SIMULATION OF VENTILATION AND SMOKE MOVEMENT

Transcription

1 SIMULATION OF VENTILATION AND SMOKE MOVEMENT Almbauer R.A., Sturm P.J., Bacher M., Pretterhofer G. Institute for Internal Combustion Engines and Thermodynamics Graz University of Technology ABSTRACT Long road tunnels still constitute an inherent risk on safety in case of fire. Smoke propagation is the most frequent reason to be killed in such accidents. Therefore special attention has to be paid on the smoke movement which mainly depends on the mean flow velocity. The ventilation system of long road tunnels should be capable to control the smoke propagation in an efficient way. Transversal ventilated road tunnels equipped with adjustable exhaust air dampers provide the necessary prerequisites. In addition, the velocity in the road tunnel has to be adjusted. The paper shows an example of the control mechanism for smoke propagation for a 10 km long road tunnel in Austria. The concept has been elaborated using a simulation model. Finally, the results from fire tests in the tunnel confirm the validity of the concept. Key words: tunnel ventilation, fire, simulation, automatic control 1. INTRODUCTION A series of at least 10 major fires in road and rail tunnels have occurred in Europe over the past decade, causing serious loss of life and significant structural damage. It was in particular the human casualties in Mont Blanc, Tauern, Kaprun and Gotthard tunnel fires (221 lives lost in four fires over a period of just two years) that have provided the impetus to intensify the studies on tunnel safety. Results show that the majority of victims in the big disasters in long tunnels have first been captured by smoke and later on they died from suffocation or direct impact from the fire. Only few people could escape through the smoke into rescue zones. The priority therefore must be to enable selfrescue of tunnel users, which is only possible with sufficient visibility in the tunnel. Even fire brigades are severely endangered when they enter a zone full of smoke. Therefore, the most important of all measures in the case of fire is to control the smoke propagation. The possibilities to influence the smoke movement depends on the technical equipment of tunnels. The new European Directive entitled Safety in European Road Tunnels proposes that all tunnels longer than 500 m belonging to the Trans European Road Network should have harmonized safety requirements. One of the technical requirements concerns ventilation: Single tunnels with bidirectional traffic shall have transverse and/or semi-transverse ventilation with exhaust possibilities. Longitudinal ventilation pushing smoke in one direction shall be used in these tunnels only when traffic conditions allow uncongested vehicles to drive out of the tunnel. For twin tube tunnels the propagation of smoke of gases from one tube into the other shall be prevented and stricter ventilation standards should be applied to unidirectional congested tunnels. The paper will show that the smoke propagation from one fire in long transversal ventilated road tunnels can be controlled up to a defined heat load and restricted meteorological boundary conditions. For that purpose they have to be equipped with a exhaust air system which is able to suck off the smoke on any place using adjustable exhaust air dampers. In addition a mechanism to control longitudinal wind velocity in the tunnel must be available.

2 Simulation is an important tool to investigate the capabilities of the ventilation system to control smoke. Due to the costs of real life tests it is not possible to investigate many cases. Therefore, the majority of situations have to be assessed using a simulation model capable of predicting the physical behavior of the flow and smoke propagation. 2. THERMODYNAMIC ANALYSIS OF A FIRE Thermodynamic analysis of a fire shows that the chemical reactions, which lead to heat release are: C + O2 CO2 and H2 + ½ O2 H2O. (C. Carbon, H2. Hydrogen, O2. Oxygen, H2O. Water, CO2. Carbondioxid ). Generally the oxygen is coming from the air. Of minor importance is the reaction of sulfur: S + O2 SO2, as sulfur does not occur in such big amounts. For materials like plastics or fuel the necessary air for a complete burning is some 15 times more in mass than the mass of carbon and hydrogen. All the other shares of the material do not burn. Assuming a calorific value of kj/kg for pure hydrocarbon, 2,5 kg/s have to be burned in order to get a heat release of 100 MW. This results in a primary (pure) smoke mass of approximately 40 kg/s. The volume of this undiluted smoke depends on its temperature. The smoke gas volume can be higher due to evaporated medium. Mostly it is water vapor. So the smoke gas extraction of a tunnel must be able to suck off at minimum the mass of the gaseous fractions of the pure smoke. If the smoke is cold it has a comparable density to air and a volume flow of 34 m³/s. For a smoke temperature of 1200 C the volume is five times higher (170 m³/s). If the pure smoke gas is diluted with the same mass of air (40 kg/s) the temperature is reduced to 640 C and the density is increased, leading to a volume flow of 210 m³/s. A reasonable part of the heat is transferred to surfaces due to radiation and convective heat transfer, so the effective heat load, which goes to the smoke gas is reduced. This decreases the smoke gas volume flow further. The German guidelines for road tunnels RABT [1] give the following values for the smoke gas volume flow of fires: 100 MW. 200 m³/s at 300 C. This would mean that approximately 50% of the heat is not contained in the gaseous fraction of the smoke gas. All of the above assessed values are not exact, but should give a good Exhaust Air Duct approximation of the smoke gas Street Tunnel mass flow and volume flow, V long towards the fire which can be expected. The more complex assessment is how Fig. 1: Ideal Smoke Extraction the smoke is diluted and dispersed. Considering the transport mechanism of smoke, namely diffusion (laminar and turbulent) and convection, the convective part is much more important. Even a mean velocity of 0.5 m/s restricts any transport of cold smoke in the upstream direction. So the necessary condition for a complete smoke extraction from the tunnel is to suck off as much as to produce a converging flow in the tunnel towards the open exhaust air damper from both sides of at least 0.5 m/s (for cold air). For a cross-sectional area of 50 m² this results in a minimum mass flow of cold air from both sides of each 30 kg/s (60 kg/s in total). From the above given assessment this is more oxygen than the fire needs to burn 2,5 kg/s of hydrocarbons. Concluding the assessment the

3 exhaust air ventilator has to suck off some 60 kg/s. As the ventilator transports a certain volume flow, the mass flow depends on the temperature of the smoke at the ventilator. The 60 kg/s at the maximum allowable temperature at the ventilator of 400 C results in a volume flow of some 120 m³/s. For a lower temperature the necessary volume flow is accordingly lower Influencing the air velocity in the tunnel In order to meet the described criterion the longitudinal air velocity in the whole tunnel must be influenced by forces. The relevant Austrian guideline RVS [2] proposes activating or deactivating the fresh air injection and the exhaust air extraction in certain ventilation sections of a tunnel. This is a possible way, but the influence on the longitudinal air velocity is restricted. Another possibility to affect the longitudinal flow is the installation of jet fans, as it has been done in the Montblanc tunnel. The injection of fresh air through a jet nozzle which perfectly transfers the pressure difference into flow velocity in the longitudinal direction (comparable to Saccardo nozzles), has been proposed by the authors [3], [4]. In order to control the longitudinal air velocity the forces have to be adjustable. Application of a control system with predefined power settings for the ventilators in order to influence the flow in the tunnel can only be made for a small range of boundary conditions. Such a predefined system can not react on changing meteorological conditions or on a malfunction of any ventilator etc. Closed loop control systems are able to react on varying boundary conditions. But they need a clear concept which provides a control variable. A PID controller unit adjusts the forces in the way that the target value is reached as soon as possible. The controlled variable for the flow in the tunnel is the longitudinal velocity which represents the volume flow and the mass flow, when the velocity profile and the density is known. Therefore the velocity measurement must have a high quality, that requires a high accuracy of the sensor and a permanent availability of the controlled variable in the controller unit Backlayering The temperatures of the smoke gas of fires have been investigated in several investigations. Temperature values of 1200 to 1300 C can be expected around the fire. Due to mixing with fresh air, the evaporation of water or the imperfect burning smoke gas temperatures can be lower. Depending on the temperature of the smoke gas the buoyancy effects are important and the hot smoke rises up to the tunnel ceiling. There the 3-dimensional effect of the movement of smoke into the opposite direction of the mean flow (backlayering) happens. It is an important effect, which has to be taken into account in the risk assessment. Investigations show that backlayering is a strongly stratified flow. The size of the smoke penetration depends on the smoke gas temperature and the mean flow velocity. It is a fact that for a steady state situation the mass in a tunnel section does not change. Due to continuity the sum of all mass flows over such a section must be zero. In a backlayering zone the hot smoke moves against the oncoming fresh air in the uppermost layer. Below this zone the smoke is colder and moves in the direction of the oncoming fresh air. Still it is much warmer and represents a stable stratification. The oncoming fresh air has to go underneath this hot wedge of smoke gas and is therefore accelerated. In the contact zone between smoke gas and fresh air the smoke is diluted and transported to the fire. As long as the mean flow is directed to the fire there exists a fresh air below the smoke gas. If the temperature difference between the smoke gas and the fresh air is small turbulence is able to mix both gases. Values of mechanical turbulence can be high, if there exist 3-dimensional effects like the flow around cars or trucks. In all the cases where there exists no backlayering any more, the smoke is not able to move against the oncoming fresh air.

4 The description of the 3d-effects around the fire can be concluded as follows: If there exists a backlayering, the flow is stable stratified and is therefore not mixed down to the ground. So a fresh air layer exists below the smoke. If the stratification is less stable and/or the mechanical turbulence is larger, the smoke is mixed and transported with the oncoming fresh air towards the fire. In both cases the area upstream of the fire, which is affected by the smoke, remains small as long as the fresh air moves towards the fire [5]. 3d-effects are only of interest in the nearfield of the fire. In the rest of the tunnel a 1-dimensional behaviour of the flow combined with the smoke propagation occurs. 3. SIMULATION MODEL Applying a 1d-simulation model the distributions of velocity, temperature, pressure, smoke etc. can be predicted in an adequate way in the whole tunnel system. This was the reason for developing a 1-dimensional flow simulation model called (Graz Tunnel Investigation System GRATIS) which solves the 1-dimensional conservation equations for mass, momentum, energy and passive scalars. The equations are solved transiently using an explicit time integration scheme and a Finite Volume method. This numerical method is a socalled pressure linked method, where the predicted mass flows over the volume surfaces are corrected by the pressure field. The pressure correction produces a flow which fulfills the continuity equation. The pressure can be interpreted as the static pressure. The model considers the equation of state for ideal gases and is therefore able to treat varying density according to temperature and pressure changes. Wall friction, flow resistance due to obstacles or deflection and other momentum sources are treated as sources in the momentum equation. Sources in the energy equation are heat transfer to the walls, heat release due to the fire, etc. The model runs on a standard PC and is written in FORTRAN 77, combined with a pre- and postprocessor in VISUAL BASIC. The model handles all geometrical and equipment data of a real tunnel. This includes ventilator characteristics, friction factors, drag coefficients, heat transfer coefficients etc. It is able to deal with the flow rates through dampers and the recovery of momentum due to the directed flow in the exhaust air channels. There are the equations available to treat humidity with evaporation and condensation of water. There exists also a subroutine, where a PID controller can be simulated applying any constants. Finally a fire can be simulated, where the thermodynamic interrelation of smoke gas production, temperature, density and flow characteristics are handled. Applying GRATIS the automatic control mechanism for the smoke extraction can be calculated. The influence of unfavourable boundary conditions, varying fire loads, different initial conditions and other effects can be studied. So the cases in which the tunnel equipment fails to suck off the smoke gas can be elaborated. Figure 2 shows the postprocessor of the 1d-model where the distribution of the variables are rendered. In the case shown the smoke extraction of the new bore of the Plabutschtunnel has been investigated. The results leaded to innovative solutions concerning the control mechanisms, which are described in the following section. 4. SIMULATION OF THE SMOKE EXTRACTION IN THE NEW PLABUTSCHTUNNEL The Plabutschtunnel is a 10 km long tunnel directed from north to south in the west of Graz, Austria. The old east bore was opened in the year Due to a continuous increase of traffic the second bore has been built over a time span of 4 years and was opened end of January For the year 2004 the new tunnel bore will be operated with two way traffic, as the old bore will be refurbished. Afterwards both bores will be operated with one-way traffic.

5 In both bores a transversal ventilation system is installed. The new bore is equipped with remote controlled exhaust air dampers of a size of approx. 9 m² positioned every 100 m over the whole length of the tunnel. The ventilation system consists of five sections of 2 km each. Four of the five sections are connected to the surrounding via two vertical shafts. One section is ventilated via the north portal. In the case of fire the detection system starts the emergency ventilation system. There are two systems implemented (a) a closed loop control system which adjusts the volume flow on both sides of the fire according to figure 1 and (b) a pre-adjusted control mechanism, which gives fixed answers for a fire in every tunnel ventilation section. In the following the closed loop control system is described. In case of fire the fresh air supply in the affected ventilation section is switched of. The exhaust air damper closest to the fire is opened, all the other dampers of the ventilation section are closed, and the exhaust air ventilator is switched to the maximum volume flow. The target value for the controlled variable, namely the longitudinal velocity in the tunnel, is deducted from the measurement values of the temperature and the volume flow at the exhaust air ventilator. The target value of the longitudinal velocity in the tunnel is calculated in the way to produce half of the mass flow at the ventilator. The closed loop control system adjusts the longitudinal velocity to the above described target value. The postprocessor of GRATIS (fig. 2) shows the simulation results for velocity (left upper diagram), temperature (right upper diagram), pressure (left middle diagram) and the smoke (right middle diagram). In the lower diagram the whole tunnel and the ventilation sections (divided by dotted lines) are shown. The vertical bars embrace the zooming area Graz Tunnel Investigation System GRATIS Figure 2: Simulation results for a smoke gas extraction in case of fire for the Plabutschtunnel

6 which is rendered in the four upper diagrams. The fire starts with a linear increase of its strength over five minutes. Then the strength is kept constant at a value of 30 MW. The results shown for the distributions 12 minutes after the beginning of the fire. The PID controller has already managed the velocities to be directed towards the fire with a speed of approx. 1.9 m/s. The smoke temperature reaches approx. 570 K (300 C) at the position of the damper. The smoke which was initially transported downwind the tunnel (from left to right) has almost been sucked of by the ventilation system. The exhaust air ventilator provides a volume flow of 193 m³/s in normal operation. In case of fire the volume flow depends on the position of the exhaust air damper and the temperature of the smoke gas at the damper and at the ventilator. The volume flow at the damper is highly increased when the smoke gas temperature at the ventilator is much smaller than the temperature at the damper. The variable forces for influencing the longitudinal flow in the tunnel are produced by the changing mass flows in the exhaust air and the fresh air system. In order to accelerate the longitudinal air flow in a tunnel section from north to south, the following measures have to be taken: (1) the fresh air flow in the north has to be increased (2) the exhaust air flow in the south has to be increased, (3) the exhaust air flow in the north has to be reduced, and (4) the fresh air flow in the south has to be reduced. For the acceleration into the opposite direction the measures have to be the other way round. In order to make the system more reliable the revolutions per minute of the ventilator are restricted between 100% and 35%. In addition there are four jet fans installed in the south of the tunnel, which support the longitudinal flow. The change from the maximum force in one direction to the other direction needs approx. 2 minutes. 5. VALIDATION OF SIMULATION RESULTS IN THE PLABUTSCHTUNNEL In order to check the smoke extraction of the real tunnel, several tests have been carried out. Two fire tests on the 22 nd of January showed reasonable results. The smoke gas has been completely sucked of during the two fire tests with a heat load of approx. 6 MW. Further improvements of the constants of the PID controller have been made for the final tests Figure 3: Results for the smoke gas extraction test in the Plabutschtunnel on the 28 th. Figure 3 shows the temporal evolution of the longitudinal velocity for a test without fire in the 5 th ventilation section in the south. The diagrams show (a) the measured

7 longitudinal velocity (smoothed), (b) the target value, (c) the difference between (a) and (b), and (d) the measured longitudinal velocity (without smoothing). The recording of the variables started when the fire alarm was triggered. Hence, all variables before that time had the value zero. The target value evolves according to the volume flow of the exhaust air ventilator, which starts from zero and needs approx. 90 seconds to reach its maximum. During the same time span the damper next to the fire is fully opened, all the other dampers of the ventilation section are closed, and the fresh air supply is switched of. The third diagram shows the changes for the control deviation. At the beginning the control deviation is more than 3 m/s. After approx. 60 seconds the control deviation is reduced to zero, but overshoots the target value. After approx. 3 minutes after the start of the fire the ventilation system has arrived at its target value. 6. CONCLUSIONS In case of a fire in a road tunnel the smoke extraction is of major importance for the prevention of casualties. Smoke reduces the visibility in the tunnel to a level where selfrescue and even rescue by fire brigades is impossible. The smoke dispersion in a tunnel is mainly dependent on the longitudinal velocity. 3d-effects like backlayering are of minor importance. Therefore, the simulation of the time dependent distribution of velocity, temperature and smoke using a 1d-model is the proposed method to investigate the smoke gas extraction performance of a tunnel ventilation system. The 1d simulation model GRATIS (GRAz Tunnel Investigation System) is able to predict these values by solving the conservation equations of mass, momentum, energy, water vapor, smoke, and the thermodynamic interrelations respectively. Long transversal ventilated road tunnels in Austria are nowadays equipped with remote controlled exhaust air dampers, where the total volume flow of one ventilator can be sucked off over one to three dampers. A prerequisite for a functioning operation is that the longitudinal velocity in the tunnel is adjusted in a way that the flow is directed to the fire from both sides. For the adjustment of the velocity the forces in the tunnel have to be controlled. In order to be able to deal with unfavorable boundary conditions the control mechanism should be automatic. The comparison of results from simulations and experiments show that the smoke extraction performance of a tunnel can be predicted and optimized using a 1dsimulation tool like GRATIS. 7. REFERENCES [1] RVS: Projektierungsrichtlinien RVS (Lüftungsanlagen, Luftbedarfsberechnungen), Forschungsgesellschaft für das Verkehrs- und Straßenwesen, Arbeitsgruppe Tunnelbau, April 1997, Austria [2] RABT (2003): Richtlinie für die Ausstattung und den Betrieb von Straßentunneln, Forschungsgesellschaft für Straßen und Verkehrswesen, Germany [3] Almbauer R. A., D. Öttl, and P. J. Sturm (2003): Verfahren zur Beeinflussung der Tunnellängsströmung einer befahrbaren Tunnelröhre mit Querlüftung zur Verbesserung der Rauchgasabsaugung, Österreichische Patentanmeldung 2CA 870/2002 [4] Almbauer R.A., P.J. Sturm, D. Oettl, M. Bacher (2003): A new method to influence the air flow in transversely ventilated road tunnels in case of fire, 11 th Int. Symposium on Aerodynamics & Ventilation of Vehicle Tunnel, pp , ISBN [5] Sturm P.J., Pretterhofer G., Rodler J. (2002): Field Tests and Numerical Simulations as Tools for Ventilation Design and Tunnel Safety, International Conference Tunnel Safety and Ventilation, April 8-10, 2002, TU-Graz, VKM-THD Mitteilungen, Heft/Volume 80, ISBN