Abstract
The dynamics of ignition of premixed hydrogen–air from a hot glow plug were investigated in a combined experimental and numerical study. Surface temperatures during heating and at ignition were obtained from 2-color pyrometry, gas temperatures were measured by high-speed Mach–Zehnder interferometry, and far-field effects were captured by high-speed schlieren imaging. Numerical simulations considered detailed chemical kinetics and differential diffusion effects. In addition to the known cyclic (puffing) combustion phenomenon, singular ignition events (single puff) were observed near the lean flammability limit. Detailed analysis of the results of our numerical simulations reveal the existence of multiple combustion transients within the thermal boundary layer following the initial ignition event and, at late times, sustained chemical reaction within a thermal plume above the glow plug. The results have significant implications for ignition from hot surfaces within near-flammability limit mixtures, at the edge of plumes resulting from accidental release of hydrogen, or within the containments of nuclear power plants during severe accidents.
Highlights
Combustion of premixed hydrogen-air atmospheres near the lean flammability limit exhibits a rich variety of behavior
Ignition thresholds increase with increasing hydrogen mole fraction, from 1010 K at XH2 = 5% to 1100 K at XH2 = 70%, and an additional steep increase occurs near the upper flammability limit, up to 1170 K at XH2 = 73%
As the temperatures increase above 900-1000 K, chain-branching reactions become increasingly more relevant as compared to reactions forming peroxides [39, 43]. This leads to a sharp decrease in ignition delay time with increasing surface temperature and enables ignition to occur when ignition delay time and gas residence time near the hot surface become comparable
Summary
Combustion of premixed hydrogen-air atmospheres near the lean flammability limit exhibits a rich variety of behavior. The very low (less than 10 cm/s for hydrogen concentrations less than 10% [1, 2]) burning speeds, high diffusivity of hydrogen and the negative Markstein lengths result in a strong coupling of chemistry and hydrodynamics in lean mixtures. This coupling [3] results in significant effect of flame stretch as well as the potential for thermal-diffusive and Landau-Darrieus instabilities, resulting in local extinction. Near-limit mixtures exhibit a number of unusual combustion phenomena such as flame balls [11]
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