This paper has analyzed the asymmetric flow effect of fire-induced thermal flow in a horizontal tunnel under the natural ventilation condition by conducting large eddy simulations (LES). The key objective is to reveal and to have a better understanding of the asymmetric flow effect caused by the upstream and downstream tunnel length difference. The mechanism behind it can be explained based on the conservation of mass and dynamic force analysis on the smoke and fresh air. The strength of the asymmetric flow effect is characterized by the mass flow rate of the induced longitudinal flow (net mass flow rate of a cross-section). An empirical correlation to predict the induced longitudinal mass flow rate is proposed. Furthermore, the law of smoke and air flow distribution within a horizontal tunnel is established. The proportion of smoke (or air) flowing out (or coming in) through the opening increase (or decrease) linearly with the increasing distance between that opening to the fire location. The variation of the air flow with the longitudinal fire location in a tunnel is more sensitive than the smoke flow. Results have shown that as the fire approaches the tunnel exit from the middle of the tunnel, the smoke spilling out through this opening is reduced from 50% to 40%, while the fresh air incoming from this opening is increased from 50% to 100% and vice versa.

通过进行大涡模拟，分析了自然通风条件下水平隧道中火源热流的非对称流动效应。关键目标是揭示并更好地了解由上游和下游隧道长度差异引起的非对称流动效应。可以基于对烟雾和新鲜空气的质量守恒和动态力分析来解释其背后的机理。非对称流动效应的强度以引起的纵向流动的质量流率（横截面的净质量流率）为特征。提出了一种经验相关性来预测引起的纵向质量流率。此外，建立了水平隧道内烟气和气流的分布规律。通过开口流出（或进入）的烟（或空气）的比例随该开口到火源位置之间距离的增加而线性增加（或减少）。气流在隧道中的纵向火场位置的变化比烟流更敏感。结果表明，随着大火从隧道中间靠近隧道出口，通过此开口溢出的烟雾从50％减少到40％，而从该开口进入的新鲜空气从50％增加到100％反之亦然。