Critical Ventilation Velocity Research Articles

Numerical Simulation of the Smoke Distribution Characteristics in a T-Shaped Roadway

This paper numerically analyzes the influence of heat release rate (HRR) and longitudinal ventilation velocity on smoke distribution characteristics in a T-shaped roadway when the fire source was located upstream of the T-junction. The back-layering length, critical ventilation velocity, smoke temperature, and CO concentration in the main and branched roadway were investigated and analyzed. The results showed that the ventilation velocity is the key factor influencing back-layering length, while the effect of HRR on back-layering length is gradually weakened as HRR increases. The critical ventilation velocity in the T-shaped roadway is higher than in a single-tube roadway, and the predicted model of dimensional critical ventilation velocity in a T-shaped bifurcated roadway is proposed. The correlation between average temperature (Z = 1.6 m) (both in the main roadway I and the branched roadway) and ventilation velocity fits the power function, and the variation in average temperature (Z = 1.6 m) according to HRR fits the linear formula. The relation between average concentration of CO (Z = 1.6 m) (both inside the main roadway I and the branched roadway) and longitudinal ventilation velocity follows the power relation, and the variation in average concentration of CO (Z = 1.6 m) according to HRR follows the linear function.

Study of critical velocity and back-layering length with fire sources both inside and outside a tunnel

The smoke back-layering length and the critical ventilation velocity in double fire scenarios where the fire sources were both inside and outside the tunnel were studied by small-scale tests and theoretical analysis. Results showed that the smoke back-layering length increased with an increase of the fire separating distance when the dimensionless fire separating distance was less than 3, and the values were larger than the results of a single internal fire at each fire separating distance. The dimensionless critical ventilation velocity segmentally increased with the increase of the dimensionless fire separating distance when the dimensionless fire separating distance was less than 10, and then remained constant. Moreover, the constant value was found to be close to that in the case of a single tunnel fire. In addition, an empirical model of dimensionless critical ventilation velocity was established for the double tunnel fire scenario both inside and outside a tunnel.

Fire Protection and Evacuation Analysis in Underground Interchange Tunnels by Integrating BIM and Numerical Simulation

Rescue and evacuation of underground interchange tunnels after a fire are challenging. Therefore, a method of integrating building information modeling (BIM) and a fire dynamic simulator (FDS) was proposed to analyze fire characteristics and personnel escapes in underground interchange tunnels. A BIM model of underground interchange tunnels was built, and then different formats (DXF and CAD) were generated and imported into Pyrosim software and Pathfinder software. With an increase in ventilation velocity, the CO concentration and temperature downstream of the fire source increased, and visibility decreased, according to simulation results. The critical ventilation velocity was 3.6 m/s at 30 MW. Evacuation simulation results suggested that the congestion of the transverse passage was very unfavorable for personnel escape: the escape time increased by 14.9% and 20% when the interior and entrance of the transverse passage were severely congested, while a 2.5 m wide transverse passage effectively reduced the escape time. Visibility was the first indicator that it did not meet the safety of the escape. After the tunnel’s personnel have been evacuated, the air supply or exhaust system should be started, and smoke should be expelled at a higher velocity. It is necessary to clear the passageway quickly or increase the automatic firefighting facilities when congestion is severe.

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.

Numerical investigation of critical velocity in curved tunnels: Parametric study and establishment of new model

Curved tunnels are more likely to have an accident and a fire source due to their physical characteristics. Using a 3D computational fluid dynamics tool with body-fitted grids, three different fire locations were investigated numerically in a series of curved tunnels with a turning radius of 50–1000 m and a heat release rate of 5–15 MW. Results showed that the critical velocity increased with increasing tunnel turning radius, and the straight tunnel had the highest critical velocity in all scenarios. For a tunnel with a 10 MW heat release rate, the critical velocity increased by 19 % from R = 50 to 1000 m. Furthermore, it was shown that the critical velocity was proportional to one-third power of the heat release rate. A gradual increase in critical velocity was also observed as the fire source was displaced from the tunnel's center to the walls. Furthermore, this increment increased as the fire source approached the internal curvature. During the displacement of the fire to the inner and outer walls of a tunnel with R = 100 m and a 10 MW heat release rate, the critical velocity increased by 7.7 % and 5.6 %, respectively. Therefore, the worst fire scenario occurs when a fire is close to a tunnel's internal curvature and has the largest turning radius and the maximum heat release rate. Tunnel curvature and fire location are not considered in normal dimensionless analysis (commonly used for tunnels). Thus, the numerical results were unified in a non-dimensional form, and a modified formula was developed to calculate critical velocities in curved tunnels. The proposed formula includes tunnel curvature, heat release rate, and fire location with a coefficient of determination of 0.98. Numerical simulations confirmed the predictions of the proposed formula. Compared to other models in straight and curved tunnels, the predicted correlation developed in the present study can accurately predict critical ventilation velocity in curved tunnels.

Comparative analysis on temperature characteristics of hydrogen-powered and traditional fossil-fueled vehicle fires in the tunnel under longitudinal ventilations

Vehicle fires in the tunnel are a great threat to the safe operation of the tunnel. Due to the rapid development of the hydrogen economy, the fire due to the hydrogen leakage could not be avoided and may bring great damage to the passengers and infrastructure. Due to the large difference between pool fires of traditional fossil-fueled and jet fires of hydrogen-powered vehicles, it is in doubt whether the existing longitudinal ventilation design could still be effective for the safety issue of hydrogen powered vehicles. To solve this problem, it is necessary to compare temperature characteristics of hydrogen-powered and traditional vehicle fires with and without longitudinal ventilations. In present work, we conducted a numerical investigation to discuss the different temperature distributions of traditional and hydrogen-fueled vehicle fires. Results indicate that the high temperature zone of the pool fire only exists above the ceiling of the vehicle. For hydrogen-powered vehicle fire, the high-speed hydrogen jet with the strong inertial force could push the hot smoke flows back to the ground. The ceiling temperature of hydrogen-powered vehicle fire is larger since hydrogen-powered vehicle has a larger heat release rate and the fire hazard of jet fires bring more danger compared with the pool fire. Although the temperature stratification is also obvious for the hydrogen-powered vehicle fire, the air temperature in the lower region could be heated and still high enough to bring a great damage to the passengers’ lives. This is quite different with the traditional pool fire. In addition, the critical ventilation velocity is also discussed. The theoretical equation could well predicted the critical ventilation velocity of traditional vehicle fires. For hydrogen-powered vehicle fires, the critical ventilation velocity could reach up to 6 m/s. The theoretical equation could not well predict the critical ventilation velocity of hydrogen-powered vehicle fires due to exist of hydrogen jet fires.

Numerical study on fire-induced smoke temperature characteristics in small curvature radius UTLT-like tunnels under emergency state

Extensive studies have usually focused on an ordinary single straight or large curvature radius tunnel fire. However, the curvature effect of small curvature radius curved tunnel on the fire-induced smoke temperature characteristics still needs to clarify. Therefore, a three-dimensional transient computational fluid dynamics (CFD) fire model for small curvature radius urban traffic link tunnel (UTLT)-like tunnel was established to understand the scope and extent of curvature effect on the fire-induced smoke under emergency state. The results show that, in the curved tunnel area, small curvature radius or large critical ventilation velocity results in stronger centrifugal force of the fire smoke, readily destabilizing the fire smoke downstream of the fire source. Compared with large radius of curvature and straight tunnels, due to the greater resistance of curved walls in small curvature radius tunnel, the length of smoke back-layering is longer. Simultaneously, the temperature of the high-temperature smoke on the outer wall of the ceiling cross section is higher than the inner wall. Prediction mathematical models for the maximum temperature and longitudinal temperature profiles were further developed by taking the curvature and ventilation velocity effect, which is basically consistent with experimental and simulated results.

Theoretical and numerical study on critical velocity and driving force for preventing smoke backlayering in a connection roadway fire of coal mines

H-shape tunnel is a common roadway structure in the coal mine. When a fire occurs in the connection roadway, the characteristics of smoke movement and control are different from those in the traditional single-hole tunnel. This paper studies the critical ventilation velocity and driving force for preventing smoke backlayering in a mine connection roadway fire by numerical modeling. Results indicate that when the dimensionless heat release rate (Q̇*) is less than 0.28, the dimensionless critical ventilation velocity (Vc*) varies as the 1/8 power of the dimensionless heat release rate. Beyond 0.28, Vc* remains almost constant, independent of the heat release rate. For a certain heat release rate, the critical velocity in the connection roadway is lower than that in the single-hole tunnel, which is probably attributed to the smaller ventilation resistance or the effect of the downstream flow field on the smoke backlayering front. The current study prefers to use the ventilation resistance for further analysis. Besides, a calculation model of driving force for preventing smoke backlayering in the connection roadway fire is put forward by theoretical analysis. The predictions by the proposed model are found to comply well with the numerical simulation data. The outcomings of the current study are of guiding significance for the fire prevention and smoke control in the tunnel with a similar structure.

Experimental study on temperature profile and smoke movement in a model-branched tunnel fire under longitudinal ventilation

Reduced scale experiments in a branched tunnel were carried out to study the temperature profile and critical ventilation velocity under different longitudinal ventilation conditions. Branch orientation was upstream and the branch angle was 15°. The results show that when dimensionless ventilation velocity is lower than 0.19, the maximum temperature is lower than that of traditional single tube tunnel because hot smoke could propagate into the branch. When dimensionless ventilation velocity is higher than 0.19, the maximum temperature is close to that of single tube tunnel. Because more heat is blown downstream, which is similar to ventilated single tube tunnel. A new model predicting the maximum temperature beneath the ceiling is developed. With increasing ventilation velocity, the upstream and branch temperature decay factor increase while the downstream temperature factor decrease. Critical ventilation velocity for the branched tunnel is higher than that of single tube tunnel. A correlation of critical ventilation velocity is proposed. The 1/3 power law still holds.

Experimental investigation of the thermal back-layering length in a branched tunnel fire under longitudinal ventilation

The smoke propagation in branched tunnel is more complicated because of the special bifurcation structure that has not been clarified, despite the fact that the scenario exists in practice. In this paper, the thermal smoke movement behavior in branched tunnel fire were experimentally investigated under the combined effect of bifurcation angle and ventilation velocity. The theoretical analysis was conducted to develop the smoke back-layering length equation based on the applied force of smoke propagation upstream of the fire source, and the smoke back-layering length under various longitudinal ventilation velocities was quantified. The results indicate that the smoke back-layering length decreases with the increasing of longitudinal ventilation velocity for a given heat release rate. The stronger heat release rate results in the longer smoke backflow distance under same ventilation velocity. The smoke back-layering length varied with bifurcation angle due to the bifurcation structure weakens the inertial force of longitudinal ventilation, but the affected degree varies with bifurcation angle. The experimental smoke back-layering length can be well correlated using the deduced logarithmic function. The empirical model taking the bifurcation angle into consideration is proposed to predict the smoke back-layering length in branched tunnel fire. Finally, the correlation for critical ventilation velocity of branched tunnel fire under varied bifurcation angles was also proposed based on the boundary condition of none smoke back-layering.

Thermal and fire characteristics of hydrogen jet flames in the tunnel at longitudinal ventilation strategies

Hydrogen leakage of vehicles in the tunnel is a great threat to the safety operation of the tunnel and longitudinal ventilation strategies have always been utilized to control the fire and smoke movement of rail transit, electric and fossil-fueled vehicles in the engineering field. It is in doubt whether the longitudinal ventilation strategy could still help to reduce the jet fire hazard of transportation with H2 power in the tunnel, considering the rapid development of the hydrogen energy. In present work, a numerical research on effects of longitudinal ventilation strategies on hydrogen jet flames in the tunnel is conducted. The results illustrate that longitudinal ventilation could affect the flame characteristics of jet flames greatly in the tunnel. The critical ventilation velocity increases firstly with the increase of hydrogen leakage rates and then changes little after a critical value. The predicted theoretical model of pool fires could well predict the critical ventilation velocity for hydrogen jet fires. With the increase of longitudinal ventilation velocity, maximum ceiling temperatures are decreased greatly. According to the heat releases, jet speeds and ventilation velocities, three kinds of flame bending characteristics of hydrogen jet fire could be observed due to different effects of the inertial force. At last, the stable thermal stratification could also be destroyed by large ventilation velocities but the corresponding ventilation velocity is far larger than the critical ventilation one. With the increase of longitudinal ventilation velocities, the height of thermal layer is reduced firstly and then maintained at a constant value.

A novel representation of the critical ventilation velocity for mitigating tunnel fires

Critical ventilation velocity dominated research within the field of tunnel fire safety because it is integral for designing a tunnel ventilation system. This work proposes a novel analytical model for the critical ventilation velocity. The theoretical analysis infers that the heat release rate and tunnel height controls the critical ventilation velocity. Earlier studies support this conclusion. We perform several highly accurate simulations of small-scale tunnel fires to quantify the influence of heat release rates and tunnel height. We define two non-dimensional quantities and identify a correlation through data analysis. The new model illustrates the transition where the critical ventilation velocity increases rapidly with the heat release rates however at higher heat release rates the critical ventilation velocity becomes constant. To verify and validate the model, we compare the novel formulae against the existing relationships and available experimental data. There exist numerous analytical models but several of these models are discontinuous while others do not capture the transition where the critical velocity becomes independent of the heat release rate. The novel relationship, presented in this work, eliminates these drawbacks.

Effects of False-Ceiling on Critical Ventilation Velocity and Maximum Gas Temperature in Tunnel Fires

In the present study, the effect of the use of false-ceiling on fire-induced smoke flow characteristics in tunnels is investigated using a 3D developed computational fluid dynamics tool. The critical velocity, the minimum required tunnel ventilation velocity to stop the smoke flow from moving toward the tunnel inlet (toward the upstream of the fire source), and the maximum gas temperature beneath the ceiling are selected to evaluate the smoke flow control in presence of the false-ceiling. The hydraulic height of the cross-sectional geometry of the tunnel is used as the characteristic length in order to dimensional analyze and compare the non-dimensional results. The results indicate that the use of the false-ceiling reduces the critical velocity and the smoke backlayering, while increases the maximum ceiling gas temperature. Reducing the critical velocity results in ventilation cost reduction (positive impact), while increasing the maximum gas temperature beneath the ceiling increases the risk of instrumental and life damages (negative impact). The detailed results and corresponding physical discussions are presented to clarify the reason for the significant differences between the results of the tunnels with and without false-ceiling.

Perspectives of big experimental database and artificial intelligence in tunnel fire research

Tunnel fire is one of the most severe global fire hazards and causes a significant amount of economic losses and casualties every year. Over the last 50 years, numerous full-scale and reduced-scale tunnel fire tests, as well as numerical simulations have been conducted to quantify the critical fire events and key parameters to guide the fire safety design of the tunnel. In light of the recent advances in big data and artificial intelligence, this paper aims to establish a database that contains all existing experimental data of tunnel fire, based on an extensive literature review on tunnel fire tests. This tunnel-fire database summarizes seven key parameters of flame, ventilation, and smoke in that is open access at a GitHub site: https://github.com/PolyUFire/Tunnel_Fire_Database. The test conditions, experimental phenomena, and data of each literature work were organized and categorized in a standard format that could be conveniently accessed and continuously updated. Based on this database, machine learning is applied to predict the critical ventilation velocity of a tunnel fire as a demonstration. The review of the current database not only reveals more valuable information and hidden problems in the conventional collection of test data, but also provides new directions in future tunnel fire research. The established database and methodology help promote the application of artificial intelligence and smart firefighting in tunnel fire safety.

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.

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires

Effect of blockage on critical ventilation velocity in longitudinally ventilated tunnel fires was investigated based on small-scale experiments and numerical simulations. Blockages were located right upstream of the fire or 3.5 m upstream of the fire. At each location, blockages have five blockage ratios and two orientations. Experimental data shows that the factors as blockage ratio, blockage-fire distance and blockage orientation could have coupling effect on the critical ventilation velocity. For blockage right upstream of the fire, the critical ventilation velocity decreases with the increasing blockage ratio when blockages are horizontally positioned, while there is first decrease and then increase in the critical ventilation velocity for blockage vertically positioned. A correlation was proposed to predict the critical ventilation velocity for blockages right upstream of the fire and horizontally positioned. For blockage 3.5 m upstream of the fire, the change in the critical ventilation velocity is very slight with the increasing blockage ratio for blockages vertically positioned, however, when blockages are horizontally positioned, there is an increase in the critical ventilation velocity for large blockage ratios. The change in the critical ventilation velocity with increasing blockage ratio is more obvious for blockages right upstream of the fire than that for blockages 3.5 m upstream of the fire. Numerical simulations were also conducted in the tunnel without fire. The altered tunnel flow field due to the obstruction of blockage was observed and its effect on the critical ventilation velocity was addressed. Comparison of the critical ventilation velocity between the experimental data and the results from previous studies showed that the critical ventilation velocity changed differently with the existence of blockage. The reason for this difference might be that the relative position between the blockage and the fire, and the fuel type could have coupling effect on the critical ventilation velocity.

Design of Fire Source for Railway Vehicles and Measurement of Critical Velocity in Reduced-Scale Tunnels

In this study, the authors designed a reduced-scale railway vehicle fire, which was necessary for evaluating the fire safety of railway tunnels using a reduced model. To overcome the shortcomings of the methods used in conventional reduced-scale railway tunnel tests, the authors simulated the fire source of a railway vehicle using a methanol fire source for fire buoyancy, and a smoke cartridge for smoke visualization. Therefore, the heat release mass consumption rates of various methane trays were measured using a cone calorimeter (ISO 5660). The critical ventilation velocity in the railway tunnels was obtained using the designed fire source of the railway vehicle, which was evaluated by the measured temperature at the top of the tunnel as well as laser visualization.

Modelling and experimental investigation of critical velocity and driving force for preventing smoke backlayering in a branched tunnel fire

The previous researchs of the critical velocity are mainly for single-point tunnel and limited for branched tunnel. The critical velocity together with the driving force for preventing smoke backflow in branched tunnel cannot be precisely predicted by former correlations. Experiments are conducted in this work to investigate the fire-induced smoke movement in branched tunnel under the cases of critical ventilation velocity condition. The pressure change in branched tunnel is analyzed on the basis of one dimensional theory, and the fire source and bifurcation angle was taken into account. Results show that the bifurcation angle has a significant influence on the critical velocity and the driving force for preventing smoke backlayering due to the local resistance. The pressure difference due to the plume blockage created by the fire are expressed on the basis of experimental data. Based on the theoretical analysis of pressure change, new relatively correlations considering the bifurcation angle and heat release rate are proposed to predict the critical velocity together with the required pressure rise for preventing smoke backflow in branched tunnel. The predicted critical velocity by proposed model are found to comply well with the experimental data.

A numerical study on critical ventilation velocity in slope tunnel fire under vehicle blockage

AbstractThe particularity of highway tunnel location and the limitation of space result in the closure of its environment, which makes evacuation, rescue, and firefighting activities more difficult. In this study, a fire dynamics simulator was used for numerical simulation to study the effect of jamming on the critical velocity of slope tunnel fire. This experiment carried out two sets of working conditions simulations: (1) In the straight tunnel, the heat release rate and the blockage ratio are considered and numerically simulated. (2) Under the heat release rate of 20 MW fire source, the blockage ratio and the gradient of the tunnel are considered and numerically simulated. The vehicle blockage is set to distribute in two rows and two columns upstream of the fire source, accounting for 6.6%‐22.0% of the cross‐section of the tunnel. The slopes were selected in nine cases of 0%, ±1%, ±2%, ±3%, and ±4%. The six heat release rates were 5 MW, 10 MW, 15 MW, 20 MW, 25 MW, and 30 MW, respectively. The fire occurred on the tunnel centerline. The results show that in flat tunnels, the variation trend of critical velocity with heat release rate is basically the same, and it increases with the increase of heat release rate. However, when the heat release rate is the same, the larger the vehicle blockage ratio is, the smaller the critical velocity is. It can be seen from the second experimental case that the influence of the tunnel slope on the critical velocity is different. In the downhill tunnel, the critical velocity increases with the increase of slope. In the uphill tunnel, the critical velocity decreases with the increase of slope.

Critical ventilation velocity under the blockage of different metro train in a long metro tunnel

SummaryTo address the effect of metro train blockage on the critical ventilation velocity in a long tunnel, a series of scenarios were conducted numerically through this study, including different fire sizes (5‐10 MW), metro train lengths (80‐120 m), and blockage ratios (φ, 0.50, and 0.57). It is known from the numerical results that the metro train length shows a limited effect on the critical ventilation velocity, which is because the longitudinal ventilation has become stable before reaching the fire source to prevent smoke back‐layering, and increasing the metro train length only increases the distance of stabilizing the longitudinal ventilation. The blockage ratio shows an obvious influence on the critical ventilation velocity, which is because the presence of the metro train can obviously reduce the flow cross‐sectional area of the tunnel. An empirical model is developed as well, while it is known that the critical ventilation velocity increases with the one‐third power of dimensionless heat release rate and (1‐φ). The research outcomes of this study provide a technical guide for the design of the metro tunnel and the relevant emergency management of fire rescue under fire conditions.