Study Of The Combustion Characteristics Of Tunnel Fires Under Different Levels Of Restriction

"The outline of the 14 th Five-Year" Plan proposes to build a modern integrated transport system,promote the integrated development of various modes of transport,improve network effects and operational efficiency,and accelerate the construction of a strong transport country.At present,the number of traffic tunnels and their scale of operation in China are the highest in the world.The large number of traffic tunnels has posed new challenges and needs for fire prevention and emergency evacuation of buildings and structures.In recent years,tunnel fires have occurred in various countries for various reasons(rear-end traffic accidents,fires caused by leaking hazardous chemicals in vehicles,fires caused by electrical wiring in motor vehicles,etc.)and the consequences have been severe.In tunnel fires,the uncertainty of the location and number of sources of combustion will lead to different degrees of confinement and therefore different combustion characteristics,as well as different fire hazards and rescue difficulties.Therefore,this paper will investigate the combustion characteristics of single and double sources in tunnel fires with different degrees of confinement,with the aim of laying the theoretical foundation for the design of smoke control and evacuation and rescue in tunnel fires.Firstly,this paper presents a numerical simulation of single-fire source fires in naturally ventilated tunnels with different degrees of confinement,and derives a prediction model for the mass flow rate distribution of smoke based on the dimensional analysis method;secondly,the combustion characteristics of symmetrical two-fire source fires in naturally ventilated tunnels are investigated using a combination of experimental research and theoretical analysis;finally,the combustion characteristics of asymmetrical two-fire source fires in longitudinally ventilated tunnels are investigated using numerical simulations.Finally,numerical simulations will be used to investigate the combustion characteristics of asymmetric dual-fire sources in tunnels with longitudinal ventilation.The main research areas of this project include the following three aspects:Firstly,the distribution of temperature and mass flow rate of smoke under the ceiling of a single-fire source fire in a naturally ventilated tunnel at different near-wall distances is simulated using the fire dynamics simulation software FDS(Fire Dynamic Simulator).The results show that the maximum smoke temperature rise below the ceiling decays exponentially along the longitudinal direction of the tunnel for different lateral fire locations and heat release rates.The smoke mass flow rate from a wall fire under the ceiling is significantly lower than that from other lateral fire locations.A quantitative model for predicting the average smoke mass flow rate in the one-dimensional horizontal phase of the tunnel is proposed based on a dimensional analysis,taking into account the HRR of the fire source and the proximity to the wall,indicating that for non-adherent fires,the dimensionless smoke mass flow rate is proportional to the 0.19 power of the dimensionless heat release rate.In addition,the plausibility of the predictive model is verified by comparing it with a large number of experimental results from full-scale and scaled-down experiments at different wall-fitting distances.Secondly,drawing on existing research on the combustion characteristics of single-and dual-fire source fires in naturally ventilated tunnels,a series of downsizing experiments were conducted to investigate the dynamic evolution of symmetrical dual-fire source fire plume behaviour in naturally ventilated tunnels at different fire source spacing and near-wall distances,and then to clarify the combustion rate,mass loss rate,flame morphology,flame The fire plume evolutionary law was then clarified in terms of combustion rate,mass loss rate,flame morphology,flame fusion and smoke temperature distribution under the ceiling.It was found that the mass loss rate,heat release rate and flame height of the fuel pool were negatively correlated with the horizontal near-wall distance and the distance between the two sources of fire.The flame colour of fuel ethanol combustion changed from yellow to blue-purple as the lateral fire position increased.As the fire spacing increases the maximum smoke temperature distribution curve along the tunnel longitudinal direction changes from a significant ’single peak’ to a ’single peak’ slowing to a ’double peak’,and the impact point changes with the fire spacing.The impact point varies with the fire source spacing.The maximum temperature rise under the ceilling of the near-fire area was predicted for the dimensionless fire source spacing between(0,4D],and a mathematical expression for the decay law of the dimensionless far-fire source temperature rise was constructed.Thirdly,numerical simulations were used to analyze the combustion characteristics of asymmetric dual-fire sources in tunnels at eight longitudinal ventilation wind speeds for different offset fire sources,and to study the temperature and visibility of the characteristic height layer to the human eye,smoke bifurcation and stratification conditions.The results of the study show that: the temperature and visibility of the human-eye characteristic height layer of a small fire source at different deflections at a longitudinal ventilation wind speed of 1.0 m/s are not conducive to safe evacuation and rescue in time and space;smoke bifurcation occurs under the ceiling under the coupling effect of longitudinal ventilation and sidewall restriction in the tunnel,and as the Fm deflection decreases,the maximum smoke temperature near the fire source at the longitudinal centre line differs from the trend for single-fire source and symmetrical two-fire source fires on the tunnel centreline,but.When the longitudinal ventilation wind speed is 1.5 m/s to 2.5 m/s,the smoke in the tunnel appears to be better stratified and the smoke temperature does not pose a threat to personnel even if there is a backflow.When the wind speed increases above 3.0 m/s,the smoke layer in the tunnel is completely destabilised.The combination of the above four points leads to the conclusion that for asymmetrical two-fire tunnel fires at different offset distances a wind speed of 1.5 to 2.5m/s can be used for ventilation and evacuation of the tunnel.