点式排烟诱导风速对大功率隧道火灾烟气影响数值模拟

Numerical simulation on impact of induced velocity of single-point extraction system on smoke in large tunnel fire

采用数值模拟研究了大功率隧道火灾点式排烟诱导风速对烟气分布的影响。选取热释放速率为100 MW的火源表示大功率隧道火灾。在隧道点式排烟模式下,设计5组不同的诱导风速进行模拟研究。隧道发生火灾后,将火源下游附近3个排烟阀打开,火源上游提供一定的诱导风速。对不同诱导风速下隧道内烟气蔓延距离、温度场分布和能见度分布进行分析。结果表明:大功率隧道火灾需要较大的诱导风速才能将烟气控制在排烟阀开启段,以保证人员安全疏散;隧道内火源上游温度梯度随诱导风速的递增而减小;诱导风速对隧道内能见度的影响与烟气回流状况存在相关性。

The present paper is engaged in the investigation of the impact of different induced velocities of the single-point extraction system on the smoke for a tunnel fire via numerical simulation. In proceeding with our investigation, we have developed a computational fluid dynamic code based on the Fire Dynamics Simulator （FDS） by NIST to solve the numerical form of Navier-Stokes equations available for the low speed, thermally driven flow with the stress on the smoke and heat transfer from the fire as the numerical tool. In addition, we have simulated the fire scenario in the scale of 100 MW denoting a typical HGV （heavy goods vehicle） fire in a tunnel so as to obtain the imagined severe scenario, with a simplified rectangular cross-section tunnel model. To make our study more effective, we have defined and analyzed the proper boundary conditions of the numerical tunnel model, in which five different induced velocities were adopted from 1.5 m/s to 3.5 m/s to evaluate and compare in the sphere of the same dimensional tunnel model. We have also taken the smoke con- trol strategy for controlling the tunnel fire by turning on the down- stream dampers in the vicinity of fire site with a number of three and the upstream jet fan for ventilation as the induced velocity to prevent the smoke spreading. We have also chosen the criteria of the impact of the induced velocities on the smoke. And, finally, the results needed turn out from the analysis： Under the operation of single-point extraction system, a greater induced velocity is needed to prevent the smoke spreading upwards and restrict it to the region between the fire site and the damper openings for safe evacuation in a large tunnel fire. Additionally, the gradients of upstream temperature are expected to decrease with the increase of the induced velocity. Thus, the visi- bility and smoke propagation situation can be shown as the similar trends in variation of the induced velocity. The results suggest that the impacts of the induced velocity on the upstream visibility are cor- related with the back layer.