Burning behaviors and pulsation frequency of near-wall buoyant diffusion flames

More than half of fire accidents happen in confined spaces. When a fire source is close to a solid wall, the wall may significantly restrict air entrainment, leading to flame attachment to the wall and even igniting the combustibles on it. However, studies on the effect of wall restrictions on the combustion and pulsation behaviors of near-wall fires remain notably limited.

This study employed a self-designed experimental setup to investigate the influence of heat release rate (Q and fire-wall separation distance (S) on the global and local pulsation frequencies and attachment length of near-wall flames with different aspect ratios (r). Simulations of the combustion behaviors of near-wall fires were also conducted using ANSYS Fluent, yielding the associated flow fields and temperature distributions.

Experimental results show that near-wall fires exhibit unstable attachment behavior. Vertical walls impede air entrainment on the near-wall side of the flame, creating asymmetric air entrainment; meanwhile, horizontal walls primarily exacerbate flow-field deformation. The flame exhibits two burning patterns due to the restriction imposed by the wall: a varicose mode at larger values and a sinuous mode at smaller ones. Fast Fourier transform analysis of the flame width and correlation coefficient shows that the pulsation frequency is significantly influenced by Qand S. For attached flames (S=0 cm), the local pulsation frequency remains stable at varying Qvalues and normalized heights. At small S, low-frequency pulsation is pronounced under conditions of high Q. Rectangular fire sources (r=4 and 8) and the square fire source (r=1) show significant differences in global pulsation behavior. With increasing S, the global pulsation frequency for rectangular fire sources gradually decreases, while that for the square fire source decreases at large Qvalues (≥11.4 kW).

(1) The flame evolution pattern of near-wall fires can be categorized as either varicose or sinuous mode, depending primarily on S. (2) Flame attachment probability and length increase with increasing Qand decreasing S. (3) For the square fire source with high Q(≥11.4 kW) and rectangular fire sources with r values of 4 and 8, the global pulsation frequency decreases with increasing S. (4) The characteristic diameter of near-wall fires is modified with the flame attachment probability and normalized flame attachment length to account for the effect of unstable flame attachment on air entrainment. Strong asymmetric entrainment causes significant spatial and temporal flame attachment, which increases the characteristic diameter of near-wall fires and reduces the global pulsation frequency. A unified correlation between the normalized flame pulsation frequency (Strouhal number) and the inverse of Froude number is proposed, which reasonably predicts the global pulsation frequency of near-wall fires with different S and Qvalues. Overall, the findings of this study lay a foundation for the reasonable prediction of flame evolution and spread in confined spaces.