Ignition Thresholds and Flame Propagation of Methane/Air Mixtures Ignited via Radiatively Heated Inert Particles

Junrong Ma,Changsuo Zhang

doi:10.3390/en14165173

Abstract

The prevention and evaluation of explosions requires suitable standards of measurement. As such, for this study two ignition thresholds, the ignition temperature and the minimum ignition irradiance were selected as the assessment criteria. These ignition threshold values were experimentally determined by heating stationary inert silicon carbide particles via thermal radiation with a large spot size in order to ignite quiescent methane-air fuel mixtures. A high-speed Schlieren camera was used to capture the progression of the formation and propagation of the flames throughout the experiments. The results of the experiments show that the irradiance and temperature threshold are directly and inversely proportional to the particle size, respectively. Furthermore, the irradiance and temperature thresholds have similar tendencies within the flammability limits; wherein, the minimum value corresponds to fuel mixtures at a stoichiometric ratio, and increases as the equivalence ratio shifts toward the flammability limits. Irradiance thresholds, though, are more sensitive to changes in equivalence ratio than temperature. The temperature histories of the heated particle determined that when the irradiance is lower than its ignition threshold value, the heated particle-fuel mixture system will arrive at a thermal equilibrium, rather than ignition, due to the inability of the particle to reach the ignition temperature. This study also found that longer ignition times will result in a more drastic deformation of the flame fronts caused by natural convection.

Highlights

With sufficient energy, is an ignition source for flammable gases in various working environments; wherein three laser ignition mechanisms for fuel mixtures are: (1) ignition by local temperature increase [1,2] or photochemical processes [3,4] that are caused by the fuel mixture directly absorbing the radiation; (2) ignition by plasma which was formed via focused laser radiation [5]; and (3) radiatively heated particle ignition, which is the heating of particles via continuous-wave lasers that subsequently induce ignition of the fuel mixture
The objectives of the study are to determine: (1) the minimum ignition energy and temperature that are required for the ignition of methane-air fuel mixtures by heated inert silicon carbide particles at varied particle size; (2) the distribution and trends of ignition threshold of energy and temperature at varying equivalence ratios; and (3) position of ignition kernels and factors that affect the propagation of flame fronts through qualitative observation and analyses, respectively
This study focuses on the influence of the fuel concentration and particle size on the irradiance threshold, wherein irradiance is obtained by dividing the power by spot area

Summary

Introduction

With sufficient energy, is an ignition source for flammable gases in various working environments; wherein three laser ignition mechanisms for fuel mixtures are: (1) ignition by local temperature increase [1,2] or photochemical processes [3,4] that are caused by the fuel mixture directly absorbing the radiation; (2) ignition by plasma which was formed via focused laser radiation [5]; and (3) radiatively heated particle ignition, which is the heating of particles via continuous-wave lasers that subsequently induce ignition of the fuel mixture. In contrast to sparks or other vectors of energy transfer that are used to ignite fuel mixtures, the means of energy transfer for radiatively heated particle ignition are thermal conduction and radiation from the heated particles. Dubaniewicz Jr. et al [8] found that the energy threshold of heated coal particles is higher than that of heated iron-oxide particles. Hills et al [9] observed that the energy threshold decreases as the particle size decreases when a heated single coal particle is used to ignite hydrogen-air fuel mixtures. Welzel et al [10] indicated that the energy threshold rapidly decreases as the mixture temperature increases when using a single oxidized iron particle to ignite ether air–fuel mixtures at varying temperatures. Dubaniewicz Jr. et al [11]

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