Genichiro Kushida

Effects of gravity and ambient oxygen on a gas-phase ignition over a heated solid fuel

A numerical study was made on the time-dependent ignition process of a horizontally placed solid fuel heated by external radiation. As soon as the solid starts to be heated, a hot plume with combustible fuel is ejected into the oxygen-containing atmosphere to be mixed and to react leading to a spontaneous ignition. The effects of gravity and ambient oxygen concentration on the ignition behavior were studied. The numerical model is a two-dimensional axisymmetric configuration with time-dependent heat and mass transport process and one-step exothermic reaction for gas phase, and three-step degradative reactions for solid phase. An appropriate ignition criterion was introduced to define ignition delay time and position. It was found that an increase in the gravity tends to prevent the ignition by increasing heat loss from the hot fuel-gas plume, whereas an increase in the ambient oxygen concentration enhances the ignition by accelerating chemical reaction in the plume. The two distinct types of ignition were identified in gravity vs. ambient oxygen concentration plot; the first one occurs when the oxygen concentration is relatively high and is ignited at the tip of the plume with a short ignition delay time, while the second one occurs when the oxygen concentration is low and is ignited at the inside of the plume with a relatively long delay time. The former type of ignition was found to be controlled basically by 1-D heat and mass transport process, whereas the latter type is controlled by the 2-D process caused by buoyancy-induced flow.

Formation and decay processes of Ar/He microwave plasma jet at atmospheric gas pressure

Dynamic behaviors of gas/plasma mixture generated by microwave discharge jet at atmospheric pressure have been investigated in terms of the effects of working gas dynamics on the discharge characteristics, focusing on the following four points: formation of discharge jet at the ignition phase, growing speed of plasma plume in relation to gas flow rate, acceleration and deceleration of upward movement of plasma plume, and decay process of plasma column at turn-off of microwave power. Four stages are identified at the discharge ignition. The acceleration coming from buoyancy effect and the deceleration due to gas flow turbulence have been found. The decay speed of plasma column at a power shut-off is found to be influenced by the gas flow turbulence and the recombination rate.

Lewis Number Effect on Smoldering Combustion of a Thin Solid

This article presents a simple model to describe the smoldering combustion of a thin solid burning in a narrow gap. Smoldering front under such a condition is unstable because of a similar instability mechanism to gaseous premixed combustion and produces a fingering pattern. In the present model, the Lewis number, defined as the ratio of the solid thermal diffusivity to the oxygen diffusivity, is typically less than 0.1, while the Lewis number of gaseous premixed flame is generally close to unity. It was found that the propagation speed increases with a decrease in the Lewis number, the opposite trend to the one-dimensional propagation. The fingering pattern predicted for = 0.1 shows, similarly to experimental observations, repeated processes of the split of a smoldering front into two branches and the subsequent extinction of one of the split branches.

DYNAMIC BEHAVIOR OF BUOYANCY-INDUCED FLAME OSCILLATION UNDER SWIRLING FLOW BY A USE OF NONLINEAR TIME SERIES ANALYSIS IN COMBINATION WITH SURROGATE DATA METHOD

The authors experimentally investigated the dynamic behavior of buoyancy-induced flame oscillation under a swirling flow produced by rotating a cylindrical burner tube, focusing on the characterization of the complex dynamics in flame front by a use of nonlinear time series analysis. A Wayland method (R. Wayland et al., 1993), which quantifies the degree of parallelism of trajectories in phase space constructed from time series data of flame front fluctuations, is applied as a sophisticated nonlinear time series analysis in this work. To reveal whether or not the complex dynamics is deterministic chaos, a quantitative method for discussing the null hypothesis that the irregular components of the flame front fluctuations represent a stochastic process (i.e., a surrogate data method; T. Schreiber and A. Schmitz, 1996), is applied in this work. A sophisticated nonlinear time series analysis in combination with a surrogate data method, which has not been widely applied to the study of combustion phenomena, clearly demonstrates that the dynamic behavior undergoes a significant transition from periodic oscillation to low-dimensional deterministic chaos with increasing rotational Reynolds number.

Numerical Simulations of Methane Diffusion Flame with Burner Rotation

Numerical simulations have been conducted to study methane diffusion flame with burner rotation. The burner rotational speed is from 300 rpm to 1200 rpm; the burner diameter is 1 cm and 2 cm; the injection velocity is 10 cm/s and 15 cm/s. Quantitative data obtained includes the velocity profile, the temperature profile, and the frequencies of fluctuation. From the simulation data, the flame shape is obtained from the temperature data and reasonably agrees with the flame shape in the experiment. Because the flame is buoyancy dominant, the frequency scales to the square root of the burner’s diameter. Vortex flow, found around the burner cylinder and induced by the burner rotation, causes the flame to be unstable at the burner exit for rotational speed larger than 800 rpm. Rotational speed increase also reduces the pulsation frequency, but not as significant as a change in burner diameter. The length of the burner cylinder and the fluctuations of the flame alter the vortex propagation shapes and directions, as well as the frequencies of the flame. It is clear that the burner length is the medium for generating instability.

Fluid flow and heat transfer in plate-fin and tube heat exchanger. Analysis of fluid flow around square cylinder situated between parallel plates.

プレートフィンチューブ形熱交換器の基本的な構造モデルを、プレートフィンとしての平行平板の間にチューブとしての正方形柱を垂直に設置した流路と考え、その流動および熱伝達を解明する。本報では、レイノルズ数Re=200の場合の流動の解析を行い、速度ベクトル、流跡線、等圧線、壁面におけるせん断応力の分布およびはく離線を明示し、また流れの非定常性について考察を加え、流路内の流動機構について詳細な検討を行う。