Shigeru Tachibana

A numerical analysis of the structure of a turbulent hydrogen jet lifted flame

This paper presents a direct numerical simulation (DNS) study of the flame structure of a turbulenthydrogen jet lifted flame. The diameter of the hydrogen injector is 2 mm, and the injection velocity is 680 m/s. The time-dependent three-dimensional simulation was made with full chemical kinetics and rigorous transport properties. More than 22 million grid points were used. The numerical analysis, in terms of the normalized flame index, has made clear that the lifted flame is not a single flame, but a complex flame consisting of three flame elements: (1) a stable laminar leading-edge flame, (2) a conical inner vigorous turbulent premixed flame, and (3) a number of floating diffusion flame islands, surrounding the inner premixed flame. The stable laminar leading-edge flame of ring shape is stabilized outside the turbulent jet and has a triple flamelike structure with the normalized flame index around unity, indicating that the incoming flow almost balances with the laminar burning velocity. The floating flame islands are produced by turbulent behavior and local extinction of the inner premixed flame. The detached gas volume flows downstream, continuing to burn by the molecular diffusion of oxidizers. The inner rich premixed flame is strongly turbulent by the instability of the hydrogen jet at the tip. The flame is strongly stabilized by the leading-edge flame, and the heat release layer of the flame is deviated from the hydrogen consumption layer, indicating that the turbulence modifies the inner flame structure. The respective flame elements have their own complicated three-dimensional structure, and further studies are required to understand in detail the structure and stability of the lifted flame. The present study has revealed that this kind of DNS study is very useful to investigate various very complicated flame structures, such as the lifted flame.

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.

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.

Intense Tangential Pressure Oscillations Inside a Cylindrical Chamber

Intense tangential pressure oscillations due to oscillatory combustion are generated experimentally inside a cylindrical chamber and are analyzed to understand their characteristic features. To generate the tangential mode, a coaxial injector is installed offcenter on the closed side of the chamber. Hydrogen and nitrogen-diluted oxygen are used as working gases under atmospheric conditions. The features of the side-wall pressure in the first tangential mode (1T mode) at high amplitude, whose amplitude is over 30% of the chamber pressure, differ from those of the side-wall pressure in the 1Tmode at low amplitude in the followingmanner: 1) The positive half-wave of the pressure oscillation has twin sharp peaks. 2) The amplitude of the positive half-wave (zero to positive peak) of the pressure oscillations is larger than that of the negative half-wave (negative peak to zero). The acoustical features of the signals are reproduced and investigated by conducting an analysis and anumerical simulation under similar configurations. It is found that the characteristic feature of the intense signal is originated in the nonlinearity due to the large amplitude of the pressure oscillation.

On acoustic damping of a cylindrical chamber in resonant modes

Acoustic damping of a cylindrical chamber with open and closed ends in resonant modes is analytically and numerically investigated to understand the low damping characteristic of the chamber without chocked nozzle. First, on the basis of the analytic solution of resonant acoustic modes inside a cylinder, the damping by radiation from the open end is calculated analytically using simple acoustic source modeling for velocity fluctuation. The effect of viscosity is also considered as an attenuation mechanism. The values of acoustic damping calculated for the first longitudinal and tangential modes are in good agreement with the corresponding values obtained using numerical simulation. The damping is also investigated for a configuration of the chamber with an injector installed off-center. Finally, we numerically and semi-analytically investigate the acoustic damping for a configuration that includes a hot-gas injection. The obtained mode is found to be a spinning tangential mode and the radiated wave also has a spinning feature. The damping for the spinning tangential mode is found to be larger than that for the symmetric dipole-like radiation under a uniform standing condition, but much smaller than the chamber with a chocked nozzle. Therefore, the chamber with an open end has the low damping characteristic suitable for intentionally generating oscillatory combustion.

Theoretical and Numerical Estimation of Acoustic Damping of a Model Combustion Chamber

Acoustic damping of a cylindrical chamber with open and closed ends is analytically and numerically investigated. In the analysis, based on the analytic solution of resonant acoustic modes inside cylinder, the damping from the open end is calculated by a simple acoustic source modeling for velocity fluctuation. The effect of viscosity is also considered as an attenuation mechanism. Acoustic damping calculated for first longitudinal and tangential modes are in good agreement with that obtained by numerical simulation. When a longitudinal mode exists, the directivity of radiation becomes like monopole and the damping is large. On the other hand, when a tangential mode exists, it is found that the dipole like directivity is obtained and the damping is small compared to that for the longitudinal mode. A configuration of the chamber and an injector installed off-centered is also investigated. Under non-resonant condition between injector and chamber acoustics, it is found that the acoustic radiation from the open end of the injector is negligible but the viscous effect becomes important. Finally we investigate the acoustic damping with hot gas injection numerically and semi-analytically. The obtained mode is found to be a spinning tangential mode due to the asymmetry of the mean flow field. The radiated wave has also a spinning feature and the damping is found to be much larger than that for symmetric dipole like radiation under uniform condition. The simple acoustic radiation modeling applied for the analysis also works well for this general non-uniform mean flow condition.

Active control of combustion oscillations in a lean premixed gas-turbine combustor

Active control of combustion oscillations occurring in a methane-air lean premixed model combustor is accomplished by the method of secondary fuel injection. The main flame is sustained by an axial vane swirler. The central part of the swirler is endowed a function as the secondary fuel injector. The fuel jets from the injector enhance the flame stability by producing rich stable flames in the region of the flame base. Open-loop controls by secondary fuel injection with constant flow rates have been conducted on a naturally unstable condition. The results show sensitivity to the injection amount. It indicates that the flame base is very sensitive to the additional fuel distribution. A similar discussion is made on NOx emission also. Finally, a closed-loop control has been performed by implementing the mixed H2/H∞ controller. An obvious effect of the closed-loop control on the suppression of pressure oscillations is found without loosing an advantage for low NOx emissions.

Detection of frequency-mode-shift during thermoacoustic combustion oscillations in a staged aircraft engine model combustor

We conduct an experimental study using time series analysis based on symbolic dynamics to detect a precursor of frequency-mode-shift during thermoacoustic combustion oscillations in a staged aircraft engine model combustor. With increasing amount of the main fuel, a significant shift in the dominant frequency-mode occurs in noisy periodic dynamics, leading to a notable increase in oscillation amplitudes. The sustainment of noisy periodic dynamics during thermoacoustic combustion oscillations is clearly shown by the multiscale complexity-entropy causality plane in terms of statistical complexity. A modified version of the permutation entropy allows us to detect a precursor of the frequency-mode-shift before the amplification of pressure fluctuations.

Effects of Swirl Combination and Mixing Tube Geometry on Combustion Instabilities in a Premixed Combustor

In this paper, flow fields inside a premixed combustor have been investigated by CFD analysis and PIV measurement in a preheating, non-reacting condition. Four types of premixer are examined. The design of the premixer is determined by the combination of swirlers and mixing tubes. There are two variations of triple-concentric swirlers and three variations of mixing tubes. Comparisons are made among mean velocity distributions derived from CFD and PIV. PDF analysis is performed on the data from PIV to discuss the possibility of the occurrence of flashback. Combustion rig tests have been carried out also on similar condition to see combustion instabilities depending on the choice of premixers and operating conditions. Flame is directly observed from crystal windows placed on the side and downstream of the combustion chamber. A glass rod is installed on the wall of the mixing tube so as to see light emissions inside the tube, i.e. evidence of flashback. Pressure fluctuations at the combustor liner are measured in one position. The spectra of pressure fluctuations are computed to look at the possibility of combustion oscillations. Discussions are made on the relation between the global flame structure and pressure modes. Finally, proper premixer configurations to prevent combustion instabilities are proposed.Copyright