Hiroshi Gotoda

Dynamic properties of combustion instability in a lean premixed gas-turbine combustor

We experimentally investigate the dynamic behavior of the combustion instability in a lean premixed gas-turbine combustor from the viewpoint of nonlinear dynamics. A nonlinear time series analysis in combination with a surrogate data method clearly reveals that as the equivalence ratio increases, the dynamic behavior of the combustion instability undergoes a significant transition from stochastic fluctuation to periodic oscillation through low-dimensional chaotic oscillation. We also show that a nonlinear forecasting method is useful for predicting the short-term dynamic behavior of the combustion instability in a lean premixed gas-turbine combustor, which has not been addressed in the fields of combustion science and physics.

Characterization of complexities in combustion instability in a lean premixed gas-turbine model combustor

We characterize complexities in combustion instability in a lean premixed gas-turbine model combustor by nonlinear time series analysis to evaluate permutation entropy, fractal dimensions, and short-term predictability. The dynamic behavior in combustion instability near lean blowout exhibits a self-affine structure and is ascribed to fractional Brownian motion. It undergoes chaos by the onset of combustion oscillations with slow amplitude modulation. Our results indicate that nonlinear time series analysis is capable of characterizing complexities in combustion instability close to lean blowout.

Transition from periodic to non-periodic motion of a bunsen-type premixed flame tip with burner rotation

Unsteady motions of a Bunsen-type premixed flame tip with burner rotation are experimentally investigatedfrom the viewpoint of nonlinear dynamics. The mean velocity from burner tube U is varied from 0.6 to 1.2 m/s, and the rotational speed of the burner tube N is varied from 0 to 2800 rpm. A rich methane/air mixture with the equivalence ratio of =1.43 is used. With the Lewis number Le larger than unity, an axisymmetric oscillating flame is formed between aconical flame and a plateau flame at U =0.6m/s and swirl number S =1.14, As U and N increase, but with S constant, the oscillating flame tip motion becomes unstable. This variation in the flame tip motion is shown qualitatively by drawing an attractor and evaluated quantitatively by estimating the correlation dimension. For U ≤0.8 m/s, the attractor is a limit cycle and the correlation dimension D c is estimated at about unity, indicating periodic motion. When U reaches 1.0 m/s, the trajectories of the attractor become rolled up slightly and D c approaches about 2, indicating quasi-periodic flame tip motion. With a further increase in U , the attractor becomes much more complicated and D c , is estimated as a non-integer value, indicating a deterministic chaos. These results indicate that the flame tip motion with the burner rotation under the condition of Le >1 varies from periodic to non-periodic (i.e., to chaotic). The present results also show that an analysis based on deterministic chaos theory, such as the correlation dimension, is valid for quantifying the motion of unsteady flames.

Short-term prediction of dynamical behavior of flame front instability induced by radiative heat loss

We apply nonlinear forecasting to the time series of the flame front instability induced by radiative heat loss to test for the short-term predictability and long-term unpredictability characteristic of deterministic chaos in flame front instability. Our results indicate that the flame front instability represents high-dimensional chaos generated via the period-doubling cascade process reported in our previous study [H. Gotoda, K. Michigami, K. Ikeda, and T. Miyano, Combust Theory Modell. 14, 479 (2010)], while its short-term behavior is predictable using a local nonlinear predictor based on the Sugihara-May method [H. Gotoda, H. Nikimoto, T. Miyano, and S. Tachibana, Chaos 20, 013124 (2011); G. Sugihara and R. M. May, Nature 344, 734 (1990)] as well as a generalized radial basis function network as a global nonlinear predictor. The feasibility of a new approach based on short-term prediction is also discussed in this work from the practical viewpoint of combustion systems.

Characterization of degeneration process in combustion instability based on dynamical systems theory.

We present a detailed study on the characterization of the degeneration process in combustion instability based on dynamical systems theory. We deal with combustion instability in a lean premixed-type gas-turbine model combustor, one of the fundamentally and practically important combustion systems. The dynamic behavior of combustion instability in close proximity to lean blowout is dominated by a stochastic process and transits to periodic oscillations created by thermoacoustic combustion oscillations via chaos with increasing equivalence ratio [Chaos 21, 013124 (2011); Chaos 22, 043128 (2012)]. Thermoacoustic combustion oscillations degenerate with a further increase in the equivalence ratio, and the dynamic behavior leads to chaotic fluctuations via quasiperiodic oscillations. The concept of dynamical systems theory presented here allows us to clarify the nonlinear characteristics hidden in complex combustion dynamics.

Dynamics of self-excited thermoacoustic instability in a combustion system: Pseudo-periodic and high-dimensional nature

We have examined the dynamics of self-excited thermoacoustic instability in a fundamentally and practically important gas-turbine model combustion system on the basis of complex network approaches. We have incorporated sophisticated complex networks consisting of cycle networks and phase space networks, neither of which has been considered in the areas of combustion physics and science. Pseudo-periodicity and high-dimensionality exist in the dynamics of thermoacoustic instability, including the possible presence of a clear power-law distribution and small-world-like nature.

Chaos of radiative heat-loss-induced flame front instability

We are intensively studying the chaos via the period-doubling bifurcation cascade in radiative heat-loss-induced flame front instability by analytical methods based on dynamical systems theory and complex networks. Significant changes in flame front dynamics in the chaotic region, which cannot be seen in the bifurcation diagrams, were successfully extracted from recurrence quantification analysis and nonlinear forecasting and from the network entropy. The temporal dynamics of the fuel concentration in the well-developed chaotic region is much more complicated than that of the flame front temperature. It exhibits self-affinity as a result of the scale-free structure in the constructed visibility graph.

Low-dimensional dynamical system for Rayleigh-Bénard convection subjected to magnetic field

We have numerically investigated the dynamical behavior of Rayleigh-Benard (RB) convection in an incompressible conducting fluid subjected to a magnetic field by solving a low-dimensional dynamical system. Its dynamical properties are quantified by nonlinear time series analysis based on chaos theory. The stretching and folding in the phase space for the chaos region (normalized Rayleigh number r = 28) and the intermittent chaos region (r = 166.1) of RB convection at a high magnetic Prandtl number of Pm = 10 become complex with increasing applied magnetic field, and the degeneration of chaos is induced by the limit of the strong magnetic field owing to the overwhelming Lorentz force compared with the buoyancy. The results obtained in this study show the importance of the magnetic Prandtl number to the dynamical behavior of RB convection subjected to a magnetic field.

Nonlinear Forecasting of the Generalized Kuramoto–Sivashinsky Equation

The emergence of pattern formation and chaotic dynamics is studied in the one-dimensional (1D) generalized Kuramoto–Sivashinsky (gKS) equation by means of a time-series analysis, in particular, a nonlinear forecasting method which is based on concepts from chaos theory and appropriate statistical methods. We analyze two types of temporal signals, a local one and a global one, finding in both cases that the dynamical state of the gKS solution undergoes a transition from high-dimensional chaos to periodic pulsed oscillations through low-dimensional deterministic chaos while increasing the control parameter of the system. Our results demonstrate that the proposed nonlinear forecasting methodology allows to elucidate the dynamics of the system in terms of its predictability properties.

Chaotic oscillation in diffusion flame induced by radiative heat loss

We numerically investigate the dynamic behavior of flame front instability in a diffusion flame caused by radiative heat loss from the viewpoint of nonlinear dynamics. As the Damköhler number increases at a high activation temperature, the dynamic behavior of the flame front undergoes a significant transition from a steady-state to high-dimensional deterministic chaos through the period-doubling cascade process known as the Feigenbaum transition. The existence of high-dimensional chaos in flame dynamics is clearly demonstrated using a sophisticated nonlinear time series analysis technique based on chaos theory.