Gas-dynamic factors in combustion processes

Abstract

In that chapter, it has been experimentally found that a flame of dilute natural gas–oxygen mixtures does not penetrate through the central opening of a confuser, but it penetrates only through the central opening of a diffuser, even if there are additional openings on the cone elements. The numerical modeling performed using compressible dimensionless reactive Navier–Stokes equations in a low Mach number approximation made it possible to qualitatively interpret the results. It is established that both minimum diameter of a central opening, through which the flame of the diluted methane–oxygen mix can penetrate, and minimum pressure of flame penetration decrease with an increase in the number of openings. It was experimentally shown that the penetration limit by the diameter of the orifice for a single asymmetrical obstacle is less than that of a single symmetrical obstacle. It was revealed that the penetration limit increases with an increase in the number of obstacles with asymmetrical openings. In this, the penetration limit for two and three asymmetrical obstacles can be lowered by changing the first obstacle toward the direction of flame propagation by the obstacle with a symmetrical opening. The value of effective activation energy of the dark reaction 2H2 + O2 over Pd is evaluated as E = 4.1 ± 1 kcal/mole that is characteristic of a surface process. The value is close to one determined for the dependence of the H2 fraction at the ignition limit over Pd surface in mixtures with O2 on temperature: 3.5 ± 1 kcal/mole. Under our conditions, no dark reaction on Pt wire was observed. It was shown that the rate of chain termination determines the value of the critical diameter for flame penetration through Pt or Pd cylinders; the efficiency of Pd surface in chain termination reaction is much greater than that of Pt. It is shown that in the presence of Pd catalyst surface, which does not react at flame temperature and generates catalytic centers diffusing into volume, heat release and pressure perturbation during the combustion occur out of phase; the catalytic Pt surface eliminates a certain stage of inhibition after the occurrence of the cool flame and NTC phenomenon vanishes. In the presence of the catalytic surface (Pd), which does not react at flame temperature and does not generate catalytic centers diffusing into a volume, NTC phenomenon occurs. It is established that the concentration limit of ignition of a premixed H2–air mix in the presence of CF2Cl2 at 1 atm exceeds 10%, whereas the inhibitor limit of ignition of the premixed methane–air mix makes 1% CF2Cl2. It means that CF2Cl2 is an effective inhibitor to prevent undesirable ignition of methane–air mixes at atmospheric pressure. It is experimentally shown that the concentration limits of the initiated combustion known from literature are meaningful only for previously prepared mixes. If the mixes are prepared just before the experiment, then the existence or the lack of ignition is determined by both an order of injection of mixture components, and the geometry of an installation. Therefore, the inhibition with halons can hardly be used to address the challenges of safety in mines. Thus, the influence of gas-dynamic factors on the efficiency of inhibition is revealed.