Smoke Exhaust Efficiency Research Articles

Study on the Natural Smoke Exhaust Performance of Board-Coupled Vertical Shaft in High-Altitude Tunnel Fires

Vertical shaft natural ventilation is a common smoke exhaust method in highway tunnel fires. This study investigated the vertical shaft natural smoke exhaust work in highway tunnel fires with the effect of multiple factors through numerical simulation. Using the analysis of the flow field of smoke in nearby areas of the vertical shaft and the quantitative calculation of the gas exhausted through the vertical shaft, considering the impact of shaft division and board height, an optimal vertical shaft arrangement was proposed, and the working conditions of this arrangement in low-pressure environments were discussed. The results show that dividing a single large vertical shaft into multiple small vertical shafts and appropriately adjusting the board height can reduce the incidence of vertical shaft plug holes and significantly enhance the vertical shaft smoke exhaust performance. Meanwhile, the board-coupled shaft (BCS) has excellent working ability in low-pressure environments, and when pressure drops, smoke exhaust efficiency will improve. This research offers a foundation and reference value for improving the vertical shaft smoke exhaust system in highway tunnels.

Study on the Effect of Natural Wind on the Smoke Spread Law of Extra-Long Tunnel Fires with Inclined Shafts for Air Supply and Exhaust

High-temperature smoke generated by tunnel fires is the most important factor causing casualties. To explore the influence of natural wind on fire smoke movement in an extra-long highway tunnel based on the Taihang Mountain Tunnel, the distribution law of natural wind in the tunnel was obtained by on-site monitoring of the meteorological conditions at the tunnel site. A three-dimensional fire dynamics tunnel model considering an inclined shaft smoke exhaust was established, and the influence of natural wind on tunnel temperature distribution, smoke spread and smoke exhaust efficiency was studied. The results show that the natural wind speed of the Taihang Mountain Tunnel is mainly concentrated at 0~3 m/s. The main wind direction of the natural wind on the left tunnel is opposite to the driving direction, and the distribution probability of the main wind direction in each section is 81.27% and 72.15%, respectively. The main wind direction of the right tunnel is the same as the driving direction, and the distribution probability of the main wind direction in each section is 56.78%, 69.73%, 67.32% and 64.65%, respectively. The negative natural wind can inhibit the smoke spread downstream of the smoke exhaust port, but it is not conducive to the smoke exhaust. The positive natural wind promotes the smoke spread to the downstream of the smoke exhaust port, and the larger the natural wind speed, the longer the spread length. Natural wind reduces the smoke exhaust efficiency. For positive or negative natural wind with a guaranteed rate of 70%, the smoke exhaust efficiency is reduced by 27.76% and 15.59%, respectively, compared with the condition without natural wind. The research results can provide a useful reference for the design of fire smoke exhausts and smoke control schemes in extra-long highway tunnels.

Investigation on the natural smoke exhaust performance by vertical shaft in tunnel fires under different ambient pressures

In high-altitude tunnels, due to the ambient pressure and density of air being different from those under normal pressure, the convection and heat radiation in tunnels are also different when fires occur. As a result, the smoke exhaust capacity varies under different pressures. This work aimed to numerically explore the impact of ambient pressure on the smoke exhaust efficiency at high-altitude shallow tunnels with natural ventilation by shaft. The numerical results demonstrated that the smoke exhaust capacity is enhanced with increasing ambient pressure. This is because of the increase in the air entrainment coefficient and air density, and it causes plug-holing more easily to occur under higher ambient pressures. Once the plug-holing has taken place, the fresh air can be directly exhausted through the shaft, which results in a poor smoke exhaust capacity. By accounting for the factors of the smoke layer thickness, ambient pressure, the exhausted smoke temperature and heat release rate, the Richardson number was introduced as the criterion for determining whether plug-holing occurs. As ambient pressure was increased, the critical Richardson number for plug-holing was observed to decrease, a finding supported by existing research.

Numerical investigation on smoke control with new‐style horizontal smoke baffle in an immersed road tunnel

AbstractTo investigate the improvement induced by horizontal smoke baffles during lateral smoke exhaust, an immersed road tunnel with various horizontal smoke baffles positioned below the lateral exhaust vent was studied numerically. Together with the velocity field characteristics, the temperature distribution was investigated near the lateral smoke exhaust vent, followed by the analysis of lateral smoke exhaust efficiency under different horizontal smoke baffles. Results showed that after installing the horizontal smoke baffle, there was a significant decrease in the extracted cold air, while the high‐temperature smoke in the exhaust vent increases, indicating the plug‐holing is effectively suppressed. It is known that the efficiency of smoke exhaust increases when the length exceedance ratio of the horizontal smoke baffle is smaller than 100%, while it changes slightly when the baffle length continues to increase. When the width ratio of horizontal baffle is smaller than 40%, the efficiency of smoke exhaust increases with the baffle width and then changes slightly with a wider smoke baffle. With a larger aspect ratio, the wider and shorter lateral exhaust vent is beneficial for improving the lateral smoke exhaust. Under the current conditions, the case shows the optimal smoke exhaust performance with a horizontal baffle length exceedance ratio of 100%, a baffle width ratio of 40%, and exhaust vent aspect ratio of 3:1. Finally, an empirical model is developed to describe the improvement of smoke exhaust efficiency caused by horizontal smoke baffle. These outcomes are helpful to the design of lateral smoke extraction system in road tunnels.

Numerical study on smoke temperature and exhaust efficiency in electric cable tunnel

Cable tunnels are the lifeline of cities. Once a serious fire occurs on cables, the toxic smoke generated by cable combustion spreads and escapes, causing serious harm to the surrounding environment and personnel. The heat generated can also ignite nearby combustibles, causing casualties and property damage. Based on the above issues, using fluid dynamics methods, fire dynamic simulation was used to simulate cable tunnel fire scenarios under four variables: wind speed, fire source power, smoke outlet height, and smoke exhaust airflow. The temperature distribution law of the ceiling smoke layer inside the cable tunnel, smoke generation law, tunnel smoke exhaust efficiency, and other factors were clarified, revealing the impact mechanism of the above factors on tunnel fire smoke exhaust efficiency and smoke performance improvement methods. The research results indicate that the combination mode of exhaust air volume and ventilation air speed can effectively control tunnel fire smoke, providing a reference for the fire prevention design of cables in urban cable tunnels.

Numerical Study on the Efficiency of the Point Smoke Exhaust System in V-shaped Urban Underground Road Tunnel

In order to study the exhaust effect of the point smoke exhaust system in V-shaped urban underground tunnel with different structures, effects of the slope differences on both sides of the slope changing point and the operating mode of the exhaust vent on the smoke spread distance, temperature at the height of 2 m above the road and the ceiling temperature were studied by the numerical method. The research results show that for the symmetric V-shaped tunnel, smoke can be controlled in the required range, but for asymmetric V-shaped tunnel with the slope difference on both sides, smoke cannot be easily controlled, ventilation supply is needed to improve the smoke exhaust efficiency. Point smoke exhaust system cannot effectively reduce the maximum temperature of the ceiling above the fire. When the fire source power is large, for the symmetrical V-shaped tunnel with a small slope, some fire prevention measures must be taken to protect the ceiling partition. For the asymmetric V-shaped tunnel, two exhaust vents opened on the small slope side and four opened on the large slope side are suggested for the occupant safe evacuation in tunnel.

Influence of Smoke Exhaust Volume and Smoke Vent Layout on the Ceiling Centralized Smoke Exhaust Effect in Tunnel Fires

This research focuses on the impact of smoke exhaust volume and smoke vent layout, which are two crucial factors affecting the smoke control efficiency in tunnels, on the smoke exhaust effect in tunnel fires. Numerical simulation methods are employed to investigate the impact of changing the smoke exhaust volume and the smoke vent number on the smoke exhaust performance in a curved tunnel with a ceiling centralized smoke exhaust system. This research primarily examines the length of the smoke distribution, the smoke temperature under the ceiling, the vertical visibility, and the exhausted smoke mass flow rate. The findings indicate that, in a tunnel with a single-side ceiling centralized smoke exhaust mode, an imbalance in smoke distribution occurs between the upstream and downstream of the fire source. The upstream area experiences a higher amount of smoke, while the downstream area has thinner smoke. Increasing the smoke exhaust volume yielded positive effects on smoke control, as evident in the reduced the smoke spread range, and improved the smoke exhaust efficiency. The influence of changing smoke vent number on the smoke exhaust effect was dependent on the smoke exhaust volume. When the smoke exhaust volume was excessive, altering the number of smoke vents had a minimal impact on smoke exhaust, while in cases with small smoke exhaust volumes, changes in smoke vent numbers obviously influenced the smoke control effect. Therefore, selecting an appropriate smoke exhaust volume and raising the smoke vent number can effectively optimize the performance of the ceiling centralized smoke exhaust system.

Smoke exhaust characteristics under stack effect of natural ventilation by shaft in high-altitude tunnel

The number of high-altitude tunnels is increasing year by year and natural ventilation of shafts is widely used as a critical component of the ventilation system. For the design of ventilation systems, it is particularly crucial to consider the smoke exhaust performance of shafts in low-pressure environment. Therefore, this study simulates tunnel fires with natural ventilation by shaft in high-altitude areas, aiming to analyze the impact of atmospheric pressure on smoke exhaust performance and smoke exhaust capacity by natural ventilated shafts at 50 to 100 kPa atmospheric pressure. It appears from the results that the increasing of ambient pressure can enhance the ability of air entrainment and natural smoke exhaust, and thus plug-holing is more likely. In which condition, fresh air can be expelled straight from the shaft, reducing smoke exhaust efficiency. As ambient pressure increases, the critical shaft height for plug-holing decreases. By analyzing the smoke exhausting process by shaft and smoke flow, smoke exhaust capacity can be evaluated by obtaining the mass flow rate of exhausted smoke and fresh air from the shaft under various atmospheric pressures and fire source power and. It could be said that increasing ambient pressure and shaft height can improve the smoke expulsion performance before plug-holing. However, once plug-holing happens, the increase of shaft height can no longer expel more smoke but much more air is exhausted by shaft. It is hoped that this work will serve as a guideline for ventilation systems design in high-altitude tunnels.

A prediction model for smoke exhaust efficiency of rescue stations in railway tunnels with semi-transverse ventilation

This study analyzes the effects of heat release rate (HRR), fire position, smoke exhaust velocity, shaft diameter, and ambient pressure on the smoke exhaust efficiency in a rescue station with semi-transverse ventilation. The results demonstrate that the shaft close to the fire source has higher smoke exhaust efficiency, but the fire location has little effect on the overall smoke exhaust efficiency. The smoke exhaust efficiency decreases with the increase of HRR and is proportional to the -1/3 power of the dimensionless HRR. Before the smoke is overall controlled in the rescue station, increasing the smoke exhaust velocity can improve smoke exhaust efficiency, and the smoke exhaust efficiency is proportional to the 2/3 power of the dimensionless smoke exhaust velocity. When the smoke in the rescue station is overall controlled, increasing the exhaust velocity can reduce the smoke spread distance. Moreover, the smoke exhaust efficiency is proportional to the 9/5 power of the dimensionless shaft diameter and 0.3 power of the pressure ratio. Finally, combined with theoretical analysis, a prediction model of the smoke exhaust efficiency is obtained. Therefore, this study can guide the design of the semi-transverse ventilation system in extra-long railway tunnel rescue stations at different altitudes.

CO Diffusion Study and Spatial and Temporal Variation Modeling during the Construction Period of the Plateau Railroad Tunnel.

In order to investigate the diffusion law of CO gas in the vicinity of the tunnel boring face of the plateau long tunnel, to improve the efficiency of tunnel smoke exhaust, and to derive the spatial-temporal variation model of CO concentration for predicting the concentration of CO at different times and in different cross sections under specific environments, a CO diffusion model of a tunnel in Yunnan was established by using Ansys Fluent Fluid Simulation Software, and the CO transport characteristics under different conditions were simulated by taking the ventilation time, wind speed, and location of the air ducts as the influencing factors. The results show that the wind flows from the mouth of the wind pipe after the wind speed decreases, the diffusion area increases and arrives at the face of the direction of the rebound in the jet stream of new wind, and the return wind under the joint action of the vortex produced obviously, to reach the wind pipe mouth after the tunnel wind flow field, basically tends to stabilize. When the wind pipe mouth was arranged in the arch waist, 20 m away from the boring face, the inlet wind speed was 9 m/s and the ventilation time was 30 min; the CO concentration in the tunnel was reduced to below the maximum allowable concentration value. Moreover, the concentration of CO in the tunnel at the moment of 15 min of ventilation has a nonlinear positive correlation with the change of distance L from the boring face, while at the cross section of the air outlet of the wind pipe L = 20 m, the ventilation time is from 1 to 30 min and the concentration of CO at the cross section has a nonlinear decreasing trend with the ventilation time, which can be deduced according to the different space-time change models.

Research on the Influence Mechanism of Outdoor Wind Environment on Indoor Smoke Exhaust Efficiency in the Super-High-Rise Tower Crown Based on Airpak Simulation

The high wind pressure and velocity of the outdoor environment make super-high-rise tower crown space distinct from general tall through space. This segregation causes the crown space to be particularly prone to smoke short-circuiting influenced by the outside wind environment if a fire occurs indoors, and causes deficient smoke exhaust efficiency in a fire. The goal of this study was to investigate the general principle regarding the effect of the outdoor wind environment on smoke exhaust efficiency of such spaces under the crown space. We measured external wind direction and wind pressure in the smoke exhaust in the tower crown and developed setting plans for the exhaust outlets and make-up air inlet. Airpak was used to create the external wind environment and compare simulations to see if smoke short-circuiting occurred. We analyzed the causes, summarized solutions that did not result in short-circuiting of smoke flow, and made adjustments. We provide an ideal plan for the setting direction and vent velocity of the make-up air inlet and exhaust outlet in the crown spaces of super-tall towers to improve the design of smoke exhaust systems in such spaces.

Theoretical model for predicting smoke back-layering length downstream of centralised ceiling exhaust vent in tunnel fires

Passing a certain amount of smoke through a centralised ceiling exhaust vent with an acceptable back-layering length can considerably reduce the smoke exhaust rate of a centralised exhaust system. Accordingly, this study conducted a theoretical analysis of the back-layering length of smoke downstream of the centralised ceiling exhaust vent and proposed a theoretical prediction model considering the smoke exhaust efficiency, exhaust vent layout, and the distance between the fire source and vent. This model can be applied across multiple exhaust vent layouts with a wide range of smoke exhaust rates. In addition, the theoretical prediction model was verified with a series of reduced-scale experiments. Thereafter, we compared the proposed theoretical model with existing relevant models. The results revealed that the existing models are applicable only for a specific range of the smoke exhaust rate because these models were either empirically derived for a particular range of smoke exhaust rate or developed assuming pure smoke flow in the vent. Furthermore, the proposed model was employed to quantitatively analyse the influence of the exhaust rate, exhaust vent layout, and the distance between the exhaust vent and fire source on the smoke back-layering length. Finally, we discussed the contribution of the proposed model for optimising the exhaust vent layout and reducing the smoke exhaust rate of the system.

Numerical Study on Coupled Smoke Control Using Longitudinal Ventilation and Naturally Ventilated Shafts during Fires in a Road Tunnel

Longitudinal ventilation and smoke extraction by shaft are common smoke control methods in road tunnel fires. Tunnels often adopt one of these methods in practical engineering. However, it may have a better effect to adopt the method of mixing the two smoke exhaust methods together, which has not been revealed in the previous literature. Hence, the coupled effects of longitudinal ventilation and natural ventilation with shafts on the smoke control in tunnel fires were studied in this work. Numerical simulation was carried out considering different longitudinal ventilation velocities (0–4 m/s) and 4 kinds of typical shaft arrangements (shaft lengths range of 3–12 m, shaft intervals range of 27–60 m). The smoke spread length and smoke exhaust efficiency were analyzed systematically. Results show that (1) with the increase in longitudinal ventilation velocity, the total smoke spread length firstly decreases (V < 1 m/s) and then keeps almost constant (1 m/s < V < 2 m/s), finally increasing significantly (V > 2 m/s). (2) The length of the dangerous area (over 60 °C) at human height is basically 0 for all cases (except for Scenario 4 of shaft arrangement) when the longitudinal ventilation velocity is less than 2 m/s. (3) The CO smoke flow rate through the shaft is relatively high when the longitudinal ventilation velocity is within the range of 1–2 m/s for 4 kinds of shaft arrangement scenarios. Factors such as smoke spread and smoke exhausted through the shaft are comprehensively considered to judge smoke exhaust performance. The following conclusions can be drawn: when the ventilation velocity ranges from 1–2 m/s, it has a positive impact on the smoke control in tunnel fires with natural ventilation with shafts. When the ventilation velocity exceeds 2 m/s, the total smoke spread length and the length of the danger area increase, and the smoke stratification becomes worse, which brings inconvenience to rescue work. The results can provide reference for the design of fire protection in tunnels.

Application of the Bernoulli Effect for Improving Smoke Exhaust Efficiency in Tunnel Fires

Application of the Bernoulli Effect for Improving Smoke Exhaust Efficiency in Tunnel Fires

Research on the effect of ceiling centralized smoke exhaust system with air curtains on heat confinement and plug-holing phenomenon in tunnel fires

As tunnel fires can easily cause numerous casualties, this paper proposes a combination of ceiling centralized smoke exhaust and air curtain ventilation to study the temperature distribution characteristics and smoke control effects to assist passenger evacuation and firefighting. Dimensionless Froude number was introduced for a theoretical analysis and a series of CFD simulations. The plug-holing phenomenon in the coupling effect between air curtains and ceiling centralized smoke exhaust systems was investigated by changed the parameters of the heat release rate, the smoke exhaust rate, and the jet velocity of the air curtains. The results demonstrate a mutually reinforcing effect of the combined ceiling centralized smoke exhaust system and air curtains in terms of smoke control. The optimal distance between the ceiling exhaust port and the air curtains is 30 m. The blocked holes in the coupled system are mainly located at the downstream edge of the exhaust port, which directly reduces the smoke exhaust efficiency. The smoke exhaust efficiency is more affected by the smoke exhaust rate variation than by the air curtains velocity. A model was built to predict the smoke exhaust efficiency of the coupled system when the fire source power is 20 MW. We propose a new modified critical Fr number (3.69), which can be used to predict the phenomenon of smoke plug-holing in the coupled system. These results allow the design of a fire protection system for tunnel fires with a high barrier and high smoke exhaust effect, facilitating the safe escape of people and efficient fire rescue in the event of a fire.

Quantitative evaluation method of smoke exhaust performance and application on exhaust volume optimization in tunnel fires under lateral centralized mode.

This study investigated the indicators in the quantitative method of evaluating the smoke exhaust performance, which provided a theoretical basis for the optimization of the smoke extraction system under the lateral centralized mode in tunnel fires. The criterion was proposed for plug-holing, and the theoretical models of smoke exhaust efficiency were established to distinguish whether the plug-holing occurs or not. The relationship between efficiency, effectiveness, and efficacy was analyzed from the perspective of smoke and heat exhaust. Meanwhile, this evaluation method was applied to the optimization of exhaust volume in a practical engineering through FDS numerical simulation. The results show that Ri is a vital basis for reflecting the movement form of smoke and the exhaust effect. The critical value of Ri is 1.09 when plug-holing occurs in a standard three-lane immersed tunnel, resulting in a significant reduction in the efficiency of smoke exhaust. The greater the exhaust volume of the exhaust fan, the greater the Ri value, the higher the total smoke and heat exhaust efficiency, and the better the exhaust effectiveness of smoke inlets without plug-holing. Under longitudinal ventilation, the optimal exhaust volume is 180 m3/s at 20MW and 360 m3/s at 50MW in the application case.

Numerical study on the critical criterion for predicting the plug-holing under lateral mechanical exhaust in tunnel fires: Considering the effect of smoke flow in longitudinal direction

This paper has analyzed the phenomenon of plug-holing under lateral mechanical exhaust in tunnel fires by conducting large eddy simulations (LES) with Fire Dynamics Simulator (FDS). The lateral mechanical smoke exhaust is a method of smoke controlling in tunnel fires with exhaust vents set in the tunnel sidewall. The results show that the phenomenon of plug-holing occurs at the lateral exhaust vent as the exhaust rate increases. The occurrence of plug-holing will lead more fresh air discharged through the vent, thereby reducing the smoke exhaust efficiency. The smoke layer thickness and temperature downstream the vent in the tunnel are analyzed to evaluate the exhaust effectiveness of lateral mechanical exhaust. When the plug-holing does not occur, increasing exhaust rate can effectively reduce the smoke layer thickness. While the plug-holing occurs, the increasing exhaust rate will increase the smoke layer thickness on the contrary. In terms of reducing smoke temperature in the tunnel, increasing exhaust rate has very limited effect in this study. Therefore, it is not the case that the higher the exhaust rate, the better the smoke exhaust efficiency and effectiveness. There is a critical exhaust rate under which the plug-holing can be avoided. Furthermore, considering the horizontal and vertical inertial forces of the smoke beneath the vent, a critical criterion (Ri'critical) for predicting the occurrence of plug-holing of lateral mechanical exhausting is proposed. Based on the results of different exhaust rates, heat release rates and vent sizes, the critical criterion (Ri'critical) that corresponds to the occurrence of plug-holing can be determined as 0.6.

Study of the Diffusion Law of Harmful Gases in Tunnel Construction on Plateaus and Optimization of Ventilation Parameters.

To explore the diffusion law of harmful gases near the excavation face in tunnel construction on plateaus, select the optimal ventilation parameters, and improve the ventilation efficiency, Ansys Fluent was used to set the environmental parameters according to the highland, where the tunnel is located, and simulate and curve fit the diffusion phenomenon of CO and NO2 based on the fluid control equations and species transport model. The effects of air velocity, ventilation time, and duct position on the diffusion law of harmful gases were analyzed, from which the optimal ventilation conditions were selected, and the ventilation effects under the optimal conditions were compared with those under the original conditions. The study shows that after the fresh airflow passes through the outlet, part of it flows out along the tunnel wall toward the cave entrance, and the other part interacts with the return air to form a vortex; when the air supply speed is 10 m/s and the distance from the duct outlet to the excavation face is 25 m, the maximum concentration of harmful gases decreases by 92–99% after 20 min of ventilation compared with that before optimization and the smoke exhaust efficiency increases by 2.5% per minute.

Study on smoke exhaust performance in tunnel fires based on heat and smoke exhaust efficiency under the lateral centralized mode

When a tunnel fire occurs, using quantitative indicators to evaluate the performance of smoke extraction system and determining a reasonable exhaust volume is the key to ensuring the high efficiency of the tunnel smoke extraction system. Based on the transfer mechanism in heat and mass flow rate of smoke, theoretical models are established to calculate the heat and smoke exhaust efficiency of smoke inlets in each group and the smoke extraction system under the lateral centralized smoke exhaust mode by temperature and CO concentration. FDS is used to investigate the temperature and CO concentration changes under different exhaust volumes, and the heat and smoke exhaust efficiency are obtained. The larger the exhaust volume, the higher the total heat and smoke exhaust efficiency of the smoke extraction system. However, the phenomenon of plug-holing occurs in the lower left area of the smoke inlets. With the heat release rate of 20 MW and no longitudinal ventilation, the smoke exhaust performance can be greatly improved when the single-sided exhaust volume is 180 m 3 /s, the heat and smoke exhaust efficiency at the smoke inlets can reach up to 54.2% and 47.3% respectively, and the total heat and total smoke exhaust efficiency are 51.3% and 39.7% respectively.

Effect of Opening Size on the Development of Ventilation-limited Fire

There are many factors, that affect the development of indoor fire, such as the size of the fire source, the opening or space of the room, and the nature of the combustible materials. Among them, the space of the room, has a significant impact on the development of a ventilation-limited fire. In this paper, the Fire Dynamics Simulator (FDS) software is used to analyze, the risk of fire initiation in the restricted ventilated compartment, when the size of vertical ventilation space is different. Through, a combination of experimental design, numerical simulation, and theoretical analysis, the changes in the level of carbon monoxide, visibility, temperature, Heat Release Rate (HRR) and, the smoke exhaust efficiency of natural smoke at different opening sizes are observed. It is observed that, when the ratio of inlet and outlet area reaches 2:1, the natural smoke exhaust effect is the best, however, the increasing in the opening size has little significance on the smoke exhaust effect. The research on the influence of smoke outlet size, will helps in the development of the law regarding fire prevention, smoke exhaust design, and fire rescue work of a building.