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.     

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.     

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.     

Study on the smoke evolution mechanism of a subway tunnel with a multi-door carriage fire under longitudinal ventilation

This paper has analyzed the longitudinal ventilation on the effect of the efficiency of the smoke evolution mechanism in a metro tunnel of multi-window carriage fires. These were simulated by Large Eddy Simulation (LES) with Fire Dynamics Simulator (FDS). In the past, analyses of smoke temperature under the tunnel ceiling and smoke overflow characteristics have been conducted. However, longitudinal ventilation has a different impact on temperature than natural ventilation, especially in a subway tunnel with a multi-door carriage fire. Consequently, several simulations were run in a subway tunnel (360-m long, 6.0-m wide, and 4.8-m high). The longitudinal ventilation velocity is set by 0–10 m/s with the heat release rate of 1–10 MW. The results show that there is a linear relationship between the maximum temperature and the longitudinal ventilation velocity. An empirical model considering various longitudinal ventilation velocities was developed to predict the maximum smoke temperature underneath the subway tunnel ceiling. The effects of the longitudinal ventilation velocity, the heat release rate, and the distance of the fire source on the characteristics of longitudinal temperature distribution were analyzed. What's more, smoke overflow characteristics under different longitudinal ventilation velocities have been described. The findings and results can also provide a reference for the fire risk assessment of a metro tunnel of multi-window carriage fires.     

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.     

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.     

实验探索室内自然置换通风单一浮力源的热力分层

通过气泡羽流模型,对具有单一浮力源的室内自然置换通风热力分层高度进行了模拟实验研究,讨论了有效通风面积与热力分层高度之间的变化关系。将通过INSIGHT 3G软件获得的实验数据与数学模型进行了对比分析,发现自然置换通风室内单一点源热力分层的量纲1高度随着量纲1有效通风面积的增加而增高,室内自然置换通风单一浮力源的热力分层高度仅是其量纲1有效通风面积的函数。     

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.     

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.     

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 on the influence of air supply and smoke exhaust on full transverse exhaust of long highway tunnel

This study investigated the efficacy of the full transverse exhaust method for smoke extraction in tunnel fires. It examines factors such as the number and layout of air supply and exhaust outlets, analyzing their impact on smoke spread, tunnel temperature, visibility, and airflow. The results demonstrate that the full transverse exhaust method effectively controls smoke emissions in raised highway tunnels. It limits smoke spread, reduces tunnel temperature, and effectively controls the fire-affected area. The number and layout of outlets significantly influence smoke dispersion, with fewer exhaust outlets providing better smoke control and optimizing the tunnel environment. However, insufficient outlets disrupt gas flow stability. The position of exhaust outlets affects smoke distribution, and caution is advised to prevent directing fresh air flow toward the fire. Opening an equal number of exhaust outlets on one side of the fire source yields superior smoke extraction results, reducing tunnel ceiling temperatures and minimizing risks to personnel and structures. Though stabilization may take longer, this configuration proves advantageous. The study offers valuable insights and practical guidelines for implementing the full transverse smoke control method in real-world scenarios.     

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.     

Research on the plume shape under optimal wind environment to prevent the smoke backflow and combustion gains in utility tunnel

The utility tunnels have been applied extensively to run the various pipelines in the urban areas such as the gas pipeline, electrical power cables, and the likes. Contradicting with the rapid development of the utility tunnels is the ambiguity of the fire protection code to which one critical point is whether to ventilate in the fire accident, which is hampered by the effect of wind on the combustion gain. Therefore, this paper combines the plume function with the backlayering length and critical backflow velocity to explore the plume shape, optimize the ventilation environment, and decrease its combustion gain in which shows three key features that include the concavity and convexity characteristics, instability of plume, and the balance feature. Moreover, through their derived five plume shape constraints, we acquire the optimal wind environment. Furthermore, we found that the expected length is 0.12 in optimal condition when the original critical velocity is larger than 0.43, and the other cases are 0.05 for expected length. Meanwhile, the ventilation velocity needs to be increased three to seven times. The study provides new insight into the plume flow under the wind environment and would accelerate the formalization of fire protection design for utility tunnel.     

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.     

Experimental study on the time‐dependent smoke densities at 39 locations in a multi‐floor building through a digital smoke detector system

The objective of this study was to quantitatively investigate some characteristics of the smoke transportation in multi‐floor buildings. Eight experiments were conducted for worst scenario. The effects of an open window in the burning room on the smoke transportation are also analyzed. The time‐dependent smoke densities at 39 locations in a half‐scale building with an atrium were measured through a digital smoke detector system. The results indicate that the chimney effect plays an important role in the smoke transportation in multi‐floor buildings with atriums. For the effects of the open window, the results suggest that the smoke densities at most locations in the building increase earlier when a window is open but have a smaller peak value than those results in the cases without any outer vents. It is suggested that a building without vertical atrium would be safer than those with long ones. More attention should be paid to those spaces when the fire protection systems are designed for buildings with atriums. The data of the time‐dependent smoke densities at tens of locations in the building are useful for the validation of smoke transportation models. Copyright © 2013 John Wiley & Sons, Ltd.     

A numerical study on the effects of naturally ventilated shaft and fire locations in urban tunnels

In more and more tunnels, natural ventilation mode with vertical shafts has been gradually employed. However, there are few studies investigating the influences of fire and shaft positions on natural ventilation performance currently. Therefore, this study investigated the effects of the transverse distance from fire source to tunnel sidewall, the longitudinal distance from fire source to shaft, and the transverse distance from shaft to sidewall on natural ventilation effectiveness in a tunnel fire by using Fire Dynamics Simulator. The typical characteristic parameters of smoke, such as mass flow rate, temperature distribution, and velocity vector were analyzed; besides, the phenomenon of plug‐holing was discussed. The results have shown that the mass flow rate of gas exhausted by the shaft decreases slightly with the increase of longitudinal distance from fire source to shaft. When the longitudinal distance from fire source to shaft is constant, changing the transverse distance from shaft to sidewall will have a more obvious effect on the effectiveness of exhausting smoke than changing the transverse distance from fire source to sidewall; in addition, the phenomenon of plug‐holing is more serious when the shaft is close to the sidewall.     

微凝胶调驱效果实验研究

为考察可动微凝胶这一新型调驱体系在非均质模型中的调驱效果,采用人造填砂管模型,对该体系进行了室内实验评价。结果表明,微凝胶可明显改善低渗层的分流量,具有较好的改善注入剖面效果。当注入0.3 PV浓度2 000 mg/L的可动微凝胶时,可提高采收率7.7%;浓度越高,注入量越大,驱油效果越好,但当注入量超过0.3 PV以后,采收率增加幅度已不太明显。     

温湿度平衡时间对卷烟主流烟气氢氰酸释放量影响的研究

为考察不同温湿度平衡时间对卷烟主流烟气中氢氰酸释放量的影响,在标准温湿度条件下,分别选择不同的平衡时间对6种卷烟样品处理后,应用连续流动分析仪对卷烟主流烟气中氢氰酸释放量进行测试,连续考察了120 d。结果表明:①平衡120 d内,卷烟样品抽吸口数没有明显变化;②平衡120 d内,不同盒标焦油量的卷烟主流烟气中HCN释放量没有明显变化;③平衡120 d内,不同类型卷烟主流烟气中HCN释放量没有明显变化。因此,在平衡120 d内对样品HCN进行测定,均能得到准确结果。     

竹叶黄酮提取分析方法的研究及其新进展

从竹叶黄酮的基本性质着手,根据其溶解性,显色反应等特性,选取适当的提取方法。综述了五种应用较广泛的提取新方法、新技术,及分析方法,并简要论述了各种新技术和方法的原理以及应用于竹叶黄酮工业生产方面的情况。