodic bifurcation to low-dimensional, deterministic chaos, a route that is common to a variety of dissipative nonlinear systems. The results support the recent identification of bifurcation scenarios in a laminar premixed flame combustor (Kabiraj et al., Chaos: Interdiscip. J. Nonlinear Sci. 22, 023129 (2012)) and extend the observation to a practically relevant combustor configuration.

Recurrence Analysis of Combustion Noise

Recurrence analysis of phase space trajectories reconstructed from scalar time series—a relatively new technique in the field of combustion dynamics—is proposed for analyzing combustion noise data. A demonstration based on the implementation of this analysis on combustion noise data acquired in experiments on an open bluff-body stabilized turbulent premixed flame is presented in this article. A combined analysis methodology involving conventional techniques: spectral analysis and proper orthogonal decomposition, together with recurrence analysis is found to be effective in identifying features embedded in combustion noise signals. In particular, the new perspective provides insights into the temporal transitions of pressure fluctuations between noise and periodic dynamics. The recurrence analysis technique, which is found to be instrumental in extracting the dynamical makeup of combustion noise signals in this work, is foreseen to be greatly beneficial for the analysis of experimental and numerical result...

Combustion Instability in a Swirl Flow Combustor With Transverse Extensions

The present investigation is an analysis of self-excited combustion instability in a swirl flame-based combustor with transverse extensions. Transverse extensions create the possibility of studying flame interaction with transverse acoustic oscillations. Such investigation important for understanding the phenomenon of thermoacoustic instability in annular combustors, where during thermoacoustic instability, azimuthal acoustic modes of the combustor couple with the multiple flames of the combustor. Flame and flow field dynamics during self-excited thermoacoustic instability in the single burner test-rig is presented here. These results are then compared to the dynamics of the isothermal and reacting flows in response to axial and transverse acoustic forcing. Both axial and transverse forcing led to the formation of axisymmetric shear layer vortices. Adding to the insight gained from previous investigations, these results suggest that that swirl flow dynamics in response to transverse acoustics consists of a non-trivial, direct effect of transverse acoustics on the flow field, in addition to its response to longitudinal fluctuations induced by transverse forcing.Copyright

Experimental and Numerical Investigation of a Turbulent Premixed Flame in an Anechoic Environment

The turbulent jet emanating from an unconfined, premixed burner is investigated by numerical simulation using Large Eddy Simulation (LES) and experimentally by means of optical (OH chemiluminescence), acoustic (microphone) and laser-optical measurement techniques (Laser Doppler Anemometry, Particle Image Velocimetry) for non-reacting and reacting flow, respectively. While 2D-LDA data of the non-reacting flow field are in good agreement with calculated results, 2D-PIV data of the reacting flow field, burning a methane-air mixture, differ significantly from the LES data. This is caused by a falsely chosen seeding injection location, which is apparently located too far downstream. In addition, a large percentage of the total air mass flow has to be used for proper seeding injection and, therefore, is not well mixed with the fuel from the actual fuel injection located further upstream. Consequently, this leads to a non-premixed flame which differs from the partially premixed flame obtained in case using LES. The strong seeding jet itself impacts on the velocity distribution, as well. The emitted noise spectrum has a tonal shape with peaks at the burner’s resonance frequency for the non-reacting flow which changes to broadband noise and is raised in amplitude in case of the reacting flow.

Coherence resonance in a thermoacoustic system.

We experimentally investigated the noise-induced dynamics of a prototypical thermoacoustic system undergoing a subcritical Hopf bifurcation to limit cycle oscillations. The study was performed prior to the bistable regime. Analysis of the characteristics of pressure oscillations in the combustor and fluctuations in the heat release rate from the flame-the two physical entities involved in thermoacoustic coupling-at increasing levels of noise indicated precursors to the Hopf bifurcation. These precursors were further identified to be a result of coherence resonance.

Swirl Flame Response to Traveling Acoustic Waves

The advent of annular combustors introduced a new facet of flame-acoustic interaction: flame coupling with standing and spinning azimuthal acoustic waves. Such coupling involves an acoustic field that is essentially transverse to the flame. Recent experiments on single burner test rigs have provided significant insight into flame interaction with transverse standing waves. However, experiments that focus on the spinning/rotating nature of azimuthal instabilities are still lacking. In this report, we demonstrate a methodology for studying spinning azimuthal instabilities on a single burner test rig. This methodology is based on analyzing flame response to a traveling acoustic wave generated in the combustor. We generate traveling acoustic waves in our transverse acoustic forcing test-rig by converting one end of the transverse extensions to a non-reflecting end. This is achieved through the implementation of the technique of impedance tuning. In the paper, we have discussed this implementation, followed by discussions on the effects of a traveling acoustic wave on a swirl-stabilized flame. The discussion is in the form of a comparison of flame oscillations for traveling wave and standing wave transverse forcing cases. Results show that the effect of transverse pressure oscillations dominates the flame response to traveling acoustic waves.Copyright

Experimental Study of Noise Generation by a Turbulent Premixed Flame

We focus on investigating temporal dynamics of combustion noise. Specifically, the dynamics of direct combustion noise, emitted from a premixed turbulent jet flame are studied for the first time with the help of recurrence analysis. The analysis showed that combustion noise is characterized by irregularly spaced periodic windows. Comparisons are carried out to bring out differences in the time trace of acoustic pressure for different equivalence ratios.

Characterizing the signature of flame flashback precursor through recurrence analysis

In this paper, it is shown that prior to flashback, small dynamical changes appear in the system. These changes appear as a drift in the recurrence plots and are found to be associated with a gradual increase in the determinism and recurrence rate. Thus, this study indicates that precursors to flame flashback exist and can be detected in the multidimensional phase space reconstructed from pressure measurements acquired during flashback. This observation could have broad academic as well as industrial implications.

Swirl Flame Response to Simultaneous Axial and Transverse Velocity Fluctuations

In this experimental study we investigate the impact of transverse acoustic velocity fluctuations on the flame transfer function in response to axial velocity fluctuations. A generic swirl flame is exposed to transverse acoustic velocities of varying amplitude and relative phase simultaneously with axial acoustic forcing. Results obtained indicate that transverse velocity affects flame response, and both the magnitude of transverse velocity and its phase with respect to axial forcing are important factors. In addition to this key results, considerations for experimental investigations dealing with transverse acoustic forcing have been discussed.Copyright