Study On Influence Of Water Spray On Smoke Temperature Distribution Under The Ceiling In Tunnel Fires And Critical Velocity

Tunnel protection facilities are commonly used in smoke extraction systems in tunnel fire.Water spray systems have been set up in many tunnels to control and extinguish the fire effectively.The water spray will change the smoke flow field in the tunnel fire,which leads to the changes of critical velocity of the key parameters of smoke control in longitudinal ventilation tunnel;at the same time,the water will cool the hot smoke to reduce the ceiling temperature.However,the smoke temperature and critical velocity models on the tunnel fire are mostly established for tunnel fires without fire suppression systems.Therefore,this work studies the effects of the flow rate and spray angle parameters of the water spray system on the temperature distribution and critical velocity,the smoke temperature distribution prediction model and a critical velocity model for establishing coupled water spray system parameters,and provides reference for such tunnel fire smoke control technology.The main research work is as follows:Firstly,the influence of different water spray flow rate on the smoke temperature was studied by FDS numerical simulation method,result showed that the test data for the case without the water spray system agreed well with previous model,the feasibility of numerical simulation was verified.Simulated data for the case with the water spray below smoke temperature of the cases without the water spray system,and degree of reduction is positively correlated with the water spray flow rate;thus a prediction model for maximum smoke temperature under different water spray flow rate was proposed by introducing a modified coefficient.It is found that the longitudinal smoke temperature decays faster with the water spray flow rate increases.Through the theoretical analysis and simulation results,the prediction model of longitudinal smoke temperature distribution under different flow rates was established.Secondly,studying the smoke temperature under different spray angles.Results showed that the maximum smoke temperature increases with the increase of the spray angle;and the larger the spray angle means the smaller the longitudinal smoke temperature decay rate.Thus a prediction model for maximum smoke temperature andlongitudinal smoke temperature distribution under different spray angle was proposed by introducing a modified coefficient.Finally,the numerical simulation method was used to study the effect of water spray flow rate and spray angle on critical velocity.It is found that the critical velocity decreases more obviously with the increase of water spray flow rate;and the critical velocity also increases at first and then decreases with the increase of spray angle(at the spray angle of 90°,the critical velocity is the maximum).The results showed the dimensionless critical velocity V* and the dimensionless heat release rate Q* are still in the piecewise function law under water spray system,the demarcation point is 0.2,and the exponential relationship is 1/3;the critical velocity model was further improved by introducing a modified coefficient.