Study On The Fire Smoke Spreading Characteristics In The Tunnel With One Closed End

The construction of tunnels concludes many sections wherein tunnel fire prevention and smoke suppression are usually the most vital ones.Compared with the other type of buildings,tunnel fire is more destructive and highly transmissible due to the long-narrow structure and relatively sealing environment.In recent years,the tunnel with one closed end gradually appears in building the metro system in mountainous cities due to the local landform.With such confinement,fire combustion and smoke spread are likely to behave in a very different way compared to what it behaves in the tunnel with opening portals.Currently,scientific reports or studies related to this regard are still rare.Motivated by such knowledge gap,the present work conducted a series of model-scale experiments and multiple-scaled simulations wherein the following topics are systematically explored.Firstly,the present paper starts from the investigation of fires in normal tunnels with two opening portals.Based on a large-scale of literature data collection,the coupling effect of longitudinal ventilation and vehicular blockage is explored and its influence on the ceiling smoke propagation and thermal distribution is analyzed.Further,three typical blockage modes are given based on the literature review.The reanalysis of literature data denotes significant discrepancy between the literature data and prediction models proposed in former studies once the blockage is considered.Based on the present model experiments and literature data,modified models are established to calculate the maximum temperature,ceiling temperature attenuation,smoke backlayering length,and critical velocity accounting for the blockage effect.Secondly,relying on the former research outcomes of fires in opening tunnels,a set of burning tests are then carried out in the tunnel with one closed end.Factors like blockage ratio,fire-closed end distance,and fire-blockage distance are considered as the main variables to account for the influence of blockage effect on fuel burning rate,plume shape,and ceiling temperatures.Results demonstrate that 1)burning rate of ethanol pool fire slightly varies when fire-end distance is over 0.50 m when the tunnel is empty;2)burning rate of ethanol pool fire significantly increases with the decreasing distance between fire and blockage;3)as for the two blockage ratios employed in the present paper,the larger one leads to a higher fuel burning rate.Finally,empirical model is established to calculate ceiling temperatures in a tunnel with one closed end considering different fire-blockage distance.Thirdly,investigation associated with the fire smoke spread in the inclined tunnel with one closed end is carried out.Results imply that smoke spread towards the opening portal is significantly confined in the uphill tunnel.Meanwhile,smoke gathering within the fire and closed end becomes more severe as tunnel slope increases,leading to the descend of smoke layer.On the contrary,smoke emission via the opening portal in the downhill tunnel is facilitated due to the buoyancy force.When the blockage effect is further considered,simulation results show that the maximum temperature at tunnel ceiling linearly increases with tunnel slope due to the weakened entrainment between fire plume and airflow.Correspondingly,the total length of high-temperature region increases.Regarding the coupling effect of tunnel slope and blockage,ceiling temperature decays still follows the exponential law.Formulae are then proposed to calculate the ceiling temperatures in the situations where blockage is considered or not.Fourth,effect of smoke extraction on the fire and smoke behavior in a tunnel with one closed end is addressed by utilizing the single-point smoke extraction system.Experimental results demonstrated that burning rate of ethanol pool fires indistinctly fluctuates in the condition that smoke outlet is installed upstream of the fire.When the ceiling extraction system is activated,ceiling temperature distribution and smoke propagation upstream of the fire can be divided into two regions by the smoke outlet,namely Region Ι and ΙΙ.In Region Ι,smoke extraction rate has limited impact on the temperature,while ceiling temperature rapidly decreases,and longitudinal smoke spreading length is significantly shortened with the increase of smoke extraction rate in Region ΙΙ.To quantify the smoke extraction effect,an ‘idealized’ induced air velocity and the concept of ‘virtual fire source’ are introduced.Based on the dimensional analysis,two empirical models are then correlated with the present measurements to predict the ceiling temperatures and longitudinal smoke spreading length.