Smoke Back-layering Length Research Articles

Experimental investigation of smoke temperature and movement characteristics in tunnel fires with canyon cross wind

A series of tests were conducted in a model-scale tunnel to investigate the influence of canyon cross wind on smoke temperature and movement characteristics of tunnel fires, including temperature distribution below ceiling, smoke back-layering length, and smoke stratification characteristics. Canyon cross wind speed, pool size, and fire location were considered. Experimental results show that the temperature along tunnel ceiling decays exponentially, but the variation laws of the upstream and downstream temperature decay coefficients are different with the increasing canyon cross wind speed. The dimensionless confinement velocity is defined as the ratio of the canyon cross wind speed to the critical velocity, and the relationship between the dimensionless smoke back-layering length and the dimensionless confinement velocity is obtained. The state of the smoke stratification is related to the canyon cross wind speed and the fire source location. When the fire source is subject to a relatively strong canyon cross wind or closer to the upstream tunnel portal, the smoke stratification will become more unstable. The results can provide engineering reference for smoke control and safe evacuation of mountain tunnels.

Numerical simulation Simultaneous use of longitudinal ventilation and smoke extraction from the ceiling in fires inside tunnels

Ventilation is essential to provide a smoke-free path for safe passenger evacuation and effective rescue services in case of a tunnel fire, because the closure tunnels increase consequences of accidents significantly. In the present study, the simultaneous use of longitudinal ventilation and smoke extraction from the ceiling in fires inside tunnels and physical phenomena has been investigated. Fire dynamics simulator will be used as a CFD tool. This simulation was performed to investigate the effect of the longitudinal distance of the smoke extraction system from the fire source on the smoke back-layering length and the maximum temperature in the two operating conditions used by this system downstream and upstream the fire source. In the present work, the smoke extraction system is located on the ceiling of the tunnel. The results show that using a smoke extraction system upstream the fire source will increase the maximum temperature, but using the same system downstream will reduce the temperature throughout the tunnel and prevent smoke back-layering. However, attention to the smoke extraction velocity for prevent the plug-holding phenomenon. The results also show that the simultaneous use of two smoke extraction systems at the upstream or downstream of the fire source will have a better result and The maximum temperature is reduced by 10%.

An experimental study on smoke back-layering and critical velocity in tunnel fires with canyon cross wind

This paper investigated experimentally the thermal smoke back-layering and critical velocity of the highway tunnel in alpine gorge area with canyon cross wind. A series of small-scale tests were performed using a pool fire as the fire source. The downstream tunnel portal was under the action of canyon cross wind, and the longitudinal ventilation device was set on the upstream tunnel portal. The smoke movement phenomena were studied by changing the velocities of the canyon cross wind and longitudinal ventilation, and the longitudinal position of the fire source in the tunnel. The experimental results indicated that the smoke back-layering length decreases with the increasing longitudinal ventilation velocity, and both the critical velocity and smoke back-layering length increase with the increasing canyon cross wind velocity. This is because the airflow velocity towards upstream in the tunnel increases with the canyon cross wind velocity. In contrast, the downstream smoke spread length of the fire decreases with the increasing canyon cross wind velocity, and increases with the increasing longitudinal ventilation velocity. An equation for the critical velocity was developed by taking the canyon cross wind into account.

Numerical simulation on the maximum temperature and smoke back-layering length in a tilted tunnel under natural ventilation

The present study investigates the maximum temperature and smoke back-layering length S in the downhill direction from the fire source in a tilted tunnel under natural ventilation. Numerical simulations were conducted using FDS to study the smoke flow behaviors for a fire in a tunnel with nine tunnel slopes of 0, 1%, 2%, 3%, 4%, 5%, 6%, 7% and 8%. It was found that, due to the stack effect, the smoke stagnated at a distance from the fire source in the downhill direction. The effects of tunnel slope, α, fire source heat release rate, Q̇, source-ceiling height H and tunnel width W on the maximum temperature and smoke back-layering length were studied. Results showed that the maximum temperature under the ceiling decreased with the increasing of tunnel slope or the decreasing of tunnel width. However, it increased with the increasing of heat release rate or the decreasing of source-ceiling height. A model was proposed for the maximum temperature rise. The smoke back-layering length S decreased with the increasing of the tunnel slope. Fire source heat release rate and tunnel width had no significant effect on the smoke back-layering length. And the smoke back-layering length decreased with the decreasing of source-ceiling height. Based on dimensional analysis, a simple model including the effects of both the tunnel slope and source-ceiling height H, was proposed to predict the smoke back-layering length.

Study of smoke back-layering length with different longitudinal fire locations in inclined tunnels under natural ventilation

To investigate the vehicle fire hazard at different locations in inclined tunnels, the effect of longitudinal fire locations on smoke behavior characteristics induced by inclined tunnel fires was proposed by simulation and theoretical analysis. A series of tunnel models with slope of 4% was conducted by Fire Dynamics Simulator (FDS) software for fire simulation, in which three fire scenarios were considered corresponding to the variation of fire location, upstream length and downstream length respectively. The results show that smoke back-layering length drops progressively with increasing of downstream length, and upstream inlet airflow velocity is linearly positively correlated with downstream length, while the upstream length has a limited impact on smoke movement. A new prediction equation of the dimensionless smoke back-layering length account for downstream length and tunnel slope was established, through the addition of height difference between fire source and downstream exit. The established correlation shows that the dimensionless smoke back-layering length is logarithmically related to cubic power of downstream length, and the predicted results agreed well with simulated data as tunnel slope from 3.5% to 7.5%. The results can provide beneficial suggestion in ventilation system design and emergency evacuation for inclined tunnels.

Effect of lateral concentrated smoke extraction on the smoke back-layering length and critical velocity in a longitudinal ventilation tunnel

In this study, the thermal smoke back-layering behavior and critical ventilation velocity of tunnel fires were investigated experimentally under the combined effect of lateral concentrated smoke extraction and longitudinal ventilation. The smoke back-layering length, critical ventilation velocity, and confinement velocity were analyzed and discussed. The results indicate that the smoke back-layering length decreases with increasing lateral concentrated smoke extraction velocity when the longitudinal ventilation velocity is less than the confinement velocity. However, above the confinement velocity, the smoke back-layering length increases with increasing lateral concentrated smoke extraction velocity. The critical velocity also shows an increasing trend with an increase in the lateral concentrated smoke extraction velocity. Finally, a correlation equation for the critical velocity of tunnel fires under the combined effect of lateral concentrated smoke extraction and longitudinal ventilation was proposed.

An experimental study on the transition velocity and smoke back-layering length induced by carriage fire in a ventilated tunnel

Smoke is one of the important causes of tunnel fire casualties. The critical velocity and smoke back-layering length are important parameters of smoke control in the tunnel. This paper focused on experimentally studying the characteristics of the critical control parameter (such as, critical velocity, transition velocity) and smoke back-layering length induced by carriage fire in the longitudinal ventilated tunnel, which has not been quantified in previous studies. Overall, many repeatable test conditions were involved on different fire heat release rate (HRR), the longitudinal ventilation velocity and opening sizes. It is found that, when the fire heat release rate in the tunnel is small, the critical velocity increases with the increasing heat release rate. The critical velocities will no longer change when the fire heat release rates further increases to a certain value. The relationship between dimensionless smoke back-layering length and critical velocity is piecewise function. With the longitudinal ventilation velocity increasing to a certain value, the smoke back-layering length will become not sensitive to variations with the longitudinal ventilation velocity any more. So, for this part, the new parameter called “transition velocity” implies that the dimensionless smoke back-layering length changes slowly with the dimensionless critical velocity. Then, the dimensionless correlation of smoke back-layering length and transition velocity induced by carriage fire in ventilated tunnel was proposed.

Experimental study on confinement velocity in tunnel fires with longitudinal ventilation

A series of experiments were carried out in a model-scale tunnel to investigate the confinement velocity in the tunnel fire with longitudinal ventilation. The significance of longitudinal ventilation to the smoke control in tunnel fire was comprehensively elaborated by both the variation of smoke back-layering length in the upstream and the characteristics of smoke layer in the downstream which was visualized by the laser-sheet and determined objectively by the measured vertical temperature profile. Experimental results show that the smoke back-layering length exhibits a common trend with the increasing longitudinal ventilation velocity, first decreases sharply and then becomes not very sensitive after reaching a turning point. By analyzing the experiment results, it was found that the ratio of confinement velocity, i.e., the velocity corresponding to the turning point to critical velocity was basically identical which could be determined as 0.86. In addition, a quantitative model for directly predicting the confinement velocity was proposed and compared with previous models. The study is expected to provide some references for the application of confinement velocity in actual tunnel fire-fighting.

Numerical Study of the Effects of the Jet Fan Speed, Heat Release Rate and Aspect Ratio on Smoke Movement in Tunnel Fires

In this study, the effects of the jet fan speed, heat release rate and aspect ratio on smoke movement in tunnel fires have been investigated. The jet fan speed was changed from 6.25 (25%) to 12.5 m/s (50%), 18.75 m/s (75%), and 25 m/s (100%). The heat release rate was set up from 3.9 to 6 MW and 16 MW, the aspect ratio was changed from 0.6 to 1 and 1.5, respectively. The lower the jet fan speed is, the longer the smoke back-layering length is. With a higher velocity, the smoke tends to move out of the tunnel quickly; however, smoke stratification also occurs, and this reduces visibility. This could make it difficult for people to evacuate. With a higher heat release rate, the smoke tends to move far away from the fires quickly when compared with other cases. Additionally, the higher the heat release is, the longer the smoke back-layering is. Finally, with a higher aspect ratio, the smoke back layering length in the tunnel is also longer. The smoke layer thickness is also larger than in other cases. The correlation of velocity, heat release rate and aspect ratio has been investigated to avoid the smoke back layer length in tunnel fires.

Theoretical and experimental studies on the fire-induced smoke flow in naturally ventilated tunnels with large cross-sectional vertical shafts

The study focuses on the fire-induced smoke flow in a naturally ventilated tunnel with large cross-sectional vertical shafts. For the shaft with large cross-section, the smoke is exhausted along four sides of the shaft, in which condition, the complete plug-holing occurs and the fresh air is entrained into the shaft strongly. In this work, studies on the longitudinal smoke temperature rise, smoke mass and heat exhausted as well as air entrainment from the shaft with large cross-section are performed theoretically. A method to predict the maximum smoke temperature rise downstream the shaft and the smoke back-layering length in naturally ventilated tunnel fires is proposed. Meanwhile, a series of small-scale fire tests was carried out by considering heat release rates (HRR) of the fires, shaft heights and adjacent shaft intervals. Experimental results indicate that the smoke back-layering length decreases with the increasing shaft heights, but it seems to be independent of the HRRs that are greater than 25.8 kW employed in the present work. Lastly, the validation of the proposed models is conducted by comparing with experimental results, which indicates that the models predict well the maximum smoke temperature rise on the downstream side of the shaft and the smoke back-layering length for small fires. The present study contributes to a guidance for smoke control in naturally ventilated tunnel fires with vertical shafts.

A numerical study on smoke back-layering length and inlet air velocity of fires in an inclined tunnel under natural ventilation with a vertical shaft

When a fire occurs in an inclined tunnel, the tunnel inlet air velocity induced by the stack effect will help to prevent the smoke backflow. This work presents numerical studies on the smoke flow behaviors induced by fires in inclined tunnels under natural ventilation with a vertical shaft. The tunnel slope, heat release rate (HRR) and distance from the fire source to the downhill inlet of tunnel were varied. The effect of tunnel slope on temperature distributions in the shaft and tunnel, smoke back-layering length and tunnel inlet air velocity were specifically focused. Results showed that the maximum ceiling gas temperatures are presented in the downstream of fire axis for all slopes (5%–25%). The plug-holing phenomenon occurs for small slopes and disappears for large slopes. Increasing the slope can help to diminish the plug-holing while it decreases the smoke temperature in the inner region of shaft as well. The established correlation shows that the ratio of the back-layering length to the hydraulic tunnel height (which is defined as the 4 times the tunnel cross-sectional area to the tunnel perimeter) is proportional to −1.57 power of the slope, while it is independent on the HRR and fire source location. The tunnel inlet air velocity increases with increasing the tunnel slope and HRR while it decreases as the fire source placed far away from the downhill inlet of tunnel. Based on dimensional analysis and using the hydraulic tunnel height as the characteristic length, the final equation suggests that the normalized tunnel inlet air velocity is proportional to 0.42 power of the normalized HRR and −0.57 power of the normalized fire source location and it increases exponentially with the tunnel slope.

Experimental studies on the gas temperature and smoke back-layering length of fires in a shallow urban road tunnel with large cross-sectional vertical shafts

A reduced-scale (1:10) tunnel with four vertical shafts is constructed to investigate the fire smoke propagation in a natural ventilation tunnel equipped with large cross-sectional ceiling openings. A series of fire tests are performed to explore the impact of fire HRR (Heat Release Rate), shaft height and shaft interval on the gas temperature and smoke back-layering length. It is observed that the smoke cannot fill up the entire cross-section of the shaft, but just propagates adhering to the shaft sidewalls and then spills out of the tunnel. This flow mode of hot smoke in the large cross-sectional shaft is significantly different from the general plug-holing phenomenon, especially for the fresh air entrainment. In such case the fresh air is entrained just along the shaft sidewalls rather than exhausted directly from the lower part of the tunnel. Moreover, experimental results indicate that the dimensionless ceiling gas temperature on the upstream side of the shaft appears to be independent on the shaft height, while the temperature decay rate on the downstream side increases with the shaft height. The variation of the smoke back-layering length under different conditions implies that the smoke back-layering length initially decreases with the shaft height, then keeps constant with the HRRs when the HRRs are greater than 25.8 kW. Additionally, the smoke back-layering length generally increases with the fire-shaft interval when the shaft height is lower than 0.65 m, while it levels off initially and then increases again when the shaft height is higher than 0.65 m. Experimental testing in this paper contributes to understanding of the smoke movement in the naturally ventilated tunnel with large cross-sectional vertical shafts.

A model for predicting smoke back-layering length in tunnel fires with the combination of longitudinal ventilation and point extraction ventilation in the roof

An analytical model is developed for quantifying the fire smoke back-layering length in a tunnel with a combination of longitudinal ventilation and point extraction ventilation in the roof. The distance of smoke vent to fire source is incorporated as well as mass flow rate during the whole smoke flow process according to the mass conservation principle. The model input quantities are the heat release rate of the fire source, the longitudinal velocity, the exhaust velocity, the width and the height of the tunnel, the distance of the smoke vent to the fire source and the area of the smoke vent. The quality of the model predictions is illustrated for a range of experimental conditions. After that, extensive model predictions on the back-layering length are presented to show its trends by varying the velocity of the longitudinal ventilation, the exhaust velocity and the position of the smoke vent in the roof. Discussions are given at last. It is highlighted that shortening the distance between the smoke vent and the fire source benefits shortening the back-layering length, and this phenomenon is more pronounced for higher exhaust velocity.

Effects of ambient pressure on smoke back-layering in subway tunnel fires

This paper investigates the effects of ambient pressure on smoke back-layering in subway tunnel fires with and without train blockage. A series of numerical simulations were conducted in a 1/4 small-scale tunnel with different heat release rates (40–160 kW), longitudinal ventilation velocities (0.2–0.8 m/s) and ambient pressures (60–100 kPa). The smoke back-layering lengths under different conditions are analyzed, and the results show that under the same heat release rate and ventilation velocity, the back-layering length increases with decreasing ambient pressure due to the weak inertial force of longitudinal airflow led by the low air density. The Li’s and Zhang’s models, which can well predict the smoke back-layering length under regular pressure, are modified for the reduced pressure. The constant increment of back-layering length between adjacent ambient pressures, which mainly depends on the heat release rate, is used to correct Li’s model. The smoke back-layering length under the low ambient pressure can be predicted by this modified model without train blockage. With train blockage considered, new models are developed by introducing both equivalent and virtual fire sources for predicting the smoke back-layering length under the low ambient pressure, which is shown to well reproduce the simulation results.

Study on induced airflow velocity of point smoke extraction in road tunnel fires

In the point smoke extraction system, induced airflow velocity which can restrain smoke back-layering is an important parameter to evaluate smoke control efficiency of smoke extraction systems. New formula for the induced airflow velocity and smoke back-layering length is proposed for the tunnel fires with different heat release rates through small-scale tunnel model experiments. The experiment results show that when the dimensionless back-layering length is more than four, the influence of smoke outlet area on the induced airflow velocity and smoke back-layering length can be negligible. The relationship between the dimensionless back-layering length and induced airflow velocity is greatly affected by the heat release rate. In point extraction system, the smoke back-layering length increases gradually with the increasing of heat release rate, then tends to be smooth.

Smoke Movement in a Sloping Subway Tunnel Under Longitudinal Ventilation with Blockage

Critical velocity and smoke back-layering length are two of the determining parameters to the fire risk assessment of subway tunnel. These two parameters of a sloping subway tunnel with train blockage were investigated both experimentally and numerically in this paper. To address the influences of slope, the slopes of 0, 3, 6, 9, 12, 15% in downhill subway tunnel were studied and the height (H) of the tunnel was replaced by the inclined tunnel height ( $$ H/\cos \theta $$ ). The train model with a dimension of 2 m (length) × 0.3 m (width) × 0.38 m (height) was also chosen in simulations and experiments for the tunnel blockage. Thenceforward, 30 reduced-scale experimental and 150 numerical scenarios were analyzed to predict the critical velocity and smoke back-layering length in various sloping subway tunnels. Six different heat release rates including 5.58, 11.17, 16.67, 22.35, 27.94, and 33.52 kW were considered in the experiments and five different heat release rates including 2.79, 5.58, 8.38, 11.17 and 16.67 kW were considered in the simulations. Based on the comparison in the horizontal tunnel, numerical results were quite consistent with the experiments. The results showed that train blockage influenced the smoke back-layering length, and the critical velocity increases with the tunnel slope. Finally, empirical models were developed to predict the critical velocity and smoke back-layering length in a sloping subway tunnel with train blockage.

Numerical Simulation of Smoke Back-layering Length with Different Smoke Exhaust Modes in Subway Tunnel

In the increasingly congested urban ground transportation today, China's major cities are actively promoting the urban subway public transport system. But the fire safety problem is still a major issue of concern to people. In this paper, the FDS numerical simulation method is used to study the effect of several kinds of smoke control modes in the tunnel fire in the subway tunnel. The research results show that the current used smoke control mode in the subway tunnle is not the best option, as there is a great threat to the safety in the downstream space. The combination of longitudinal ventilation and specific smoke exhaust can control the smoke most effectively, which may be considered for the fire smoke design of the subway tunnel in the future.

Numerical Investigation of Back-Layering Length and Critical Velocity in Curved Subway Tunnels with Different Turning Radius

Curved tunnels are inevitable subjected to the city underground geological conditions. Due to the catastrophic consequence of tunnel fires with high population density, the related researches on fire safety of curved tunnel are full of significance. Therefore, a series of curved subway tunnels with turning radius of 300–1000 m were investigated numerically by FDS 5.5.3 in terms of the smoke back-layering length and critical ventilation velocity under the heat release rate of 5–10 MW. Theoretical analysis shows that the curved tunnel with the local resistance has an advantage of preventing smoke spreading compared with straight tunnel. The simulation results also indicated that both the smoke back-layering length and the critical ventilation velocity increased with the rising turning radius, and the straight tunnel has the largest values. In fact, the local resistance impact factor for the smoke back-layering length in the curved tunnel, $$ k_{f} $$ , was controlled by turning radius $$ R $$ and ventilation velocity $$ V $$ . The dimensionless critical velocity increased slightly from $$ 0.638Q^{*1/3} $$ to $$ 0.669Q^{*1/3} $$ when the turning radius increased from 300 m to 1000 m. Without considering the influence of turning radius (local resistance), previous models cannot be applied to the curved tunnel. The improved prediction models about smoke back-layering length and critical velocity with the factor of turning radius could provide a technical guideline for the tunnel ventilation designs.

Experimental study on thermal smoke backlayering length with an impinging flame under the tunnel ceiling

Model scale tests were performed with varying tunnel width and height in order to study the effect of tunnel cross section and ventilation velocity on smoke backlayering length in case of large heat release rate. The results show that in tests with the same fuel and ventilation velocity, the backlayering length trends to increase with increasing tunnel cross-sectional area after the tunnel cross-sectional area reaches a certain value, under which the critical velocity is obtained. The reason should be as the tunnel height or width increases, the blockage effect of flame inside the tunnel is reduced, thus leading to a smaller local ventilation velocity under the ceiling to confine the backlayering and ultimately a larger backlayering length is formed. In order to establish an integral model of backlayering length considering the effect of both tunnel height and width, new dimensionless forms of backlayering length and ventilation velocity, including hydraulic diameter of tunnel, are introduced. The proposed equation can correlate well with all the data from the model scale tests with different fuels, ventilation velocities and tunnel cross sections, and also Runehamar full scale tunnel tests.

Large Eddy Simulation on the Effect of Smoke Exhaust Openings Arrangement on the Smoke Spread in Tunnel Fires

This paper reveals effect of smoke exhaust openings arrangement on the smoke spread performance in the tunnel fire of semi-transverse smoke extraction mode. Results show that the smoke back-layering can be significantly affected by the upstream openings arrangement in tunnel fires. As the more exhaust openings added to upstream, the smoke back-layering length becomes stronger and smoke spread rate remarkably increase. Moreover, the maximum smoke temperature above the fire source also remarkably increase due to combined effect of longitudinal air flow and the amount of smoke exhaust increased in upstream from the fire. Consequently, the fire risk to the blocked occupants and the tunnel structure increase in upstream. This research can contribute to the design of fire safety facilities and management of fire emergency in tunnels with similar conditions.