Smoke Front Research Articles

Fire Hazard Analysis on Different Fire Source Locations in Multi-Segment Converging Tunnel with Structural Beams

To investigate the fire risk in a complex tunnel with varying cross-sections, sloped structures, and dense upper cover beams, this study considered four fire source positions: the immersed tube section, confluence section, highway auxiliary road section, and four-lane sections of the main line. It also considered four beam spacings: 1 m, 1.8 m, 3.6 m, and 7.2 m. The Fire Dynamics Simulation Software FDS was utilized to create a comprehensive tunnel model. The analysis focused on temperature and visibility changes at a 2 m height under a 20 MW fire condition for different fire source positions. These changes were then compared with critical danger values to assess the safety of evacuating personnel within the tunnel. Subsequently, this study proposed corresponding emergency rescue strategies. The findings indicated that when the beam grid spacing exceeded 3.6 m, the upper dense beam gap showed a robust smoke storage capacity, leading to a reduced distance of high-temperature smoke spread. However, this increased smoke storage disrupted the stability of the smoke layer, resulting in a heightened smoke thickness. The location of the ventilation vent at the entrance of the immersed tunnel section caused a non-uniform ventilation flow under the girder, deflecting the smoke front towards the unventilated side and decreasing visibility in the road auxiliary area. In comparison to scenarios without a beam lattice, the presence of a beam lattice in the tunnel amplified fire hazards. When the beam lattice spacing was 3.6 m or greater, the extent of the hazardous environment, which is unfavorable for personnel evacuation, expanded. With the exception of the scenario where the fire source was located in the highway auxiliary roadway, all other conditions surpassed 150 m, which is roughly one-third of the tunnel length. Consequently, more targeted strategies are necessary for effective evacuation and rescue efforts.

A Study on the Behavior Characteristics of Air Supply during Tunnel Fires under Natural Ventilation with Multiple Vertical Shafts

This study investigates the behavior of air supply in tunnels with multiple vertical shafts during fire incidents, focusing on natural ventilation dynamics. Numerical simulation is utilized to analyze the effect of different variables on air supply within vertical shafts. The findings reveal that the position of the smoke front significantly influences the direction and flow rate of gases during fire development. The mass flow rate of air supply during the stable fire development stage is influenced by the geometric size and positioning of vertical shafts, with shafts closer to the fire source exhibiting higher air flow rates. To address this issue, this study introduces a predictive model for estimating air flow rates in vertical shafts. This model exhibits a high level of accuracy when compared to simulations, offering a reliable method for predicting air flow rates based on the geometric characteristics of vertical shafts. Overall, this research contributes to understanding the complexities of air supply in tunnels with multiple vertical shafts, aiding in the improvement of natural ventilation strategies during fire incidents.

An experimental investigation into the fire behaviors and smoke characteristics of continuous spill fires in road tunnels

Continuous fuel spill fires are a common accident scenario which needs to be considered for the safe operation of tunnels, given the potential threat posed to trapped persons. In this paper, a series of continuous spill fire experiments on a concrete surface were studied using a 1/8 reduced-scale tunnel model. Gasoline was used as the discharge fuel and the discharge rate varied from 1.5 to 5.2 ml/s. The burning area, steady regression rate, smoke front arrival time, and ceiling temperature were analyzed. The results show that the burning area spreads slowly following an initial rapid spill stage, with the spread process following a pattern of exponential decay. The steady regression rate is found to be approximately 63% of that of pool fires for the same burning area. At the same heat release rate, the smoke front moves faster for spill fires than pool fires because of the larger burning area, and a correlation is built to predict the smoke front arrival time. Finally, a ceiling temperature rise prediction model is developed which considers the spread and burning processes of the fire. The results are of practical importance in the quantitative risk assessment of liquid fuel fire accidents in tunnels, and their implications for personnel evacuation.

Experimental evaluation and real-time forecast of smoke propagation in tunnels under intervention of sub-critical longitudinal ventilation

Sub-critical longitudinal ventilation can restrict the back-layering length in the upstream region and maintain suitable smoke stratification in the downstream region of a tunnel fire. In this study, experiments using a 1/15 scale, 80-m long, 1.2-m wide, and 0.5-m high model tunnel were conducted to investigate the effects of sub-critical longitudinal ventilation velocity and heat release rate (HRR) on the key parameters describing smoke propagation. The smoke front temperature near the fire source decreased significantly as the longitudinal ventilation velocity increased. The longitudinal ventilation velocity and HRR were incorporated into a regression model describing the smoke front velocity, which was applied to the evaluated fire scenarios using an ensemble Kalman filter to account for intervention of sub-critical longitudinal ventilation. The smoke front temperatures at discrete locations were assimilated to estimate the heat exchange correction coefficient and air entrainment ratio of smoke front in real-time and dynamically correct the forecasted smoke front temperature and velocity accordingly. When sub-critical longitudinal ventilation intervened, the smoke front parameters could be forecasted with a maximum lead time of 60 s and an average error within 5%. Therefore, the proposed forecasting method can inform the selection of disposal measures in early tunnel fire emergencies.

Brine-water experimental study on the propagation of stratified smoke flow in tunnel fires under subcritical longitudinal ventilation

In the early stage of tunnel fires, longitudinal ventilation velocity is often smaller than the critical velocity to preserve the smoke stratification and therefore facilitate the evacuation. A series of brine-water experiments are conducted to investigate the unsymmetrical propagation of the stratified smoke flow under subcritical ventilation. A light attenuation technique is used to measure the distribution of the reduced gravity in the tunnel. The ventilation velocity V and the source buoyancy flux per unit width B0 are two dominant parameters for the flow. The reduced gravities at both sides of the source are quantitatively compared, and then a model is proposed to estimate the reduced gravities under different B0 and V. In terms of the propagation velocity, two flow regimes are observed. When V/B01/3 is smaller than 0.56, both the propagation velocity of the downstream smoke front, ud, and that of the backlayering smoke front, ub, are independent of time; moreover, ud is smaller than the propagation velocity of the smoke following the downstream smoke front. However, when V/B01/3 is larger than 0.56, ud is still independent of time, but ub decreases with time; furthermore, ud is approximately equal to the propagation velocity of the smoke following the front. Prediction models regarding the propagation velocities at both sides of the fire are established. As the backlayering flow advances forward, its thickness approximately linearly decreases. The maximum thickness of the backlayering flow is correlated with V/B01/3. The study might be useful for determining the ventilation and evacuation strategies in tunnel fires.

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.

Study on control of upstream smoke propagation in road tunnels by using a side-wall sprinkler system

Side-wall sprinkler systems used for fire suppression in road tunnels have been found to redistribute the longitudinal air flow within the tunnel, with the most notable feature being an acceleration of the airflow near the ceiling. The present work is driven by the motivation of utilizing the acceleration effect to block the upstream smoke propagation in cases when the ventilation rate is sub-critical. A nozzle prototype that has practical applications in road tunnels was selected for a side-wall water spray system in a 300 m road tunnel, and a total of 60 cases were modelled using the CFD code FDS (version of 6.7.3) to investigate its smoke blocking effect. Experiments and correlations in the literature have validated the accuracy of the FDS method. The confinement velocity reduction rate, which is determined as the reduction of the ventilation rate required to stop the smoke front point at a given position, was found to be positively affected by both the water spray rate and the ventilation rate and was successfully correlated using dimensionless momentums of these two factors. By comparing the confinement velocity reduction rates with the increases in the ceiling velocity in the absence of tunnel fire, utilization efficiencies of the spray-induced accelerations were further obtained.

Study on smoke propagation in tunnel construction of a hydropower station: A full-scale fire experiment

In this study, full-scale fire experiments were conducted in a hydropower station to investigate smoke propagation during tunnel construction. The flame height, smoke temperature and stratification, smoke descent and spread velocity were analyzed via measurements and on-site observations. The initial combustion stage was largely affected by ignition source during tunnel construction for diesel pool fire, and the average flame height in the fully developed stage could reach 1.4–2.1 m in experimental fire scenarios. The gradient of the smoke temperature evolution near the fire was the opposite for the upstream and downstream regions. The longitudinal temperature distribution was concentrated in a small range at the heights of the smoke layer, and gradually decreased by air entrainment as the height decreased, while further increasing in the lower half of the tunnel height in the near-fire region under heat radiation from the fire source. Moreover, distinct and stable smoke stratification formed during the fully developed combustion stage, and the smoke layer interface was at approximately half the tunnel height. Smoke descent was aggravated in the decay stage of combustion, and the fire risk remained high after the fully developed period. The smoke front spread velocity was empirically determined for the full-scale tunnel fire scenarios. Conclusions from full-scale experiments can support smoke control design and on-site fire emergency response plans for hydropower stations.

Numerical analysis of the performance of a PID control based real-time mechanical ventilation system to prevent smoke back-layering in tunnel fires

Longitudinal mechanical ventilation systems are widely employed to prevent smoke back-layering in tunnel fires. Correlations for the critical velocity of ventilation in tunnels have already been extensively studied, for given fire and tunnel (cross-sectional) dimensions. This work describes the potential of a novel concept of an automatic ventilation system based on a PID (Proportional-Integral-Derivative) controller for preventing smoke back-layering under a priori unknown fire size. A few different tunnel dimensions and heat release rates (HRR) are addressed in order to illustrate how the ventilation velocity adapts automatically. Two methods to identify the location of the smoke front are proposed, based on temperature information. Simulation results show that the proposed approach can control the smoke well, preventing back-layering in real-time. It has excellent adaptability for the unknown fire size, which is considered an advantage compared to a system based on constant ventilation velocity. The smoke front becomes stable more quickly for higher HRR, while it takes longer to stabilize for increased tunnel height. This relates to the flame height: sufficiently high flames (compared to the tunnel height) induce a fast rise in temperature, which is the input parameter for the PID system. The tunnel width therefore has little impact. The values for the critical velocity and for the ventilation velocity at maximum smoke back-layering length (BL), as obtained with the proposed method, agree well with existing correlations. The influence of the set-point value for temperature is briefly discussed.

Full-scale experimental study on fire under natural ventilation in the T-shaped and curved tunnel groups

To assess the fire risk during the construction of the pumped storage power station, full-scale fire experiments in the ventilation and safety tunnel were conducted to research temperature distribution and smoke diffusion under ventilation. The temperature profile and airflow velocity of air flow under different conditions were recorded and analyzed. The results show that the airflow velocity was mainly influenced by the large space like powerhouse and transformer chamber under natural ventilation. By analyzing smoke propagation characteristic parameters including the maximum ceiling temperature rise, smoke front arrival time and temperature variation with time, the fire-induced smoke hazard under different fire locations was assessed. The research results can provide reference data for smoke exhaust design and help developing the emergency evacuation plan for the large underground space building with similar structure.

Experimental and simulation studies on hybrid ventilation in interlayer of underground depot with superstructure

With the modernization of urban development, land resources are becoming increasingly scarce. A new type of building model comprising an underground depot and a superstructure has been constructed. To study the hybrid ventilation (HV) in interlayer of the underground depot with a superstructure under monorail line section fire, full-scale numerical simulation and reduced-scale (1:50) experiment were performed. Five ventilation velocities were used under a certain heat release rate. The temperature distribution and stratification in the interlayer were examined, and the movement of the fire smoke was analyzed. The smoke layer was visualized using a laser sheet and the smoke layer thickness determined by N-percentage rule (N = 10, 20, 30) was compared with the results of visual observation. At the same time, CFD simulations were conducted to measure the height of smoke layer and the smoke front propagation. Besides, the distribution of carbon monoxide (CO) in the interlayer was also simulated. The results show that N value of 30 could properly determine the smoke layer thickness in relatively stable stage of combustion. HV had a better effect on smoke control than natural ventilation (NV) did. At a ventilation velocity of 1.4 m/s, the smoke layer in the interlayer remained in a steady state until its exclusion from the building. CO could be effectively controlled and did not spread all over the interlayer in HV systems. The structure of HV system can be adjusted according to the actual conditions of the various sizes and types of buildings. Thus, the HV system shows good adaptability and can be directly applied to the design and installation of new buildings such as underground garages and depots. The results of the fire dynamic simulations and experimental tests exhibited considerable agreement.

Smoke flow in full-scale urban road tunnel fires with large cross-sectional vertical shafts

In this study, a series of full-scale fire tests are performed in a tunnel with large cross-sectional vertical shafts. The heat release rates and fire locations are considered, and the general smoke development, smoke flow pattern in the shaft, smoke temperature distribution, and smoke front velocity are analysed. It is found that complete plug-holing occurs in the tunnel with large cross-sectional vertical shafts. The smoke flows into the shaft from the bottom sides and spreads out along the shaft sidewalls. The smoke cannot fill the shaft cross-section, even at the shaft outlet. In the early and full-development stages of a fire, a well-stratified smoke layer exists, whereas in the decay stage and after burning out, the visibility significantly reduces. The smoke can be controlled within 80–125 m on each side of the fire source, and it takes approximately 10 min for the visibility to self-recover naturally for common car fires after burning out. The results of the full-scale experiment indicate that the shaft has acceptable performance in smoke control for the test scenario investigated. According to an analysis of the tunnel environment during a common car fire, as well as the evacuation movement, it can be concluded that the occupants can escape safely. The experimental results are used to verify the smoke control performance of the vertical shaft and provide guidance for the design of naturally ventilated tunnels with vertical shafts.

Full-scale experimental study on fire-induced smoke movement and control in an underground double-island subway station

The research carried out full-scale fire experiments for smoke movement and control in an underground double-island subway station. Four kinds of fire scenarios were designed in the station hall and platform respectively, and series of thermocouples were arranged in the public space. Several vital fire parameters were analyzed, including the vertical temperature distribution, the longitudinal temperature distribution, smoke layer height and smoke front arrival time. Besides, the effectiveness of exhaust system was evaluated for fire emergency. The results showed that the smoke flow characteristics under different fire scenarios exhibited diversely. Under natural ventilation, the maximum temperature excess near fire source was 18 ℃ and 21℃ in the station hall and platform fire scenarios separately, and the smoke temperature attenuated gradually with the longitudinal distance from fire. Besides, the smoke layer exceeded the height of 4.0 m and 2.8 m when fire in the station hall and platform. After activating the exhaust system in the B-end, it was obvious that the smoke layer height increased and the temperature excess decreased. Additionally, the smoke front arrival time was influenced by the fire source locations and ventilation conditions. In the above scenarios, the smoke diffusion velocity was empirically determined as 0.11–0.18 m/s and 0.21–0.35 m/s when a fire in the station hall and platform respectively. The conclusions from the on-site fire experiment could provide data support for the fire accident emergency in an underground double-island subway station.

Full-scale experimental study on fire-induced smoke propagation in large underground plant of hydropower station

In this study, full-scale fire experiments in a large underground plant of a hydropower station were conducted to investigate smoke propagation under natural ventilation, and the overall temperature profile was measured through distributed thermometric cables. The smoke temperature variation with time, maximum temperature distribution in the upper and lower space, smoke stratification, and smoke front spread velocity were acquired based on full-scale measured data. Results indicate that under the fire sizes of 4, 6, and 10 fuel pans with combustion areas of 2, 3 and 5 m2, the temperature rise of the upper smoke layer in the fire cross section against the ambient are 1.7, 2.3, and 3.8 ℃, respectively, while they are 6.7–7.6, 9.0–10.8, and 12.9–13.0 ℃ above the ground of 0–3.5 m in the lower space of the plant. Owing to continuous heat loss to the surrounding, the maximum temperature in the smoke layer decayed as the smoke propagated longitudinally, and the attenuation trend decelerated or reversed in front of the end wall by enhanced accumulation of smoke and heat. The smoke stratification is distinct in the large underground plant and the smoke layer interface with fresh air descended within the height of 20–30 m. Moreover, the smoke front spread velocity were empirically determined for the three fire sizes. Conclusions from the full-scale fire experiments can provide on-site data and propagation characteristics for smoke control design, as well as scientifically guide the emergency response plan during fire accidents in large underground plant of hydropower stations.

Numerical study on overall smoke control using naturally ventilated shafts during fires in a road tunnel

This paper studies the overall smoke control of natural ventilation systems with vertical shafts during fires in a common road tunnel by numerical modelling. The variables studied include the heat release rate, longitudinal fire location along the tunnel, length of shafts and the interval between two shafts. Simulation results indicate that the total smoke spread length on both sides of fire source is closely independent of the heat release rate and longitudinal fire locations. For a given dimensionless shaft interval (the ratio of the shaft interval to shaft length), with the increase of shaft length, the smoke spread length firstly increases, reaching a maximum at 12 m, and then decreases significantly until 18 m. For a fire less than 30 MW, the first shaft pair on both sides of fire source prevents the critical-temperature smoke (270 °C) from spreading beyond this shaft. For a 100 MW fire, in the cases with shorter shaft lengths (Lshaft≤9 m), the critical-temperature smoke can't be controlled between the first shaft pair. The gas temperature at human height (1.8 m) is less than 60 °C in all cases with shafts. Downdraught occurs when the smoke front stabilizes at the bottom of a shaft and the buoyancy force could be too low to overcome the kinetic pressure of the air flow flowing into this shaft, consequently destroying the structure of smoke layer. In most scenarios, the total exhaust area of shafts that is required to exhaust all the smoke is about 100 m2. The first shaft pair plays a critical role to exhaust the smoke, and its exhaust efficiency is also affected significantly by the shaft length. This study investigates how to control the smoke by using vertical shafts in a road tunnel fire and the conclusions are useful to tunnel fire protection engineering.

Analysis of smoke movement in the Hsuehshan tunnel fire

In 2012, a serious fire accident occurred in the Hsuehshan southbound road tunnel, which resulted in two fire fatalities and 34 injuries. A pickup truck was hit by a bus from behind; gasoline leaked from the bus and started the fire. Smoke was diverted from the southbound tunnel into the service tunnel and the northbound tunnel via two connection tunnels. Fire dynamics simulator (FDS) software was used to simulate the smoke movement in the 13 km long southbound road tunnel, employing more than 21 million computational cells. The heat release rate curve used in the simulation was based on a single bus fire which reached a maximum value of 30 MW. The smoke front movement obtained from the computer simulation was compared with that determined from the closed circuit TV (CCTV) recordings, and the agreement between the simulation results and CCTV recordings was sound. The critical velocity for controlling backlayering of a 30 MW bus fire was also determined from simulation, which agreed well with the value estimated from empirical correlations of model experiments. This research showed that during a fire, doors to connection tunnels should close automatically, and the service tunnel should only be used for emergency rescue purpose, instead of evacuation of people.

Ceiling smoke front velocity in a tunnel with central mechanical exhaust system: Comparison of model predictions with measurements

The front velocity of transient ceiling becomes an important consideration for tunnel fire safety, which provide a prediction of travel time of ceiling jet. The previous model predictions of ceiling jet front velocity were developed in a natural ventilation tunnel. The effect of central mechanical exhaust system on the ceiling jet front velocity have not been revealed in the past. This work experimentally revealed smoke front velocity of ceiling jet in a tunnel under the effect of central mechanical exhaust system, and the relative location between fire source and central exhaust system were considered. It is found that, the relative direction of fire source and ceiling mechanical extraction opening has a great influence on the smoke front velocity of ceiling jet. Some previous models (Jones, Benjamin, and CFAST) all overestimate the smoke flow velocity of ceiling front; the predicted ceiling jet front velocity deduced by Jones and Benjamin have the maximum excessive deviation of about 60% and 98%, respectively, but the CFAST model is closer to the measurements with relatively higher front velocity. The Hinkley’s model can be well used to predict it with different central mechanical exhaust system.

水平天井面を軸対称に拡大する火災初期の煙先端部位置の近似解析解

The approximate analytical solution of the location of smoke front below horizontal ceiling at early time in fire is derived by the handling as the axi-symmetric problem. At first the approximate analytical solution of the average temperature of smoke layer is derived in assuming that the progress velocity of smoke front is proportional to 1/2 power of time based on the mass preservation. Then the integral equation to predict the location of smoke front is derived in this smoke average temperature. As this equation is not able to solve analytically, an approximate analytical equation is derived by approximation of integrand and being subject to temperature less exceeding initial inflow temperature. Particularly, in case of the treatment of center sprang out, the location of smoke front is proportional to 1/2 power at early time then to 3/4 power at time. The prediction is compared to the experimental data. It is shown that the location of smoke front is predicted by the approximate analytical solution.

Smoke Flow Visualizations for the Study of Surface Dielectric Barrier Discharge Induced Flow Field

The flow field induced by surface dielectric barrier discharge (SDBD) is visualized using illumination of smoke seeded to the flow. A planar SDBD of 50 mm span is created on a flat surface between thin copper electrodes separated by layers of kepton tapes which act as dielectric; AC voltage of 25 kV (peak to peak) is supplied across the electrodes. The smoke is illuminated by a planar laser sheet, and the time resolved visualizations are captured using a high speed camera operated at a recording rate of 2000 frames per second. The time evolution of the starting vortex and the steady wall jet that eventually gets established on the surface are recorded. The speed of the smoke front at the beginning of actuation and the starting vortex speed are quantified from the visualizations, which are used for parametric study of the SDBD flow actuator. The DBD actuator is found to induce a maximum velocity of 8 m/s close to the electrode at AC frequency of 2000 Hz. It is found that the starting vortex speed increases...

Mathematical Model and Experiments of Axisymmetric Spread of Smoke Front under Ceiling

Mathematical Model and Experiments of Axisymmetric Spread of Smoke Front under Ceiling