Sprinklers Modeling for Tunnel Road Fire Fighting

Transcription

1 Sprinklers Modeling for Tunnel Road Fire Fighting P. Ciambelli, M.G. Meo, P. Russo, S. Vaccaro Department of Chemical and Food Engineering, University of Salerno - ITALY 1. INTRODUCTION Loss of lives and strong damages to tunnel structures consequent to fires in European tunnels, placed a focus on fire safety issues concerning tunnels. Beside traditional means of fire control and protection, new innovative measures such as water mist and sprinkler systems have been proposed. However, their employment, although accepted as good standard in other applications [1], is controversial for tunnel fires as its benefit is strongly connected to fire scenario, ventilation conditions and fire reliable detection [2]. Ventilation, instead, is by far the best known mitigation system and, then, it is widely applied in existing tunnels although there is no guarantee that it will efficiently work in all fire scenarios. Computational Fluid Dynamics (CFD) may be usefully employed to assess the performance of different mitigation measures and to design properly fire protection systems. The present authors in previous studies [3,4] used a CFD code, JASMINE, to assess the effectiveness of several fire protection systems in a m long tunnel where a car incident evolved in a severe gasoline pool fire. They found that the only natural ventilation was not sufficient to assure safety conditions for escape and rescue operations and that, among the various tunnel forced ventilation systems (axial and transverse), the axial proved to be very effective for immediately creating upstream the fire a safe route for evacuation and rescue, although very unsafe conditions took place downstream the fire. The authors also investigated the mitigation effect on the hypothesized tunnel fire of the presence of a sprinkler system alone [5]. They found that it helps in confining the fire and in reducing the spread and the temperature of the hot gases along the tunnel on both sides with respect to the fire site, and strongly limits tunnel structural damages. Negative effects of the presence of sprinklers were found on the oxygen concentration close to the fire location and on the visibility distance. The focus of the current study was to investigate the mitigation effect of the simultaneous presence of a sprinklers system and of a system on the hypothesized tunnel fire consequent to an car crash and gasoline spilling from a tank truck. 2. CASE STUDY A gasoline pool fire in a tunnel was assumed as scenario. The tunnel is a two-tubes (each tube one-way) road tunnel located between Pontecagnano and Salerno along the Italian highway A3. It is about m long, with an arched cross section of 12 m x 7 m and it is equipped only with natural ventilation and has no fire mitigation systems. A 1 x 1 m 2 pool fire with a burning rate of.4 kg/m 2 s was assumed at the tunnel centre, with a very fast growth rate (up to 17 MW in 9 s) followed by a constant HRR phase until 1 min. 3. NUMERICAL SIMULATIONS 3.1. The CFD-Sprinkler model The JASMINE code by FRS/BRE, uses CFD to describe the heat and mass transfer processes associated with the dispersion of combustion products from a fire. An Eulerian-Lagrangian sprinkler particle-tracking model, SPARTA, is coupled to the CFD model to allow to predict the interaction of the water spray with the combustion gas. The computed motion, heating and IX-4, 1

2 31st Meeting on Combustion evaporation of the droplets are fully coupled to the gas phase CFD calculations to incorporate momentum, mass and heat transfer between gas phase and droplets [1, 6, 7]. The simulations were run using the modified k- model for turbulence and the eddy break-up model for combustion. The total energy available for convection was assumed to be 7% of the real HRR to take into account the energy loss for radiation. An extended domain was employed to ensures realistic simulation of the plume outside the tunnel. A grid with variable cell size was set. A 5 s time step was set for the simulations without sprinklers and 2 s for the simulations with sprinklers. The maximum number of iterations per time step was 1. A forced with a global 6 m 3 /s air flow rate was simulated by two ceiling axial fans located close to the entrance portal. A sprinklers system set up in the tunnel central zone was hypothesized by 5 pendant automatic bulb-activated heads with a flow rate of 9 dm 3 /min each arranged in a 2 (across) by 25 (along) grid, spaced 5 m x 1 m. The sprinklers were assumed to be Early Suppression Fast Response, extended coverage type, going into action if they were heated to their actuation temperature (8 C). During the simulations in the case of the sprinklers system alone, they resulted symmetrically activated along the tunnel progressively in 12 s, while in the case of the combined sprinklers and system all the heads downstream and those restricted to 4 m upstream resulted activated after 9 s The simulation results Simulations results yielded fire-induced temperatures, species and pollutants concentrations. The visibility distance was estimated assuming a smoke particulate yield of.5 kg/kg of fuel. The performance of the single and combined mitigation systems ( and sprinkler) in managing and containing the consequences of the hypothesized tunnel fire was evaluated by comparing values of calculated gas temperature, CO, CO 2 and O 2 concentrations, visibility and radiation heat flux at the breathing height of 2 m, on the tunnel centreline, at the presumed distances walked down by pedestrians escaping from the tunnel centre and at the pertaining times with the accepted safety values for tunnel users. The presumed distances walked down by escaping pedestrians and at the pertaining times were calculated considering an escape delay of 9 s after the fire ignition and a pedestrian flight velocity of 1.2 m/s Accepted safety criteria for tunnel users The survival aims for tunnel users are: i) pollutants and toxic species concentrations below dangerous values (IDLH is 5 ppm for CO 2 and 15 ppm for CO [8]); ii) minimum 16 vol % O 2 concentration to allow breathing without impairing thinking and coordination [9]; iii) smoke concentration such that illuminated signs should be discernible at 1 m [1]; iv) gas temperatures not exceeding 6 C, or 5 C when water based suppression are activated [1]; v) radiation heat flux below 5 kw/m 2 [1]. 4. RESULTS Simulations results are reported in Fig. 1 as space profiles of the gas temperature at different times (t = 2, 3, 4 and 5 min) after the fire start during the early phase of the fire and in Fig. 2 as time profiles of temperature (Fig. 2a and 2b), oxygen concentration (Fig. 2c and 2d) and visibility (Fig. 2e and 2f). These time profiles, however, are not obtained at a fixed distance from the fire but, for each time, at the distance, both upstream and downstream the fire, presumably (according to the assumption reported under section 3.2) walked down by pedestrians who were close to the fire location when the fire started. In Fig. 2 there is also a red hatched zone, which represents for each variable the region where the survival criteria, IX-4, 2

3 Italian Section of the Combustion Institute reported under the section 3.3, are not fulfilled. Therefore, if the value of the variable is in the red hatched zone it gives rise to unsafe conditions for people. The results in Fig. 1 show how the keeps very low the temperature upstream the fire, but at the same time it rapidly increases the temperature along the whole tunnel portion downstream the fire by carrying over there the combustion gases. With the sprinklers system alone a global beneficial effect on confining the fire, cooling the gases and reducing their spread along the tunnel occurs, as it is clearly shown particularly by the temperature profiles in Fig. 1c and 1d. When the forced ventilation and the sprinklers are simultaneously operated, Fig. 1 shows very safe conditions upstream the fire (a little better than with the only forced ventilation) but unsafe conditions take place downstream the fire. Actually, the gas temperature is much lower than with the only forced ventilation but in any case much higher than that necessary to survive. The combined effect of the sprinklers and of forced ventilation limits the temperature of the smoke layer throughout the tunnel zone downstream the fire. Compared to the longitudinal ventilation alone, the automatic sprinklers greatly cool the hot layer along all the downstream zone when the fire is fully developed (t = 4 min, 5 min). However, while the sprinklers system alone was able to contain most of the combustion products within the fire zone cooling the smoke layer, the combined added effect of the ventilation system extends the smoke movement along the tunnel in a shorter time. At t = 4 min, with the combined ventilationsprinklers systems, the combustion gases spread downstream throughout the tunnel, and at the height of 2 m the hot black smoke completely fills the tunnel. a) b) sprinklers sprinkers tunnel length - distance from fire (m) c) tunnel length - distance from fire (m) d) tunnel length - distance from fire (m) tunnel length - distance from fire (m) Fig. 1 Temperature profiles along the tunnel centreline at 2 m from the floor. a) t = 2 min; b) t = 3 min; c) t = 4 min; d) t = 5 min. The survival possibility, yielded by the various fire protection systems, for people located close to the fire location when the fire starts can be assessed on the basis of data shown in Fig. 2. Specifically, for what concerns gas temperature, only the simultaneous action of forced ventilation and sprinklers assures safe conditions upstream the fire (Fig. 2 a) but none of the IX-4, 3

4 31st Meeting on Combustion protection systems guarantees safety flight downstream the fire (Fig. 2 b). The oxygen concentration upstream the fire results safe for all the protection systems but for the sprinklers alone (Fig. 2 c) while downstream the fire it is unsafe whatever the protection measure but the natural ventilation (Fig. 2 d). For what concerns the visibility, irrespective of the fire protection system, very unsafe conditions for escaping people are found downstream the fire (Fig. 2 f). Upstream the fire, instead, the visibility is always below the safety values for natural ventilation and sprinklers alone (Fig. 2 e). However, and the combination of and sprinklers give rise to safe values of visibility except in the time interval 6-1 s, that is when people who stands close to the fire when it starts, initiates the escape. This would mean that such a people could never initiate the escape. UPSTREAM THE FIRE DOWNSTREAM THE FIRE T ( C) 1 T ( C) 1 a) b) O 2 concentration (v/v) O 2 concentration ( C) c) visibility distance (m) d) visibility distance (m) natural ventilation sprinklers sprinklers + safety value e) f) Fig. 2 (a, b) Temperature, (c, d) oxygen concentration and (e, f) visibility profiles along the time at the respective escape distance from the fire location (at 2 m from the floor, at the tunnel centreline). a), c), e): escape in the upstream direction; b), d), f): escape in the downstream direction. Red hatched: unsafe conditions. Compared to the alone, the simultaneous presence of sprinklers cools IX-4, 4

5 Italian Section of the Combustion Institute down the temperatures, but in any case at values above the safety limit of 5 C. Similarly, the oxygen concentration is below the 16 vol. % minimum concentration downstream the fire up to the exit portal. The smoke downdrag occurs mainly at the location of the sprinklers, in the tunnel section downstream the fire where the activation temperature is reached in very short time because of the asymmetry of the airflow conditions, thus affecting the visibility distance. The presence of sprinklers alone and when combined with the forced ventilation has a further positive effect: it strongly limits tunnel structural damages because very high gas temperatures occurs only in a small region and for short time. This can be appreciated from Fig. 3 where the profiles along the tunnel of the ceiling temperature at 5 min from the fire start, i.e. when the fire is fully developed, are reported by (Fig. 3). 1 sprinklers sprinklers + T ( C) tunnel lenght - distance from fire (m) Fig. 3 Ceiling gas temperatures along tunnel length at the tunnel centerline, t = 5 min. 5. CONCLUSIONS Numerical simulations of a proposed tunnel have indicated the benefits and the limits of a combined system of mechanical smoke management () and sprinklers. Parametric studies could be performed with different ventilation and sprinkler systems to help optimise the design of future tunnel fire mitigation strategies. 6. REFERENCES 1. Miles, S., Kumar, S., Chong, K.: Fourth International. Conference on Tunnel Fires, Basel, Switzerland, December, p. 329 (22). 2. Fielding, L.: International Symposium on Catastrophic Tunnel Fires, Borås, Sweden, November, p. 23 (23). 3. Ciambelli, P., Meo, M.G., Russo, P., Vaccaro, S.: Third International Conference on Tunnel Safety and Ventilation, Graz, Austria, May, p. 324 (26). 4. Ciambelli P., Meo M.G., Russo P., Vaccaro S.: Proceedings of the 29 th Meeting on Combustion of The Italian Section of The Combustion Institute, I15.1 (26).. 5. Ciambelli, P., Meo, M.G., Russo, P., Vaccaro, S.: Fifth International Seminar on Fire And Explosion Hazards, Edinburgh, UK, April, in press (27). 6. Miles, S., Kumar, S.: Fifth International Conference on Tunnel Fires, London, UK, October, p. 23 (24). 7. Kumar, S.: First International Symposium on Safe and Reliable Tunnels. Innovative European Achievements, Prague, Czech Republic, February, p. 97 (24). IX-4, 5

6 31st Meeting on Combustion 8. NIOSH-CDC, Documentation for Immediately Dangerous to Life or Health Concentrations (IDLH). Available from: 9. Rom, W.: Environmental and Occupational Medicine. Little Brown, Boston, (1992). 1. Opstad, K., Stensaas, J.P.: Second International Symposium on Safe and Reliable Tunnels. Innovative European Achievements, Lausanne, Switzerland, May (26). IX-4, 6