Raman Sujith - Academia.edu (original) (raw)

Papers by Raman Sujith

Proceedings of the Combustion Institute, 2017

Abstract This paper proposes and compares two nonlinear time-domain models of self-excited thermo... more Abstract This paper proposes and compares two nonlinear time-domain models of self-excited thermoacoustic oscillations of laminar premixed flames. Both models are hybrid formulations, where the flame and its immediate vicinity are resolved with a reactive flow simulation, while the acoustic field is modeled with a low-order model that is coupled to the reactive flow simulation. Firstly, a flame model based on the fully compressible Navier–Stokes equations is investigated. In this case, the flame simulation is coupled to the low-order model via the characteristic wave amplitudes at the inlet boundary. Secondly, the flame is resolved with a low Mach number reactive flow simulation. In order to include two-way thermoacoustic feedback, this flame model is coupled with an acoustic network model via the global heat release rate and the fluctuation of the axial velocity at a reference position upstream of the flame. A bifurcation analysis using the plenum length as bifurcation parameter is conducted. Both models exhibit complex nonlinear oscillations and are in good agreement with each other. Therefore, we conclude that the coupling of a linear acoustic model and a nonlinear flame model via reference velocity and global heat release rate is sufficient to accurately capture thermoacoustic oscillations of the configuration investigated. This implies that the most important nonlinearities can be attributed to hydrodynamic effects and flame kinematics. Furthermore, the study corroborates that premixed flames respond predominantly to fluctuations of the upstream flow velocity.

We investigate combustion dynamics using complex networks. We observed a transition from low-ampl... more We investigate combustion dynamics using complex networks. We observed a transition from low-amplitude, aperiodic, combustion noise to high-amplitude, periodic, thermoacoustic instability via intermittency in the system dynamics of a turbulent lifted jet flame combustor while varying the location of the burner inside the combustor. As we change the burner location further past the condition of thermoacoustic instability, intermittency is again observed and finally, the flame blows out. The complex networks are derived from the time series data of acoustic pressure which represent the system dynamics using the visibility algorithm. We show that time series data of acoustic pressure during combustion noise exhibit a scale-free behavior. The scale-free nature of combustion dynamics disappears at the onset of thermoacoustic instability. We show that the system dynamics during thermoacoustic instability can be represented as a regular network. Further, we show that the transition from th...

2018 AIAA Aerospace Sciences Meeting, 2018

Swirl flows are often used for flame stabilization in gas turbine combustors. However, when these... more Swirl flows are often used for flame stabilization in gas turbine combustors. However, when these combustors are operated at lean fuel/air ratios, they are prone to thermoacoustic instability. In this study, we experimentally investigate the effect of distribution of the blockage in the inlet flow on the transition of the combustion dynamics from combustion noise to thermoacoustic instability. We acquire unsteady pressure fluctuations and heat release rate fields (CH * chemiluminescence) by capturing the flame images to investigate this transition in the thermoacoustic system. We utilize a turbulence generator with two different configurations to modify the inlet flow dynamics to achieve passive control of thermoacoustic instability. To that end, using flow restrictors, we induce blockage in the inlet flow upstream of the swirler, perpendicular to the bulk flow direction. We observe that in one case, there is a reduction in the amplitude of the periodic oscillations while in the other case, there is a suppression of thermoacoustic instability. In the former case, the blockage in the inlet flow stream is more distributed, while in the latter, the same degree of blockage is clustered into one region of the inlet flow stream. The field of the local instantaneous acoustic energy production (p ′ (t)q ′ (x, y, t)) shows the presence of coherence during the occurrence of thermoacoustic instability for the experiments without any blockage. This emerging coherence is disrupted with the inclusion of the blockage. In the case with clustered blockage, the coherence is suppressed significantly, while for the case with the distributed blockage, it is reduced

52nd AIAA/SAE/ASEE Joint Propulsion Conference, 2016

Volume 4B: Combustion, Fuels, and Emissions, 2019

Experiments are performed in a partially-premixed bluff-body stabilized turbulent combustor by va... more Experiments are performed in a partially-premixed bluff-body stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff-body.

Chaos: An Interdisciplinary Journal of Nonlinear Science, 2021

Many fluid dynamic systems exhibit undesirable oscillatory instabilities due to positive feedback... more Many fluid dynamic systems exhibit undesirable oscillatory instabilities due to positive feedback between fluctuations in their different subsystems. Thermoacoustic instability, aeroacoustic instability, and aeroelastic instability are some examples. When the fluid flow in the system is turbulent, the approach to such oscillatory instabilities occurs through a universal route characterized by a dynamical regime known as intermittency. In this paper, we extract the peculiar pattern of phase space attractors during the regime of intermittency by constructing recurrence networks corresponding to the phase space topology. We further train a convolutional neural network to classify the periodic and aperiodic structures in the recurrence networks and define a measure that indicates the proximity of the dynamical state to the onset of oscillatory instability. We show that this measure can predict the onset of oscillatory instabilities in three different fluid dynamic systems governed by different physical phenomena.

Journal of Engineering for Gas Turbines and Power, 2019

Experiments are performed in a partially premixed bluff body-stabilized turbulent combustor by va... more Experiments are performed in a partially premixed bluff body-stabilized turbulent combustor by varying the mean flow velocity. Simultaneous measurements obtained for unsteady pressure, velocity, and heat release rate are used to investigate the dynamic regimes of intermittency (10.1 m/s) and thermoacoustic instability (12.3 m/s). Using wavelet analysis, we show that during intermittency, modulation of heat release rate occurring at the acoustic frequency fa by the heat release rate occurring at the hydrodynamic frequency fh results in epochs of heat release rate fluctuations where the heat release rate is phase locked with the acoustic pressure. We also show that the flame position during intermittency and thermoacoustic instability are essentially dictated by saddle point dynamics in the dump plane and the leading edge of the bluff body.

International Journal of Spray and Combustion Dynamics, 2018

An experimental study on a turbulent, swirl-stabilized backward facing step combustor is conducte... more An experimental study on a turbulent, swirl-stabilized backward facing step combustor is conducted to understand the spatiotemporal dynamics during the transition from combustion noise to thermoacoustic instability. By using a turbulence generator, we investigate the change in the spatiotemporal dynamics during this transition for added turbulence intensities. High-speed CH* images of the flame (representative of the field of local heat release rate fluctuations ([Formula: see text]( x, y, t))) and simultaneous unsteady pressure fluctuations ([Formula: see text]( t)) are acquired for different equivalence ratios. In the study, without the turbulence generator, as the equivalence ratio is reduced from near stoichiometric values, we observe an emergence of coherence in the spatial dynamics during the occurrence of intermittency, enroute to thermoacoustic instability. As the turbulence intensity is increased using the turbulence generator, we find that there is an advanced onset of the...

International Journal of Aeroacoustics, 2016

In this study, we attempt to provide a repertoire of measures to forewarn the onset of impending ... more In this study, we attempt to provide a repertoire of measures to forewarn the onset of impending flow-induced mechanical oscillations via online health monitoring. To illustrate the principles, the flow of air through a pipe terminated by a circular orifice plate is investigated at various flow velocities using a suitably placed pressure transducer. It is observed that the regimes corresponding to the production of a tone is presaged by operating conditions that display temporarily intermittent bursts of periodic pressure oscillations that emerge from a background of lower-amplitude aperiodic fluctuations. The various model-free measures prescribed in this paper serve as efficient precursors by characterizing these intermittent states, which can potentially arise in aeroacoustic systems when the flow is highly unsteady or turbulent.

Combustion Theory and Modelling, 2016

Recently, there has been a growing interest in understanding and characterising intermittent burs... more Recently, there has been a growing interest in understanding and characterising intermittent burst oscillations that presage the onset of combustion instability. We construct a deterministic model to capture this intermittency route to instability in a bluff-body stabilised combustor by coupling the equations governing vortex shedding and the acoustic wave propagation in a confinement. A feedback mechanism is developed wherein the sound generated due to unsteady combustion affects the vortex shedding. This feedback leads to a variation in the time of impingement of the vortices with the bluff body causing the system to exhibit chaos, intermittency, and limit cycle oscillations. Experimental validation of the model is provided using various precursor measures that quantify the observed intermittent states.

Journal of Fluids and Structures, 2016

The aeroelastic response of a NACA 0012 airfoil in the flow regimes prior to flutter is investiga... more The aeroelastic response of a NACA 0012 airfoil in the flow regimes prior to flutter is investigated in a wind tunnel. We observe intermittent bursts of periodic oscillations in the pitch and plunge response, that appear in an irregular manner from a background of relatively lower amplitude aperiodic fluctuations. As the flow speed is increased, the intermittent bursts last longer in time until eventually transitioning to a fully developed periodic response, indicating the onset of flutter. The repeating patterns in the measured response are visualized using recurrence plots. We show that statistics of the recurrence states extracted from these plots can be used to develop model-free precursors that forewarn an impending transition to flutter, well before its onset.

44th AIAA Aerospace Sciences Meeting and Exhibit, 2006

Volume 4B: Combustion, Fuels and Emissions, 2014

The dynamic transitions preceding combustion instability and lean blowout were investigated exper... more The dynamic transitions preceding combustion instability and lean blowout were investigated experimentally in a laboratory scale turbulent combustor by systematically varying the flow Reynolds number. We observe that the onset of combustion-driven oscillations is always presaged by intermittent bursts of high-amplitude periodic oscillations that appear in a near random fashion amidst regions of aperiodic, low-amplitude fluctuations. The onset of high-amplitude, combustion-driven oscillations in turbulent combustors thus corresponds to a transition in dynamics from chaos to limit cycle oscillations through a state characterized as intermittency in dynamical systems theory. These excursions to periodic oscillations become last longer in time as operating conditions approach instability and finally the system transitions completely into periodic oscillations. Such intermittent oscillations emerge through the establishment of homoclinic orbits in the phase space of the global system whi...

Journal of Sound and Vibration, 1995

In this paper an approach is presented for obtaining exact analytical solutions for sound propaga... more In this paper an approach is presented for obtaining exact analytical solutions for sound propagation in ducts with an axial mean temperature gradient. The one-dimensional wave equation for ducts with an axial mean temperature gradient is derived. The analysis neglects the effects of mean flow, and therefore the solutions obtained are valid only for mean Mach numbers that are less than 0•1. The derived wave equation is then transformed to the mean temperature space. It is shown that by use of suitable transformations, the derived wave equation can be reduced to analytically solvable equations (e.g., Bessel's differential equation). The applicability of the developed technique is demonstrated by obtaining a solution for ducts with a linear temperature profile. This solution is then applied to investigate the dependence of sound propagation in a quarter wave tube upon the mean temperature profile. Furthermore, the developed analytical solution is used to extend the classical impedance tube technique to the determination of admittances of high temperature systems (e.g., flames). The results obtained using the developed analytical solution are in excellent agreement with experimental as well as numerical results. Analytical solutions were also obtained for a duct with an exponential temperature profile and also for a temperature profile that corresponds to a constant convective heat transfer coefficient at the wall.

Combustion Science and Technology, 2008

There have been conflicting opinions in literature, about the consequences of ignoring the linear... more There have been conflicting opinions in literature, about the consequences of ignoring the linear coupling terms in finite-dimensional thermoacoustic models. We try to clarify this issue in this paper. We study consequences of ignoring the linear coupling terms on the non-normal ...

Annual Review of Fluid Mechanics, 2018

Nine decades of rocket engine and gas turbine development have shown that thermoacoustic oscillat... more Nine decades of rocket engine and gas turbine development have shown that thermoacoustic oscillations are difficult to predict but can usually be eliminated with relatively small ad hoc design changes. These changes can, however, be ruinously expensive to devise. This review explains why linear and nonlinear thermoacoustic behavior is so sensitive to parameters such as operating point, fuel composition, and injector geometry. It shows how nonperiodic behavior arises in experiments and simulations and discusses how fluctuations in thermoacoustic systems with turbulent reacting flow, which are usually filtered or averaged out as noise, can reveal useful information. Finally, it proposes tools to exploit this sensitivity in the future: adjoint-based sensitivity analysis to optimize passive control designs and complex systems theory to warn of impending thermoacoustic oscillations and to identify the most sensitive elements of a thermoacoustic system.

Journal of Fluid Mechanics, 2013

Recent investigations by L. Magri and M. P. Juniper have brought to light certain errors in the p... more Recent investigations by L. Magri and M. P. Juniper have brought to light certain errors in the paper by Balasubramanian and Sujith (Journal of Fluid Mechanics, vol. 594, 2008, pp. 29–57). The revision is available in a separate note (Journal of Fluid Mechanics, vol. 733, 2013, pp. 681–683), which shows that these ducted diffusion flames exhibit a transient growth of order 1 to 10. These results are in contrast to those presented by Balasubramanian & Sujith (2008), who found the transient growth to be of order 1051{0}^{5} 105 to 1071{0}^{7} 107.

Combustion Science and Technology, 2014

Combustion instability is a nonlinear process with interaction between combustion and acoustics. ... more Combustion instability is a nonlinear process with interaction between combustion and acoustics. The nonlinearity in the combustion process is observed in the flame dynamics. In our study of a ducted laminar premixed V-flame, we varied the distance between the flame anchor and the duct exit. We observed that the thermoacoustic system bifurcates from a stable state to a frequency locked state, followed by quasi-periodicity, period 3 oscillations, and finally chaotic oscillations. During the occurrence of these dynamical states, the role of flame dynamics is investigated using high speed imaging. We observed wrinkle formation, its propagation and growth along flame front, rollup of the flame, separation, and annihilation of flame elements during instability. From the present study it is found that each dynamical state is characterized by a particular sequence of flame behavior, highlighting the role of nonlinear flame dynamics in establishing the observed dynamical state.

International Journal of Spray and Combustion Dynamics, 2010

A bifurcation analysis of the dynamical behavior of a horizontal Rijke tube model is performed in... more A bifurcation analysis of the dynamical behavior of a horizontal Rijke tube model is performed in this paper. The method of numerical continuation is used to obtain the bifurcation plots, including the amplitude of the unstable limit cycles. Bifurcation plots for the variation of nondimensional heater power, damping coefficient and the heater location are obtained for different values of time lag in the system. Subcritical bifurcation was observed for variation of parameters and regions of global stability, global instability and bistability are characterized. Linear and nonlinear stability boundaries are obtained for the simultaneous variation of two parameters of the system. The validity of the small time lag assumption in the calculation of linear stability boundary has been shown to fail at typical values of time lag of system. Accurate calculation of the linear stability boundary in systems with explicit time delay models, must therefore, not assume a small time lag assumption....