Longitudinal Ventilation Velocity Research Articles

Experimental study on thermal smoke layer thickness with various upstream blockage–fire distances in a longitudinal ventilated tunnel

In order to study the effect of varied blockage-fire source distances (rectangular cylinder shape) on the smoke layer thickness in a longitudinal ventilated tunnel, a series of experiments were conducted in a 1/6 model tunnel platform. Varied longitudinal ventilation velocities, fire heat release rates, and blockage-fire distances D were considered. The vertical region temperature was measured by using a thermocouple tree, and the integral ratio method was used to calculate the smoke layer thickness. It was determined that the smoke temperature at any specified height decreased with increasing longitudinal ventilation velocity. The smoke layer thickness increased with increasing longitudinal ventilation velocity irrespective of the presence or absence of vehicular blockage, and the smoke layer thickness in the presence of vehicular blockages was smaller than that in the absence of blockages.

Experimental Investigation on the Mass Loss Rates of Thin-Layered n-Heptane Pool Fires in Longitudinally Ventilated Reduced Scale Tunnel

ABSTRACTThin-layered n-heptane pool fires are burned with varied pool depths under longitudinal ventilation velocities ranging between 0.5–2.5 m/s in a reduced scale tunnel model. The combined effects of ventilation, pool size, and depth are investigated on the heat release rate, temperature distribution, and mass loss rate of fire. The gas temperature distribution and heat release rate results indicate that the critical ventilation velocity is achieved around 1 m/s in the scaled model, corresponding to 3.6 m/s in the real scale tunnel. It is observed that the gas temperature downstream of the fire increases at 2.5 m/s ventilation due to an enhancing effect of oxygen supply to the fire and increased flame deflection towards the leeward side of the pan. Results show that maximum heat release rate and total heat release normalized by fuel amount tend to occur at critical ventilation velocity. The measured mass loss rates show a considerable increasing trend with pool depth.

Effect of blockage-heat source distance on maximum temperature of buoyancy-induced smoke flow beneath ceiling in a longitudinal ventilated tunnel

Longitudinal ventilation is the commonly mode in tunnel for smoke control. This paper presents experimental study on the effect of blockage-heat source distance on maximum temperature of buoyancy-induced smoke flow beneath tunnel ceiling in a ventilated tunnel. Experiments were carried out in a scale model tunnel with a blockage placed at different position upstream of the fire source. The temperature beneath the tunnel ceiling is measured by K-type thermocouples considering different heat release rate and longitudinal ventilation velocity. Results show that Li model can well predict the experimental data when there is no blockage in the tunnel. With upstream blockage of the heat source and different blockage-heat source distance D considered, including D=0m, the maximum temperature beneath the tunnel ceiling first increases to the maximum value when the blockage-heat source distance D=0.5m, then decreases as blockage-heat source distance increases. An empirical model including blockage-heat source distance (from 0m to infinity) is proposed and verified to well collapse and predict the experimental values.

Numerical analysis of smoke dispersion against the wind in a tunnel fire

In a tunnel fire, the dispersion of smoke in the opposite direction to the longitudinal imposed air flow remains the principal factors prejudicial to users which are blocked upstream of the fire. This phenomenon, usually referred to as “backlayering”, is a key parameter that one must better understand to ensure proper evacuation of users and emergency intervention. This paper carried out full-scale numerical simulation using Fire Dynamic simulator (FDS) to analyze the behavior of backlayering flow in a longitudinally ventilated tunnel fire. FDS predicted backlayering lengths are compared to calculated values using the Hu model. A reasonably good agreement has been obtained. Furthermore, the predicted maximum smoke temperatures are compared to those given by the Kurioka model. CFD results show that the backlayering flow takes CO towards the tunnel entry, which can be deadly to users immediately even at low concentrations. The inertia and buoyancy forces produced by ventilation and fire, respectively, affect the interface height. The backlayering arrival time increases with longitudinal ventilation velocity while it decreases with heat release rate of the fire. An equation is developed to predict the backlayering front arrival time against the distance to fire.

CFD analysis of smoke backlayering dispersion in tunnel fires with longitudinal ventilation

SummaryIn a longitudinally ventilated tunnel fire, the backlayering flow propagated in the opposite direction to the air current is the most fatal contaminations to users which are blocked upstream of the fire. In the present paper, numerical simulations were conducted using Fire Dynamic Simulator, which is based on large eddy simulations to estimate the backlayering arrival time in a longitudinally ventilated tunnel fire. The effect of a vehicle obstruction on the backlayering arrival time will be also investigated. For this, a vehicle model occupying about 31% of the tunnel cross section is simulated upstream of the fire source with its location relative to the tunnel floor is varied. The numerical investigation shows that the inertia and the buoyancy forces produced by ventilation and fire, respectively, affect the backlayering spread. The backlayering arrival time increases with the longitudinal ventilation velocity while it decreases with the fire heat release rate. When a vehicle obstruction existed within the tunnel, the numerical results show an increase of backlayering arrival time. This increase is significantly more important with the fire distance when the vehicle obstruction approaches the tunnel floor. Two correlations are developed, with and without obstruction in the tunnel, to predict the backlayering arrival time against the distance to fire. Copyright © 2016 John Wiley & Sons, Ltd.

Smoke back-layering flow length in longitudinal ventilated tunnel fires with vertical shaft in the upstream

Smoke back-layering flow length is the length of the reversed smoke flow upstream of the fire when the longitudinal ventilation velocity is lower than the critical velocity. This paper experimentally investigates the smoke back-layering flow length of longitudinal ventilated tunnel fires with a vertical shaft in the upstream (0.5–4.0m from the fire source) using a 1/10 reduced-scale subway tunnel model. Experimental results show that the vertical shaft in the upstream can control the smoke back-layering flow length within a relatively limited range, compared to the tunnel without vertical shaft. Moreover, for the cases that the fire source is not located exactly below the vertical shaft, the most appropriate dimensionless distance between the vertical shaft and fire source is 3, resulting in the shortest smoke back-layering flow length. By introducing a concept of virtual fire source below vertical shaft, a new empirical model was further deduced to predict the smoke back-layering flow length. Its predictions fit reasonably well when the dimensionless longitudinal flow velocity is less than 0.19. Beyond that, the predictions are little higher than the experiments, which is because some of the assumptions in this model are invalid under higher longitudinal ventilation velocity.

Thermal smoke back-layering flow length with ceiling extraction at upstream side of fire source in a longitudinal ventilated tunnel

This paper investigates experimentally the thermal buoyant smoke back-layering flow length beneath the tunnel ceiling with combination ceiling extraction and longitudinal ventilation, especially focusing on the condition that the ceiling extraction is employed at upstream side of the fire source, which has not previously been reported. Experiments are conducted in a model tunnel. A square gas burner was used to simulate the fire source, whose side dimension is 0.3m. The heat release rates (HRRs) of fire source were 30kW, 40kW and 50kW. Ceiling extraction opening with circular outlet of 0.3m diameter was located at 1m upstream from the fire source. Six different longitudinal ventilation velocities with six different ceiling extraction flow velocity were considered. Thermocouples are employed to measure the longitudinal temperature profile of the thermal smoke flow beneath the tunnel ceiling, to determine the upstream back-layering flow length. It was found that the upstream back-layering flow length increases with increase in heat release rate, meanwhile decreases with increase in longitudinal ventilation velocity or ceiling extraction velocity. A new non-dimensional factor was proposed to predict the thermal smoke back-layering flow length, which is shown to be in good agreement with the experimental results.

A Revised Correlation on Downstream Temperature Distribution of Fire-Induced Flow with Mechanical Longitudinal Ventilation

To evaluate the stability of smoke stratification and fire evacuation effectively, the temperature distribution downstream the fire beneath the ceiling of tunnels with mechanical longitudinal ventilation needs to be estimated. Based on Li model, a revised correlation is proposed by concerning the effects of the mechanical longitudinal ventilation velocity and the width of the tunnel. Ten full-scale field test data in three real tunnels from Hu et al. are employed to correlate the revised correlation. The applicability of the new revised model is validated by comparing with the field test data and previous models. Results indicate that the new model can agree with the field test data well. The Mean Absolute Percentage Error (MAPE) of the revised model is found to be 19.99%. Great improvement of the prediction accuracy is achieved as compared with the previous models.

Prediction of smoke back-layering length under different longitudinal ventilations in the subway tunnel with metro train

Due to the small width and the large train blockage ratio in subway tunnel, the smoke back-layering will be different from that in the wider road tunnel with small vehicle blockage ratio. In the train blockage region of tunnel, the velocity of longitudinal ventilated air-flow interacting with the back-flowed smoke gas is different from that in the upstream tunnel without train blockage. Then the back-flowed smoke gas might be prevented in the train blockage region with higher ventilation velocity, otherwise it would be stopped in the upstream tunnel without train blockage but with lower ventilation velocity. They were taken into consideration separately and an equivalent fire source was introduced by dividing the dimensionless heat release rate of fire source into two parts in the cases where the smoke back-layering length is longer than metro train length. A series of full-scale numerical simulations are carried out with FDS to investigate the smoke back-layering length in subway tunnel with different train lengths and longitudinal ventilation velocities. The simulation results indicate that the influence of metro train length on the smoke back-layering is great and cannot be ignored any more. A global correlation model is proposed based on the dimensionless analysis and simulation results.

Effect of tunnel cross section on gas temperatures and heat fluxes in case of large heat release rate

Tests with liquid and solid fuels in model tunnels (1:20) were performed and analysed in order to study the effect of tunnel cross section (width and height) together with ventilation velocity on ceiling gas temperatures and heat fluxes. The model tunnel was 10 m long with varying width (0.3 m, 0.45 m and 0.6 m) and height (0.25 m and 0.4 m). Test results show that the maximum temperature under the ceiling is a weak function of heat release rate (HRR) and ventilation velocity for cases with HRR more than 100 MW at full scale. It clearly varies with the tunnel height and is a weak function of the tunnel width. With a lower tunnel height, the ceiling is closer to the base of continuous flame zone and the temperatures become higher. Overall, the gas temperature beneath the ceiling decreases with the increasing tunnel dimensions, and increases with the increasing longitudinal ventilation velocity. The HRR is also an important factor that influences the decay rate of excess gas temperature, and a dimensionless HRR integrating HRR and other two key parameters, tunnel cross-sectional area and distance between fuel centre and tunnel ceiling, was introduced to account for the effect. An equation for the decay rate of excess gas temperature, considering both the tunnel dimensions and HRR, was developed. Moreover, a larger tunnel cross-sectional area will lead to a smaller heat flux.

Numerical study of the effect of blockage on critical velocity and backlayering length in longitudinally ventilated tunnel fires

In a tunnel fire, one of the protective strategies of users and vehicles which are blocked upstream of fire is provided by longitudinal ventilation systems which serve to limit the rise of smoke flow (backlayering). The presence of these vehicles creates an obstruction which affects the plume fire behavior and the smoke movement. The longitudinal ventilation velocity from which the backlayering disappears, usually designated by “critical ventilation velocity”, is a key parameter to ensure proper evacuation of users and emergency intervention. This study performs numerical simulation using Fire Dynamic Simulator (FDS) to estimate the effect of an obstacle blockage according to its location relative to the tunnel floor on the backlayering flow behavior and the critical velocity. An obstacle occupying about 31% of the tunnel cross section is placed symmetrically upstream of fires by changing its location relative to the tunnel floor. The validity of the FDS Numerical results is firstly demonstrated through a comparison, in terms of critical velocity, with experimental results available for public in the literature. Results which are based on CFD modeling show that the effect of obstacle blockage brings about a decrease of the critical velocity compared than to those obtained with an empty tunnel. This decrease depends on the obstacle location relative to tunnel floor. It is slightly greater when the distance between the bottom of obstacle and the tunnel floor increases. Further, when the obstacle exists in tunnel, the backlayering length become much smaller compared to those predicted in an empty tunnel.

Experimental Study on the Influence of Different Transverse Fire Locations on the Critical Longitudinal Ventilation Velocity in Tunnel Fires

A set of fire experiments were conducted in a small-scale tunnel of length 8 m, width 1 m and height 0.5 m, to investigate the influence of different transverse fire locations on the critical longitudinal ventilation velocity for suppressing the smoke backflow in tunnel fire. Six pool sizes were separately used in the experiment, and the burning rate of these pool fires were in the range of 0.05–0.5 g/s. Experimental results showed that for ethanol pool fire, the mass burning rate of fire source kept almost unchanged in the process of the fire moving from the central tunnel to the sidewall, while the maximum temperature rise under the ceiling increased exponentially with the decrease of the distance between the fire and the sidewall, and reached about 1–1.7 times of the fire at the longitudinal centerline. Because critical longitudinal ventilation velocity is associated with the temperature in the impingement area, the critical velocity was investigated in the experiments as well. The dimensionless critical velocity varied with the burning rate, and located in the range of 0.1–0.24 in the experiment. When fires occurred at the center of the tunnel, there was a good agreement between the critical longitudinal ventilation velocity and the model of Wu. The critical longitudinal ventilation velocity would increase exponentially with the decrease of the distance between the fire and the sidewall. When fires neighbored sidewall, the critical longitudinal ventilation velocity was agreed well with the value calculated using the “mirror” method based on Wu’s model.

An experimental study on critical velocity in sloping tunnel with longitudinal ventilation under fire

Experiments based on Froude scaling law were conducted in a 1/10 reduced-scale model tunnel to investigate the effect of slope on critical velocity in tunnel with longitudinal ventilation under fire. Methanol pool fire was used as the fire source. Smoke movement in the model tunnel with different combination of tunnel slope (−3%, −1.8%, −1%, 0%, 1%, 1.8% and 3%) and longitudinal ventilation velocity was studied. The longitudinal distribution of temperature and velocity were measured. Critical longitudinal ventilation velocity for arresting smoke back flowing in the model tunnel was investigated. Critical velocity in horizontal tunnel obtained in this study was used to compare with that of previous studies. The experimental results agreed well with the model proposed by Wu and Bakar. Correlation between the critical velocity and the slope of the tunnel is proposed based on the experiment results. As the tunnel slope increases from downhill to uphill, critical velocity decreases nearly at a rate independent of the heat release rate of the fire source. The correlation proposed in this studied agrees well with the equation adopted in the Subway Environment Simulation Computer Program (SES). However, it has some discrepancy with the expressions proposed by Atkinson and Ko et al. based on their experimental results. The cause of the discrepancy is attributed to different configuration of the experiments.

A study of bifurcation flow of fire smoke in tunnel with longitudinal ventilation

In this paper, numerical simulation was conducted using Fire Dynamics Simulator (FDS) to study bifurcation flow of smoke in tunnel with longitudinal ventilation. Through analyzing the effect of longitudinal ventilation velocity on smoke flow field, temperature distribution within smoke layer and height of smoke layer, generation mechanism of smoke bifurcation flow was revealed. Longitudinal ventilation would increase entrainment of smoke with air and dilute the smoke at the center of tunnel ceiling, and would enhance inertial force of horizontal flow and weaken driving force of reverse flow of ceiling jet by sidewall. A low temperature region below tunnel ceiling was formed when longitudinal ventilation exceeded a critical value. There was little smoke in the low temperature region, so smoke bifurcation flow was occurred. The occurrence of the bifurcation flow mainly depends on the horizontal inertial force and the driving force.

Studies on buoyancy driven two-directional smoke flow layering length with combination of point extraction and longitudinal ventilation in tunnel fires

This paper investigates the buoyancy-driven smoke flow layering length (both upstream and downstream) beneath the ceiling with combination of point extraction and longitudinal ventilation in tunnel fires. A theoretical model is developed based on previous back-laying model with only longitudinal ventilation, with modified actual heat release rate, as well as modified upstream and downstream opposing longitudinal air flow velocities by the induced flow velocity due to point extraction. Experiments are carried out in a reduced scale model tunnel with dimensionless of 72m×1.5m×1.3m. A LPG porous gas burner is used as fire source. The smoke flow layering length both upstream and downstream are identified based on temperature profiles measured along the ceiling, for different experiment conditions. CFD simulations with FDS are also performed for the same scenarios. Results show that with combination of point extraction and longitudinal ventilation, the smoke flow layering length is not symmetric where it is longer downstream than that upstream. The upstream smoke layering length decreases, while the downstream layering length increases with increase in longitudinal ventilation velocity; and they both decrease with increase in point extraction velocity. The predictions by the proposed theoretical model agree well with the measurements and simulation results.

Experimental Study on Suppression of n-Heptane Pool Fire with Water Mist under Longitudinal Ventilation in Long and Narrow Spaces

This paper studies suppression of n-Heptane pool fire with water mist under different longitudinal ventilation velocities in a long and narrow space. Through the suppressing experiments with two different spraying positions and seven ventilation speeds, it is found that the extinguishment of fires has a close relationship with the fuel temperature. Moreover, the fuel temperature, when the fire is extinguished by spraying at different positions, is almost the same within the measurement ranges, and it tends to decrease as the wind speed rises. Mechanism studies show that the coupling effect of dilution and replacement of oxygen and fuel surface cooling is the main mechanism in n-Heptane fire extinguishment of water mist under longitudinal ventilation. In addition, the ability to efficiently cool the fuel plays a determinant role in the n-Heptane pool fire suppression.

Experimental Study on Effect of Longitudinal Ventilation Velocity and Slope on Smoke Movement in Tunnels

The slope effects on the movement of internal smoke cannot be ignored. Fire experiments in a small scale tunnel are conducted in order to observe the smoke movement in a tunnel with three kinds of slope (0%, 3% and 5%) under different longitudinal ventilation velocity, then find the correlation between smoke diffusion velocity and longitudinal ventilation velocity, and slope in tunnel by numerical fitting and dimensional analyses, finally conclude:(1) as longitudinal ventilation velocity increases, smoke diffusion velocity gradually decreases; (2) when longitudinal ventilation velocity is smaller, with the increase of slope, smoke diffusion velocity is gradually increasing; (3) when longitudinal ventilation velocity is large, slope to smoke diffusion velocity is not affected. These quantitative conclusions provide fire Prevention of the tunnel in city with a reference.

Fire-induced flow temperature along tunnels with longitudinal ventilation

The temperature distribution of fire-induced flow beneath the ceiling of tunnels or corridors with longitudinal ventilation needs to be estimated in order to properly design the fire detectors and provide adequate fire protection to the tunnel structure. The authors’ previous correlation for naturally ventilated tunnels is evaluated to correlate the temperature distribution of fire-induced flow under mechanically ventilated tunnels. The effects of the longitudinal ventilation velocity and the fire heat release rate are introduced into this correlation by replacing the reference flow excess temperature and its position with the maximum flow excess temperature and its position, respectively. Experimental data ranging from reduced-scale to laboratory-scale tunnels from two past studies are employed. Supplementary data are obtained from numerical simulations. Several past empirical correlations for the maximum flow excess temperature and its position are validated with adjusted coefficients by these data. The evolved correlation for the temperature distribution of fire-induced flow along mechanically ventilated tunnels is finally evaluated and modified. Results indicate that this correlation could predict the temperature distribution with engineering accuracy.

Effect of Ventilation and Geometrical Parameters of the Burning Object on the Heat Release Rate in Tunnel Fires

A 1/13 scaled model tunnel is constructed in order to investigate the blockage effect (the ratio of the model cross sectional area to the tunnel cross sectional area) on the heat release rate inside the tunnel with different ventilation velocities. Scaling is done based on Froude number. In the experiments, wood sticks assembled in different geometrical configurations are burned with various longitudinal ventilation velocities inside the model tunnel. Combustion gases are sampled to calculate the heat release rate. A statistical model is developed using the analysis of variance (ANOVA) method. According to the statistical model, 79.8% of the variation in heat release rate is attributed to changes in blockage ratio, 10.6% to changes in thickness, and 4.5% to changes in velocity.

Full-scale experimental study on fire suppression performance of a designed water mist system for rescue station of long railway tunnel

Rescue stations have been newly designed in some super-long mountain railway tunnels of China such as over 30 km long, as a new type of strategic counteract design for fire accident in tunnels. Water mist fire suppression systems are needed to be installed in such rescue stations to control the train fire. However, there is no design engineering experience for it. In this article, a full-scale experimental facility was built to study the fire suppression performance of water mist system in such railway tunnel rescue station. A series of full-scale tests was preliminarily conducted considering the fire position was on top of, inside, and under the train carriage with gasoline pool fire and wood crib fire used as fire sources. A design of water mist suppression system for such a fire scenario was presented, and its performance was tested. Experimental data on temperature, CO concentration, and radiation flux were collected and analyzed. The results showed that the water mist system could control fires, cool the air temperature in the rescue station, and strongly reduce the radiant heat flux in the vicinity of the fire source under various ventilation conditions. However, the visibility of the rescue station was much deteriorated after the injection of water mist, so that emergency lights and evacuation lights are necessary for personnel evacuation. The changes of fire extinguishing time with longitudinal ventilation velocities showed that longitudinal ventilation could provide both favorable and unfavorable conditions for water mist fire extinguishment. So, to achieve efficient fire suppression performance of water mist system, attention might be paid to the controls of tunnel ventilation system in the event of a fire in a railway tunnel rescue station. The design of the water mist system and the full-scale experimental data on the fire suppression performance presented in this article may serve as a reference for such engineering applications.