Tunnel Fire Research Articles

Experimental study on flame tilt angles of two unequal fires in a longitudinal ventilated tunnel

In historical tunnel fire accidents causing major casualties and property losses, the special fire phenomenon of multiple fires burning was usually accompanied. Other than the longitudinal ventilation in tunnel, the interaction burning of multiple fires will also make the flame tilt. Flame tilt angle is an important parameter to characterize the magnitude of acting forces on fires and it determines the possibility of ignition and flame spread. Former studies were mainly focused on multiple equal fires with same sizes, very limited attention has been paid on multiple unequal fires. Due to the different air entrainment mechanisms, the proposed correlations for flame tilt angle of multiple equal fires might be unsuitable for the multiple unequal fires. This paper presents an experimental study on flame tilt angles of two unequal square gaseous fires of propane in a 1/10 scale tunnel with longitudinal ventilation. Two square burners with different side lengths were located parallel and along the longitudinal centerline of tunnel. The effects of heat release rate of each burner, the burner edge spacing and the ventilation velocity were explored. Using the image processing method, the flame heights and tilt angles of upstream fire and downstream fire were determined. The results showed that due to the shielding effect of upstream fire acting on the downstream fire, the flame height of downstream fire is larger than the upstream fire, while the flame tilt angle of downstream fire is lower than the upstream fire. In addition, the flame heights of upstream and downstream fires are reduced with increasing the ventilation velocity, while the flame height of downstream fire is weakly affected by the separation distance. As the ventilation velocity increases, both the upstream and downstream flame tilt angles increase first and then maintain unchanged. The separation distance has little influence on the upstream flame tilt angle, while it affects the downstream fire more. Among the impact factors, the longitudinal ventilation plays a leading role of the flame tilt behaviors of two fires in tunnel. Based on the force analysis, it is found that the pressure thrust between two fires is much smaller than the buoyancy force and ventilation force. As a result, different correlations for predicting the flame tilt angles of two unequal fires in tunnel are developed based on the coupling effect of buoyancy force and ventilation force of each fire.

Assessing the Quality of Concrete Tunnel Lining Exposed to Tunnel Fire through Residual Compressive Strength

Compressive strength performance of concrete after exposure to the elevated temperature is important for evaluating and repairing concrete structures. This paper presents an experimental study to determine the residual compressive strength of concrete used in tunnel lining after exposed to tunnel fire. Two types of concrete tunnel lining segments are evaluated in this study. One of it was constructed using a patented fire-resistant concrete (MYC) containing high volume fly ash and nanosilica (HVFANS). Another concrete tunnel lining segment was constructed using concrete containing silica fume normally used in the current construction, coded as SPC concrete. The drilled core results show that, after exposure to tunnel fire temperature up to around 1045°C, the compressive strength of MYC has dropped to 66% of the design strength. In comparison, the SPC concrete showed a decrease in compressive strength to 62% of design strength. The experimental results confirmed that the SPC segments have shown slightly lower residual compressive strength compared to the MYC segments. However, the MYC tunnel segment shows high resistance to the spalling of cover concrete compared to the SPC tunnel segment. Therefore, it can be said that the residual strength alone is not sufficient to compare the damage of concrete exposed to tunnel fire; the spalling damage observation is similarly important as it is one of the important serviceability criteria for designing concrete structures.

Simulation Research on Effects of Ambient Pressure on Plug-Holing Phenomenon in Tunnel Fires with a Shaft

This paper studied the effects of ambient pressure on the plug-holing phenomenon in tunnel fires with a shaft by a Fire Dynamics Simulator. The influence of ambient pressures on the smoke movement, temperature distribution, critical Richard number (Ric) and critical shaft height for plug-holing were analyzed in detail. A new prediction formula of smoke flow velocity considering different pressures was modified. A prediction formula of smoke temperature distribution beneath the ceiling under different pressures was developed. As a result, a prediction model of Richard numbers to determine whether the plug-holing occurs was proposed by combining smoke flow velocity and smoke temperature distribution. The critical Richard numbers (Ric) and critical shaft height (hc) increases as the pressure decreases. Outcomes in this study can provide references for the design of a natural ventilation system in tunnel fires at a higher altitude.

Experimental study on vertical temperature distribution of the two-layer smoke flow in tunnel during construction

For a tunnel fire during construction, the formation of two-layer smoke flow underneath the ceiling accelerates the smoke descent threatening the personnel evacuation. As an important indicator of smoke diffusion, the gas temperature distribution inside tunnel is worthy of analysis in depth. However, the vertical temperature profiles within this kind of two-layer smoke flow inside tunnel have not been experimentally studied. To fill the gap, this paper conducts a series of fire experiments in a 1/20 reduced-scale tunnel model. Multi-variables including fire heat release rate, fire position, lifting-up height of fire source, inclined shaft slope, are considered. The visible flame shape, O2 volume fraction and gas temperature are recorded and measured. The burning of the propane gas persists steadily for 20 mins without the occurrence of self-extinction. The structural evolutions of the smoke layer are predicted by FDS simulation and traced by the laser sheet. The smoke descent process in the main tunnel is divided into three stages. The effects of fire source position, inclined shaft slope, fire heat release rate and lifting-up height on the gas temperature distributions inside tunnel are analyzed. The measured temperature rise and the perpendicular distance from the ceiling are normalized by ΔTmax and LT, respectively. At various longitudinal positions (1.82 ≤ │X-dm│/Hef ≤ 10.68), the normalized temperature distributions follow the same curve, independent of the values of Qm, dm and hm. The self-similarity of the vertical temperature distribution within the two-layer smoke flow are sufficiently proved. It is found that ΔT/ΔTmax with z/LT shows a two-regional variation. Based on over 1,740 temperature data from 36 experimental cases, an experimental equation is proposed to describe the gas temperature rise in the region between the fire source and the closed end. The proposed model shows good agreement with the measured data, which can help the researchers to better assess the smoke layer height inside tunnel.

Research on Automatic Patrol Inspection Technology Scheme for Safe Operation of Super Long and Long Span Highway Tunnel

Firstly, this paper conducts systematic research on tunnel fire and traffic incident detection technologies, and develops a high-precision monitoring and detection implementation plan for ultra-long and large-span tunnels; Secondly, in terms of tunnel fire detection, video thermal imaging technology is used as the main detection method, combined with dual wavelength flame detectors, to achieve fire open fire monitoring and pure smoke detection, making up for the blind spots of traditional fire detection, and improving the accuracy, timeliness, and reliability of monitoring. In terms of traffic incident detection, based on traditional traffic incident detection technology and advanced technical means, the system mainly focuses on lightning visual integration detection, combined with artificial video monitoring, to comprehensively judge the status of tunnel traffic incidents, thereby reducing the false alarm rate of incident detection. Finally, based on the technical research scheme, an intelligent automatic patrol robot was developed using the mobile automatic patrol mode. Using this as a carrier, various sensor modules and emergency equipment were built to replace manual 24-hour uninterrupted full coverage patrols, achieving automatic patrol, automatic reporting, analysis, early warning, and emergency response for ultra-long tunnels, reducing the cost of manual patrols and improving the efficiency of patrols, Enhanced emergency response capabilities of operating units.

Experimental study on collaborative longitudinal ventilation of smoke control for branched tunnel fires considering different branch angles

For branched tunnel fires, the main tunnel and the branch should be collaboratively ventilated to keep the branch and upstream of the main tunnel free of smoke. In this paper, reduced scale experiments were conducted to study the collaborative longitudinal ventilation in branched tunnel fire, considering three branch angles (15°, 45°, and 60°) and five ethanol pool fire sizes. The fire source located at the intersection. The results show that the longitudinal controlling ventilation velocities in the main tunnel and branch are related to the critical ventilation velocity of branched tunnel fire when only the main tunnel is ventilated, vc,m. Two critical ventilation modes were proposed: (a) the values of ventilation velocities in the branch and the main tunnel are equal; (b) the ventilation velocity in the main tunnel equals to vc,m, and the ventilation velocity in the branch is smaller. Under ventilation mode A, when ventilation velocity is approximate 42 % of vc,m, the smoke can be controlled. Under ventilation B, when ventilation velocity in the main tunnel equals to vc,m, and the ventilation velocity in the branch is within 0.10 m/s–0.22 m/s, the smoke could be controlled. However, the heat release rates in ventilation mode A could be larger than that under mode B by 33 %—99 % regarding the same pan size. Hence, in practice, the collaborative ventilation modes, in which the ventilation velocities in the branch and the main tunnel are close, should be avoided. This work could help to guide the emergency rescue in relevant fire accidents.

Experimental Study on the Smoke Temperature Decrease near Natural Vents Induced by Air Entrainment in a Tunnel Fire

Previous studies have shown that there is a large difference in the temperature of smoke in the upstream and downstream of a vertical shaft, but no quantitative study on the subject has been conducted. In this paper, a model experimental test was conducted to study the effect of a natural vertical shaft on the distribution of smoke temperature in tunnel fires. A laser sheet was used as an assisting tool to show the smoke flow field. A series of tests were conducted with shaft heights varying from 0.2 m to 1.2 m and a heat release rate (HRR) of 9 kW, 17 kW, 29 kW, and 65 kW. The entrainment phenomenon around the vertical shaft was analyzed, and the results showed that the amount of fresh air entrainment is the main reason for the temperature decrease in upstream and downstream smoke temperatures. Based on the experimental results, an empirical model for predicting the amount of fresh air entrainment and a prediction model of the downstream smoke temperature were proposed, and the predicted values are in good agreement with the experimental values.

A comparative study on smoke temperature profile and corresponding different-exponential form correlation in single-line and bifurcated tunnel fires

A series of model-scaled tunnel fire tests were performed to investigate the temperature attenuation characteristics of thermal smoke. The internal temperature profiles of single-line tunnel and T-shaped tunnel were measured and compared. Results indicate that within the current measurement range, for the fire scenario with same fire power, the maximum smoke temperature of T-shaped bifurcated tunnel is slightly lower than that of the single-line tunnel under natural ventilation, which can be attributed to the reduction of smoke accumulation and the increase of air entrainment induced by bifurcation region. The longitudinal temperature decay of fire smoke in entire spread region shows an obvious region-variation feature. The temperature attenuation rate in the region near fire source is larger significantly than that of one-dimensional spreading region. Compared with the representation in the form of single exponential correlation, the double-exponential correlation can better characterize the temperature decay process in entire spreading region of fire smoke. While, if the single-exponential representation is adopted, the correlation formula is piecewise. Based on the analysis and physical observations of the longitudinal decay rate of smoke temperature in different spreading regions, a modified double-exponential correlation for longitudinal temperature attenuation is obtained. By comparing with the model-scaled measurement results in current work and the full-scaled experimental data of previous scholars, the proposed correlation is reasonably verified to be within an acceptable error range. This work is anticipated to serve as a resource for firefighters to evaluate the temperature profile and decay characteristics of fire smoke in tunnel-like space.

Smoke movement and control in longitudinal ventilated tunnel fires with cross-passages

This paper studies the characteristics of smoke movement and control in longitudinal ventilated tunnel fires with cross-passages through numerical modeling. Previous reports mainly focused on fires in single-hole tunnel, however, tunnels with cross-passages have been widely applied in real life, the cross-passages between main tunnels provide additional path for air and fire smoke, and significantly affect smoke temperature and back-layering length, which has not been studied in the past. Hence, full-scale simulations were conducted considering different heat release rates, fire locations, longitudinal ventilation velocities, and cross-passage intervals in this study. It shows that the smoke temperature distribution with or without cross-passages is almost the same under natural ventilation condition, while the temperature gradually increases with the decrease of interval of cross-passages under longitudinal ventilation condition. Due to the more air leakage, the smoke back-layering length increases with decreasing interval of cross-passage. The air leakage mass through the cross-passage was then characterized by the interval of cross-passages and fire location. Considering fire position and interval of cross-passages, the actual ventilation velocity is used to establish a new smoke back-layering length model. A basic understanding was provided by new findings in this study for smoke movement and control in tunnel fires with cross-passages.

Numerical study on the smoke movement and control in main roadway for mine fires occurred in branch

Mine exogenous fire is the main disaster in coal mines. Owing to the complicated structure and the ventilation network, the smoke movement and control in branched roadway fires of coal mines is more complicated than that in traffic tunnel fires. In this study, the smoke backlayering length and critical velocity in a main roadway for fires that occurred in a branch were studied with varying fire locations. The results suggest that the smoke from the branch does not spread along the width centerline of the main roadway, but forms an early “snake-shaped” structure. The variation of dimensionless backlayering length with the dimensionless variable ln(Q˙*1/3/V*) is divided into two regions with different slopes by the line of L* = 1.6. Besides, branched roadway fires have a lower backlayering length and critical velocity compared to single-hole tunnel fires. These two parameters increase with decreasing the fire-node distance. Combined with dimensionless analysis and simulation results, calculation models considering fire location were proposed to estimate the smoke backlayering length and critical velocity. The credibility of prediction models is validated by comparing them with simulation results. The outcomes of the current study guide smoke control in similar-structured mine roadways and traffic tunnels.

Optimization and determination of the parameters for a PID based ventilation system for smoke control in tunnel fires: Comparative study between a genetic algorithm and an analytical trial-and-error method

A PID-based longitudinal ventilation system for smoke control in tunnel fires is numerically illustrated to yield excellent performance. However, the three parameters in the PID algorithm significantly affect the performance. Two methods are proposed to optimize the parameters: (1) an analytical trial-and-error method (ATEM), based on an order-of-magnitude analysis; (2) a heuristic method: system identification and genetic algorithm (SI-GA). An SI-GA without a differential controller was also used for the comparative study, labeled as SI-GA*. The results show that all methods can obtain near-optimum parameters under their optimization goals. The ATEM is more time consuming, but it is more straightforward to manipulate the performance metrics directly than with the SI-GA method. The optimized parameters are tested with different fire sizes and different tunnel dimensions. The results show that the system with the optimized parameters can control the smoke well for large enough fire. In general, the system performs better for the stabilization time of the smoke front when using the parameters obtained from ATEM, but it achieves shorter maximum back-layering distances with the parameters obtained from SI-GA and SI-GA*. The latter two also give stronger fluctuations, but lower average values, of the ventilation velocity. Nevertheless, SI-GA* has more application potential due to the absence of a differential controller.

Highway Tunnel Fire Traffic Control Plan

Tunnel fire accidents occur from time to time, due to its special structure, will cause huge losses, this paper by studying the division of tunnel fire, put forward the principles and specific measures of traffic control, and carry out zoning control of tunnel fire traffic, and prepare a plan for the traffic control of the affected area and the fire tunnel area.

Numerical Study on Coupled Smoke Control Using Longitudinal Ventilation and Naturally Ventilated Shafts during Fires in a Road Tunnel

Longitudinal ventilation and smoke extraction by shaft are common smoke control methods in road tunnel fires. Tunnels often adopt one of these methods in practical engineering. However, it may have a better effect to adopt the method of mixing the two smoke exhaust methods together, which has not been revealed in the previous literature. Hence, the coupled effects of longitudinal ventilation and natural ventilation with shafts on the smoke control in tunnel fires were studied in this work. Numerical simulation was carried out considering different longitudinal ventilation velocities (0–4 m/s) and 4 kinds of typical shaft arrangements (shaft lengths range of 3–12 m, shaft intervals range of 27–60 m). The smoke spread length and smoke exhaust efficiency were analyzed systematically. Results show that (1) with the increase in longitudinal ventilation velocity, the total smoke spread length firstly decreases (V < 1 m/s) and then keeps almost constant (1 m/s < V < 2 m/s), finally increasing significantly (V > 2 m/s). (2) The length of the dangerous area (over 60 °C) at human height is basically 0 for all cases (except for Scenario 4 of shaft arrangement) when the longitudinal ventilation velocity is less than 2 m/s. (3) The CO smoke flow rate through the shaft is relatively high when the longitudinal ventilation velocity is within the range of 1–2 m/s for 4 kinds of shaft arrangement scenarios. Factors such as smoke spread and smoke exhausted through the shaft are comprehensively considered to judge smoke exhaust performance. The following conclusions can be drawn: when the ventilation velocity ranges from 1–2 m/s, it has a positive impact on the smoke control in tunnel fires with natural ventilation with shafts. When the ventilation velocity exceeds 2 m/s, the total smoke spread length and the length of the danger area increase, and the smoke stratification becomes worse, which brings inconvenience to rescue work. The results can provide reference for the design of fire protection in tunnels.

Effects of Fire Location and Forced Air Volume on Fire Development for Single-Ended Tunnel Fire with Forced Ventilation

Single-ended tunnels are a typical structure and an important part during tunnel construction. In the case of a fire in a single-ended tunnel, forced ventilation is commonly used to create a safe area near the excavation face. This work is aimed at examining the effects of fire location and air volume on fire development for single-ended tunnel fires with forced ventilation. A single-ended tunnel was built in Fire Dynamics Simulator (FDS), and twenty simulation tests were carried out. In the simulation, the distribution of flow field, temperature, and CO concentration in the tunnel were measured and analyzed. The results show that three regions can be identified based on airflow directions and velocity: (1) turbulent flow zone, (2) turbulent flow transition zone, and (3) steady flow zone. It was found that the maximum ceiling temperature rise decreases first with the distance between the fire source and the excavation face (XL), and then increases with a further increase in XL. The simulation results also showed that CO can easily accumulate on the ventilation duct side at the fire source position and the opposite side of the ventilation duct 5.0–15.0 m downstream of the fire source. Both the CO concentration and the maximum ceiling temperature rise decrease with increasing air volume, while the larger forced air volume will result in a higher risk for the downstream regions. The present results are of practical importance in firefighting and personnel evacuation in single-ended tunnels with a forced ventilation system.

A Study on the Evacuation of an Extra-Long Highway Tunnel Fire—A Case Study of Chengkai Tunnel

The smoke from tunnel fires spreads over long distances and is difficult to vent. Smoke accumulation leads to high temperatures, low visibility, and high concentrations of toxic gases, which greatly hinders the evacuation of people inside the tunnel. In this paper, a representative extra-long highway tunnel—Chengkai Tunnel—is selected as the engineering background, and a tunnel model is built using FDS and Pathfinder software to simulate the fire scenario and evacuation scenario under different longitudinal wind speeds. The concept of safe evacuation reliability is proposed to describe the relationship between the ASET (available safe egress time) and the RSET (required safe egress time). The simulation results show that with the increase in longitudinal wind speed, the ASET upstream of fire source increases first and then remains unchanged, while ASET downstream of fire source increases first and then decreases. The ASET upstream of the fire source is affected by visibility, while the ASET downstream of the fire source is affected by visibility when the wind speed is low, and is affected by temperature as the wind speed increases. The bottleneck effect is an important reason for the long evacuation time of people. The blockage time is a power function of the evacuation movement time, and increasing the width of the cross passage can improve the evacuation efficiency of the tunnel. The increase in the number of evacuees will reduce the reliability of the safe evacuation of personnel. Among all simulated scenarios, a longitudinal wind speed of 2.5 m/s has the highest safe evacuation reliability, with 0.79, 0.92, and 0.99 for scenarios R1, R2, and R3, respectively. Excessive wind speed reduces the safe evacuation reliability downstream of the fire source.

Numerical Study of Smoke Distribution in Inclined Tunnel Fire Ventilation Modes Considering Traffic Conditions

Japan and Europe have adopted significantly different ventilation modes with regard to traffic conditions (with or without congestion). This study focuses on the smoke behavior of different ventilation modes by conducting a 3D CFD analysis of smoke distribution and CO concentration variation and then comparing these modes based on two groups (relatively low target velocity: 0 and 1 m/s; relatively high target velocity: 2 and 3 m/s). The considered fire size is 30 MW for four longitudinal gradients (0, 2, 4, and 6%). In the simulation results, velocities of both 0 and 1 m/s reveal good performance in maintaining the stratification of smoke and ensuring the safety of the environment in 10 min in the occurrence of traffic jams. However, in 15 min, the smoke conditions change. Thus, it is difficult to distinguish which ventilation mode (0 and 1 m/s) shows relatively better performance during traffic congestion. When traffic is uncongested, the comparison of Um = 2 and 3 m/s reveals that a target velocity of Um = 2 m/s (lower than critical velocity) can also prevent the risk of smoke on the upstream side because no descending phenomenon is observed. Moreover, Um = 2 m/s causes the relatively slow propagation of descending smoke, increasing the possibility of evacuation once a second traffic incident occurs on the downstream side of the fire source.

A DIDE-FPP Composite Algorithm for Accurate Tunnel Fire Location

We propose a DIDE-FPP composite algorithm to improve the spatial location accuracy of tunnel fires based on the distributed individuals differential evolution (DIDE) algorithm and the four-point positioning (FPP) method. Using the DIDE algorithm to solve the multimodal optimization problems in tunnel fire location can locate more peaks and improve peak finding, and the FPP method is used to process the peak points located using the DIDE and achieve the spatial location which cannot be achieved otherwise using the DIDE method only. We used 20 multimodal test functions to evaluate the performance of the DIDE-FPP algorithm in peak finding and solving MMOPs. Through experimental comparisons with 13 other existing advanced methods, the comprehensive performance of the proposed DIDE-FPP composite algorithm shows advantages to some extents. Additionally, the combined value of PR (peak ratio) and SR (success rate) on up to 20 experimental functions is relatively high. The spatial positioning accuracy of a tunnel fire warning system using this positioning method can reach the centimeter level.

Experimental Study of Scale Effect in Tunnel Fires at Different Sealing Ratios

Fully or partially sealing the openings of tunnels to accelerate the self-extinction of fires provides a promising firefighting tactic to beat large fires in a long tunnel. So far, most experimental studies on the characteristics of fire with different sealing ratios have been conducted in reduced-scale tunnels. However, whether the findings in a reduced-scale tunnel can be converted to its full-scale prototype tunnel based on scaling laws has not yet been adequately studied. A series of experiments with heat-release rates of 15.8, 31.6 and 63.2 kW were conducted with sealing ratios ranging from 0% to 100% in a prototype tunnel measuring 20 m long, 0.9 m wide and 0.46 m high. The experimental results were compared with those from a 1/2 reduced-scale tunnel measuring 10 m long, 0.45 m wide and 0.23 m high. It showed that temperature rise along the tunnel in the 1/2 reduced-tunnel could be significantly underestimated. The differences in temperature rise increased monotonously with distance away from the fire seat, and they were as high as 70% at the tunnel portals, irrespective of the heat-release rates and sealing ratios. The study showed that the scale effect of fires was not sensitive to the Reynolds number of flows in tunnels. The minimal sealing ratio for the self-extinction of fires in the prototype tunnel was 85%, whilst it was 75% in the 1/2 reduced-scale tunnel, and the study revealed that the fires were much easier to extinguish in the 1/2 reduced-scale tunnel than those in the prototype tunnel, where the fires can sustain in a lower oxygen concentration. The study demonstrated that scaling laws could be invalid for tunnel fires with different sealing ratios and that results observed in reduced-scale tunnels should be further verified when applied to full-scale prototypes.

Study on Tensile Behavior of Reinforcing Bar with High Temperature Exposure for Damage Recovery Diagnosis in Tunnel Fire

Study on Tensile Behavior of Reinforcing Bar with High Temperature Exposure for Damage Recovery Diagnosis in Tunnel Fire

Can heavy rainfall affect the burning and smoke spreading characteristics of fire in tunnels?

Extreme rainfall events are increasingly common under the current trend of global warming. This work investigates how a heavy rainfall on one exit affects the fire burning and the smoke spread in tunnels. Several reduced-scale tests are designed with various rainfall intensities (up to 60 mm/h, equivalent to 232 mm/h in nature), raindrop sizes (1.0-1.5 mm, equivalent to 4-6 mm in nature), and tunnel fire heat release rates (2.1-6.7 kW, equivalent to 2-6 MW in real scale). Experiments show that heavy rainfall on one exit can induce a longitudinal airflow inside the tunnel, and the induced airflow is caused by the increased pressure at the rainfall exit. The airflow pushes the flame tilting towards the no-rainfall portal, and the correlation models of the flame length and flame inclination are characterized by considering the induced airflow and rainfall. The rainfall-induced airflow has a limited effect on the burning rate of pool fires, but it can change the ceiling temperature and limit the smoke back-layering toward the rainfall portal. In contrast, the ceiling temperature distribution towards the no-rainfall portal is found not sensitive to rainfall, which can be well described by an empirical model.