The Smoke Propagation Mechanism And Control Method For Fires In Subway Tunnel With Roof Openings

Natural ventilation by setting openings at the top of the tunnel section is a new way of subway ventilation, which not only save the energy consumption of ventilation in the tunnel section, also save the investment of ventilation facility. From the point of engineering application, the important issues for this ventilation method is how to meet the requirement of fire smoke and ventilation by setting the minimum opening numbers. In this paper, theoretical analysis, reduced-scale experiment, and numerical simualtion method were carried out to investigate the smoke diffusion characteristics and smoke control. This research can further improve the theory of natural ventilation in the tunnel section and also provide theoretical support for fire control standards for natural ventilation in the tunnel section.The smoke control criterion NFPA130and PIARC were adopted to constitute the smoke control standard in tunnel fires with natural ventilation and the integrated visibility was obtained from theoretical analysis. The fire plume characteristics in horizontal tunnel was derived and then the dimensionless smoke flow, smoke temperature and smoke concentration were defined based on the plume characteristics. From the definition of these dimensionless smoke parameters, the relationship between smoke flow and smoke temperature or smoke concentration were developed.The experimental tests in reduced-scale tunnel and numerical simulation in full-scale tunnel were carried out to investigate the effect of some factors on smoke diffusion characteristics such as:heat release rate, tunnel cross-section, tunnel gradient, smoke curtain, shaft distance, shaft size, fire location, train blockage, piston wind and so on.The results of smoke diffusion characteristics in tunnel fire with natural ventilation showed that the smoke parameters changed sharply under the shaft as the smoke exhaust through natural ventilation shaft. The ceiling temperature presented the decrease trend in the longitudinal direction in tunnel, while the ceiling concentration presented the decrease trend in the longitudinal direction in fire section of the tunnel and had almost the same value in non-fire section. In fire section, the thickness of smoke layer were almost the same, while in non-fire section the parameter increased firstly and then decreased to0with the distance from fire source increase. In tunnel fires with natural ventilation, the ceiling temperature and visibility at the height of people head could make a danger, while the rest of smoke parameters didn’t play the role in passenger escape. The heat release rate didn’t have the effect on the dimensionless smoke parameters and the thickness of smoke layer, which indicates the definition of dimensionless parameters were reasonable in tunnel fires with natural ventilation. The train blockage and piston wind didn’t have the significant effect on smoke parameters in tunnel. However the tunnel cross-section, tunnel gradient and smoke curtain had the very significant effect on smoke parameters in tunnel. Moreover, the shaft distance and shaft size could affect the smoke parameters in non-fire section of the tunnel, but didn’t have the significant effect on smoke parameters in fire section.The3-d numerical simulation was used to investigate the worst fire location in full-scale tunnel through the study on train location, fire source location in the carriage and the carriage location in the train. The result showed that the worst fire location was that the widest boundary of door or window in the train was under the inner boundary of the shaft and the fire source located the related boundary in the fire carriage.Based on the smoke diffusion characteristics in reduced-scale tunnel and full-scale tunnel, the theoretical analysis method was adopted to develop the formulas to predict the smoke diffusion characteristic in tunnel fires with natural ventilation. The formulas include ceiling temperature decay, smoke exhaust from shaft, smoke temperature distributions, smoke diffusion distance, critical smoke diffusion distance, the visibility in tunnel and so on.On the basis of smoke diffusion characteristics and the model from theoretical analysis, the engineering design method was established for subway tunnel fires with natural ventilation. The way of natural ventilation couldn’t be used in the single-line tunnel as the small tunnel cross-section. In the tunnel in which the smoke curtain was not set up, the shaft distance of30m is reasonable when the ceiling temperature at the distance from fire source was beyond180℃, while the shaft distance ought to be selected in the range of50m to140m when the ceiling temperature was less180℃. Moreover, in the tunnel in which the smoke curtain was set up, the shaft distance ought to be select in the range of130m to220m. The relationship between the shaft distance and the minimum shaft length could be obtained from the related formulas.