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Raman I Sujith
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Raman I Sujith
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Raman I Sujith
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Sujith, Raman I.
Sujith, Raman
Sujith, R.
Sujith, R. I.
Sujith, Ri
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21 results
Now showing 1 - 10 of 21
- PublicationOnset of thermoacoustic instability in turbulent combustors: An emergence of synchronized periodicity through formation of chimera-like states(25-01-2017)
;Mondal, Sirshendu ;Unni, Vishnu R.Thermoacoustic systems with a turbulent reactive flow, prevalent in the fields of power and propulsion, are highly susceptible to oscillatory instabilities. Recent studies showed that such systems transition from combustion noise to thermoacoustic instability through a dynamical state known as intermittency, where bursts of large-amplitude periodic oscillations appear in a near-random fashion in between regions of low-amplitude aperiodic fluctuations. However, as these analyses were in the temporal domain, this transition remains still unexplored spatiotemporally. Here, we present the spatiotemporal dynamics during the transition from combustion noise to limit cycle oscillations in a turbulent bluff-body stabilized combustor. To that end, we acquire the pressure oscillations and the field of heat release rate oscillations through high-speed chemiluminescence images of the reaction zone. With a view to get an insight into this complex dynamics, we compute the instantaneous phases between acoustic pressure and local heat release rate oscillations. We observe that the aperiodic oscillations during combustion noise are phase asynchronous, while the large-amplitude periodic oscillations seen during thermoacoustic instability are phase synchronous. We find something interesting during intermittency: patches of synchronized periodic oscillations and desynchronized aperiodic oscillations coexist in the reaction zone. In other words, the emergence of order from disorder happens through a dynamical state wherein regions of order and disorder coexist, resembling a chimera state. Generally, mutually coupled chaotic oscillators synchronize but retain their dynamical nature; the same is true for coupled periodic oscillators. In contrast, during intermittency, we find that patches of desynchronized aperiodic oscillations turn into patches of synchronized periodic oscillations and vice versa. Therefore, the dynamics of local heat release rate oscillations change from aperiodic to periodic as they synchronize intermittently. The temporal variations in global synchrony, estimated through the Kuramoto order parameter, echoes the breathing nature of a chimera state. - PublicationInterplay between random fluctuations and rate dependent phenomena at slow passage to limit-cycle oscillations in a bistable thermoacoustic system(01-03-2019)
;Unni, Vishnu R. ;Gopalakrishnan, E. A. ;Syamkumar, K. S.; ;Surovyatkina, ElenaKurths, JürgenWe study the impact of noise on the rate dependent transitions in a noisy bistable oscillator using a thermoacoustic system as an example. As the parameter - the heater power - is increased in a quasi-steady manner, beyond a critical value, the thermoacoustic system undergoes a subcritical Hopf bifurcation and exhibits periodic oscillations. We observe that the transition to this oscillatory state is often delayed when the control parameter is varied as a function of time. However, the presence of inherent noise in the system introduces high variability in the characteristics of this critical transition. As a result, if the value of the system variable - the acoustic pressure - approaches the noise floor before the system crosses the unstable manifold, the effect of rate on the critical transition becomes irrelevant in determining the transition characteristics, and the system undergoes a noise-induced tipping to limit-cycle oscillations. The presence of noise-induced tipping makes it difficult to identify the stability regimes in such systems by using stability maps for the corresponding deterministic system. - PublicationPredicting the amplitude of limit cycle oscillations in thermoacoustic systems with vortex shedding(01-01-2018)
;Seshadri, Akshay ;Induja, P. ;Unni, Vishnu R.Thermoacoustic instability is a plaguing problem encountered in many combustion systems. The large amplitude acoustic oscillations, a noted aspect of this instability, can have a detrimental effect on the performance of the system, and sometimes even cause lasting damage to the system components. The aim of this study is to estimate the amplitude of the limit cycle oscillations observed during thermoacoustic instability corresponding to the axial acoustic modes using pressure measurements acquired during stable combustion. Firstly, an equation is derived which describes the slow-varying amplitude of oscillations in certain reduced-order models of combustion systems involving vortex shedding. Subsequently, a procedure is detailed, wherein this equation is used in conjunction with the measured pressure time series and some information about the system to predict the instability amplitude. The estimation capability of this technique is then tested using acoustic pressure data obtained from laboratory scale bluff-body and swirl stabilized combustors. It is observed that the estimated amplitudes are in good agreement with actual values. - PublicationPattern formation during transition from combustion noise to thermoacoustic instability via intermittency(25-08-2018)
;George, Nitin B. ;Unni, Vishnu R. ;Raghunathan, ManikandanGas turbine engines are prone to the phenomenon of thermoacoustic instability, which is highly detrimental to their components. Recently, in turbulent combustors, it was observed that the transition to thermoacoustic instability occurs through an intermediate state, known as intermittency, where the system exhibits epochs of ordered behaviour, randomly appearing amidst disordered dynamics. We investigate the onset of intermittency and the ensuing self-organization in the reactive flow field, which, under certain conditions, could result in the transition to thermoacoustic instability. We characterize this transition from a state of disordered and incoherent dynamics to a state of ordered and coherent dynamics as pattern formation in the turbulent combustor, utilizing high-speed flame images representing the distribution of the local heat release rate fluctuations, flow field measurements (two-dimensional particle image velocimetry), unsteady pressure and global heat release rate signals. Separately, through planar Mie scattering images using oil droplets, the collective behaviour of small scale vortices interacting and resulting in the emergence of large scale coherent structures is illustrated. We show the emergence of spatial patterns using statistical tools used to study transitions in other pattern forming systems. In this paper, we propose that the intertwined and highly intricate interactions between the wide spatio-temporal scales in the flame, the flow and the acoustics are through pattern formation. - PublicationOnline detection of impending instability in a combustion system using tools from symbolic time series analysis(01-09-2015)
;Unni, Vishnu R. ;Mukhopadhyay, AchintyaIn this paper, we introduce a novel technique (anomaly detection) for the online detection of impending instability in a combustion system based on symbolic time series analysis. The experimental results presented in this paper illustrate the application of anomaly detection to a combustor in which the flame is stabilized either by a bluff body or by a swirler. The detection unit works on the principle that in the transition region from combustion noise to thermoacoustic instability, combustion systems exhibit peculiar dynamics which results in the formation of specific patterns in the time series. Further, tools from symbolic time series analysis is used to recognize these patterns and then define an anomaly measure indicative of the proximity of system to regimes of thermoacoustic instability. - PublicationExperimental investigation of bifurcations in a thermoacoustic engine(01-06-2015)
;Unni, Vishnu R. ;Yogesh Prasaad, M. S. ;Ravi, N. T. ;Iqbal, S. Md ;Pesala, BalaIn this study, variation in the characteristics of the pressure oscillations in a thermoacoustic engine is explored as the input heat flux is varied. A bifurcation diagram is plotted to study the variation in the qualitative behavior of the acoustic oscillations as the input heat flux changes. At a critical input heat flux (60 Watt), the engine begins to produce acoustic oscillations in its fundamental longitudinal mode. As the input heat flux is increased, incommensurate frequencies appear in the power spectrum. The simultaneous presence of incommensurate frequencies results in quasiperiodic oscillations. On further increase of heat flux, the fundamental mode disappears and second mode oscillations are observed. These bifurcations in the characteristics of the pressure oscillations are the result of nonlinear interaction between multiple modes present in the thermoacoustic engine. Hysteresis in the bifurcation diagram suggests that the bifurcation is subcritical. Further, the qualitative analysis of different dynamic regimes is performed using nonlinear time series analysis. The physical reason for the observed nonlinear behavior is discussed. Suggestions to avert the variations in qualitative behavior of the pressure oscillations in thermoacoustic engines are also provided. - PublicationMultifractal characteristics of combustion dynamics(01-01-2015)
;Unni, Vishnu R.Traditionally, blowout is described as loss of static stability of a combustion system whereas thermoacoustic instability is seen as loss of dynamic stability of the system. The above description follows from the analysis of the stability of the flame. At blowout, the system transitions from a stable reacting state to a no-reaction state indicating the loss of static stability of the reaction. At instability, the flame is dynamically unstable and the reaction rate exhibits periodic oscillatory behavior. However, this simple description of stability margin is inadequate since combustors exhibit various nonlinear behaviors at the transition regimes for either phenomenon. Recently, it was shown that combustion noise, the ‘stable regime’ according to the concepts of stability, is by itself dynamically complex and exhibits multifractal characteristics. Considering this, researchers have already described the onset of combustion instability as loss of multifractality. In this work we will provide a multifractal description for lean blowout in combustors with turbulent flame, thereby bringing in a common framework to describe both thermoacoustic instability and lean blowout. Further, we will also introduce a method for predicting blowout based on the multifractal description of blowout. - PublicationFlame dynamics during intermittency in a turbulent combustor(01-01-2017)
;Unni, Vishnu R.The dynamic characteristics of the intermittency exhibited by turbulent combustors using LPG as fuel before and after the occurrence of thermoacoustic instability were compared. The unsteady pressure fluctuations analysis suggested that the intermittency before and after thermoacoustic instability are of type II. The flame dynamics during either state were also compared through a high speed Mie scattering images acquired simultaneously with unsteady pressure. During both regimes of intermittency the flame switches between an oscillation where the flame oscillates in a periodic manner due to the inherent turbulent fluctuations and an oscillation where the flame exhibits periodic roll-up as a consequence of the periodic vortex shedding at the dump plane. The flame dynamics during intermittency before and after thermoacoustic instability varied. The flame along the inner shear layer was stabilized by the stagnation point flow behind the bluff body during intermittency before the occurrence of thermoacoustic instability while the flame along the inner shear layer was stabilized by the recirculation zone created by the bluff body during intermittency after the occurrence of thermoacoustic instability. - PublicationBursting and mixed mode oscillations during the transition to limit cycle oscillations in a matrix burner(01-04-2019)
;Kasthuri, Praveen ;Unni, Vishnu R.We investigate the route to self-excited thermoacoustic instability in a laminar flow multiple flame matrix burner. With an increase in the equivalence ratio, the thermoacoustic system that is initially quiet (stable operation) transitions to limit cycle oscillations through two distinct dynamical states, namely, bursting oscillations and mixed mode oscillations. The acoustic pressure oscillations transition from quiescence to large amplitudes during bursting oscillations. Such high amplitude bursting oscillations that occur well ahead of the onset of limit cycle oscillations can potentially cause structural damage. The thermoacoustic system exhibits hysteresis. The transition to limit cycle oscillations is replicated in a phenomenological model containing slow-fast time scales. - PublicationCoupled interaction between unsteady flame dynamics and acoustic field in a turbulent combustor(01-11-2018)
;Godavarthi, Vedasri ;Pawar, Samadhan A. ;Unni, Vishnu R.; ;Marwan, NobertKurths, JürgenThermoacoustic instability is a result of the positive feedback between the acoustic pressure and the unsteady heat release rate fluctuations in a combustor. We apply the framework of the synchronization theory to study the coupled behavior of these oscillations during the transition to thermoacoustic instability in a turbulent bluff-body stabilized gas-fired combustor. Furthermore, we characterize this complex behavior using recurrence plots and recurrence networks. We mainly found that the correlation of probability of recurrence (C P R), the joint probability of recurrence (J P R), the determinism (D E T), and the recurrence rate (R R) of the joint recurrence matrix aid in detecting the synchronization transitions in this thermoacoustic system. We noticed that C P R and D E T can uncover the occurrence of phase synchronization state, whereas J P R and R R can be used as indices to identify the occurrence of generalized synchronization (GS) state in the system. We applied measures derived from joint and cross recurrence networks and observed that the joint recurrence network measures, transitivity ratio, and joint transitivity are useful to detect GS. Furthermore, we use the directional property of the network measure, namely, cross transitivity to analyze the type of coupling existing between the acoustic field (p ′) and the heat release rate (q ′) fluctuations. We discover a possible asymmetric bidirectional coupling between q ′ and p ′, wherein q ′ is observed to exert a stronger influence on p ′ than vice versa.
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