Effects of a transverse external wind on natural ventilation during fires in shallow urban road tunnels with roof openings

Fire experiments using a 1:12 scale model tunnel were conducted to evaluate effects of a transverse external wind on the performance of roof openings in a shallow urban road tunnel under fire conditions. A particular focus was placed on clarifying the characteristics of the spread of smoke in the tunnel under the condition of a transverse external wind blowing above the roof openings. Two types of median barriers, pillars and walls, were examined as dividing structures in the model tunnel, and the heat release rate was selected as the experimental parameter. The following conclusions were obtained. Compared to conditions with no external transverse wind, in the model tunnel with the pillar median structure, the smoke spreading distance was shortened when the transverse external wind was in the positive direction, and the distance remained the same when the wind was in the negative direction; in the model tunnel with the wall median structure, the smoke spreading distance was shortened when the wind was in the positive direction. Furthermore, the smoke spreading distance in each case was constant and independent of the heat release rate of the fire under the scope of the experimental conditions used in this study.     

Characteristics of smoke extraction by natural ventilation during a fire in a shallow urban road tunnel with roof openings

The characteristics of the spread of smoke were investigated for a fire occurring in a shallow urban road tunnel with roof openings in its ceiling. In this type of tunnel, the smoke produced by a fire is ventilated through the openings in the ceiling given the natural buoyancy of hot smoke. A fire experiment was conducted using a 1/12 scale model tunnel to ascertain whether natural ventilation via the roof openings was sufficient to maintain a safe evacuation environment for tunnel users. The distance from the fire to the tip position of the spreading smoke and the thickness of smoke layers along the ceiling were investigated by changing the heat release rate and using two types of median structure as experimental parameters. The two types of median structure dividing the tunnel into two road tubes were pillars and walls. It was clarified that the smoke spreading distance was constant and independent of the heat release rate of the fire under our experimental conditions. Moreover, it was confirmed that the thickness of the smoke layers in the tunnel thinned out quickly due to the natural ventilation.     

Possibility of using roof openings for natural ventilation in a shallow urban road tunnel

Naturally ventilated urban vehicular tunnels with multiple roof openings have increased in China. Unnecessary gas (polluted air or fire smoke) are expected to be exhausted out through openings. Whether its safety standards can be satisfied or not still needs to be verified. In this paper, a safe CO concentration was firstly discussed, and a heat risk level of very high to extreme up to 46 °C was given. Secondly, a real 1410 m tunnel was proposed, and a 1/10 scale model tunnel was reproduced. Ambient winds of 0.95 m/s in prototype and 0.3 m/s in model were considered. Under normal traffic test, a track circuit was constructed with model vehicles moving on it to form traffic wind, and once the air velocity was larger than 0.31 m/s, the airflows were found to be not relevant to the Reynolds number. The traffic winds were weakened by openings. For three of all tested traffic, the actual air velocities were larger than the required ones, so its air qualities were satisfied. In firing test, two sets of burning experiments were conducted with which the heat release rates (HRR) were 8.35 kW and 13.7 kW. Large amounts of smoke were exhausted out of openings, and the high-temperature was not significant. Full-scale numerical simulations were carried out to verify the experimental results respectively using Fluent 6.0 for normal traffic and FDS 4.07 for firing. The simulations were compared well with the experiments. Further FDS simulations show that the openings’ mass flow rates are influenced little by ambient temperature; with the increasing length of the buried section, much smoke accumulate inside leading to a high temperature; having 4–5 openings in one shaft group is oversize in the actual engineering design.     

Full-scale experiment research and theoretical study for fires in tunnels with roof openings

In order to assess the possibility of exhausting smoke through passive roof openings and the influence of smoke on personnel in the tunnels, full-scale fire experiments in tunnels with roof openings are carried out, which were rarely reported in the previous references. The data of smoke propagation, smoke sedimentation, velocity field and temperature field are measured. On the basis of the smoke longitudinal propagation laws, the prediction model of calculating backlayering distance is built. The Kurioka model and the built mathematical models are validated by those experiments. All the experimental data presented in this paper can be further applied for verification of numerical models, and bench-scale experimental results. Those full-scale experimental results and theoretical analysis can also be used for directing tunnel fire research, which afforded scientific gist for fire protection and construction of road tunnel with roof openings.     

Performance validation of a hybrid ventilation strategy comprising longitudinal and point ventilation by a fire experiment using a model-scale tunnel

We examined the exhaust performance of a hybrid ventilation strategy for maintaining a safe evacuation environment for tunnel users in a tunnel fire. The hybrid ventilation strategy combines the longitudinal ventilation strategy with the point ventilation strategy which is a type of transverse ventilation strategy. The model tunnel developed by this study was scaled to 1/5 the size of a full-scale tunnel. The model-scale experiment was performed taking into consideration Froude's law of similarity. Measurement items were the distribution of temperature and concentration of smoke inside the tunnel, longitudinal wind velocity, mass flow of smoke in the point ventilation duct, and the heat release rate of the fire source. The following main conclusions were obtained. The smoke height was constant even when varying the extraction rate of smoke from the ceiling vent. The backlayering length and critical velocity of the smoke flow in the hybrid strategy could be predicted by the methodology developed by using the longitudinal strategy. The hybrid strategy maintained a safe evacuation environment on both sides of the tunnel fire.     

Studies on buoyancy driven two-directional smoke flow layering length with combination of point extraction and longitudinal ventilation in tunnel fires

This paper investigates the buoyancy-driven smoke flow layering length (both upstream and downstream) beneath the ceiling with combination of point extraction and longitudinal ventilation in tunnel fires. A theoretical model is developed based on previous back-laying model with only longitudinal ventilation, with modified actual heat release rate, as well as modified upstream and downstream opposing longitudinal air flow velocities by the induced flow velocity due to point extraction. Experiments are carried out in a reduced scale model tunnel with dimensionless of 72 m×1.5 m×1.3 m. A LPG porous gas burner is used as fire source. The smoke flow layering length both upstream and downstream are identified based on temperature profiles measured along the ceiling, for different experiment conditions. CFD simulations with FDS are also performed for the same scenarios. Results show that with combination of point extraction and longitudinal ventilation, the smoke flow layering length is not symmetric where it is longer downstream than that upstream. The upstream smoke layering length decreases, while the downstream layering length increases with increase in longitudinal ventilation velocity; and they both decrease with increase in point extraction velocity. The predictions by the proposed theoretical model agree well with the measurements and simulation results.     

Modeling for predicting the temperature distribution of smoke during a fire in an underground road tunnel with vertical shafts

In this study, fire experiments using a 1:20 model-scale tunnel were conducted to investigate the performance of natural ventilation in an underground road tunnel with six vertical shafts. The experimental parameters were the heat release rate of a fire source and the height of the shafts, and nine experiments were conducted in total. Furthermore, simple models were developed for predicting the temperature distribution of the smoke flowing under the tunnel ceiling. The following results were obtained: (1) In the experiments, the form of the smoke exhausted from the shaft became plug-holing when the shaft height was 1.0H_t, and became boundary layer separation when the height was 0.24H_t. (2) The average efficiency of heat exhaust was 0.16 when the form was plug-holing, and was 0.12 when the form was boundary layer separation. (3) When the form was plug-holing, the ratio of entrainment of fresh air became almost constant regardless of Ri. On the other hand, when the form was boundary layer separation, the ratio of entrainment of fresh air was smaller than that under the condition of plug-holing. (4) The temperature distribution under the tunnel ceiling predicted by the models agreed with that measured by the fire experiments in all cases.     

Experimental study of fire growth in a reduced-scale compartment under different approaching external wind conditions

In order to clarify the fire growth process in compartments under external wind conditions, detailed fire tunnel experiments were conducted in a reduced-scale compartment. The approaching external wind velocity was set to 0.0, 1.5 and 3.0 m/s, and the location of the fire source was changed between the downwind corner, upwind corner and center of the compartment. The experiments considered the effect of wind on a through-ventilation situation. The temperatures of the air and the wall surfaces in the compartment and the temperatures of the flames ejected from the opening were measured. The fuel mass loss rate and the heat flux from the opening were also recorded. Different fire growth characteristics are shown under different wind and fire source conditions. The temperature rises faster and burnout time is reduced under windy conditions. It is found that external wind has two opposing effects. One is to promote combustion within the compartment and thus raise the temperature, the other is to blow away and dilute the combustible gases in the compartment and decrease the temperature, or hasten its extinction. When the approaching wind velocity is high, the external plume is greatly inclined to the downwind side, and the flame becomes larger, thus increasing the risk of the fire spreading to neighboring buildings. The dimensionless temperature of the external flame was a little lower than the results indicated by Yokoi's experiments without wind.