Longitudinal Ventilated Tunnel Research Articles

Heat transfer and mathematical model of flame geometry of rectangular-shaped pool fire in longitudinal ventilation tunnel

Fire is one of the most serious threats in operation of the tunnel. The thermal radiation disaster of fire is dependent on flame height and inclination angle which are affected by the longitudinal ventilation. The coupling effect of aspect ratio of rectangular container (n = 1, 2, 4, 8, 16) and longitudinal wind on flame geometry of heptane pool fire are revealed. The flame height decreases with an increase in the wind, while the inclined angle increases in longitudinal winds of 0–2.49 m/s. The variation of flame geometry is interpreted using the mechanical equilibrium of horizontal inertial force and gas uprising buoyancy induced by liquid fire. According to air entrainment theory and heat feedback theory, the flame height and inclined angle of rectangular-shaped fire change with characteristic scale ratio (n+1n) in mathematics. The measurements of flame height and inclined angle are analyzed and several quantitatively mathematical equations incorporating heat release rate, characteristic scale ratio, non-dimensional burning rate, non-dimensional air flow are proposed. The predicted results of flame geometry are fully validated by experimental measurements. The current finding is convenient for establishing radiation model of pool fire in air-blown condition.

Numerical investigation of the temperature characteristics beneath the ceiling driven by a strong plume in a longitudinal ventilated tunnel

AbstractPrevious studies have mainly focused on the situation that the tunnel's continuous flame region was lower than the ceiling height. For tunnels with longitudinal ventilation, the temperature characteristics of strong fire plumes are still unclear. In this paper, the maximum temperature rise beneath the ceiling for strong fire plume conditions in a small‐scale tunnel is studied using fire dynamics simulator. Results show that when the effective height (the distance from the burner surface to the ceiling of the tunnel) is 0.65 m, the maximum temperature rise beneath the ceiling in this work shows a good correlation with Li's model and Kurioka's model. However, as the effective height decreases to 0.55 and 0.50 m, the maximum temperature rise would be significantly lower than the previous model. Therefore, a dimensionless coefficient is introduced to modify the maximum temperature rise model for a strong fire plume, which involves the effective height coefficient, heat release rate, and longitudinal ventilation. A calculation model for the maximum temperature rise beneath the tunnel ceiling in the condition of strong fire plumes is established. The temperature attenuation data along the tunnel are given by statistics. The relationship between the dimensionless temperature rise and the dimensionless position parameter is established, and a unified model under various heat release rates is obtained.

Effect of different configurations of multi-blockages on the fire characteristics in tunnel fires with longitudinal ventilation

A tunnel vehicle fire is usually accompanied by traffic jams and less attention has been paid to the effect of multi-blockage on the fire characteristics in longitudinal ventilated tunnel fires. Thus, this study has revealed this point based on a full-scale model tunnel(100 m in length, 12 m in width, and 6 m in height) by Fire Dynamic Simulator(FDS) considering the different configurations of blockages, namely transverse and longitudinal. Results show there exist significant differences between two typical configurations of blockages and the distances between blockages have great effects on the fire characteristics in the cases with transversely arranged blockages. Moreover, the empirical models for maximum temperature rise and longitudinal temperature decay are developed and predict the numerical results well.

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

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

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

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

Investigation on the maximum ceiling temperature of the weak plume impingement flow in tunnel fires under longitudinal ventilation

This paper numerically investigated the characteristics of the fire plume behavior and the ceiling maximum temperature with increasing fire source height, in the longitudinal ventilated tunnel fires. Results show that with the continued increase of the longitudinal velocity, the fire plume gradually tilts to the downstream of the tunnel, causing significantly decrease of the ceiling temperature. With the elevation of the fire source, a circulation area forms at the leeward side of the fire source, the pulling effect of the circulation area and the accumulated smoke layer can protect the fire plume from the effect of ventilation airflow, which can weaken the fire plume inclination. To quantify the influence of the longitudinal flow, the dimensionless velocity v* is adopted and divided into three regions, i.e., v*= 0, 0 < v*≤ 0.19 and v*> 0.19. When 0 < v*≤ 0.19, a velocity influence factor f (v) is added to the original equation to represent the effect of ventilation velocity for increasing fire source locations and the ceiling maximum excess temperature varies as the 2/3 power of the dimensionless heat release rate and linear function of dimensionless ventilation velocity. When v* > 0.19, a concept of virtual fire source is proposed to quantify the effects of the inclined fire plume on the maximum temperature and the ceiling maximum excess temperature varies as the 2/3 power of the dimensionless heat release rate and exponential function of dimensionless ventilation velocity. Besides, the applicability and reliability of the predictive correlations are further verified by comparing to the experimental data of different scales.

Effect of single-side centralized exhaust on smoke control and temperature distribution in longitudinal ventilation tunnel fires

In the centralized exhaust tunnel, the smoke exhaust fan room is generally built on both sides of the tunnel to discharge toxic smoke from the tunnel. Because the construction of the fan room is complex and expensive, only one fan room can be built in some short-centralized exhaust tunnels, such as the Mawan Cross-Sea Passage in Shenzhen, China. In this paper, a 1:10 single-side centralized exhaust small-scale tunnel was built to study smoke and temperature distribution beneath ceiling under different longitudinal velocities, smoke exhaust volumes and HRRs. The results showed that the smoke in a single-side exhaust tunnel can be effectively controlled at the appropriate smoke exhaust volume with the longitudinal speed at 0.5 m/s; temperature of smoke is less than 80℃ after passing through exhaust outlet. Based on experimental results, a smoke control scheme for a ceiling point exhaust tunnel with one fan room was obtained. The temperature decay beneath ceiling accords with law of exponential attenuation. A calculation model for temperature decay in a ceiling point exhaust tunnel with one fan room was also proposed. The error between the experimental results and the prediction model in this paper is less than 20%.

Experimental Study on the Influence of Fire Source Elevation on Smoke Temperature Profile Driven by Buoyancy in a Full-scale Mountain Tunnel

ABSTRACT Previous studies have not sufficiently focused on the fire characteristics of different fire source elevations in a longitudinal ventilated tunnel fire, especially for a full-scale tunnel. In this work, a set of full-scale burning experiments were conducted to investigate the maximum temperature under the ceiling and longitudinal hot smoke temperature decay characteristics by considering the effect of heat release rate (HRRs), ambient wind velocity, and fire elevation. Experimental results show that the maximum temperature profile under the tunnel ceiling is mainly dependent on the interactions of HRRs, wind velocity, and fire elevation. The comparison of the maximum temperature model predicted by previous studies with these experimental data was conducted, which demonstrates that Li’s model is more appropriate to describe the maximum temperature under the ceiling. Moreover, the dimensionless longitudinal temperature decay rate of hot smoke induced by fire exhibits an obvious increasing trend with the fire elevation increases. A modified model of longitudinal temperature profile along the tunnel ceiling was put forward by considering the effect of fire elevation. The current work will provide some references for fire protection design and arrangement of alarm devices in an ambient ventilated tunnel fire caused by different vehicle heights.

Maximum temperature of ceiling jet flow in longitudinal ventilated tunnel fires with various distances between fire source and cross-passage

Maximum temperature rise is an important parameter in tunnel fires. It is noted that classical models of maximum temperature rise in previous studies are limited to single tunnel. However, in real situations, there is cross-passage between two neighboring tunnels. The studies about the effect of distance between fire source and cross-passage on the temperature rise is limited. In this study, numerical simulations on the maximum temperature rise of ceiling jet flow in tunnel fires under longitudinal ventilation with various distances (Lf) between fire source and cross-passage are conducted by using FDS for a cross angle of 30°. Results show that maximum temperature rise decreases with either the increasing of longitudinal ventilation velocity or the decreasing of heat release rate. The cross-passage has an effect on the fire plume flow velocity under the ceiling, which further affects the maximum temperature. For small ventilation velocities (1 m/s and 2 m/s), the maximum temperature rise is much lower than that of a single tunnel without cross-passage, and there is no significant difference between various Lf. For large ventilation velocities (3 m/s and 4 m/s), the maximum temperature rise decreases with the increasing of Lf from 0 to 4.5 m, and for Lf>4.5 m, the maximum temperature rise has no significant change anymore. It is noted that for the large ventilation velocities, the maximum temperature rise of Lf = 0 is close to that of a single tunnel without cross-passage. The effect of cross angle on the maximum temperature rise is also researched which shows that maximum temperature rise increases with the increasing of cross angle. A correlated model is proposed to predict the maximum temperature rise by taken Lf into account.

The maximum gas temperature rises beneath the ceiling in a longitudinal ventilated tunnel fire

This paper studies the maximum gas temperature rises beneath the ceiling in tunnel fires. The fire scenarios with higher longitudinal ventilation velocities are of key concern. This work was done in a 1/10 model-scale tunnel and propane gas burner was used as fire source. The variables include longitudinal ventilation velocity and heat release rate. In this study, the dimensionless longitudinal ventilation velocity varies from 0 to 0.84 and the dimensionless heat release rate is less than 0.15 in all tests, corresponding to a scenario of small fire. It is assumed that the maximum gas temperature rises in the longitudinal ventilated tunnel fires almost equals that in tunnels without longitudinal ventilation by use of the length of inclined path of plume as the effective tunnel height. By analyzing the length of inclined path of smoke plume that correlates the plume inclination angle, a piecewise model for predicting maximum gas temperature rises was developed, using the dimensionless longitudinal ventilation velocity of 0.42 as the dividing point. The comparisons between the calculations from this new model and the experimental results in this study as well as some data collected from other tests show a good agreement.

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 the effect of lateral concentrated smoke extraction on smoke stratification in the longitudinal ventilated tunnel

SummarySmoke is the main cause of death in tunnel fires. It is one of the important measures to maintain smoke stratification in the early stage of tunnel fire. This article focused on experimentally studying the combined effect of lateral concentrated smoke extraction and longitudinal ventilation on the smoke stratification, which never be revealed before. The velocity of the smoke layer and air layer, vertical temperature distribution, and the flow patterns of the smoke were measured. It was found that the longitudinal ventilation and lateral concentrated smoke extraction would affect the flow of the smoke and change the shear velocity between the smoke layer and air layer, then, the patterns of the smoke layer will be affected. And the flow patterns with Froude (Fr) number can be classified into three categories: (a)Fr &lt; 0.6, with stable smoke stratification; (b) 0.6 &lt; Fr &lt; 0.85, with a stable smoke stratification but the blurring interface; and (c) Fr &gt; 0.85, the smoke layer is completely unstable. The result can provide a reference for ventilation design of immersed tube tunnels.

Experimental investigation on carriage fires hazards in the longitudinal ventilated tunnels: Assessment of the smoke stratification features

In tunnel fires, toxic and harmful smoke is more likely to cause many casualties. The smoke stratification is an important parameter for the smoke control in tunnel fires, which has practical significance for the personal emergency escape in the early stage of tunnel fire. This work focused on the stratification stability of the smoke layer induced by carriage fires in the longitudinal ventilation tunnel. The experiment is carried out in a model tunnel with a size of 22 m (length) × 1.2 m (width) × 0.8 m (height). In this experiment, the anemometers were used to measure to flow shear velocity of the upper smoke layer and the lower layer of cold air. In addition, vertical temperatures of different heights were measured using vertically arranged thermocouple trees. The laser sheet is used to show the buoyant smoke flow, and many methods are used to calculate and compared the thickness of the smoke layer under different longitudinal velocities conditions. Finally, it was found that the buoyant stratification pattern was divided into three regimes, stable stratification, stable stratification but with some vortexes existing and unstable stratification where a strong mixing existed between the upper buoyant smoke flow and the lower air flow. This work is a significant supplement to the buoyant stratification of smoke layer from previous results.

Ceiling heat flux and downward received radiation heat flux induced by weak and relative strong fire plume in ventilation tunnels

The heat flux beneath the ceiling in tunnel fires is an important parameter, which have seldom been reported before in a longitudinal ventilation tunnel. A series of model tunnel experiments were conducted to investigate the ceiling heat flux and downward received radiation heat flux induced by weak and relative strong fire plume in ventilation tunnels. Overall, 96 repeatable test conditions were involved on different heat release rates, longitudinal ventilation velocities and the source-ceiling heights. The correlation for predicting the heat flux distribution beneath the tunnel ceiling is developed based on dimensionless analysis correlation, and the sidewalls of the tunnel configuration present are concluded to increase the thermal heat feedback and further increase the ceiling heat flux. For the effect of longitudinal ventilation velocity, the position and value of the ceiling maximum heat flux will be varied with longitudinal ventilation velocity, and the source-ceiling height, the prediction model of the ceiling maximum heat flux under the effect of longitudinal ventilation was proposed. Meanwhile, with the increasing of the longitudinal ventilation velocity, the downward received radiation heat flux first increase and then keep relatively stable.

Effects of ambient pressure on the critical velocity and back-layering length in longitudinal ventilated tunnel fire

Nowadays, the critical velocity and back-layering length are the key parameters in longitudinal ventilation design. However, most studies research them at standard air pressure but ambient pressure decreases at high-altitude area and the reduced ambient pressure could affect the smoke movement characteristics in a tunnel fire. In order to investigate the effect of ambient pressure on the velocity and back-layering length in longitudinal ventilated tunnel, theoretical analysis was carried out first and a series of numerical simulation were conducted with varying heat release rate and ambient pressure. Results show that Li’s model is also reliable under various ambient pressures. The critical velocity under various ambient pressures would become larger with an increase in the heat release rate and would remain stable after the heat release rate reaches a certain value. At smaller heat release rate, the length of counterflow would be higher under reduced ambient pressure while it remains the same when the HRR is large. This could provide reference for tunnel ventilation design at high-altitude areas.

Effect of vehicular blocking scene on smoke spread in the longitudinal ventilated tunnel fire

In order to investigate the effect of vehicular blocking scene on the smoke spread in the longitudinal ventilated tunnel fire and improve the effectiveness of smoke control scheme, 30 simulation conditions were carried out. The effect of different blockage area ratio, blockage position, and blockage length on the smoke spread were included. The temperature, velocity, and smoke spread characteristics beneath the tunnel ceiling were analyzed. The results showed that the maximum temperature and smoke spread range beneath the tunnel ceiling was affected significantly by the blockage area ratio. The critical velocity was different when the blockage vehicle was located on upstream or downstream of the fire. When the critical velocity was the same, as the blockage length increased, the maximum temperature beneath the tunnel ceiling decreased at first and then increased gradually.

Experimental study on the maximum temperature and flame extension length driven by strong plume in a longitudinal ventilated tunnel

In order to study the maximum temperature and flame extension length when flame impinges on the tunnel ceiling, experiments were carried out with different longitudinal velocities, heat release rates and ceiling-source heights considered. The results showed that: (1) The maximum temperature without ventilation was not sensitive to heat release rate and ceiling-source height, and the longitudinal velocity was a key factor for the reduction of the maximum temperature. A new hypothesis based on the decease of real effectual heat release rate in a ventilated tunnel was given and a new model to predict the maximum temperature at the flame impingement region was proposed. (2) The correlation of the flame extension length was built with the modified flame height and heat release rate. And, the cylindrical and ellipse flame shape hypotheses were used to verify the experimental data in this study. It was found that the theoretical value of the flame extension length agreed well with the corresponding experimental results. In addition, the cylindrical flame hypothesis was more consistent with experimental data, compared to ellipse flame hypothesis.

Effect of ceiling centralized mechanical smoke exhaust on the critical velocity that inhibits the reverse flow of thermal plume in a longitudinal ventilated tunnel

Toxic fire smoke in a tunnel fire is an important factor causing casualties. The critical ventilation velocity that inhibits reverse transportation of toxic thermal smoke in a longitudinal ventilated tunnel is an important design parameter. The evolution characteristics of the critical velocity in a tunnel with the coupling effect between ceiling centralized mechanical smoke exhaust and longitudinal ventilation have never been studied before. A series of small scale tunnel experiments were conducted, ten different ceiling mechanical exhaust rates (0–2.7 m/s) and twelve fire heat release rates (1.5–18 kW) are considered. It is found that, the ceiling centralized mechanical smoke exhaust will affect the critical velocity that inhibits the reverse flow of smoke in a tunnel. Due to the effect of ceiling mechanical smoke exhaust, the critical velocity decreases with increasing mass flow rates of mechanical smoke exhaust. The critical Froude number increases with increasing exhaust mass flow rate for a given dimensionless fire heat release rate. A new empirical model for predicting the critical ventilation velocity with the coupling effect between ceiling centralized mechanical smoke exhaust and longitudinal ventilation is proposed, which agree well with the measurement.

Water spray flow rate effect on smoke temperature distribution under the ceiling in tunnel fires with longitudinal ventilation

A series of fire tests with various water spray flow rates were conducted in a small-scale longitudinal ventilated tunnel to investigate the influence of flow rates of a water spray system and different longitudinal wind speed on maximum smoke temperature and longitudinal smoke temperature distribution. Then, the test data for the case without the water spray system were compared with previous studies and agreed well with previous model which overestimates smoke temperature of the cases with the water spray. Results show that the maximum smoke temperature decreases and the longitudinal smoke temperature decays faster with the increase of water spray flow rate. Thus an improved model for maximum smoke temperature is proposed by introducing a modified coefficient λrepresenting the water spray flow rate effect. It was found that longitudinal smoke temperature decays exponentially and the decay coefficient increases with increasing water spray flow rate. Therefore considering the effect of water spray flow rate, a modified coefficient Kdown is introduced into the exponential model to predict the longitudinal smoke temperature distribution.