Quenching of the Combustion Wave

The highest rate of cooling (10 -10 K/s) was reported for a relatively thin sample quenched with a high-speed water jet (Khusid et al, 1992). However, the high water velocity (up to 150 tn/s) necessary to achieve high rates of quenching often destroyed the sample and interaction occurred between the water and reacting sample. 

Evstigneev, 1992), Ni-Al (Lebrat et al., 1994), and Ti-Si (Rogachev et al., 1995). Some of these results are discussed here. 

The product grain growth rate was determined from measurements of the grain size in the different quenched regions obtained by SEM observations. For example, a sequence of microstructures taken from different zones of Ti-C-Ni quenched samples is shown in Fig. 63. The results of statistical analysis of such microstructures for Ti-C, TiB2, and Ti-C-Ni systems are presented in Fig. 64, and illustrate an initial rapid increase in grain size that approaches a constant value in 

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Design of explosion suppression systems is clearly complex, since the effectiveness of an explosion suppression system is dependent on a large number of parameters. One Hypothesis of suppression system design identifies a limiting combustion wave adiabatic flame temperature, below which combustion reactions are not sustained. Suppression is thus attained, provided that sufficient thermal quenching results in depression of the combustion wave temperature below this critical value. This hypothesis identifies the need to deliver greater than a critical mass of suppressant into the enveloping fireball to effect suppression (see Fig. 26-43). 

Nitrogen and Ta in the combustion wave form the intermediate product TaN with a single phase if the process is quenched after the combustion wave has passed (44). If the sample is heated by the combustion wave in nitrogen, repeated combustion takes place with the formation of the TaN phase. It should be noted that experiments at ambient nitrogen pressure (—40 atm) produce a hexagonal modification of TaN, but those at high pressures (—3000 atm) yield a cubic one. The cubic modification is synthesized by the combustion of Ta in liquid nitrogen (44). 

The microstructure of the initial titanium-graphite mixture is shown in Figure 62a. When the temperature in the combustion wave reaches 1660°C, titanium melts. It was determined from quenching results that a thin film ( 0.1 / m) of the Ti melt spreads over the solid carbon surface with simultaneous formation of titanium carbide grains (Fig. 62b). Small rounded TiC particles were observed to appear within the liquid rather than in the form of a continuous product layer (Rogachev et al, 1987). To illustrate this fact further, the typical microstructure formed during combustion reaction of titanium melts with graphite whiskers (10 /.im in diameter) is shown in Fig. 62c. 

Khusid, B. M., Khina, B. B., Kuang, V. Z., and Bashtovaya, E. A., Numerical investigation of quenching of the state of a material in a wave of SHS during a two-step reaction. Combust. Explos. Shock Waves, 28,389 (1992). 

For the boron-nitrogen system, because the high gas pressure is required for the synthesis, it is difficult to apply any dynamic method for investigation of the microstructural transformations, which occur in the combustion front. Thus, the static quenching technique was used [26, 23, 27]. The idea of this method is to extinguish the combustion wave and quickly cool the sample it is necessary to freeze all zones with the characteristic microstructure, chemical and phase structure of the reactants, intermediates, and final products. For quenching to take place, the heat loss from the reaction front at some point must exceed the critical... 

The second type of quenching method is based on the gradual increase of the heat loss with movement of the combustion front through the sample. The prototype of these methods was considered for experiments by Belyaev and coworkers [29] to determine the critical diameter of combustion of condensed matter. During movement of the combustion front from the base of the cone to its top, the specific heat loss increased, reaching the critical value in some cross-section of the cone where the extinction occurred. The most widely used method is the quenching of combustion waves in a wedge-shaped cutout of a massive copper block [27]. 

To understand the process of ferrosilicon nitriding in the combustion mode, the method of stopping the combustion wave by rapid nitrogen pressure release and filling the apparatus by argon was used. The microstructure analysis of quenched samples with the use of X-ray microanalysis and SEM analysis allowed us to... 

A distinctive peculiarity of high-grade ferrotitanium is the lowest melting temperature among examined ferroalloys (1085 °C). To understand the mechanism for nitriding of ferrotitanium, we stopped the combustion wave by quenching. 

Buckmaster ]., The quenching of deflagration waves. Combust. Flame, 26 151-162,1976. 

Combustion, Flames and Explosion in Gases , 2nd Editn, Academic Press, NY (1961), 322—23 (Dust quenching occurs at a critical value of the surface area of the dust per unit vol of the suspension, and depends on the nature of the salt. Better results are obtained with salts having a mp under 200°. Alkali halides are better than carbonates, potassium better than sodium, fluoride better than iodide and better than chlor-ide. If the dust concentration is high enough, even detonation waves can be extinguished)... 

The ability of the burning surface of a powder to intensify reflecting acoustic waves was studied in detail with respect to rocket combustion chambers, and a theoretical analysis of this question was performed by S. S. Novikov and Yu. S. Riazantsev. The critical conditions of powder quenching by a decrease in the pressure became the subject of research, and this question was also developed theoretically.13 The ignition of powders was studied experimentally,14 and different... 

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