Study Of Smoke Transport Characteristics And Smoke Extraction Strategies Induced By Thermal Pressure In Underwater Tunnels

With the rapid development of China’s national economy and the rapid process of urbanization,urban underground space has been further developed and the number of urban rail transit projects has been on the rise.Compared to traditional road tunnels,the structure of underwater tunnels is more complex,as shown by the existence of inclined tunnels on both sides and nearly horizontal tunnels in the middle part,and the construction lengths are longer than those of general road tunnels,with more complex structures and more elements affected by fires,which will cause huge casualties and property damage.The lack of in-depth research on typical fire scenarios in underwater tunnels and the unclear evolutionary mechanism of smoke propagation at this stage have resulted in relatively few key parameters and effective smoke extraction methods for fire protection design.Based on this,this thesis combines theoretical analysis and numerical simulations to investigate the temperature distribution patterns of inclined and horizontal tunnel sections in underwater tunnels under natural ventilation,the smoke spread characteristics and critical air velocity under the synergistic effect of a single smoke extraction method/longitudinal ventilation and lateral smoke extraction,with the following main conclusions:Numerical simulation of tunnel fires was carried out to carry out the variation laws of maximum temperature rise and longitudinal temperature decay in the inclined tunnel section and horizontal tunnel section under the influence of different inclination angles of the inclined tunnel section,different longitudinal positions of the fire source and different heat release rates of the fire source.The results show that: in the horizontal tunnel section,the maximum temperature rise is not greatly affected by the longitudinal position of the fire source because of the loss of supplementary air passing through the inclined tunnel section;as the inclination angle of the inclined tunnel section increases,the maximum temperature rise in the horizontal tunnel decreases,and both the maximum temperature rise and longitudinal temperature decay in the horizontal tunnel section are studied in depth,a prediction model relating the maximum temperature rise and the heat release rate of the fire source in the horizontal tunnel section is given,and the horizontal The longitudinal temperature decay in the tunnel section shows a double exponential decay.Based on the characteristics of the inclined tunnel section,a prediction model for the maximum temperature rise in the inclined tunnel section is developed based on the velocity of the induced airflow,and a prediction model for the longitudinal temperature decay is developed based on the mass flow rate of the makeup air.The factors affecting the induced airflow wind speed include: the rate of heat release from the fire source,the inclination angle of the inclined tunnel section and the distance from the left side of the tunnel opening.The smoke spread under a single exhaust method was analysed separately for longitudinal ventilation and lateral exhaust,which are currently commonly used in underwater tunnels.The results show that the critical wind speed is much higher than the velocity at the lateral smoke vent when the lateral ventilation is turned on alone,so the design of the wind speed for the lateral smoke vent should be based mainly on the critical wind speed at the lateral smoke vent.When longitudinal ventilation is turned on alone,it is found that the fan air speed decays faster as the inclination angle increases.A theoretical model is established for different fan smoke exhaust air speeds and slopes with respect to the air speed when reaching the horizontal tunnel section,and the relationship between fan smoke exhaust air speed and smoke counterflow length is discussed in the case of a larger heat release rate from the fire source,and an empirical prediction model for fan air speed and critical air speed is established.A numerical simulation method was used to analyse the smoke extraction effect of the coordinated smoke extraction methods with different slopes and different smoke extraction air velocities for the longitudinal and lateral smoke extraction methods of the underwater tunnel.The results show that when longitudinal ventilation and lateral smoke extraction act together,the original critical wind speed of lateral smoke extraction increases and the smoke settling conditions are different in the upstream and downstream of the fire source,i.e.the smoke layer height is lower in the upstream of the fire source due to the influence of longitudinal ventilation and higher in the downstream of the fire source due to the influence of mainly lateral smoke extraction.At the same time,the integrated frequency method was used to analyse the height of the smoke layer downstream of the fire source near the inclined tunnel opening,and it was found that the inclination angle and lateral smoke exhaust wind speed did not have a significant effect on the overall height of the smoke layer downstream of the fire source,but when the inclination angle was larger and the smoke exhaust wind speed was higher,the height of the smoke layer would increase significantly.