Studies On Characteristics Of Fire Combustion And Smoke Movement In Tunnels Under Different Environmental Conditions

With the development of economy and the increase of people's travel demand,the number of traffic tunnels is increasing.While tunnel traffic brings convenience to us,tunnel fires also caused great losses of life and property.A deep understanding of the characteristics of fire combustion and smoke movement in tunnel fires under different environmental conditions makes practical significance for the prevention and control of tunnel fires and protecting the safety of personnel and property in tunnel fires.In view of the shortcomings of current tunnel fire research,small-scale experiments,theoretical analysis,numerical simulation and experimental comparison were used in this study to investigate the characteristics of fire combustion and smoke movement in tunnel fires under the effects of longitudinal wind,sidewall restriction and lower ambient pressure.Based on small-scale tunnel fire experiments,the combined effects of longitudinal wind and sidewall restriction on the characteristics of flame shape and downstream heat transfer of a propane burner fire were studied.Firstly,the flame tilt angles of the two different fire source positions(centerline fire and sidewall fire)were both found first increasing rapidly and then increasing slowly with the increase of wind speed,and the difference of the flame shape between the two fire source positions was compared and analyzed.The evolutions of the flame drag length with the wind speed for the two fire source positions were expounded and compared,and the force mechanism of the flame was analyzed.Using the flame length under no wind condition as a reference,the correlations of flame drag length for different fire source positions were established.Secondly,under larger wind speeds,the downstream radiative heat flux and temperature rise were both found to first increase and then decrease with increasing distance from the fire source.The peak radiative heat flux and peak temperature rise both appeared near the front end of the flame drag.The downstream radiative heat flux distributions as well as temperature rise distributions were compared for the two different fire source positions,respectively.Based on the solid flame model,two radiation models for the two fire source positions were proposed.The correlations of the peak temperature rises and their location for the two fire sources positions were propposed,and the variations of the temperature attenuation beyond the peak temperature rise were given.Based on small-scale tunnel experiments,the combined effects of longitudinal wind and sidewall restrictions on the fuel burning rates and thermal feedback mechanism of n-heptane pool fire were studied.Firstly,combined with the flame shape,the evolutions of the fuel burning rates under two fire source positions(centerline fire and sidewall fire)were analyzed.It was found that the fuel burning rates increased rapidly with the increase of wind speed,then quickly decayed;then increased slowly and tended to be stable.The variations of fuel burning rates with wind speed for the two fire source positions were compared and explained.Secondly,the radiative thermal feedback power and conductive thermal feedback power received by the fuel were calculated,respectivly,and the evolutions of the radiative and conductive thermal feedback power with wind speed for the two fire source positions were analyzed by comparing.The internal relationship among the fuel burning rates,thermal feedback and flame shape was revealed.Finally,according to the total thermal feedback power received by the fuel,the ratios of three kinds of thermal feedbacks were calculated and compared for the two fire source positions.Combining numerical simulations,theoretical analysis and experimental comparison,the effects of ambient pressure on the characteristics of smoke movement and smoke control under longitudinal ventilation were studied.Firstly,the effects of ambient pressure on smoke movement stage divisions and longitudinal ceiling smoke temperature distribution were revealed,and a correlation of the starting position of one-dimensional horizontal spreading stage was established.Using the temperature rise at the starting position of one-dimensional horizontal spreading stage as a reference,a unified correlation for the dimensionless ceiling smoke temperature rise attenuation under different ambient pressures and heat release rates was obtained.Secondly,the development of smoke mass flow rate along the tunnel was studied.A correlation of averaged smoke mass flow rate for one-dimensional horizontal spreading stage was proposed,and compared well with the experimental results from different ambient pressures and experimental scales.Finally,the effects of ambient pressure on smoke backlayering distance and critical ventilation velocity were studied.Based on the dimensional analysis and simulation results,a correlation of dimensionless critical ventilation velocity for different ambient pressures was proposed.