We present a novel and an efficient way to mitigate oscillatory instability in turbulent reactive flows. First, we construct weighted spatial correlation networks from the velocity field obtained from high-speed particle image velocimetry. Using network measures, we identify the optimal location for implementing passive control strategies. By injecting micro-jets at this optimal location, we are able to reduce the amplitude of the pressure oscillations to a value comparable to what is observed during the state of stable operation. This approach opens up new avenues to control oscillatory instabilities in turbulent flows.
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.
Tackling the problem of emissions is at the forefront of scientific research today. While industrial engines designed to operate in stable regimes produce emissions, attempts to operate them at "greener" conditions often fail due to thermoacoustic instability. During thermoacoustic instability, hazardous high amplitude periodic oscillations lead to failure of these engines in power plants, aircrafts and rockets. Yet, identifying the onset of thermoacoustic instability remains elusive due to spatial variability and the continuous evolution of spatiotemporal patterns in the reacting flow field. Here, we show experimental evidence of early manifestation of the onset of thermoacoustic instability at certain zones. Our findings allow us to identify a critical threshold that enables us to distinguish stable operating regimes from hazardous operations. This opens new perspectives for predicting the onset of thermoacoustic instability and could be a step forward to "greener" operations. The developed methodology is applicable for other systems exhibiting phase transitions.
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.
The objective was to study the effect of change in swirl intensities, S=0.4, 0.7 & 1 of the annular swirling flow on the exit flow field of an unconfined annular swirl burner operated at isothermal (only dry air) and reacting flow (premixed methane air mixture) conditions. Reynolds number at the burner's annular exit based on its hydraulic diameter (D) was kept constant at 4000. Exit flow field at isothermal conditions was measured using planar particle image Velocimetry rig and processed using commercial software. The percentage decay in the magnitude of peak value of axial velocity obtained from its radial profile at a height of 4D from the burner exit with the change in swirl intensity of 1, 0.7, 0.4 and 0 was 65%, 55%, 47.2% and 13.5%. The jet spreading angle was 6.5 o for S=0, 8.4 o for S=0.4, 9.8 for S= 0.7 and 14.2 o for S=1. Recirculation zone was observed only for S=0.7 and 1.0. The width of the recirculation zone was 3D (S=0.7) and 3.4D (S=1) respectively. The normalized reverse mass flow rates estimated were 0.027 for S = 0.7 and 0.058 for S = 1.0. The magnitude of turbulence intensities at wake shear layer was much higher than the jet shear layer due to the presence of recirculation zones for S= 0.7 and 1.0. The integral length scales calculated were varied in the range of 0.06D0.18D for all swirl intensities. Reaction front was identified by deconvoluting the time mean OH* chemiluminescence using Abel inversion method. The flame became shorter and wider with increase in swirl number which was in consonance with the observation of increase in size of recirculation flow in the isothermal flow. The equivalence ratios at which the lean blow out observed were 0.58, 0.6 and 0.62 for S=0.4, 0.7 and 1.
The tonal sound production during thermoacoustic instability is detrimental to the components of gas turbine and rocket engines. Identifying the root cause and controlling this oscillatory instability would enable manufacturers to save in costs of power outages and maintenance. An optimal method is to identify the structures in the flow-field that are critical to tonal sound production and perform control measures to disrupt those “critical structures”. Passive control experiments were performed by injecting a secondary micro-jet of air onto the identified regions with critical structures in the flow-field of a bluff-body stabilized, dump, turbulent combustor. Simultaneous measurements such as unsteady pressure, velocity, local and global heat release rate fluctuations are acquired in the regime of thermoacoustic instability before and after control action. The tonal sound production in this combustor is accompanied by a periodic flapping of the shear layer present in the region between the dump plane (backward-facing step) and the leading edge of the bluff-body. We obtain the trajectory of Lagrangian saddle points that dictate the flow and flame dynamics in the shear layer during thermoacoustic instability accurately by computing Lagrangian Coherent Structures. Upon injecting a secondary micro-jet with a mass flow rate of only 4% of the primary flow, nearly 90% suppression in the amplitude of pressure fluctuations are observed. The suppression thus results in sound pressure levels comparable to those obtained during stable operation of the combustor. Using Morlet wavelet transform, we see that the coherence in the dominant frequency of pressure and heat release rate oscillations during thermoacoustic instability is affected by secondary injection. The disruption of saddle point trajectories breaks the positive feedback loop between pressure and heat release rate fluctuations resulting in the observed break of coherence. Wavelet transform of global heat release rate shows a redistribution of energy content from the dominant instability frequency (acoustic time scale) to other time scales.
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.
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