This paper is aimed at studying the down-reaching flame behaviors of tank fires with large ullage heights. Experiments were first conducted using a gas burner in a transparent quartz glass cylinder to simulate the large ullage and the experimental data was used to validate the computational fluid dynamics (CFD) model. Subsequently the effects of ullage height, fuel velocity and burner diameter on the flame behaviors were examined systematically. Both experimental and numerical results showed that, for lower fuel velocities, the down-reaching flame height (h_down) is restricted by the ullage height. As the fuel velocity continues to increase exceeding a critical value, independent of the ullage height, h_down starts to decrease. For a given fuel velocity, h_down increases with an increase of the burner diameter owing to enhanced air entrainment. A detailed analysis of the flow field and oxygen concentration inside the tank at the steady burning stage was also carried out. Based on the numerical results and dimensionless analysis, a piecewise function was proposed to predict the down-reaching flame height and validated against the experimental data.

本文旨在研究大高度坦克火灾的向下蔓延火焰行为。首先使用透明石英玻璃圆筒中的燃气燃烧器进行实验来模拟大的空量，并使用实验数据来验证计算流体动力学（CFD）模型。随后系统地研究了空缺高度、燃料速度和燃烧器直径对火焰行为的影响。实验和数值结果均表明，对于较低的燃料速度，向下到达的火焰高度（h _down）受到损耗高度的限制。当燃料速度继续增加超过临界值时，与空缺高度无关，h _down开始减小。对于给定的燃料速度，由于空气夹带的增强， h _down随着燃烧器直径的增加而增加。对稳定燃烧阶段罐内流场和氧气浓度进行了详细分析。基于数值结果和无量纲分析，提出了分段函数来预测向下火焰高度，并根据实验数据进行了验证。