为了探究近壁面浮力扩散火焰非稳态燃烧行为和振荡特性, 该文开展了3种长宽比火源在不同火源壁面间距(0~4 cm) 和热释放速率(3.8~17.1 kW) 下的燃烧实验与数值模拟, 重点分析了火焰振荡频率和动态附壁特征参数的变化规律。研究结果表明：火焰附壁概率和附壁长度随着火源长宽比和热释放速率的增加而增加。对于长宽比为1的方形火源, 当热释放速率小于11.4 kW时, 全局振荡频率随着火源壁面间距的增加而增大; 当热释放速率大于或等于11.4 kW时, 全局振荡频率随火源壁面间距的增加而减小, 与长宽比为4和8的矩形火源的变化趋势一致。基于镜面模型分析, 获得近壁火源特征直径, 并进一步建立了无量纲全局振荡频率与Froude数倒数的无量纲关系式。该研究可为受限空间防火设计提供一定的理论依据。

Objective: 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. Methods: 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. Results: 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). Conclusions: (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.