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Buoyant unstable behavior in initially spherical lean hydrogen-air premixed flames within a center-ignited combustion vessel have been studied experimentally under a wide range of pressures (including reduced, normal, and elevated pressures). The experimental observations show that the flame front of lean hydrogen-air premixed flames will not give rise to the phenomenon of cellular instability when the equivalence ratio has been reduced to a certain value, which is totally different from the traditional understanding of the instability characteristics of lean hydrogen premixed flames. Accompanied by the smoothened flame front, the propagation mode of lean hydrogen premixed flames transitions from initially spherical outwardly towards upwardly when the flames expand to certain sizes. To quantitatively investigate such buoyant instability behaviors, two parameters, “float rate (ψ)” and “critical flame radius (Rcr)”, have been proposed in the present article. The quantitative results demonstrate that the influences of initial pressure (Pint) on buoyant unstable behaviors are different. Based on the effects of variation of density difference and stretch rate on the flame front, the mechanism of such buoyant unstable behaviors has been explained by the competition between the stretch force and the results of gravity and buoyancy, and lean hydrogen premixed flames will display buoyant unstable behavior when the stretch effects on the flame front are weaker than the effects of gravity and buoyancy.

Energy attenuation behavior on propane premixed combustion on account of nonmetallic spherical spacers (NSSs) was investigated in a constant volume combustion bomb with two accelerative perforated plates. The flame evolution process is captured by a photography device with schlieren. Energy attenuation effects of NSSs were explored on flame propagation, overpressures, and pressure oscillation. Normal combustion accomplished with flame acceleration, jet autoignition, deflagration, and weak detonation is observed, respectively. NSSs show a good energy attenuation effect on propane normal combustion, jet autoignition, and deflagration evolution behavior, however, during the evolution of weak detonation, the attenuation effect is relatively weak. The mean velocities difference, as well as explosion overpressure decay rate caused by NSSs, decreases from (45.70 m/s and 26.48%) to (2.70 m/s and 18.15%) when the initial pressure increases from 0.1 to 0.5 MPa, namely, attenuation effect of NSSs on propane explosion decreases with increasing initial pressure.