Research On Smoke Flow Characteristics Under The Effect Of Lateral Smoke Exhaust And Critical Criterion Of Plug-holing The Long-narrow Space

In recent years,with the increase of long-narrow buildings year by year,the probability of fire risk in long-narrow space is increasing.However,because of its special space,the long-narrow building is not conducive to rescue and smoke exhaust,so it is necessary to install a smoke exhaust system in these buildings.If the height direction during construction is limited and the top independent flue cannot be set,the lateral mechanical smoke exhaust system can be used in buildings such as subways and tunnels.In recent years,lateral mechanical smoke exhaust system is widely used in subsea tunnels,underground stations and other buildings.Therefore,it is necessary to study the smoke flow under the action of lateral mechanical smoke exhaust and critical criterion of plug-holing in long-narrow space,so as to provide reference for the improvement of lateral mechanical smoke exhaust effect in long-narrow in the future.Firstly,a full-scale tunnel model is established by using numerical simulation method,and the exhaust effects of ceiling and lateral mechanical smoke exhaust system are compared.The results showed that compared with ceiling mechanical smoke exhaust,lateral mechanical smoke exhaust is more likely to occur plug-holing.And the velocity vector distribution and the smoke inhalation style of the two smoke exhaust methods are quite different.The smoke exhaust efficiency of this two smoke exhaust methods decreases with the increase of the exhaust velocity,and the downward trend is gradually flattened.By comparison,under the same conditions,the ceiling mechanical smoke exhaust efficiency is better than the lateral mechanical smoke exhaustSecondly,a long tunnel model is established to study the smoke flow characteristics under the influence of lateral mechanical smoke exhaust in the tunnel.The main conclusions are as follows: the lateral mechanical exhaust will break the onedimensional horizontal spread stage,so the smoke flow under the influence of lateral mechanical exhaust is divided into five stages,that is,the mechanical exhaust stage is added after the one-dimensional horizontal spread stage.Lateral mechanical exhaust will accelerate the upstream smoke flow and reduce the upstream smoke layer thickness.The entrainment coefficient of one-dimensional horizontal spread stage increases with the increase of exhaust velocity and HRR,and the influence of ceiling mechanical smoke exhaust vent on one-dimensional horizontal flow stage upstream is greater than that of lateral mechanical smoke exhaust.Finally,to reveal the key factors that influencing the smoke exhaust effect and the plug-holing phenomenon,a set of numerical simulations were conducted in a subway station with the lateral mechanical exhaust.Results show that with the increase of exhaust velocity,the phenomenon of plug-holing will occur at the lateral exhaust vent.The occurrence of plug-holing can lead to the decrease of the exhausted smoke concentration.Furthermore,by introducing a virtual smoke vent that perpendicular to the original lateral one,a critical Froude number for predicting the occurrence of plugholing of lateral mechanical exhausting is proposed.Based on the results of different exhaust velocities,heat release rates and smoke vent heights,the critical Froude number that corresponds to the occurrence of the plug-holing can be determined as 0.5for the lateral mechanical exhaust.That is to say,if the Froude number is greater than0.5,the plug-holing occurs,vice versa,it doesn’t occur.