Longitudinal Ventilation Velocity Research Articles

Experimental study on heat transfer of tunnel fire under the influence of longitudinal ventilation and water mist system

SummaryIn this paper, the influence of longitudinal ventilation and water mist on the heat transfer of tunnel fire is carried out in the reduced‐scale model tunnel, and the results show that: Radiation heat flux dominates the heat transfer of the tunnel ceiling in the downstream of fire source, and the proportion of radiation heat flux decreases with the increase of longitudinal ventilation velocity. For the tunnel floor in the downstream of fire source, the proportion of radiation heat flux increases first then maintains in a stable value with the increase of longitudinal ventilation. The water mist will strengthen the combustion of the fire source to a certain extent, but for the tunnel space, the water mist and water vapor have the effect of absorbing and attenuating the radiation heat flux and total heat flux, and the attenuation effect will increase with the higher pressure of the water mist.

Experimental investigation on fire smoke bifurcation flow in longitudinal ventilated tunnels

SummaryInvestigation of smoke bifurcation flow has been receiving more attentions, however, delicate quantitative analyses on different regions of the bifurcation flow have rarely been addressed. In this study, a series of small‐scale experiments were conducted to investigate smoke bifurcation flow in longitudinal ventilated tunnels. Results show that when longitudinal ventilation velocity increases to a certain value, the smoke bifurcation phenomenon emerges, and a low‐temperature region forms in the center of the tunnel. Similar to the natural conditions, smoke development under relatively strong ventilation can also be subdivided into four regions. With the increase of ventilation velocity, the ceiling impact region, side wall impact region, and convergence region of two smoke streams move further downstream, indicating that the bifurcation phenomenon becomes more evident. A simple model is proposed based on theoretical analysis and experimental phenomenon to predict two characteristic lengths of smoke bifurcation flow: the offset distance of ceiling impact region and the length of low‐temperature region. Both characteristic lengths increase with ventilation velocity and can be well correlated with the dimensionless ventilation velocity defined in Equation (2) ( V′). The results of this work could provide references for both tunnel ventilation designers and fire science researchers.

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.

Experimental study on smoke characteristics of bifurcated tunnel fire

The fire risk of bifurcated tunnels is increasing as rapid development of the urban underground traffic in China. Because of the special bifurcation structure, the temperature distribution of bifurcated tunnel fire is different from the traditional single tube tunnel fire. However, so far, there are few studies on this aspect of bifurcated tunnel fire. Therefore, it is of great significance to study the smoke characteristics of bifurcated tunnel fire. In this paper, a series of experiments were carried out in a 1:10 scale bifurcated tunnel to study the effects of the branch and the longitudinal ventilation velocity on the smoke characteristics in the bifurcated tunnel. The main conclusions are as follows: (1) When the fire source is located at the bifurcated point, the existence of the branch will make the temperature of the fire source area rise significantly. However, it has no influence on the temperature variation of the tunnel when the fire source is located at upstream of the bifurcated point. (2) Whether the fire source is located at upstream of the bifurcated point or at the bifurcated point, as the longitudinal ventilation velocity increases, the ceiling temperature of the central axis of the tunnel decreases, and the position of the maximum temperature point moves towards downstream. (3) The maximum temperature in the tunnel decreases linearly with the longitudinal ventilation velocity whether there is a branch or not. (4) When the fire source is located at upstream of the bifurcated point, the temperature on the central axis of the branch ceiling rises up firstly and then reduces with the increase of the longitudinal ventilation velocity. At the entrance of the branch, the temperature increases firstly and then decreases, and this phenomenon is more obvious with the higher longitudinal ventilation velocity.

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 passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires

SummaryThis paper investigates the effects of passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires. A series of numerical simulations were conducted in a 1/5 small‐scale tunnel with the different heat release rates (50‐100 kW), longitudinal ventilation velocities (0.5‐1 m/s), passenger blockage lengths (2‐6 m), and ratios (0.17‐0.267). The typical smoke flow properties in different tunnel fire scenarios are analyzed, and the results show that under the same heat release rate and longitudinal ventilation velocity, the smoke back‐layering length, maximum smoke temperature, and downstream smoke layer height decrease with increasing passenger blockage length or ratio. The Li correlations can well predict the smoke back‐layering length and maximum smoke temperature in tunnel fire scenarios without the passenger blockage. When the passenger blockage exists, the modified local ventilation velocity that takes the blockage length and ratio into account has been proposed to correct the Li correlations. The smoke back‐layering length and maximum smoke temperature with the different blockage lengths and ratios can be predicted by the modified correlations, which are shown to well reproduce the simulation results.

Experimental and numerical study of maximum ceiling temperature in a tunnel

Maximum ceiling temperatures in a tunnel with different ventilation velocities with three heat release fires are studied experimentally and theoretically. This article investigates the ventilation velocity effects on maximum ceiling temperature combustible materials around ignition source in tunnel fires. Several fire experimental tests are conducted with longitudinal ventilation velocity changes in a small-scale tunnel (23 m in length, 2 m in width, and 0.98 m in height), where three heat release fires (237, 340, and 567 kW) and their corresponding values in the real tunnel are 20, 30, and 50 MW, respectively. This article modifies the current temperature prediction model taking the ignition materials near the fire source into account in tunnels. Results show that the ceiling maximum temperature increases, corresponding to the burn time when other experimental conditions remain unchanged for a given fire heat level source. The ceiling temperature reduces quickly when the ventilation velocity is increased from 0.5 to 2.0 m/s. Moreover, this article proposes an equation that can be used to estimate the ceiling maximum temperature variation value with three heat release fires in tunnels. Finally, experimental results are also compared with the tunnel ceiling temperature attenuation equations established by Alpert, Heskestad, and Ingason. The equation proposed in this article appears to provide better estimates of ceiling temperature variation than the Kurioka model developed in their scaled experiments. The prediction agrees well with the experimental and measured data by the modified equations of this article.

A numerical study of the low-temperature zone in tunnel fires with strong longitudinal ventilation

In this paper, a numerical study was conducted to study the low-temperature zone induced by the strong longitudinal ventilation in tunnel fires by using Fire Dynamics Simulator (FDS). Six different longitudinal ventilation velocities at a range of 2m/s to 6m/s, two different heat release rates (5MW and 8MW) and two tunnel widths (5m and 10m) were considered in the simulations. Results showed that the bifurcation flow formed with the increasing of longitudinal velocity, and there would be a low-temperature zone. The low-temperature zone, results from comprehensive effect of inertia force and thermal buoyancy, expands with the enhancement of longitudinal ventilation. A prediction model was proposed and it revealed that the length of low-temperature zone is in proportion to the -0.44 power of Q*/V*3.

XPERIMENTAL STUDY OF THE INFLUENCE OF BLOCKAGE ON CRITICAL VELOCITY AND BACKLAYERING LENGTH IN A LONGITUDINALLY VENTILATED TUNNEL

This study has been conducted to analyze the effect of vehicular blockage on critical velocity and backlayering length experimentally through a 3 m model tunnel (1:50 of a standard tunnel length). Three vehicle sizes in two or three arrays, occupying 3-15% of tunnel crosssection in six scenarios, were used. Based on this study, critical velocity decreases in the case that ventilation velocity reaches the fire source directly. However, critical velocity increases when the path of ventilation flows are blocked by blockages due to an increase in heat release rate through vehicular obstructions. In addition, the influence of blockages on backlayering length and its dimensionless analysis by the ratio of longitudinal ventilation velocity to critical velocity is also presented. It is found that the backlayering length follows an exponential relation with and without blockages. Moreover, the analysis of backlayering length with respect to the blockage section length shows that as backflow occurs along the ceiling and flows over blockage section, its heat transfer to surrounding increases leading to shorter backlayering length due to the enhanced inertial force and heat loss.

Influence of ambient pressure on critical ventilation velocity and backlayering distance of thermal driven smoke in tunnels with longitudinal ventilation

Thermal driven smoke backlayering in tunnels with longitudinal ventilation is a huge threat to the trapped people during fires. To explore the influence of ambient pressure on smoke backlayering distance and critical ventilation velocity of thermal driven smoke flow, fire simulations with varying ambient pressures and heat release rates (HRRs) were carried out in a full scale tunnel with longitudinal ventilation. The results suggest that for a certain HRR and longitudinal ventilation velocity, the smoke backlayering distance increases with the decrease of ambient pressure due to the weaker inhibiting effect of longitudinal airflow, which is characterized by the decrease of the ratio of the horizontal inertial force of longitudinal airflow to thermal buoyancy of the smoke backlayering. Moreover, a parallel analysis for critical ventilation velocity was also made, and it has been found that the critical ventilation velocity increases with the decrease of ambient pressure for a certain HRR. Based on the dimensional analysis, a correlation of dimensionless critical ventilation velocity is proposed and is compared with previous correlations at normal pressure. Finally, the credibility of correlation are validated by comparing with experimental results from large and full scale tunnel fire tests.

Study on the smoke back‐layering and critical ventilation in the road tunnel fire at high altitude

SummaryA series of numerical simulations were conducted in order to investigate the characteristics of smoke back‐layering and critical ventilation in the road tunnel at high altitude with reduced ambient atmospheric pressures. The results indicated that the smoke back‐layering length decreases with the reduction of ambient pressure. Meanwhile, the dimensionless critical longitudinal ventilation velocity decreases with one‐third power of the factor of ambient pressure at high altitude. By modifying the traditional dimensionless fire heat release rate with ambient pressure, new models were deduced to predict the smoke back‐layering length and critical ventilation velocity in the road tunnel at high altitude.

A theoretical and experimental study on the buoyancy-driven smoke flow in a tunnel with vertical shafts

In this study, a series of small-scale experiments was carried out in a model scale tunnel with dimensions of 20 m (Length) × 2 m (Width) × 1 m (Height) to investigate the characteristics of buoyancy-driven smoke flow in a tunnel with vertical shafts. Different shaft settings and four different longitudinal ventilation velocities were tested in the experiments. A theoretical model for the mass flow rate of buoyancy-driven smoke flow in the shaft was developed and validated. The gas temperature along the tunnel ceiling and smoke stratification were subsequently analyzed and discussed. The results showed that more shafts, greater shaft heights and greater shaft cross sectional areas can significantly increase the smoke extraction rate, and the total smoke mass flow rate in the shafts increases with the increasing ventilation velocity. The local pressure loss coefficient at the shaft inlet may not be a fixed value. An average value of 1.0 for this coefficient was recommended for engineering estimation and design of rectangular-shaped natural shafts. The presence of vertical shaft is beneficial to the smoke stratification and could increase the height of the smoke layer interface, especially for the downstream of the shaft.

Experimental study of the effectiveness of a water mist segment system in blocking fire-induced smoke and heat in mid-scale tunnel tests

A 1/3-scale model tunnel installed with a water mist segment system is built to study the effectiveness of a water mist segment system in blocking fire-induced smoke and heat. Interaction of water with fire is avoided. A series of 14 tests are performed for a wide range of settings, including variable fire loads, nozzle quantities, water pressures and longitudinal ventilation velocities. Temperature distribution is presented and discussed for different ventilation conditions. A method for qualitatively analyzing the effectiveness of the water mist segment in blocking fire-induced smoke and heat is proposed, and the smoke blocking phenomenon resulting from the water mist segment system is discussed and explained by an order of magnitude analysis. Important parameters, including the total water flow rate (nozzle numbers), water pressure, heat release rate, and the distance from fire source to the water mist segment, are suggested for improving the smoke blocking efficiency of water mist segment systems in tunnels. The effects of the water mist segment on life safety are discussed at last.

Experimental investigation on temperature profile with downstream vehicle in a longitudinally ventilated tunnel

The vehicle being ignited in tunnel fire could disturb the temperature distribution and result in more serious hazards. However, the temperature profile about the tunnel fire with an ignited vehicle has not been clarified. This paper conducted a series of 1/6 small scale experiments with different heat release rates to investigate the vehicle ignition time delay and temperature profile. The longitudinal ventilation velocity varied from 0.5 m/s to 2.0 m/s, and the blockage-fire distance varied from 0.35 m to 1.6 m. The temperature and the ignition time delay were measured. Results show that the ignition time delay is shorter with shorter blockage-fire distance. The ignited vehicle results in the temperature hump and larger high temperature region at downstream. A modified model to predict the maximum temperature beneath the tunnel ceiling by considering the combined effect of longitudinal ventilation and blockage-fire distance is proposed. Finally, the predictions obtained by proposed model are compared with the experimental data and previous experiments.

Review of Research on critical velocity in tunnel fire

The critical velocity is the key for tunnel fire control. If the longitudinal ventilation velocity is greater than the critical velocity when the fire occurs, the upstream of the fire source is smokeless, and the smoke will flow to the downstream of the fire source, which can effectively control the fire spread and provide valuable time for personnel to escape and fire fighting. The researches of domestic and foreign scholars are used to investigate the influencing factors of critical velocity. the results show that the main influencing factors of critical velocity are fire heat release rate, tunnel section geometry, obstacle and slope in tunnel, etc. In this paper, the influencing factors are summarized, and some problems that need to be studied in tunnel fire are put forward.

Investigation on smoke temperature distribution in a double-deck tunnel fire with longitudinal ventilation and lateral smoke extraction

A set of experiments is carried out in a 1/15 reduced-scale double-deck tunnel to investigate the effect of longitudinal ventilation and lateral smoke extraction on the temperature distribution. The factors of the longitudinal ventilation velocity and the location of fire source are considered under the heat release rate (HRR) of 23 kW. The thermal behavior of the tunnel is observed. The experimental results show that the upstream ceiling temperature decreases with the increase of longitudinal ventilation velocity, while the downstream ceiling temperature increases with the increase of longitudinal ventilation velocity. With the increasing of distance away from fire source, the dimensionless smoke temperature rise reduces exponentially along the tunnel ceiling. The upstream or downstream temperature of the upper tunnel at the location with the same distance away from the fire source is higher than that of the lower tunnel, and the maximum temperature difference can be reached 93 °C. With the longitudinal ventilation velocity at 0.516 m/s, the downstream smoke layering length are 8.35 m and 7.65 m longer than the upstream smoke layering length in the upper and lower tunnel, separately.

Effect of blockage-induced near wake flow on fire properties in a longitudinally ventilated tunnel

Effect of blockage-induced near wake flow on fire properties in a longitudinally ventilated tunnel was studied. Numerical simulation was conducted in a tunnel under longitudinal ventilation velocity in the range of 1.0–4.0 m/s. Vehicular blockage was positioned in the upstream of the fire source, with four kinds of blockage ratios (0.24, 0.40, 0.56, 0.72). Temperature contours were used to describe the fire plume region in the tunnel with longitudinal ventilation. It is found that the fire flame goes five stages and transits leaning direction from downstream to upstream of the fire with increasing blockage ratio and ventilation velocity, behaving differently from that in just a longitudinally ventilated tunnel without vehicular blockage. With the increasing ventilation velocity and blockage ratio, the vertical temperature 5 m downstream of the fire decreases and temperature difference in the vertical direction gradually weakens. However, in the upstream 2.5 m from the fire, after the temperature tending to be uniform in the vertical direction, it is high in the middle height and low at the top and bottom. At ventilation velocity larger than 2.5 m/s, the temperature beneath the tunnel ceiling decreases with the increasing blockage ratio downstream of the fire, while there is increase in the upstream. A new expression with the factor blockage ratio included is proposed to predict the maximum temperature beneath the tunnel ceiling, showing good agreement with simulation data.

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.

Driving force for preventing smoke backlayering in downhill tunnel fires using forced longitudinal ventilation

The critical ventilation velocity plays an important role in smoke control in longitudinally ventilated tunnel fires. The magnitude of the critical velocity has been widely studied. In the present study, we carried out theoretical analyses and numerical simulations to investigate the driving force necessary for achieving the critical velocity in downhill tunnels. Theoretical models suggest that buoyant source location has a considerable effect on the condition for achieving the critical velocity. For a constant convective heat release rate of the fire, the driving force necessary for achieving the critical velocity increases with the height difference between the fire source and the outlet of the smoke. According to FDS simulation results, with the increase of the height difference between the fire source and the smoke outlet, the pressure loss induced by the stack effect increases significantly; however, the variation in the ventilation critical velocity is insignificant. Therefore, the variation in the driving force is mainly because of the stack effect rather than the variation in the critical velocity. In addition, the effects of the fire occurrence on the longitudinal ventilation velocity and pressure distribution in the tunnel are also numerically studied. If the pressure rise of the jet-fan keeps constant, the longitudinal velocity declines remarkably after the fire occurrence as a result of the pressure loss induced by the fire and the pressure loss induced by the stack effect. The phenomenon presented in the paper has implications for both tunnel ventilation and smoke control in inclined tunnels.

Experimental studies on fire-induced temperature distribution below ceiling in a longitudinal ventilated metro tunnel

A series of experiments were carried out in a small-scale model tunnel with dimensions of 13m(Length)∗0.32m(Width)∗0.48m(Height) to investigate the fire-induced temperature distribution below ceiling in a longitudinal ventilated metro tunnel. The different temperature distributions with varying longitudinal ventilation velocity and fire size at both upstream and downstream side have been compared and analyzed. The relative smoke flow directions to the longitudinal ventilation flow in the upstream and downstream are different, obvious “convection shear phenomenon” occurred only in the upstream side. And it was shown that smoke temperature distribution in the upstream back layer seemed to be much more sensitive to the ventilation velocity than that in the downstream flow. Based on the experimental data, a modified coefficient 1/γ which was related to the ventilation velocity and fire size was introduced into the upstream temperature decay equation. A new modified temperature decay model for the temperature decay at upstream side of fire source was proposed. Moreover, the comparison of the modified coefficient value indicated that Tang’s model underestimated the rate of temperature decay in the upstream side of fire source, the modified coefficient in his study was apparently smaller.