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Fire Behaviors and smoke transportation law of tunnel fires under confined portal boundaries
RISE - Research Institutes of Sweden, Safety and Transport, Safety. (Tunnel Fire Dynamics Group)
2019 (English)Report (Other academic)
Abstract [en]

An increasing number of tunnels have been built around the world. They play an important role to relieve traffic congestions and facilitate goods transportation. However, in the event of a fire in tunnels, the consequences can be serious due to its narrow-long structure. The previous studies about tunnel fire dynamics and mitigation measures are mostly based on good ventilation conditions in tunnels, such as longitudinal ventilation and natural ventilation with the premise that a tunnel has two open portals. However, the studies about the characteristics of tunnel fires under confined portal boundaries with complete or incomplete sealing at both portals are rare. Typical fire scenarios can appear in a subway train, a building corridor, an underground utility tunnel, a mining tunnel, a tunnel during construction and the application of sealing tunnel portals for fighting large tunnel fires and so on. The knowledge of tunnel fire dynamics for tunnels under good ventilation conditions is probably not applicable to the scenarios of tunnel fires under confined portal boundaries. Conducting the studies of tunnel fires under confined portal boundaries is of great significance for better understanding the characteristics of this type of tunnel fires and developing tunnel fire mitigation measures. Therefore, by combining model-scale tunnel experiments and theoretical analyses, this thesis studies the fire behaviors and smoke transportation law of tunnel fires under confined portal boundaries. The main research contents include:

1.Scaling effects of mass loss rate per unit area (MLRPUA) for well-ventilated pool fires are studied by summarizing large amounts of experimental data from the literature together with theoretical analyses. As a further extension of tunnel fire similarity theory, it provides the basis and reference for later model/medium-scale tunnel experiments. Results show that when a small-scale pool fire (D<1 m) occurs in the open, increasing wind velocity tends to increase the MLRPUA, especially for pools with D<0.2 m. This is because the ventilation significantly increases the conductive and convective heat feedbacks (leading role). But when small-scale pool fires occurs in tunnels with a short distance between the pool surface and ceiling (Hef/D<3), the radiative heat feedback from the tunnel ceiling is probably dominating, leading to a much higher MLRPUA than that in the free burn. When subjected to longitudinal flows, the MLRPUA decreases due to the reduced radiation effect from the ceiling. With the increase of pool diameter, the influence of wind on the MLRPUA decreases gradually, no matter whether the pool occurs in the open or in a tunnel. Finally, when the pool diameter exceeds 1 m, the radiation from flame itself is probably predominant. The MLRPUA is not significantly affected by increasing wind velocity and most likely fluctuates within 30% for a wide range of wind velocities based on the test data collected.

2.The flame behaviors and the maximum gas temperature rise beneath the ceiling in an enclosed tunnel are studied using a model-scale tunnel. Results show that when a fire (small fire) is not located at the tunnel center, the flame inclines towards the closer tunnel end due to the asymmetric flow field on both sides of the flame. The flame inclination angle keeps increasing when the fire is moving away from the tunnel center. Furthermore, when a fire is in Region I (0< ≤0.64), the maximum gas temperature rise decreases with the increasing dimensionless fire distance due to the increasing flame inclination angle. When a fire is in Region II (0.64< <1), the maximum gas temperature rise increases with the increasing dimensionless fire distance due to the heat feedback of returned hot smoke bounced from the end wall. By introducing a concept of equivalent ventilation velocity based on the flame inclination mechanism, a prediction model of maximum gas temperature rise beneath the ceiling in Region I is developed. Beyond that, an extra correction factor is proposed to the improved model in Region II with a consideration of heat feedback of returned hot smoke bounced from the end wall. Besides, further dimensional analysis indicates that the normalized maximum gas temperature rise follows an exponential attenuation law with the dimensionless fire distance.

3.The coupling control effects of sealing ratio and initial sealing time on the fire development (large fire) are studied using a model-scale tunnel. Results show that sealing tunnel portals can decrease the mass loss rate of fuel and gas temperature inside the tunnel, no matter whether the sealing is complete or incomplete. The earlier the initial sealing time is, the better the fire can be controlled. For the incomplete sealing, when the sealing is implemented during the violent burning stage, the sealing not only does not limit the fire growth but also exacerbates the tunnel fire, producing an extremely high CO concentration at tunnel portals and a longer ceiling flame jet. This will result in a huge threat to the rescue service at tunnel portals. Besides, if the tunnel portals are sealed incompletely, it will leave a small area for the exchange of smoke and air. The smoke will not continue to spread horizontally after leaving the tunnel portals under the action of inertial forces. In order to maintain the combustion of fuel, the fresh air from external environment flows into the tunnel vigorously and quickly from the gap and then uplifts the smoke out of the tunnel portals, which is also an important phenomenon for firefighters and needs to draw their attentions.

4.The critical conditions for the occurrence of under-ventilated tunnel fires and the combustion mechanisms under confined portal boundaries are studied by using both model-scale and medium-scale tunnels. Results show that the critical equivalence ratio for the occurrence of under-ventilated tunnel fires is within 0.53 - 0.6, which is less than the theoretical value of 1. This is related to the occurrence of vitiation, consequently reducing the level of oxygen around the flame by diluting the O2 concentration. The low ventilation rate and vitiation result in a low O2 volume fraction around the flame, and then the MLRPUA starts to decrease and at the same time the air mass flow into the tunnel becomes almost constant. Also, an oscillating MLRPUA and lifted flame are observed in the model-scale tests. Consequently, the ventilation rate approaches and even reaches the amount required for complete combustion of vaporized fuel. This means that the insufficient combustion in early under-ventilated tunnel fires has converted to sufficient combustion (from the perspective of the change of equivalence ratio, the fire has converted from under-ventilated to well-ventilated). As a result, no significant increase in CO production in under-ventilated fires is observed in both test series.

5.The critical conditions for the occurrence of self-extinguishment and influencing factors in under-ventilated tunnel fires are studied in a model-scale tunnel during construction. The tunnel consists of an inclined access tunnel and a horizontal main tunnel. Results show that when a fire is in the horizontal main tunnel, the critical equivalence ratio for self-extinguishment is within 0.28 - 1.38 for the propane gas burner and 1.11 - 3.6 for the fibre board soaked with heptane. The difference is related to the burning behavior of the different fuels used. Moreover, the critical O2 volume fraction is about within 12 - 15% when the fires self-extinguish. When a fire is at the closed end of the horizontal main tunnel, the stratification of smoke is destroyed after hitting the closed end, and then the smoke seems to spread over the entire cross section of the tunnel. The smoke spread velocity is proportional to the ventilation rate. However, when a fire occurs at the closed end of the inclined access tunnel, the fire does not self-extinguish, even when the ventilation rate is 0 m3/s. The corresponding smoke spread velocity is higher than that in the horizontal main tunnel. This is probably related to the increasing component of buoyancy in the longitudinal direction in the inclined access tunnel. Besides, no insignificant vitiation behind the fire is found. These two characteristics in the inclined access tunnel increase the temperature of smoke flowing out of the tunnel portal and in turn promote the natural ventilation and increase the O2 volume fraction.

Place, publisher, year, edition, pages
Borås: RISE Research Institutes of Sweden , 2019. , p. 170
Series
RISE Rapport ; 2019:58
Keywords [en]
Tunnel fire, model-scale tunnel, Scaling effects, Confined portal boundaries, Under-ventilated, Self-extinguishment, Equivalence ratio, Critical conditions, Combustion efficiency, Mass loss rate, Gas temperature, CO volume fraction, O2 volume fraction
National Category
Building Technologies Infrastructure Engineering Transport Systems and Logistics Other Engineering and Technologies not elsewhere specified
Identifiers
URN: urn:nbn:se:ri:diva-42069ISBN: 978-91-88907-85-1 (print)OAI: oai:DiVA.org:ri-42069DiVA, id: diva2:1378536
Note

This report contains a copy of the translated version of Yongzheng Yao’s PhD

thesis. He has been educated at both University of Science and Technology of China and RISE Research Institutes of Sweden. On 26 Oct, 2019, he has successfully defended his thesis. His formal thesis in Chinese has been published by University of Science and Technology of China. The thesis consists of a significant amount of work that Yao conducted at RISE. To facilitate wider use of the results, he has translated it into this English version, which is published here as a RISE report.

Available from: 2019-12-16 Created: 2019-12-13 Last updated: 2019-12-16Bibliographically approved

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