The intrinsic instability of premixed flames due to hydrodynamic effects and diffusive-thermal effects is presented in this paper. Hydrodynamic effects caused by the thermal expansion through flame fronts have a destabilizing influence. Diffusive-thermal effects caused by the interaction between mass and thermal diffusion have a destabilizing (stabilizing) influence when the Lewis number is lower (higher) than unity. To obtain the dispersion relation, a sufficiently small disturbance is superimposed on a planar flame. The growth rate increases and the unstable range widens as the unburned-gas temperature becomes higher, which is because of the increase of the burning velocity of a planar flame. To obtain the cellular flame, a disturbance with the critical wavelength is superimposed. Owing to intrinsic instability, cellular fronts form, and cells move laterally at Lewis numbers lower than unity. The burning velocity of a cellular flame normalized by that of a planar flame decreases as the unburned-gas temperature becomes higher. This is because of the decrease of the temperature ratio of burned and unburned gases. In large space, the burning velocity of a cellular flame depends strongly on the size of space. Since long-wavelength components of disturbances play an important role in the flame behavior, the burning velocity increases as the space size becomes larger. Finally, we show cellular fronts of methane-air premixed flames on a flat burner and expanding spherical flames of hydrogen-air mixtures in a chamber.
Thermo-acoustic instability is induced by interaction between fluctuations of pressure and heat release rate to satisfy Rayleigh's criteria. Some important mechanisms to couple pressure fluctuation and heat release rate are introduced. Then, two fundamental coupling mechanisms, pressure coupling and velocity coupling, are investigated experimentally by using novel technique developed by the authors, laser irradiation method. In the research the transition process through a laminar flat flame, local deformation of the flame, generation of corrugated structure and transition to the turbulent motion is observed in detail by high-speed camera. Simultaneously, time dependent pressure measurement have been done to compare with flame structure change. The results suggested that even with flat flame, pressure fluctuation would occur by pressure coupling, but strong pressure fluctuation occurs with corrugated structure induced by acoustic vibration. The critical condition to start such corrugated structure have been discussed and it was pointed out that the generation of concave structure at the flame tip can be a trigger of strong acoustic vibration initiation.
Dynamical systems theory conveys striking and useful information for clarifying the hidden nature of irregular temporal fluctuations obtained in a wide spectrum of experiments in the fields of biological medicine, electronic information, mechanical engineering, and many other natural sciences. It extensively covers sphere ranging from the quantification of important nonlinear properties, which yields a physical description of the dynamical structure in phase space, to practical applications such as detection and control. This article presents the availability of dynamical systems theory involving three analytical methods: colored recurrence plots, local predictor, and translation error, to deal with nonlinear dynamics of combustion instability in a laboratory-scale gas-turbine model combustor.
Combustion instability has become a main technical challenge in the development of low-emission combustors of aero and land-based gas-turbine engines as it may cause catastrophic damages to the combustion chambers. Therefore, combustion stability is one of the major performance requirements of the current gas-turbine engines. In this article, two recent studies on gas-turbine combustion instabilities, one is of a simple model gas-fuel combustor and the other is of an aero-engine liquid-fuel combustor, are introduced. As a key aspect, the effects of temporal and spatial variations of equivalence ratio on the combustion instabilities are discussed.
The control of knock is necessary for further improvement of thermal efficiency of gasoline engines. Knock is considered as the pressure oscillation caused by autoignition in end gas before the completion of flame propagation. In order to control the knock, it is important to understand these successive phenomena well. This article shows authors' recent investigations of knock phenomena using a constant volume vessel. The constant volume vessel was used in order to clarify fundamentally how flame propagation and chemical reactions of the end gas contribute the knock phenomena. The influences of mixture temperature on flame propagation and knock phenomena and knocking process were described in this article.
The paper sheds light on a fundamental issue of knocking combustion, i.e., mechanisms of pressure wave generations and autoignition in end-gas region. The results from our recent numerical simulations, where a knocking combustion is modeled using a one-dimensional constant-volume reactor, are introduced and discussed with a survey of relevant previous studies. The importance of the negative temperature coefficient on the mechanisms of strong pressure wave generations and the location of autoignition position in end-gas region is clearly highlighted. The paper also presents a strategy for efficiently handing detailed chemical kinetics mechanisms in computational fluid dynamics simulations.
Currently, various energies such as fossil fuels and nuclear power are considerable expenditures. Resource depletion and environmental issues are taking place by activation of various problems and economic activity due to population growth in developing countries. However, in order for society to develop, energy, such as natural energy, is required have been studied for this purpose. We are grappling with new solid bioenergy development named Biocoke. This Biocoke has new solid fuel characteristics with high density and high compressive strength. Firstly, this article reported on new bio-solid fuel which has different density and observed its combustion phenomenon. Through the observation of combustion for bio-solid fuel with different density from 0.8∼1.4 g/cm3, we quantitatively derived ignition limit of biosolid fuel about the difference of density. Thus, we confirmed time continuing the flame and char combustion and observed the situation of spread of combustion into the inside of solid fuel. Finally, Kinki University launched the Great East Japan Earthquake Reconstruction Support Office in August 2012. Its two main themes are of the reconstruction assistance "From Zero to Plus" and rehabilitation support from the disaster of "To Zero from Minus". We suggested that Biocoke is stabilized for a long time from various data. Especially, we was carried to the dissolution test into water from Biocoke including of adhere radioactive Cs-147. Emission of radioactive material from Biocoke reduced until 25%.
Sub-chambers were used to initiate the detonation wave by injecting a flame-jet into the detonation tube. A configuration of the sub-chamber was divided into five types by changing the number of sub-chambers and the directions of flame injection. The detonation tube had a cross area of 50 × 50 mm, a length of 775 mm and equipped a window section for a visualization of the flowfield. The test gas was a stoichiometric premixed gas of hydrogen and oxygen and the initial pressure was constant as 80 kPa. The experimental results showed that the detonation transition distance was about 60% shorter than the case of spark ignition without flame-jet. Increasing the number of sub-chambers and colliding flame-jets emanating from counter positions resulted in decreasing the detonation transition distance. Schlieren photographs showed that the flame-jets promoted the formation of leading shock wave ahead of the turbulent flame in a short distance. Pressure measurements on the end wall of the detonation tube indicated that the shock or compression waves were reflected repeatedly in the cross-section, leading to a pressure wave and flame interaction. For the flame-jet initiation using the sub-chambers, generating turbulent flow in the premixed gas was of importance to promote the detonation transition.