Study on the Combustion Characteristics of Fires inside the Subway Train with Multiple Lateral Openings

側向多開口地鐵列車車廂火災燃燒特性研究

Student thesis: Doctoral Thesis

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Award date2 Jul 2021

Abstract

With the rapid development of China's economy and the increasing advancement of the urbanization process, the urban rail transit based on the subway plays an increasingly important role in the modern transportation system. However, once a fire accident occurs in the subway system, especially in the long-narrow space of the train carriage, it is very likely to cause serious casualties and bad social impact. Predecessors have carried out a lot of research on the fire dynamics in the long-narrow confined space of the tunnel, while the research on the fire of the long-narrow train carriage in the subway system is relatively few. Traditional tunnel-like long-narrow confined structures are basically open at both ends, which is obviously different from the characteristics of train carriage with multiple lateral openings. It is not clear whether the previous mature theoretical model of fire dynamics is applicable. Therefore, based on the previous research, this thesis adopts the method of combining scaling experiment and theoretical analysis to carry out a systematic quantitative study on the train carriage fires under various scenarios. The main research contents include:

1.The characteristics of the fire plume under the special confined structure of the train carriage are studied, and the transverse/longitudinal one-dimensional temperature distribution in the long-narrow train carriage is clarified. Through a series of experiments carried out on the experimental platform of the reduced-size subway train carriage, it is found that the fuel mass loss rate per unit area and the maximum smoke temperature obtained in the train carriage are far greater than those predicted by the theoretical model of the traditional tunnel scenario. Based on the experimental results, a prediction model of the maximum smoke temperature rise under the ceiling with special confined structure is established. Further study shows that the transverse and longitudinal temperature distribution under the carriage ceiling conforms to the law of exponential attenuation, but the transverse temperature is more sensitive to the influence of the lateral door status than the longitudinal temperature. Based on the previous exponential function model, a modified empirical model for predicting the longitudinal and transverse temperature distribution under the carriage ceiling is proposed, and the index of the exponential function is proportional to the dimensionless heat release rate to the power of n.

2.The flame extension characteristics of the long-narrow train carriage fire are revealed, and the global analysis of the two-dimensional temperature distribution under the carriage ceiling under the condition of lateral opening is completed. The flame extension length generated by the carriage fire under various conditions is experimental studied. Based on the strong plume condition of the carriage fire and the theoretical basis that the heat release rate per unit flame volume is invariable, the flame volume shape is analyzed and the dimensionless heat release rate under the ceiling is introduced, further analysis indicates that the normalized ceiling flame length varies as 1/4 power of the dimensionless ceiling heat release rate. In addition, results show that the influence of the status of the lateral door on the distribution of two-dimensional temperature field under the carriage ceiling is gradually significant with the increase of the fire scale, which is mainly manifested in that the opening of the lateral door makes the high temperature area under the ceiling shift towards the side with doors. Further analysis of the two-dimensional temperature field under the carriage, it is found that the longitudinal temperature distribution at different transverse distances from the fire source also follows the exponential attenuation law, and the attenuation coefficient has an exponential function relationship with the dimensionless transverse distance. A two-dimensional temperature prediction model under the carriage ceiling including the transverse and longitudinal coordinates is established.

3.The variation law of the temperature characteristics of a single train carriage fire under the condition of lateral opening is studied, and the influence mechanism of the lateral opening condition on the temperature distribution in the train carriage is revealed. A long-narrow confined compartment platform with closed ends and multiple lateral openings is used to simulate the structure of a single subway train carriage. A series of experimental results show that the temperature inside the carriage is not uniformly distributed in space, and presents a trend of attenuation in three-dimensional direction. The lateral opening condition mainly affects the temperature in the area near the fire source, and the classic model based on the long-narrow confined space opened at both ends underestimates the maximum smoke temperature in the carriage. In the longitudinal direction, keeping the openings near the fire source closed and symmetrically opening the openings away from the fire source can significantly increase the temperature in the carriage and expand the fire hazard, while asymmetrically opening the openings on the side of the fire source can effectively reduce the temperature caused by the fire. In the experimental scenario with all the lateral opening open, the temperature attenuation law under the ceiling is related to the fire power, and the attenuation rate and critical attenuation distance are linear and power functions with the fire power, respectively. Moreover, a prediction model of temperature attenuation under the carriage ceiling considering the fire power is established.

4.A reduced-size experimental platform for the long-narrow space fire of subway tunnel and train carriage is developed, and the characteristics of smoke flow and flame extension in the double long-narrow confined space caused by the carriage fire are initially explored. It is found that in the case that the fire source is close to the lateral door, the flame overflow phenomenon will appear when the fire power is small. With the further increase of the fire power, the flame will overflow intermittently in the whole height direction of the lateral door. In the case that the fire source is located in the middle of the carriage, a larger fire source power is required to cause the flame overflow phenomenon. At this time, the flame overflow from the upper part of the lateral door is mainly the horizontal extended flame after the impinging of the strong fire plume on the ceiling. In addition, the traditional theoretical model of smoke temperature rise in strong fire plume in a single tunnel can not well predict the maximum smoke temperature under the carriage ceiling of this special scenario. The critical value of the maximum smoke temperature to distinguish the impinging zone of continuous flame and intermittent flame is obtained, and the prediction correlations for the dimensionless maximum smoke temperature under the tunnel ceiling and the carriage ceiling are established. The longitudinal temperature attenuation rate under the carriage ceiling in the scenario of double long-narrow space is greater than the results of the single long-narrow confined space, but its attenuation rate is obviously less affected by the fire source power.

    Research areas

  • Double long-narrow confined structure, Subway train, Carriage fire, Multiple lateral openings, Strong plume, Maximum temperature, Two-dimensional temperature attenuation, Flame extension, Spill fire plume, Reduced-scale experiment