ISSN 10613862, International Journal of SelfPropagating HighTemperature Synthesis, 2013, Vol. 22, No. 2, pp. 88–92. © Allerton Press, Inc., 2013.
Unsteady InfiltrationMediated Combustion in Conditions of Regulated QuasiIsobaric Flow of Gaseous Reagent S. V. Kostin and P. M. Krishenik Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia email:
[email protected];
[email protected] Received December 14, 2012
Abstract—Unsteady infiltrationmediated combustion of Ti powder in air was explored in conditions of reg ulated quasiisobaric flow of gaseous reagent. Variation in the parameters/geometry of air flow was found to affect the structure of reaction zone, burning velocity, composition of combustion products, and extent of conversion. Keywords: SHS, infiltrationmediated combustion, fire safety DOI: 10.3103/S1061386213020040
solid Ti and then the metal gets started to react with components of the surrounding atmosphere (air). Consumption of nitrogen becomes noticeable already at 500–550°C and sharply grows above 600°C. Oxida tion with atmospheric oxygen becomes predominant around 800°C. Reaction of Ti with water vapor, Me + H2O → MenOm + [H]Me + H2, proceeds at any tem perature to yield porous oxide films. Dissolution of hydrogen in Ti powder may proceed until saturation [19].
INTRODUCTION As is known, nonequilibrium reacting systems can undergo selforganization into complex regular struc tures whose symmetry may strongly differ from that of initiating perturbation [1]. The formation of such structures in diffusional thermal flames was postulated in [2] and qualitatively substantiated in [3]. The spin ning mode of unsteady combustion over the surface of heterogeneous reactive powder compacts was reported and analyzed in [4, 5]. The fingering instability in smoldering combustion of filter paper in a flow of air was reported in [6] and then modeled [7] in terms of diffusional thermal model [3]. A fluiddynamic nature of fingering insta bility in coflow infiltration combustion was suggested and experimentally substantiated in [8, 9]. The first 3D mathematical models for unsteady infiltration combustion were suggested in [10, 11] and respective experimental observations of cellular com bustion in conditions of natural infiltration were reported in [12–14]. The effect of gas transport to the reaction zone on the evolution of cellular combustion was studied in [14, 15]. Restricted gas supply was found [12–14] to result in the decay of frontal combustion into more complex structures with other values of burning velocity and extent of conversion. The process was also found to be accompanied by adsorption/desorption of impurity gases (such as hydrogen) that may restrict the delivery of gaseous reagent to the reaction zone [16, 17]. According to [18], the interaction of Ti powder with air proceeds as follows. At 450–500°C the oxide film on the surface Ti particles begins to dissolve in
In this work, we explored unsteady infiltration mediated combustion of Ti powder in air in conditions of regulated quasiisobaric flow of gaseous reagent with special emphasis on the effect of heat/mass trans fer conditions on the structure of reaction zone, burn ing velocity, combustion products, and extent of con version. (a)
(b)
Fig. 1. Combustion geometries used in our experiments: configuration I (a) and configuration II (b).
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UNSTEADY INFILTRATIONMEDIATED COMBUSTION (a)
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(b)
Fig. 2. Ti layer 3 mm thick, gap 4 mm: (a) frontal combustion wave in geometry I, mean front velocity 7–8 mm/s; (b) cellular combustion mode in geometry II, mean cell velocity 0.7–0.8 mm/s.
EXPERIMENTAL Experiments were carried out in 64 × 120 mm rect angular steel vessels (Fig. 1) containing a uniform layer of Ti powder (d < 100 µm, density 1.6 g/cm3, porosity 0.6). Using steel plates, the ledge height could be made to have a value of 5, 7, and 11 mm. Using 100 × 100 sil ica plates, we could organize the following conditions for admission of ambient air as a gaseous reagent: mode I (reactor I) with an open end at the side oppo site to the igniter (Fig. 1a) and mode II (reactor II) with two open ends (Fig. 1b). The ambient temperature was varied between 18 and 22°C while relative humidity, between 46 and 65%. Combustion products were characterized by opti cal microscopy (MBS9 apparatus) and XRD. In control experiments, we determined composi tion of the gases evolved upon holding starting Ti pow der at 1500°C: 98.86% H2, 0.5% CO2, 0.5% CO, and
