| Due to the particularity of the long and narrow structure,once a fire occurs in the tunnel,fire rescue and personal evacuation will face huge challenges.The hightemperature and toxic smoke is the main cause of casualties.Longitudinal ventilation is an effective method to control the smoke.When the ventilation velocity is lower,the smoke will quickly spread along the tunnel ceiling after impinging,and demonstrate the characteristics of one-dimensional spread.While the ventilation velocity increases to a certain value,the smoke bifurcation may occur in downstream area of the fire source,and a low-temperature zone without smoke will appear on the tunnel ceiling.When the fire source is located in the center of the tunnel(center fire),the ceiling smoke will flow in a "V" shape and eventually converge,while the fire occurs at the sidewall of the tunnel(wall fire),the ceiling smoke will flow in an "S" shape.Therefore,the smoke flow structure will show great difference as the difference of longitudinal ventilation velocity and the transverse position of the fire source.The critical velocity is the main index for the design of longitudinal ventilation.In practical engineering,for the sake of conservativeness,the longitudinal velocity is usually over-designed compared with the calculated value from theoretical model.When the longitudinal velocity is too large,the smoke bifurcation will occur downstream of the fire source.Once the shaft or the smoke exhaust vent is installed on the low-temperature zone,the smoke will not be effectively exhausted.Therefore,the bifurcation characteristics of smoke under longitudinal ventilation have important theoretical significance and practical value for guiding the smoke control system design for tunnels.Through the small-scale tunnel,a series of experiments have been carried out to study the maximum temperature beneath the ceiling when the smoke bifurcation occurs.The experimental results of the center fire were used to compare with maximum temperature prediction model proposed by Kurioka.There will be large errors between the experimental results and the calculated values from Kurioka’s model.Based on the small-scale experimental results,a prediction model for the maximum temperature beneath the ceiling with smoke bifurcation is proposed.When the heat release rate of the fire source is small,the Kurioka’s prediction model is similar as the prediction model proposed in this paper.When the heat release rate of the fire source is large,the prediction results of the two models will be significantly different.In order to analyze the influence of the lateral position of the fire source,experiments on the maximum temperature beneath the ceiling with smoke bifurcation under the condition of wall fire source were carried out,and a prediction model of the maximum temperature of the wall fire was proposed.At the same time,the "mirror image" principle was adopted to deal with the wall fire source,it is found that there is a big difference between the results based on the mirror image principle and the experimental results.Finally,through numerical simulation by CFD code FDS,the effect of longitudinal ventilation on the smoke bifurcation of wall fire under different tunnel widths was studied.Aiming at the characteristic of the "S" shape of smoke bifurcation under the condition of wall fire,the "S" flow of the ceiling smoke was divided into regions,the location of the smoke impacting the ceiling and the location of the smoke impacting the side wall were analyzed.The theoretical model of the longitudinal offset distance of the impinging area and the longitudinal length of the low-temperature zone was proposed.The results show that as the longitudinal velocity increases,the area where the smoke impinging the ceiling and the sidewall will continue to shift downstream,and the low-temperature zone will expand.Based on the FDS simulation results,the prediction models of the three characteristic lengths to describe the smoke bifurcation of wall fire are proposed. |
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