Study on temperature decay characteristics of fire smoke backflow layer in tunnels with wide-shallow cross-section

•Smoke temperature decay characteristics were studied experimentally and numerically.•Smoke transverse temperature decay was influenced significantly by aspect ratio.•Factors to affect smoke longitudinal temperature decay were discussed in detail.•A piecewise model to predict smoke temperature decay was proposed and verified. The temperature distribution characteristics in tunnel fire scenarios are not only crucial for evaluating the hot damage of tunnel structure, but also pivotal fundation to study the smoke kinetic characteristics. In the present work, the scaled-model tests and the CFD simulations were adopted to investigate the effects of aspect ratio, longitudinal ventilation velocity and fire heat release rate on smoke temperature decay characteristics at both transverse and longitudinal directions in tunnel fires. Results indicate that: Tunnel aspect ratio has a significant influence on smoke temperature rate at both transverse and longitudinal directions. The increase of aspect ratio will increase the contact area between the smoke layer and the ceiling, enhancing the heat loss of the smoke backflow layer. As a result, smoke temperature decays faster in a wide-shallow cross-section tunnel. The longitudinal temperature decay rate in the wide-shallow tunnel (AR = 9.3) is almost eight times as large as that in the normal tunnel (AR = 1.1). The increase of fire heat release rate will reduce the smoke temperature decay rate at both transverse and longitudinal direction; As the ventilation velocity increases, the longitudinal temperature decay rate increases while the transverse temperature decay rate decreases. The definition of smoke backflow ratio was proposed to establish the fire smoke temperature decay model of the smoke backflow layer, which considers the effects of aspect ratio, longitudinal ventilation velocity and fire heat release rate. By comparing the test results in previous research work, the new model shows relatively good applicability in the full-scaled tunnel and is valid for a wide range of testing conditions.