Seishiro Fukutani

Estimation of the stability of droplet as nucleus of soot formation in hydrocarbon flames

The reaction path in the nucleation of soot formation in hydrocarbon flames has been studied based on the thermodynamic equilibrium. The possibility that hydrocarbons condense into liquid droplets even at temperatures above their boiling points has been investigated and the following theoretical explanations have been obtained. A system that contains charged molecules is more unstable energetically than one that contains no charged molecules. This electric energy decreases as the molecules in the vapor phase condense into droplets because the electric energy is inversely proportional to the charged particle diameter. The condensed droplet then grows spontaneously to a size which can be computed as a function of the vapor pressure, using Kelvins equation. The physical properties required to compute the diameter were estimated approximately for hydrocarbons up to circumanthracene, C 40 H 16 , The diameter of a droplet of hydrocarbon in equilibrium with its vapor was evaluated at various vapor pressures and temperatures. Droplets charged can be freed from evaporation, increase in size as polymerization reactions proceed, and grow to soot nuclei. It is ascertained experimentally that heavy hydrocarbons are less abundant than light hydrocarbons. The paths along which nucleation proceeds were proposed with due consideration to the above points.

Flame structure of an axisymmetric hydrogen-air diffusion flame

A detailed model including a full scheme of combustion reactions and the governing equations of fluid mechanics was designed for two-dimensional hydrogen burning systems, and applied to a Burke-Schumann type hydrogen-air diffusion flame to elucidate its flame structure and combustion reaction mechanism. The same flame was also experimentally investigated and radial profiles of OH radical concentration and rotational temperature through the flame were determined by a conveniently improved line-of-sight absorption method. Simulation suggested that a well-developed diffusion flame occurs from heights about 5 mm from the burner mouth, in very good agreement with the experimental results. At each horizontal section of the well-developed diffusion region, the calculated rates of the chemical reactions showed considerable values within an annular zone about 5 mm thick, in which there is a point where H2 and O2 are simultaneously exhausted, and in whose vicinity the temperature becomes maximum. This result confirmed one of the Burke-Schumanns predictions, but radial displacements of about 1 mm between the peeks of the rates of different reactions and also between the maximum OH radical concentration and the maximum temperature were found in both experiments and calculation, showing that the reactions do not occur in an infinitely thin region, contradicting one of the Burke-Schumanns assumptions. At the lowest part of the flame, below the burner tip, the issuing air and the H2 diffusing downward meet and react like premixed gas mixtures; the whole flame was found to be held at the burner rim by large heat release rates at that region. The experimental results, in very good agreement with the simulation, showed that the maximum OH concentration through the flame occurs at a height around 2 mm, confirming the premixed-like intense combustion reactions taking place at the lower part of the flame.

First Principles Calculation for Cu Gettering by Dopantor Dopant-Vacancy Complex in Silicon Crystal

For finding the effective gettering center of cupper (Cu) atom in silicon (Si) crystal, an interaction between interstitial Cu atom and dopant (B, Sb, As, P), C or O atom was studied with first principles calculation. It was found that only B could be an effective gettering center. This result indicates that heavily B doped p/p+ epitaxial wafers will show a sufficient gettering efficiency for Cu contamination. In order to design the gettering center of Cu in n/n+ epitaxial wafers, the interaction between vanancy (V)-Sb, V-As or V-P complexes and Cu was investigated. It was found that these complexes could be effective gettering centers. The mechanism of Cu gettering by oxide precipitates was also studied with further calculations. It was found that the stabilities of Cu atom in β-crystobalite or in strained Si were not superior to that in strain-free Si. The trapping at the interface or the interaction with emitted interstitial Si (I) from oxide precipitates should be the possible mechanisms for Cu gettering.

Propagation of unsteady hydrogen premixed flames near flammability limits

The structure and propagation mechanism of spherically propagating hydrogen-air premixed flames were investigated under the conditions of various fuel concentrations and initial temperatures to elucidate the essential factors influencing propagation of the flames near the lean flammability limits. Measurements of the burning velocity indicated that flames with adiabatic flame temperature lower than 890 K cannot extend up to 100 mm in diameter. In addition, they showed appreciable dependence of the burning velocity on the initial temperature. The hydrogen flames were also simulated using a model consisting of a full set of combustion reactions of hydrogen and the governing equations for spherical expansion of gases. The structure and the propagation mechanism of hydrogen flames near the lean flamability limits differ from the flames having large burning velocity such as unsteady stoichiometric hydrogen flames. As a result of decreasing adiabatic flame temperature, the reactions belonging to the high-temperature reaction mechanism of hydrogen flames cannot be well activated. The low-temperature reactions are also depressed because of insufficient amount of hydrogen atoms which usually diffuse from the high-temperature regions, and the temperature of the gas mixtures is thermally raised. Since a wide temperature range without heat release yields a large temperature gradient due to an exponential temperature change, a large amount of heat release is required after that range to keep large burning velocity. Flames which cannot satisfy this condition have inevitably small burning velocity. When unburned gases are initially heated, the thermally-heated regions become narrower and the temperature gradients in the following high-temperature regions are reduced, and consequently stable combustion can be maintained even if heat release rate is small in the main reaction zones.