Influence of longitudinal fire location on smoke characteristics under the tunnel ceiling

The critical ventilation velocity is almost the most well‐investigated fire phenomenon in the tunnel fire research field whereas previous studies have always investigated it when the fire source is distant from the downstream tunnel exit. Fortunately, a recent study provided a set of data on the critical ventilation velocity for tunnel fires occurring near tunnel exits by small‐scaled experiments, nevertheless, with a lack of further analysis. To demonstrate the relationship of the critical ventilation velocity and the distance between the fire and tunnel exit more explicitly and detailedly, a quantitative and graphical study was carried out and a correlation was presented in this paper. Inspired by this, a set of small‐scaled experiments were carried out to investigate the influence of different longitudinal fire locations on maximum smoke temperature under the tunnel ceiling. Results show that unlike the critical ventilation velocity, the maximum smoke temperature was not obviously affected by longitudinal fire location. Copyright © 2014 John Wiley & Sons, Ltd.     

Experimental investigation on fire smoke bifurcation flow in longitudinal ventilated tunnels

Investigation 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.     

Influence of multisource fire on temperature distribution and natural smoke exhaust effect in urban tunnels with a shaft

The combustion characteristics of multisource fire and single-source fire are quite different, and there is little research on the influence of multisource fire on the natural smoke extraction effect of shaft in urban tunnels. Therefore, in this article, the method of numerical simulation was used to study the influence of fire power and distance between two fire sources on the natural smoke extraction effect of shaft and the temperature distribution in tunnel in the case of multisource fire. Typical characteristics of smoke are analyzed, such as mass flow rate, temperature distribution, velocity vector, and CO concentration. The simulation results show that when there is a certain distance between the fire sources, the two flames are inclined and close to each other. The smoke temperature under the ceiling is higher under multiple fire sources than that under single fire source. In addition, when one of the fire sources is located at the downstream of the shaft, the smoke emission in the shaft is relatively high. As the distance between fire sources continues to increase, the smoke exhaust rate basically remains stable, and an empirical relationship between smoke exhaust rate and fire source location is established.     

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

To 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.     

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

A 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.     

Effects of passenger blockage on smoke flow properties in longitudinally ventilated tunnel fires

This 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.     

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

In 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.     

Study on the heat exhaust coefficient and smoke flow characteristics under lateral smoke exhaust in tunnel fires

The heat exhaust coefficient and smoke flow characteristics under lateral smoke exhaust in tunnel fires were studied in this paper. Through the dimensional analysis, the dimensionless relationship between the heat exhaust coefficient, heat release rate, exhaust vent size, and exhaust velocity was obtained. In addition, this paper also studied the effect of the lateral exhaust vent on the smoke flow field. Results showed that the lateral smoke exhaust caused strong air entrainment on the downstream of the exhaust vent and boundary layer separation on the upstream of the exhaust vent. As the exhaust velocity increased, the degree of air entrainment gradually increased, and the smoke layer near the exhaust vent gradually became thinning and plug‐holing phenomenon occurred; meanwhile, the boundary layer separation would be suppressed or disappear, but the increase of the heat release rate would enhance the boundary layer separation. As the exhaust vent got narrower, the air entrainment downstream of the exhaust vent was reduced, and the boundary layer separation also got weaker.     

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

Smoke 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 < 0.6 , with stable smoke stratification; (b) 0.6 < Fr < 0.85 , with a stable smoke stratification but the blurring interface; and (c) Fr > 0.85 , the smoke layer is completely unstable. The result can provide a reference for ventilation design of immersed tube tunnels.     

Study on fire smoke flow characteristics in the ventilation network and linkage control system in coal mines

In coal mining, smoke flow from tunnel fires can easily cause a large number of deaths in the ventilation network. But the optimal smoke flow path control methods and automatic control system were lacked. In order to improve the efficiency of fire emergency rescue, the control mechanism and regional linkage control system for fire induced smoke flow in ventilation network was studied. Based on a ventilation system in coal mines, different fire scenarios for smoke flow were analysed using ventilation simulation software (VSS). Smoke flow control methods were simulated under different ventilation modes, a contrastive analysis was conducted for the respective effects and the optimal smoke flow path control methods were confirmed in different fire scenarios. A new type of ventilation facility, regional monitoring sub-stations and remote linkage control platforms were developed for smoke control. A reliability evaluation model for the control system was established by Bayesian network. The failure of the linkage control is 98.9%, the monitoring sub-station is 64.4%, the sub-station communication is 43.9%; thus, a double insurance of the control process must be realised. Since its application, the proposed regional linkage control system has been repeatedly tested through fire drills, and good results have been obtained.     

Study of smoke movement characteristics in tunnel fires in high-altitude areas

Understanding smoke temperature distributions and transport characteristics is of great importance to control and exhaust thermal-driven smoke. However, previous studies have focused on this problem in plain areas, whereas ambient pressure decreases as elevation increases. This study investigates the influence of ambient pressure on the hot gas temperature distribution and movement characteristics in a tunnel fire. A series of numerical simulations are carried out in a vehicle tunnel with various heat release rates (HRRs) and ambient pressures. The results show that the maximum temperature and longitudinal temperature distribution under the tunnel ceiling increase with decreasing ambient pressure due to less heat loss caused by lower air density. In addition, the vertical temperatures of the smoke are slightly higher under lower ambient pressure, and this phenomenon makes the smoke spread slightly faster while the smoke layer thickness remains nearly the same under different ambient pressures. The results can provide a reference for tunnel lining design and ventilation arrangements in high-altitude areas.     

圆筒形地下立体停车库火灾火源位置与火势蔓延规律研究

选取一个典型的圆筒形地下立体停车库作为研究对象，运用star-ccm+软件，采用数值模拟的方法对车库内自然通风条件下的火灾场景进行模拟计算。分别选择负一层、负六层以及负十层作为起火点，计算得出：火源位置越深入地下，着火车位顶棚中心温度越高，最高可达942℃，与起火点同层的车位顶棚中心温度均高于200℃。当某车辆发生火灾后，其相邻车辆、上方车辆、相对车辆将分别在起火后的240、312、322 s左右被引燃；火源位置越往下，引燃附近车辆的时间越短。同时，通过计算值与理论值的对比，讨论了计算火源附近车辆被引燃时间理论公式的适用性。     

Experimental study of the combustion characteristics of methanol‐gasoline blends pool fires in a full‐scale tunnel

The combustion characteristics of methanol‐gasoline blends pool fires were studied in a series of full‐scale tunnel experiments conducted with different methanol and gasoline blends. The parameters were measured including the mass loss rate, the pool surface temperature, the fire plume centerline temperature, the ceiling temperature, the smoke layer temperature profile, the flame height, and the smoke layer interface height. The gasoline components were analyzed by GC‐MS. The effects of azeotropism on the combustion characteristics of the different blends were discussed. On the basis of the results of the fire plume centerline temperature, the ceiling temperature, and the flame height, it shows that the tunnel fire regime gradually switches from fuel controlled to ventilation controlled with increasing gasoline fractions in the blends. The fire plume can be divided into 3 regions by the fire plume centerline temperature for the different blends. The N‐percentage rule to determine the smoke layer interface height is found to be applicable for tunnel fires with different blends for N = 26.     

Smoke characteristics of coal-lined tunnel fires

Characteristics of smoke particulates generated from a coal fire in a ventilated model tunnel were investigated by laser optical transmission and by electron microscopy. Average particle diameter and mass concentration of the smoke were determined as a function of the temperature and stoichimoetry of the coal tunnel fire. Smoke particle sizes ranged from 0.2 to 0.9 μm, with larger particle sizes associated with higher smoke concentrations. These coal smoke data are relevant to several aspects of underground mine safety including the development and location of smoke detection instruments, the understanding of the fire (toxic fume) hazard, and the development of new fire protection and control techniques.     

Numerical analysis of smoke flow under the effect of longitudinal airflow in a tunnel fire

This study investigated the influence of the longitudinal airflow on the smoke propagation in a tunnel by large-eddy simulation, which is now widely applied to study the turbulent flow. The smoke movement characteristics were studied in detail, with varying the longitudinal airflow in the tunnel. Six fire scenarios have been simulated with Fire Dynamics Simulator (FDS) and the results of the longitudinal distribution of CO concentration, temperature distribution, interface height, stratification, and the efficiency of smoke extraction in the tunnel have been analyzed to evaluate the different fire cases. FDS predicted a CO concentration distribution compared to calculated values using the Hu model. Furthermore, the predicted maximum smoke temperatures are compared to those given by the Kurioka model. A reasonably good agreement has been obtained for both models. The obtained results showed that the increase of the forced airflow velocity has for results a loss of stratification and significant decrease in the efficiency of extraction.     

Effect of altitude on vertical temperature distribution and longitudinal smoke layer thickness in long tunnel fires

Based on large eddy simulation, a series of long tunnel fire experiments with different heat release rates (HRRs) and altitudes were carried out. The vertical temperature and thickness of fire smoke are studied. The simulation results show that the higher the altitude, the lower the flame temperature rise, while the change of smoke plume temperature rise is opposite. The movement of smoke in the tunnel can be divided into four regions, and the smoke layer thickness in the longitudinal direction of the tunnel corresponds to the latter three regions. The thickness in Region II increases along the longitudinal direction, the thickness in Region III is a constant value, and the thickness in Region IV increases along the longitudinal direction. Besides, the change of altitude only has an effect on the smoke layer thickness in Region IV. Then, by considering the altitude, HRR, and smoke layer thickness, and using dimensional analysis, an empirical formula for predicting the smoke layer thickness under the influence of altitude in Region IV was established.     

Evaluating methods for preventing smoke spread through ventilation systems using fire dynamics simulator

Fires in enclosures equipped with mechanical ventilation remain one of the key issues for fire safety assessment in multifamily homes and industries. Therefore, a wide variation of methods for preventing smoke spread through the ventilation system exist and are applied, in performance‐based designs. Through the use of the heating, ventilation and air conditioning (HVAC) model in the fire dynamics simulator, several different common and less common methods for preventing smoke spread in the ventilation system were tested. The effects on smoke spread with changing building leakage and fire growth rates were also investigated. The results were evaluated by determining the total soot spread from the fire room to other compartments connected to the ventilation system, as well as soot/thermal load on the fans and system in general. The maximum and average heat release rate was also of interest and hence compared between systems. It was found that, while many methods perform similar, a few proven methods, such as fire and smoke dampers, performed very well with very little smoke spread to the rest of the system. The study should be considered as an introduction to implementing a similar methodology in specific cases because different ventilations systems will present very different challenges and weaknesses. Copyright © 2016 John Wiley & Sons, Ltd.     

Use of zone models on simulating compartmental fires with forced ventilation

Performance of three fire zone models BR12, CCFM.VENTS and CFAST in simulating forced ventilation fires with low heat release and high ventilation rates were studied experimentally. A fire chamber of length 4.0 m, width 3.0 m height 2.8 m with adjustable ventilation rates was used. Burning tests were carried out with wood cribs and methanol to study the preflashover stage of a compartmental fire and the effect of ventilation. The mass loss rate of fuel, temperature distribution of the compartment and the air intake rate were measured. The heat release rates of the fuel were calculated from the measured mass loss rate. The smoke temperature was used as the validation parameter. A scoring system is proposed to compare the results predicted by the three models. An empirical expression for calculating the smoke temperature is assessed. Lastly, the Computational Fluid Dynamics technique is also used for comparing the simulated fire environment.     

通风方式对化工车间火灾影响的实例分析

在分析化工企业火灾危险性的基础上,建立了化工车间火灾危险度分析数学模型。根据工程实际,分别对正常工业通风和火灾后事故通风工况进行了火灾危险度计算。计算结果表明事故通风及防排烟系统启动后,能有效的降低车间的火灾危险性等级。研究结果用于指导化工厂通风系统设计及提高消防安全管理水平。     

Characterization of the fire environments in central offices of the telecommunications industry

Eight free burning and two sprinklered fire tests were performed with electrical cable trays and live digital switch racks in a large enclosure to simulate telecommunications central office (TCO) fires started by electrical overheating. Very‐slow‐growing (non‐flaming), slower‐growing (partially flaming) and low‐intensity‐faster‐growing (flaming) fires releasing gray‐white, gray, and black smoke, respectively, were observed in the tests. Under quiescent conditions present in the unvented enclosure fire tests for cables, very‐slow‐growing fires were detected in about 1452 s, whereas the slower‐growing fires were detected in about 222 s by commercial fire detectors. Under ventilation conditions typical of TCOs, detection times were very similar for the five types of commercial TCOs fire detectors used in the tests. The average detection times for slower‐growing fires (cable fires) and low‐intensity‐faster‐growing fires (digital switch rack fires) were 242±17% and 249±11%s respectively. The TCO procedures to reduce smoke damage from fires (on fire detection, inlet ventilation flow is turned off and exhaust flow is turned on) were found to be beneficial. The extent of smoke damage decreased significantly with an increase in the exhaust flow rate. The chloride ion mass deposition suggested that equipment recovery would be possible in the smoke environment if the cable vapor concentration could be reduced below about 3 g/m³. The metal corrosion rate was found proportional to the 0.6th power of the smoke concentration, similar to that found for the corrosion of metal surfaces exposed to aqueous solutions of HCl and HNO₃ and for acid rain with no protective layer at the surface. Sprinkler water was found to wash down the smoke deposits on the surfaces with little indication of corrosion enhancement. Copyright © 2003 John Wiley & Sons, Ltd.