火灾工况下半敞开式隧道自然通风与混合通风对比研究

Comparative study on natural and hybrid ventilation of a semi-opened tunnel in a fire

针对半敞开式城市公路隧道自然通风模式存在一定的火灾安全隐患问题,提出在顶部开口处加装排烟风机的混合式通风来保障烟气不侵入半敞开段(安全区域)。以某实际隧道为研究对象,采用FDS软件,使用LES方法模拟自然通风和混合式通风两种模式下的烟气扩散,火源释热率7.5 MW,火源区网格尺寸0.083 m。结果表明,在300 s的安全疏散时间下,自然通风的半敞开段内仍滞留大量烟气,混合模式下机械排烟量大于112.5 m3/s时,烟气即被有效控制在半敞开段之外。通过风洞效果对比试验证明,与纯自然通风相比,顶部加装排烟能有效抑制烟气下沉。

The paper would like to propose an improved hybrid venti lation method with smoke exhaust fans installed at the top openings in a semiopened section of the half open urban transportation tunnel in hoping to reduce the smoke exhaust from spreading into the safety zones. For our research purpose, we have taken an authentic tunnel with twin oneway tubes as our study sample, each of the two being divided into 5 inlaid sections by 4 groups of multiopening tubes. In the tunnel, the fire source is supposed located in the middle at the longest buried section on the ground, with its two adjacent semi opened sections taken as the safety zones. And, then, we had been trying to adopt LES technique to simulate its smoke spreading manner respectively by way of natural and hybrid ventilation in a supposed safety egress time of 300 s by using a FDS software. The heat releas ing rate of the said hybrid ventilation efficiency was supposed to be 7.5 MW. In our experiments, we have chosen the firesource mesh size as 0.083 m, with half of the smokefire supposed coming from the nonfire source. The number CFL was always supposed to be low er than 1 and the quasisteady time step was about 0.005 s. The sim ulated results show that under the temperature of natural ventilation the smoke under the ceiling tends to disperse away quickly from the fire source both in the upwind and downwind directions which doesn＇t seem to be threatening to the passengers inside the two safety zones, though a lot of smoke was still gathered in the zones at 300 s, which is likely to bring about a threat to the passengers or vehicles passing through the tunnel. Here we have laid out five different simulated mechanical smoke exhaust rates. It turned out obvious that it was possible for the smoke in the safe zones to disperse gradually with the increase of the smoke exhausting rate, and later it would be getting totally clear off until 112.5 m3/s. Being restricted tO the experimental conditions, a simple comparative test has been conducted in a wind tunnel. At this point, we have done observations over the smoke spreading and propagation respectively if the smoke exhaust at the top with the fan working on spreading or not. It indicated that as com pared with the cheer natural ventilation, the mechanical smoke ex haust with fans working at its top could effectively prevent the smoke exhaust from subsiding. Thus, it can be seen that this study is quali fied to provide a theoretical basis for designing semiopened urban tunnels and also highly valuable for roadengineering application.