Vishnu R. Unni

Experimental investigation of bifurcations in a thermoacoustic engine

In 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.

Recurrence networks to study dynamical transitions in a turbulent combustor

Thermoacoustic instability and lean blowout are the major challenges faced when a gas turbine combustor is operated under fuel lean conditions. The dynamics of thermoacoustic system is the result of complex nonlinear interactions between the subsystems-turbulent reactive flow and the acoustic field of the combustor. In order to study the transitions between the dynamical regimes in such a complex system, the time series corresponding to one of the dynamic variables is transformed to an ε-recurrence network. The topology of the recurrence network resembles the structure of the attractor representing the dynamics of the system. The transitions in the thermoacoustic system are then captured as the variation in the topological characteristics of the network. We show the presence of power law degree distribution in the recurrence networks constructed from time series acquired during the occurrence of combustion noise and during the low amplitude aperiodic oscillations prior to lean blowout. We also show the absence of power law degree distribution in the recurrence networks constructed from time series acquired during the occurrence of thermoacoustic instability and during the occurrence of intermittency. We demonstrate that the measures derived from recurrence network can be used as tools to capture the transitions in the turbulent combustor and also as early warning measures for predicting impending thermoacoustic instability and blowout.

Online detection of impending instability in a combustion system using tools from symbolic time series analysis

In 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.

Multifractal characterization of combustion dynamics

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.

On the emergence of critical regions at the onset of thermoacoustic instability in a turbulent combustor

We use complex network theory to investigate the dynamical transition from stable operation to thermoacoustic instability via intermittency in a turbulent combustor with a bluff body stabilized flame. A spatial network is constructed, representing each of these three dynamical regimes of combustor operation, based on the correlation between time series of local velocity obtained from particle image velocimetry. Network centrality measures enable us to identify critical regions of the flow field during combustion noise, intermittency, and thermoacoustic instability. We find that during combustion noise, the bluff body wake turns out to be the critical region that determines the dynamics of the combustor. As the turbulent combustor transitions to thermoacoustic instability, during intermittency, the wake of the bluff body loses its significance in determining the flow dynamics and the region on top of the bluff body emerges as the most critical region in determining the flow dynamics during thermoacoustic instability. The knowledge about this critical region of the reactive flow field can help us devise optimal control strategies to evade thermoacoustic instability.

Predicting the Amplitude of Limit-Cycle Oscillations in Thermoacoustic Systems with Vortex Shedding

Thermoacoustic instability is a plaguing problem encountered in many combustion systems. The large-amplitude acoustic oscillations, which are a noted aspect of this instability, can have a detrimen...

Suppression of thermoacoustic instability in a swirl-stabilized combustor by inducing blockage in the inlet flow stream

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)q̇(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

Performance optimization of tunable standing wave thermoacoustic engine by varying the stack parameters and resonator length: An experimental study

Thermoacoustic heat engine (TAHE) converts thermal power (heat) into acoustic power. TAHE has been gaining significant interest because of its non-fuel specific, low cost and high reliability (due to reduced moving parts) compared to conventional IC engines. The performance of TAHE depends upon the various parameters such as stack position, stack length and resonator length. Previously, we built a fixed TAHE which converts heat energy to electrical energy with an efficiency of 2 %. However, the performance of the fixed engine was not fully optimized. To investigate further, another novel tunable TAHE has been built with the goal of optimizing the efficiency by tuning three critical parameters, namely stack position, stack length and resonator length. This paper shows the influence of stack parameters (stack position and stack length) and resonator length on the performance of the thermoacoustic heat engine. The performance is measured in terms of the pressure amplitude generated inside the TAHE using air as the working fluid. It is observed that the stacks position considerably affects the performance. Further, from experiments, it is observed that the maximum acoustic power is generated when the stack is positioned closer to a pressure antinode.