Experimental studies of the process of initiation and spread of the dangerous fire factors while burning of the tube stock in the tunnel

The present article is concerned with the analysis of the experimental results performed with full-size large-scale prototypes of tube stock in subways in case of the fire in the tunnel. The process of the fire spread in the passenger compartment and along the tube stock is investigated.


Introduction
The experience of subways operation provides evidence that the fires in them occur rather regularly, while in case of the operational imperfections and the absence of required level of the fire protection the fires can result in catastrophic consequences, accompanied by mass mortality of the people and large material loses.
Dangerous factors of the fire (DFF) affecting the people and assets include [1]: 1) Flame and sparks; 2) Heat flow; 3) increased temperature of the environment; 4) increased concentration of the toxic combustion products and thermal decomposition; 5) Reduced concentration of oxygen; 6) Decrease of visibility in the smoke. Dangerous factors of the fire, according to [2] include: -increased temperature; -smoke; -reduced concentration of oxygen; -toxic combustion products and products of thermal decomposition. Critical duration of the fire is determined as achievement by one of the DFF its ultimate value. From the obtained results of the calculations or the experimental investigations of the values concerned with the critical duration of the fire the minimal one is chosen.
The data known from the literature [3] mean that the main reason of combustion of the tube stock in the subway is malfunction of the electrical equipment.
Therefore, as the possible points of the fire appearance the following ones were adopted: -operator's cab of the tube stock; The object of investigations is the process of the fire spread along the tube stock in the single-track in the interstation tunnel of subway and conditions of DFF formation at the escape routes. By the implementation of the full-scale experiments in the experimental tunnel and using methods of physical modeling it was investigated the process of the fire spread into the cabin of the transit vehicle as well as along the tube stock [4,5]. In addition, temperature measurements were performed as well as gas analysis of the combustion products.
In order to study possible ways of the fire spread into the transit vehicle and its affect on the partition the flame experiment was performed inside the operator's cabin of the transit vehicle of «Е» serial number. For combustion of the cabin 0.5 kg of cotton waste and 1 liter of kerosene were applied.
Development of the fire occurred in the following way: 1-2 minemission of smoke into the passenger cabin over set-in of the door; 7 minincrease of the smoke emission into the passenger cabin; 17 minintensive smoke emission into the passenger cabin; 26 minflame yield into the passenger cabin (under the ceiling); 28 mindestruction of the windshield in the operator's cab; 31 minfalling down of the burning of ceiling lining in the passenger cabin.
According to the description of the experiment one can assume that in the period from 7-17 minutes both as in the passenger cabin as in the tunnel attaining of the critical DFF values happened: loss of visibility, reduced concentration of oxygen and formation of the toxic combustion and thermal decomposition products.
In other experiments [6-9] 1 kg of cotton waste wetted in 0.5 liter of kerosene was applied as a combustion source. Air flow rate of 1,5 m • s -1 was provided with the help of ventilation installation mounted in the butt of the tunnel. Four fire experiments were performed in the tunnel full-sized mockup; in each of them operators cabin was stuffed with different fire load.

Results of experiments
A distinctive feature of these experiments from the previous ones was the fact that fire-resistant partition was place between the operator's cabin and passenger cabin, that prevented DFF spread to the passenger compartment of the transit vehicle. However, destruction of the wind-shield in the operator's cabin resulted in formation of the DFF in the tunnel and in the transit vehicle.
Insignificant excess of temperature about 60 о С during 5 minutes in the tunnel was registered in the experiment at the level of operating zone opposite the first door of the injured transit vehicle relative to the seat of fire. However, no crucial value of temperature was attained in this experiment. The value of 70 о С was taken as a crucial one in accordance with [2,10].
Moreover, while performing the experiments a dynamics of spread of the toxic substances by escape routes was studied. The following toxic substances were analyzed: CO, HCN, HCl. In the experiment the values of CO concentration were determined exceeding that one maximum permissible concentration (MPC) in the case of accident. It was implied that MPC for CO in this case was adopted as that one if a man can escape from the dangerous area and take part in the accident elimination [11 -13]. For example, inside the injured transit vehicle the crucial MPC level for CO was attained at 16.5 minute, while in the tunnel at the juncture of the injured and adjacent transit vehicles it was attained at 25-th minute. At the same time no MPC level of CO was attained at the escape routes.
The values of HCN and HCl concentrations inside the cabin of injured transit vehicle (33,0 mg•m -3 and 33,0 mg•m -3 , respectively) and in the tunnel (28,0 mg•m -3 and 35,0 mg•m -3 , respectively) already in the first ten minutes indicate at the possibility of dangerous effect of these substances on the people during the fire. While combustion of the under-carriage equipment inside full-size tunnel mockup accumulator box was taken as a combustion source representing the largest fire load in the under-carriage equipment (92,4 kg•m -2 ).
Just as in the first variant temperature, fields and composition of the gas environment in the injured, adjacent transit vehicles and in the tunnel studied for the fire-resistant design of the field.
Results of the experiment demonstrated that the maximal temperature at the distance of 1.5 m from the floor of transit vehicle (thermocouple was placed just above the fire set area) was attained for 24 minutes and its maximum value (60 о С) was below the crucial one, as determined in [2]. In the other points of measurements, the temperature did not exceed 40 о С.
From the viewpoint of support of the safe evacuation in this experiment a special interest is connected with formation of the toxic gases concentration. Figure 1 represents data on CO concentration at the escape routes. As it is seen from the represented data concentration of the toxic gas is not especially dangerous while the passenger's evacuation. Analysis of results concerned with the changes of HCN and HCl concentrations at the certain parts, showed that: -HCN concentration for the first ten minutes inside tunnel is much smaller the accident MPC, that is of 16 мг•м -3 [11] under isolated effect on a human for 15 minutes; -the changes of HCl concentration within the first ten minutes near the injured transit vehicle did not go beyond the permissible limit [31]. Near adjacent transit vehicle HCl concentration at the 15-th minute of a stable combustion of the accumulator box and wire insulation increased up to 55 mg•m -3 , that is as more than twice bigger than the value set in [2].  In the study of the floor fire-resistance for the transit vehicle «Е» the wooden accumulator box was used as a fire load of the simulated fire seat.
The firing was performed with the use of 1 kg of the cotton waste wetted with 1,5 litres of kerosene and placed on the boxes of the storage batteries. Combustion was developed in the following way: 1 minsmoke emission from combustion of the cotton waste; Results of experiments demonstrated that at the time of attaining maximum temperature values coating of the floor from the cabin side was heated up to 250-270 о С in several points. An intensive thermal decomposition of the plywood takes place in this moment of time with a considerable smoke emission. Decrease of the oxygen content in the cabin was of 2%, while content of carbon oxide was of 1 %, that is quite permissible for the safe escape of passengers. Combustion went beyond the sizes of the transit vehicle, next, the paint caught fire on the side surface, the temperature at 100 mm from the surface of the door attained 400-500 ° С, that makes it impossible to pass this part of the route in the tunnel by passengers along the injured transit vehicle. Change of temperature at 100 mm from the surface of the door in the lower area is presented in figure 2. As it is seen from the data presented in the figure, the crucial value of temperature was attained to the 15-th minute after beginning of combustion.  Figure 2. Change of temperature at 100 mm from the surface of the door in the lower zone: 1-temperature at 100 mm from the door surface in the lower zone; 2-crucial value of the temperature.
Next object of the study was the process of fire development in the tunnel. Temperature conditions were investigated by the results of fire experiments with transit vehicles in the mockup of the part of tunnel of М1:1 size. Thermocouples were arranged in the tunnel mockup at the height of 1,5 m above the level of track concrete that corresponds to the level of the operation zone. The doors of injured transit vehicle were opened from the side where thermocouples were arranged. Patch array of thermocouples at the escape routes inside the tunnel is presented in figure 3.