| With the rapid development of tunnel construction in China,tunnel fire safety has become an urgent problem.In order to study the smoke propagation and confinement under different ventilation longitudinal condition,A series of propane fire experiments were conducted in a1/20 reduced-scale tunnel with the mass rates from 0.12 m~3/h to 1.90 m~3/h,corresponding to the theoretical heat release rate(HRR)of 2.8 k W to 44.7 k W in reduced-scale tunnel or 5 MW to 80 MW in full-scale tunnel,and tunnel fire characteristics including heat release rate,temperature distribution,back-layering and critical velocity were studied.A tunnel fire is easily affected by the ventilation condition,due to the closure structure,especially for a long-narrow tunnel,and it is well-accepted that ventilation condition has a marked impact on heat release rates of pool fires or cribs fires in tunnel.However,the results from the systematic calibration in this paper on the HRRs of propane fires in tunnel at different ventilation conditions show that the ventilation also has a significant impact on HRRs of propane gas fire.In addition,it is questionable to simply estimate HRRs based on the gas supply and directly estimate the combustion type based on the relationship between the fuel mass loss rate and the air flow rate in tunnel fires.Temperature distribution is not only related to the fire size,but also to the tunnel wall conditions.Based on dimensionless analysis,and comparison with empirical formula,reduced-scale and full-scale fire tests,it is reasonable to believe that there is a relationship between temperature and scale correlations.In this paper,a prediction model of vertical temperature distribution is proposed by dimensionless processing and based on the distribution function of vertical velocity in fluid mechanics.The length of back-layering was determined both by temperature rising and laser sheet in this paper and the former will overestimate the length of back-layering,with the effect of heat radiation.Based on the dimensionless analysis of the back-layering length,it was found that when the tunnel height is the characteristic length,the wall condition has an important influence on the back-layering length,while when the hydraulic diameter is the characteristic length,the effect of wall condition on the length of back-layering is weaken,and related to the shape of the tunnel.Based on the study of critical velocity,a new concept,i.e.,sub-critical velocity,is proposed in this paper.The sub-critical velocity allows the length of back-layering to be greater than zero as far as the presence of back-layering has no detrimental impact on the safe egress of tunnel users,i.e.,the back-layering height should not be lower than 1.8 m,and the radiant heat flux of the gas layer should not be higher than 2.5 k W/m~2.The results showed that sub-critical velocity is around 45～60%of critical velocity.The adoption of sub-critical velocity in the design of longitudinal ventilation system in tunnel not only significantly reduces the cost of fans to drive the airflow,but also minimizes the impact of ventilation on heat release rates. |
PDF Full Text Request