Investigation Of The Effectiveness Of A Water Mist Segment System In Blocking Fire-induced Smoke And Heat In Long Tunnels

There is an abundance of tunnels in China because of its large area of mountains and hills.The heavy traffic in tunnels leads to high fire risk,while the narrow and long configuration of tunnels resulting in confined ventilation would lead to high temperature concentration and toxic gases once a fire breaks out.Researches find that the catastrophic consequences of destructions of vehicles and tunnels and great numbers of casualties from fire disasters are mostly contributed to high temperature and poisonous gases.Therefore,it is of critical importance to control the spread of high temperature and polluted smoke.Water based firefighting systems,which could be divided into water spray systems and water mist systems according to the water particle size,are most often used in building fires,but seldom used in tunnels except in Japan and Australia.Although the water spray/mist could largely decrease the temperature,the smoke damage and impair in visibility remains unsolved.A water mist segment is constituted of several rows of nozzles and used as a compartment configuration installed in tunnels at a certain interval.Instead of applying the water mist directly on the fire,using water mist segments to physically confine the smoke in a section and absorb heat could protect the evacuated personals from high temperature and polluted smoke.The effectiveness of water mist segment system would be studied from four aspects in below.The first part is experimental study.A 1/3 scale model tunnel was constructed,and the effectiveness of the water mist segment system was studied through varying heat release rates,water pressures,nozzle quantities and ventilation conditions.Based on the performance of water mist segment systems under natural ventilation,the non-dimensional number R_M representing the ratio of water mist segment momentum to smoke momentum was put forward.The larger R_M is,the better the smoke blocking effect of the water mist segment system.The second part is CFD simulation based on the test scenarios,i.e.,model validation.The model tunnel is reproduced based on the test conditions,and sensitivity analysis is conducted to guarantee the effectiveness of parameters.The process of comparing and contrasting the test data and simulation data shows the effectiveness of CFD simulation.And the information from temperature fields and flow fields are also achieved from the simulation.The third part is theoretical analysis.Based on the smoke descent research before,two retrofits are further considered,namely,the horizontal movement of smoke,and the cumulative momentum of smoke with time.A theoretical model predicting the critical length of water mist segment to block the smoke when smoke is continuously produced is put forward based on the mechanical analysis of smoke particle and the momentum analysis of smoke cloud.This theoretical model could calculate the change of horizontal smoke velocity with interaction length with water mist segment,predict the critical length of water mist segment to just block the smoke,and interpret the phenomenon why smoke cannot be blocked forever by water mist segment systems.The forth part is theoretical model verification and application research based on CFD full-scale simulations.Firstly,the theoretical model is verified,the function of smoke generation time to the non-dimensional number R_M is extracted,and the critical non-dimensional number R_MC is calculated.Then a water mist segment is designed based on R_MC in the CFD full-scale simulations.The results showd that,the water mist segment designed from R_MC to just block a 30 MW fire smoke could effectively confine the 100 MW fire smoke and high temperature between two water mist segments when combined with a semi-transverse ventilation system.In summary,experimental study,theoretical study and numerical simulation are combined to study the smoke blocking effect of the water mist segment system in tunnels.First of all,important parameters are achieved through experimental study,and a non-dimensional number R_M,which is the ratio of water mist segment momentum to smoke momentum was put forward to represent the smoke blocking effect.Then,theoretical model predicting the critical length of water mist segment to block the smoke when smoke is continuously produced is put forward,and R_MC,the critical value of R_M,is calculated.Finally,the effectiveness of improved water mist segment system in blocking smoke is verified by full-scale CFD simulations.The research results showed that,the water mist segment designed from R_MC to just block small fire smoke could effectively confine the large fire smoke and high temperature between two water mist segments when combined with a semi-transverse ventilation system,and greatly improve the tunnel fire safety.