Exploring the survival of premixed hydrogen flames below the lean flammability limit Articles uri icon

publication date

  • July 2025

start page

  • 114315-1

end page

  • 114315-10

volume

  • 279

International Standard Serial Number (ISSN)

  • 0010-2180

Electronic International Standard Serial Number (EISSN)

  • 1556-2921

abstract

  • Ultra-lean hydrogen flames, which can ignite unintentionally due to leaks near a heat or power source, pose
    significant safety risks. This study investigates why flames propagate at equivalence ratios below the theoretical
    flammability limit (¿l = 0.255), where the equilibrium temperature equals the crossover temperature. To find
    the answer, we use detailed chemistry to numerically study the conditions that explain recent experimental
    observations of flame propagation in confined channels at equivalence ratios ¿ < 0.2.
    Our simulations consider a two-dimensional geometry of two parallel plates separated by a small distance to
    form a straight channel. Adiabatic and isothermal boundary conditions are considered at the walls to evaluate
    the effect of heat losses on the survival of the flame. The flame curvature, caused by the confinement within
    the narrow channel, leads to the formation of a high-temperature region near the center of the channel. This
    region is surrounded by unburned gas flowing close to the channel walls. The reaction is then sustained by
    the hydrogen that diffuses from the low-temperature region to the reactive front. This behavior is unique to
    fuels or fuel blends with sufficiently high mass diffusivity and does not occur when the Lewis number is near
    or above unity. A new scaling, that accounts for the flame curvature to define the characteristic velocity and
    lengths scales, is proposed to describe the flame dynamics at equivalence ratios near the flammability limit.
    According to our calculations, self-sustained 2D hydrogen flames may exist at equivalence ratios as low as
    ¿ = 0.15, a threshold determined by the existence of a stationary flat flame that is unaffected by heat losses.

subjects

  • Mechanical Engineering

keywords

  • hydrogen premixed flames; flame propagation in channels; ultra-lean hydrogen flames; hydrogen flames flammability limits