| Summary: | 碩士 === 國立中正大學 === 化學工程研究所 === 89 === This thesis is to study the nano-scale SHS combustion behavior by the image acquisition and micro-imaging technique. In the first part of the thesis, correlation dimension (Dc), computed from the fluctuating combustion wavefront time series, was used to characterize the complexity of the combustion wavefront. The influences of parameters such as nano reactant powders and diluents are carried out. In the second part, the micro-imaging technique was first established to observe the combustion wavefront microscopically. With this tool, the microscopic wavefront behavior was studied under the influences of reactant and diluent particles.
In the first part, the chaotic time series technique was first introduced. Influences of nano reactant particles were studied by designing the carbon reactant being a mixture of nano and micro carbon powders. Denote x as the content of nano carbon powders. Because all the combustion reactions were carried out with diluent, the combustion velocity was greatly reduced. When more diluent was included in the reactant powders, the combustion could fail to propagate and extinguish. It is well known that when the combustion progresses with the possibility of extinguishment, the combustion behavior becomes extremely complicated and unpredictable. Thus, it corresponds to a higher fractal dimension. When x increases, both the combustion velocity and temperature were raised. Combustion reaction became more violent and combustion behavior became more complex as well. Dc increased with the increasing x. When different diluent titanium carbide and alumina were used, combustion with titanium carbide had a smaller Dc than that with alumina. This is due to the lower thermal conductivity of alumina. With a smaller thermal conductivity, the combustion progressed even slower and this resulted in a more complicated combustion behavior. The particle size of diluent powder also influences the combustion. For titanium carbide diluent with two particle sizes —325 mesh and 3 mm, Dc was higher for —325 mesh titanium carbide. The reason is also due to large diluent particle slowing down the combustion. But, the result was reverse for alumina diluent (-325 mesh and 0.05 nm)! When nano-sized diluent particles were used, the complexity of combustion was no longer determined by reducing combustion velocity. It was dominated by the mixing inhomogenity due to the participation of nano particles.
In the second part, the combustion was successfully observed with a x750 magnification. The combustion system was Ti + Si àTiSi. First, when particle size of Ti was kept at 15 mm, the local combustion with the smaller silicon particles (10 mm) showed a brighter combustion wavefront, larger preheat zone and smaller and dispersed ignitions, because smaller reactant particles had higher reactivity, higher reaction rate and larger reaction heat released. Next, when Ti was changed into 75 mm, with lower reactivity, combustion with the 40-mm silicon powders exhibited higher brighter wavefront and larger preheat zone, contradicting to the previous results. This is mainly due to the reduction of overall reactivity. Energy needed to be accumulated longer to ignite the reactants. Therefore, at the verge of ignition, larger silicon powders produced more molten silicon locally and hot spot appeared in a larger scale than that of 10-mm silicon. Next, when particle size of alumina diluent was changed from 0.05 mm to 10 mm, the wavefront became dimmer and more local hot spots were observed. Finally, for a local defect in reactant pellet, the combustion was also greatly affected.
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