Applicability Of Fr-based Scaling Law In Tunnel Fire Experiment

In recent years,tunnel fire has caused great casualties and property losses.A large number of scholars have carried out tunnel fire experiments to study the fire mechanism.When carrying out tunnel fire experiment,many scholars have adopted the reduced-scale tunnel experiment because of the high cost of full-scale experiment.However,for the field of tunnel fire,the scaling law used in the reduced-scale tunnel experiment has not been verified systematically.This paper systematically verifies the applicability of scaling law in reducedscale tunnel through experiments and numerical simulation.In order to explore the applicability of scaling law based on the Froude number in natural smoke exhaust tunnel fire,this paper established 1/10 scale tunnel(L)and 1/20 scale tunnel(S),and carried out tests corresponding to full scale 5,10 and 20 MW.For natural smoke exhaust tunnel,the experimental results show that the scaling law is basically applicable to the maximum temperature rise above the fire source,and the error is within the acceptable range.For the maximum temperature rise along the tunnel,the error increases continuously with the distance from the fire source,and the error can reach 50%-70% at the tunnel exits.Therefore,the maximum temperature rise of far fire field obtained from the reduced-scale tunnel is obviously underestimated compared with the full-scale tunnel results,and the smoke concentration in the reduced-scale tunnel is seriously overestimated.The Reynolds number in the experiment is about 2800-35000.For the 2.8k W test in tunnel S,the Reynolds number is less than 4000,while the Reynolds number under 5.6k W and 11.2k W is high enough,and the flow field is fully turbulence.Although the ratio of the two tunnels is only 1:2,the scaling law leads to a great error in temperature rise,smoke concentration and speed.This means that maintaining Froude number and maintaining turbulence still cannot guarantee the applicability of scaling law.In addition,for natural smoke exhaust tunnel,the velocity in the upper and lower areas is large enough to meet the requirements of turbulence,and there will always be a central area(similar to the neutral plane)between the two areas,and the velocity is almost 0which doesn’t meet the requirements of turbulence.For the blocked tunnel,the scaling law can predict the temperature rise above the fire source and self-extinction time of 100% blocked tunnel,but it is not applicable for the partial blocked tunnel.The reduced-scale tunnel experiments will underestimate the critical sealing rate and temperature rise along the tunnel,but overestimate the critical oxygen concentration.Then,the FDS numerical simulation is carried out,and the simulation results are in good agreement with the experimental results.But the simulation results show that although the error of the maximum temperature rise above the fire source between tunnel S and tunnel L is small,the error is significant compared with the full-scale tunnel and increases with the reduction of tunnel size.The simulations and experiments show similar rules,that is,the reduced-scale tunnel will underestimate the temperature rise and wind velocity along the tunnel,and overestimate the smoke concentration.In addition,a series of verification conditions are carried out.Firstly,the low HRR test(1MW)is carried out,which proves that the error of scaling law is not caused by the approximation dissatisfaction of Boussinesq.Secondly,the comparative tests in which the floor is open are carried out,which proves that the strong entrainment effect of air supply on the flue gas layer is not the key factor affecting the scaling law.Finally,the simulated tests in which thermal radiation is shut off are carried out to verify the influence of the internal error of scaling law.The results show that there is still significant error in the temperature rise along the tunnel while the thermal radiation is shut off.This study systematically verified the applicability of the scaling law in tunnel fire,and provided a reference for the future fire experiment of reduced-scale tunnel.