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The design of Chinese high-speed railway tunnels, characterized by their high arched ceiling and large sections, presents unique challenges in terms of heat and mass transfer behaviors. These architectural features significantly influence the dynamics of smoke movement, resulting in distinctive patterns of longitudinal flow and temperature distribution of smoke during a fire, which differ markedly from those observed in conventional highway tunnels.
To investigate the specifics of smoke flow dynamics, this study embarked on full-scale fire experiments conducted within the Baijiashan tunnel of the Yuxiang high-speed railway between Chongqing and Qianjiang. These experiments were instrumental in capturing critical data on flame height and longitudinal distribution of smoke temperature under various fire scenarios. Building on this empirical foundation, the study analyzed the combustion stages and calculated the longitudinal smoke velocity for each fire scenario examined.
Previous literature has highlighted that the heat release rates in full-scale experiments were calculated based on the equivalent diameter of fire sources, with values of 0.38, 1.01, and 2.52 MW, respectively. It was observed that as the heat release rate increased, there was a corresponding significant uptick in the longitudinal velocity of the smoke. Within the confines of a high-speed railway tunnel, the vertical temperature distribution of fire smoke exhibits a distinct top-hat pattern. This characteristic distribution remains consistent farther from the fire source. Furthermore, this study delineates the boundary between one-dimensional shooting flow (Region Ⅱ) and critical flow (Region Ⅲ) within the context of a Chinese high-speed railway tunnel, identified as x/H ≈3.85 based on experimental data. The study also probes into the suitability of existing models for predicting the longitudinal flow and temperature distribution of fire smoke in long, narrow spaces such as high-speed railway tunnels. It was found that owing to the extended length of one-dimensional shooting flow (Region Ⅱ), models that assume constant smoke thickness fall short in accuracy within this region.
This study revealed that despite the larger net height and cross-sectional area of high-speed railway tunnels compared to those of conventional railway tunnels, the heat release rate of the fire source critically influences the speed at which smoke spreads longitudinally. Moreover, the evolution of vertical temperature distribution is determined by the convective heat transfer coefficient beneath the ceiling and the ceiling jet thickness. As the smoke spreads, the smoke velocity evolution, along with the convective heat transfer coefficient and ceiling jet thickness, gradually stabilizes. This stabilization contributes to the stable top-hat temperature profile observed vertically. Leveraging Froude scaling, a new model for the longitudinal attenuation of smoke temperature rise in Region Ⅲ of a Chinese high-speed railway tunnel has been developed and validated for x/H ≥3.85. The insights gained from this work enrich the experimental research on the characteristics of longitudinal smoke flow in Chinese high-speed railway tunnels. Moreover, the field data obtained on the longitudinal flow and temperature distribution of fire smoke offers theoretical support for evaluating how the longitudinal spread of fire smoke affects personnel evacuation strategies in Chinese high-speed railway tunnels.
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