Study On Natural Ventilation Smoke Gas Characteristics And Smoke Control Of Sloped Tunnel Fires With Roof Openings

The natural ventilation by setting opening at the top of tunnel section has become a hot spot in the academic research recent years due to its advantages of saving energy.The research on smoke characteristics and smoke control in natural ventilation tunnel is of great significance for ensure the life safety of personnel during the fire.The research objects of scholars in the past mainly focus on the natural ventilation in a horizontal tunnel.There is less study of sloped tunnels in the actual projects.The high temperature flue smoke can be influenced by the slope,which will make the smoke diffusion significantly different from that of a horizontal one during a fire.In this paper,a sloped tunnel with natural ventilation during the fire is studied.In this paper,the smoke characteristics and shaft size of natural ventilation exhaust in sloped tunnel fire was studied with the combination of experimental and the theoretical analyses.The fire scenario simulated in the experiment is that the fire point is located between two shafts,and the shaft is far from the tunnel exit.Firstly,the smoke characteristics in sloped tunnel was analyzed according to the reducedscale model,and result suggested the fire plume deflected to the uphill side influenced by the slope.The distribution of smoke temperature beneath the ceiling was no longer distributed among the fire location;the smoke temperature on the uphill side was significantly higher than that on the downhill side and the smoke temperature decay rate on the uphill side was slower than that on the downhill side.Besides,the smoke under the shaft on the downhill side was thinner and been exhausted easily,while it on the uphill side was denser and been exhausted difficultly.Secondly,the influence factors were studied based on the experimental results,with the combination of theoretical analysis.The results suggested,the fire location,HRR,shaft height had no influence to fire plume deflection angle and the distribution of temperature beneath the ceiling.However,the smoke characteristics were greatly affected by sloped,it can be refined as: fire plume deflection angle increased with the increase of slope.In the range close to the fire source,the smoke temperature beneath the ceiling decreases significantly with the increase of slope.And in the range far from the fire source,there is nearly no influence from fire location to the distribution of temperature on the uphill side beneath the ceiling,but it on the downhill side increased with the decrease of slope.Combined with the theoretical analysis,a calculation formula of the fire plume deflection angle,the maximum smoke temperature beneath ceiling and the temperature beneath the ceiling was adopted.When the smoke could not be exhausted completely from the shaft,by the influence of environment gas on the tunnel portal,the smoke which spread out of the fire location subsided and then flow back to the fire location.This could negatively impact the personal safety.Therefore,the minimum critical shaft height which can exhausted the smoke completely was studied.And in the simulated situation of this experiment,the following conclusions were obtained: With the increasing shaft length,the critical shaft height decreased first and then remained stable,and the smoke could exhausted completely with 0cm shaft height when the shaft increased to a certain length;HRR had barely no influence to the critical shaft height;the critical shaft height increases with the increase of tunnel slope,and the increases of the distance between fire location and the shaft at uphill side.In the end,based on the experimental results,with the combination of theoretical analysis,a prediction formula of critical shaft height in a sloped tunnel was established.