隧道内氨气泄漏扩散分布特征数值模拟

Numerical simulation for the distributive features of ammonia leakage and diffusion inside the tunnel

为分析运输过程中液氨罐车在隧道内泄漏的危险性,利用Fire Dynamics Simulator(FDS)软件模拟氨气在隧道内的扩散过程,发展了隧道内氨气泄漏扩散体积分数分布特征经验公式。采用大涡模拟处理湍流流动,以便兼顾计算精度和计算效率。考虑储罐车发生泄漏后停止不动,液氨在泄漏瞬间转变为气体,模拟在连续点源泄漏情况下的氨气射流及扩散过程。结果表明,高体积分数危险区域主要集中在隧道顶棚附近,更高截面的体积分数处于爆炸极限的区域更长。泄漏源与洞口之间的隧道中段区域的体积分数梯度相对两端较小,此中段区域也是人员安全高度截面最高氨气体积分数发生位置。最大泄漏量情况下氨气在沿纵向扩散过程中平均运动速率保持在0.63~1.06 m/s,扩散速率随纵向距离增加而降低。顶棚氨气体积分数升高程度随纵向距离增加呈幂函数降低,体积分数沿纵向衰减规律适用于其他泄漏量的情况。后期工作可考虑开展缩尺试验,并同时考虑通风条件等因素对氨气泄漏扩散的影响研究。发展的氨气在隧道内泄漏扩散的体积分数分布经验公式可为氨气事故后果评价、应急处置等工作提供参考。

To analyze the risk of leakage of the liquid ammonia tankers in the tunnel with the transportation process going on,the present paper has made a simulated study for the diffusion process of the ammonia gas in the tunnel via a fire dynamics simulator software. We have developed an empirical formula of ammonia leakage and diffusion volume fraction distribution featured for the tunnel interior application,with an eddy simulator being adopted to register the turbulent flow speed and efficiency. At the same time,we have also simulated the ammonia jet and diffusion process in the case of the continuous point source leakage,given that the tank cars can usually be stopped in case there happens any leakage,for the liquid ammonia in that case is likely to turn into the gaseous state as soon as the leakage takes place.Technically speaking,the length of the model tunnel is 1 000 m,whereas its net width is 12. 8 m. As to the four scenarios of different leakage rates,they can be used to obtain volume fraction distribution of the net height of 6. 2 m independent of the leakage volume. The results of our simulated experiment show that the high volume fraction risk area lies mainly near the tunnel ceiling,with the longer explosion limit area lying at a greater height.However,the volume fraction gradient in the middle part of the tunnel between the leakage source and the portal tends to be rather small,which just serves as a place where highest ammonia volume fraction is situated at the safety height. And,there,the average velocity of the ammonia gas can be kept at about 0. 63-1. 06 m/s in the longitudinal diffusion process at the maximum leakage rate,though the diffusion rate may get lower with the increase of the longitudinal distance. Furthermore,the increase of the ammonia volume fraction may also tend to decrease at the ceiling with the power function getting smaller in the longitudinal distance. Besides,the longitudinal attenuation of the volume fraction can also be applied to the other leakage rates,so that a scale model of the ammonia leakage should be set up in the future. Thus,it is also possible to work out the effects of the ventilation condition and other factors on the leakage and diffusion of the ammonia gas. In so doing,the volume fraction distribution model of the ammonia gas leakage and diffusion in the tunnel can be expected to provide a valuable reference for evaluating the consequences and emergency treatment of the ammonia accident.