Critical Ventilation Research Articles

Assessment of Ventilation Potential and Construction of Wind Corridors in Chengdu City Based on Multi-Source Data and Multi-Model Analysis

The establishment of urban ventilation corridors (UVCs) aims to mitigate the urban heat island effect. While most studies focus on the construction and assessment of the environmental benefit of UVCs, they often overlook the analysis of UVCs’ topological features. This research integrates multi-source data including 3D urban buildings, historical meteorological observations, high-resolution remote sensing, and land use planning, combined with multiple models, including geographic information system spatial analysis, circuit theory, and complex networks. Based on an assessment of urban ventilation potential, the circuit model was applied to extract UVCs aligned with the prevailing wind direction for both summer and winter seasons. Complex network modeling was employed to analyze the topological features of the ventilation network. From the analytical results, a multi-level wind corridor system for Chengdu was quantitatively developed. The results indicate that the city’s overall ventilation resistance is high, with notable spatial clustering, and the southeastern region faces substantial ventilation obstructions. A total of 143 critical ventilation nodes were identified, with the number of air inlets and outlets in summer being significantly fewer than in winter. However, the cooling effect of ventilation corridors in the prevailing summer wind direction is superior to that in winter. The ventilation network comprises 16 communities with distinct ventilation characteristics, exhibiting moderate connectivity, lacking small-world properties, and showing congestion and instability.

A Mixed Convection Model for Estimating the Critical Velocity to Prevent Smoke Backlayering in Tunnels

AbstractA novel mathematical model for the critical ventilation velocity to prevent smoke backlayering in tunnels is presented, addressing limitations of prior approaches. The basis of the model is a rigorous characterisation of the physical processes by the characteristic quantities. Empirical parameters within the new model are determined, to align with results from both full-size and small-scale tunnel experiments. Data from numerical simulations (CFD, Computational Fluid Dynamics), validated by known test data, are then used to estimate the effects of tunnel slope and other parameters on the critical velocity. The model is seen to approximate the critical velocity well, following all trends identified by test data and CFD parameter studies. The empirically calibrated equation permits prediction of the critical velocity beyond the narrow range of tunnel geometries where known results already give an answer. The resulting equation has practical application for tunnel design.

Experimental study on the effect of longitudinal ventilation on spill fire characteristics in sloped tunnels

A series of spill fire tests were carried out in a reduced-scale tunnel (6 m × 0.6 m × 0.45 m), considering different discharge rates (60–140 ml/min), ventilation rates (0∼1.26 m/s), and tunnel slopes (3 %, 5 %, 7 %). Some crucial characteristic parameters during the quasi-steady combustion stage of tunnel spill fires were analyzed, including the diffusion shape of the liquid fuel, mass burning rate per unit area, flame morphology and plume temperature, and the critical ventilation velocity. The results show that as the tunnel slope increases, the diffusion shape of the liquid fuel transitions from circular to an ellipse shape and then to rectangular or linear shape. Moreover, the mass burning rate per unit area decreases with increasing ventilation rates at low ventilation (V < 0.5 m/s), while it remains almost constant at high ventilation rates (V ≥ 0.5 m/s). This could be attributed to variations in flame tilt behavior under different ventilation conditions. Predictive models for the flame tilt angle and the vertical temperature distribution of tunnel spill fires under the effect of ventilation were further established and verified. Furthermore, the critical ventilation velocity of tunnel spill fires remains essentially unchanged as the discharge rate (or fire source power) increases. This phenomenon was explained by analyzing the resistance of the spill fire plume and the buoyancy generated by the smoke temperature. This work could serve as a critical reference for assessing disaster risks and executing emergency rescue operations during tunnel spill fires.

Badanie efektu dławienia w pożarze tunelu

As per the emergency ventilation strategy, air velocity of 3 m/s in case of the longitudinal ventilation is sufficient for smoke control in all fire conditions. Numerical experiments were carried out with FDS software to estimate the numerical value of the critical velocity. Numerical models were realized in 0-6% slope tunnels with a 1% step for 5, 10, 20, 30 and 50 MW fires for four types of fuel: gasoline, diesel fuel, oil and firewood. The paper notes that the dynamic pressure induced by a strong fire is much higher than the static pressure of tunnel jet fans. As a result, following the algebraic summation of positively-directed ventilation flows and the negatively-directed flows induced by fire, an intense back layering occurs, which casts doubt on the suitability of the specified emergency ventilation strategy when designing the fire ventilation. The critical ventilation speed of 3 m/s cannot cope with the traction caused by fire, expressed by the ascending movement of the high-temperature and low-density combustion products. The paper discusses the numerical modelling results with an adiabatic underground heat exchange model and presents typical tunnel fire modelling plans, which correspond to an inclined tunnel for ascending and descending ventilation flows as well as a horizontal tunnel. The article gives the regularities obtained by the numerical models of changes in the variables of average air temperature and density, average carbon monoxide, average carbon dioxide and soot concentrations. According to the emergency ventilation strategy, critical velocity is an important value and a major determinant of back layering prevention in sloping tunnels. Although many papers have been devoted to this problem, the obtained results differ much. The present paper shows that strong fires induce much greater dynamic pressures than the static pressures of the tunnel jet fans are. Consequently, the flows caused by these forces, as they move in different directions, following their algebraic summation, cause a strong back-layering in case of positive ventilation flows, i.e., when the ventilation flow is descending and the fire seat is found at a lower point compared to the air supply portal. The new results can be used to develop fire ventilation plans as well as life-saving and emergency control solutions in the operating tunnels for personnel and rescuers.

Numerical Simulation of the Smoke Distribution Characteristics in a T-Shaped Roadway

This paper numerically analyzes the influence of heat release rate (HRR) and longitudinal ventilation velocity on smoke distribution characteristics in a T-shaped roadway when the fire source was located upstream of the T-junction. The back-layering length, critical ventilation velocity, smoke temperature, and CO concentration in the main and branched roadway were investigated and analyzed. The results showed that the ventilation velocity is the key factor influencing back-layering length, while the effect of HRR on back-layering length is gradually weakened as HRR increases. The critical ventilation velocity in the T-shaped roadway is higher than in a single-tube roadway, and the predicted model of dimensional critical ventilation velocity in a T-shaped bifurcated roadway is proposed. The correlation between average temperature (Z = 1.6 m) (both in the main roadway I and the branched roadway) and ventilation velocity fits the power function, and the variation in average temperature (Z = 1.6 m) according to HRR fits the linear formula. The relation between average concentration of CO (Z = 1.6 m) (both inside the main roadway I and the branched roadway) and longitudinal ventilation velocity follows the power relation, and the variation in average concentration of CO (Z = 1.6 m) according to HRR follows the linear function.

The Windcatcher: A Renewable-Energy-Powered Device for Natural Ventilation—The Impact of Upper Wing Walls

In recent years, there has been increased interest in natural ventilation solutions as a means to achieve sustainable and energy-efficient building design. Windcatchers, ancient Middle Eastern architectural elements, have surfaced as viable passive cooling devices in modern architecture, thereby enhancing interior air quality and reducing the reliance on mechanical ventilation systems. Integrating upper wing walls (UWWs) is hypothesized to augment a windcatcher’s effectiveness by optimizing wind capture, air circulation, and thermal regulation. Therefore, this study aimed to explore the influence of incorporating a two-sided windcatcher with UWWs, with a particular emphasis on the effect of the UWW angle on ventilation performance within building spaces. To achieve this aim, a series of numerical simulations were conducted to assess the synergy between the windcatcher and the wing wall configuration with varying UWW angles and under varying wind speed conditions. As the first step of the research methodology, the CFD model was validated through a comparison between the numerical results and the experimental data. The findings showed good agreement between these methods. In the next phase, windcatchers with different UWW angles spanning the range of 0° to 90° were subjected to rigorous evaluation. The results revealed that the configuration with a 30° angle exhibited the optimal performance concerning critical ventilation parameters encompassing the airflow rate, air change rate, and mean age of air. Finally, the selected configuration underwent an evaluation under diverse wind speed conditions, which affirmed that even under low-wind-speed conditions, the windcatcher provides ventilation levels that align with the standard requirements.

Experimental comparison on the smoke movement in horizontal and downhill tunnel fires under subcritical ventilation

In longitudinally ventilated tunnel fires, subcritical ventilation is widely used in the early stage of accidents to maintain smoke stratification. Due to the stack effect, the smoke movement in inclined tunnels is different from that in horizontal tunnels. In this study, a series of brine-water experiments are carried out to compare the characteristics of buoyant contaminants flow in horizontal and downhill tunnels under subcritical ventilation, which has not been experimentally investigated. The results suggest that subcritical ventilation is an effective longitudinal ventilation strategy in the early stage of horizontal tunnel fires, as it can retain the smoke stratification and an acceptable back-layering flow. Moreover, the critical ventilation condition can be resumed after the ventilation pressure increases to the critical value. However, if subcritical ventilation is first adopted in downhill tunnel fires, no stable back-layering flow will be observed, the buoyant contaminants consistently flow out from the higher exit, ultimately leading to the establishment of a “stack-dominated flow regime”. Under this circumstance, even if the ventilation pressure increases to the critical value or slightly higher, the buoyant contaminants upstream of the source still cannot be pushed back. Moreover, the reduced gravity of the smoke in the tunnel increases as the ventilation pressure rises. Therefore, subcritical ventilation is not suitable for downhill tunnel fires. The study is helpful for understanding the smoke movement in horizontal and inclined tunnel fires under subcritical ventilation.

Analytical Model of Critical Ventilation Flow Rate for Accidental Hydrogen Leakage in a Confined Space

The determination of the critical ventilation flow rate is significant for risk control and standard development during accidental hydrogen leakage in a confined space with hydrogen-related equipment. This paper presents an analytical model for calculating the critical ventilation flow rate through the quantification and constraint solution of the ventilation effect and ventilation cost. The experimental method was used to investigate the effects of nozzle diameter and stagnation pressure on the diffusion and ventilation of horizontal hydrogen leakage in a cuboid chamber. Ventilations from 30 to 180 m3/h were carried out through the rectangular vent. It was shown that the peak concentration of the measuring point was positively correlated with the stagnation pressure and the nozzle diameter. The experimental data were used to verify the analytical model by calculating the effective ventilation time. This study demonstrates that the critical ventilation flow rate can be increased significantly at higher stagnation pressures and larger nozzle diameters. Furthermore, the discrepancy of critical ventilation flow rates under different nozzle diameters will be enhanced with the increase of stagnation pressure. For a stagnation pressure of 0.4 MPa, the critical ventilation flow rate under a 4 mm nozzle even increased by 52% relative to the 2 mm nozzle.

Study of critical velocity and back-layering length with fire sources both inside and outside a tunnel

The smoke back-layering length and the critical ventilation velocity in double fire scenarios where the fire sources were both inside and outside the tunnel were studied by small-scale tests and theoretical analysis. Results showed that the smoke back-layering length increased with an increase of the fire separating distance when the dimensionless fire separating distance was less than 3, and the values were larger than the results of a single internal fire at each fire separating distance. The dimensionless critical ventilation velocity segmentally increased with the increase of the dimensionless fire separating distance when the dimensionless fire separating distance was less than 10, and then remained constant. Moreover, the constant value was found to be close to that in the case of a single tunnel fire. In addition, an empirical model of dimensionless critical ventilation velocity was established for the double tunnel fire scenario both inside and outside a tunnel.

Characteristics of the spatial and temporal evolution of the environmental parameters for belt fire in underground coal mine roadway

The high temperature and toxic gases produced by burning belt in mine are a serious threat to the miners’ safety. A full-scale fire experiment was conducted in an exercise roadway, as a basis for which a physical model was established. The spatial and temporal evolution of the environmental parameters was obtained in the U-shaped working face, and the effect of wind speed on the fire parameters was studied. It was found that the smoke concentration and temperature from large to small are the downwind side of the transport roadway, working face and return airway. The structure of smoke layer near the working face is disorganized, which is related to the local turning structure of the roadway. The smoke reverse occurs later than the spread of smoke downstream of the fire area. The thickness and maximum temperature of counterflow layer decreases linearly with the increase of ventilation speed. Increasing the critical ventilation speed can control the smoke reserve and increase the risk downstream of the fire area. The harmfulness of wind speed to smoke reserve and spread in single roadway and complex network is then discussed. In addition, the influencing factors of fire evolution in confined space are put forward.

Experimental study on tail cavity structure and pressure characteristics of underwater vehicle with tail jet

The tail of an underwater vehicle with a tail jet often creates an attached tail cavity when moving at high speed. The strong coupling between the tail cavity and high-speed jet affects the kinematics characteristics of underwater vehicles. This paper compares the influence of Froude number (Fr) and ventilation coefficient (CQv) on the tail cavity at area ratio A/Ae = 92.5 and 236.7 by water tunnel experiment. It was found that at a small Fr, the critical ventilation coefficient of the transition from intact (IC) to partially broken (PBC) and PBC to pulsating foam (PFC) at A/Ae = 92.5 is greater than that of A/Ae = 236.7. The cavitation number (σc) at A/Ae = 92.5 is generally smaller than that of A/Ae = 236.7. With increased CQv, PBC's dimensionless time-averaged length (Lc‾/D) decreases sharply and then gradually flattens. The Lc‾/D of A/Ae = 236.7 is larger than that of A/Ae = 92.5. The fluctuation amplitude of the PBC length is smaller than that of the PFC, and the fluctuation frequency (fLc) of the PFC length is about 120 Hz. The change in the tail cavity will affect the pressure characteristics of the vehicle's bottom. With the increase of CQv, the peak frequency of the bottom pressure fluctuation (fpeak) will continue to rise in the PBC until the transition to the PFC, and the fpeak will remain at about 120 Hz. The correlation coefficient is 0.74 between the fLc and the fpeak. In addition, the pressure oscillation intensity at the vehicle's bottom (pbrms) is small in the IC region. When the tail cavity changes from PBC to PFC, pbrms increases gradually at first, then tends to be flat.

Fire Protection and Evacuation Analysis in Underground Interchange Tunnels by Integrating BIM and Numerical Simulation

Rescue and evacuation of underground interchange tunnels after a fire are challenging. Therefore, a method of integrating building information modeling (BIM) and a fire dynamic simulator (FDS) was proposed to analyze fire characteristics and personnel escapes in underground interchange tunnels. A BIM model of underground interchange tunnels was built, and then different formats (DXF and CAD) were generated and imported into Pyrosim software and Pathfinder software. With an increase in ventilation velocity, the CO concentration and temperature downstream of the fire source increased, and visibility decreased, according to simulation results. The critical ventilation velocity was 3.6 m/s at 30 MW. Evacuation simulation results suggested that the congestion of the transverse passage was very unfavorable for personnel escape: the escape time increased by 14.9% and 20% when the interior and entrance of the transverse passage were severely congested, while a 2.5 m wide transverse passage effectively reduced the escape time. Visibility was the first indicator that it did not meet the safety of the escape. After the tunnel’s personnel have been evacuated, the air supply or exhaust system should be started, and smoke should be expelled at a higher velocity. It is necessary to clear the passageway quickly or increase the automatic firefighting facilities when congestion is severe.

Experimental study on collaborative longitudinal ventilation of smoke control for branched tunnel fires considering different branch angles

For branched tunnel fires, the main tunnel and the branch should be collaboratively ventilated to keep the branch and upstream of the main tunnel free of smoke. In this paper, reduced scale experiments were conducted to study the collaborative longitudinal ventilation in branched tunnel fire, considering three branch angles (15°, 45°, and 60°) and five ethanol pool fire sizes. The fire source located at the intersection. The results show that the longitudinal controlling ventilation velocities in the main tunnel and branch are related to the critical ventilation velocity of branched tunnel fire when only the main tunnel is ventilated, vc,m. Two critical ventilation modes were proposed: (a) the values of ventilation velocities in the branch and the main tunnel are equal; (b) the ventilation velocity in the main tunnel equals to vc,m, and the ventilation velocity in the branch is smaller. Under ventilation mode A, when ventilation velocity is approximate 42 % of vc,m, the smoke can be controlled. Under ventilation B, when ventilation velocity in the main tunnel equals to vc,m, and the ventilation velocity in the branch is within 0.10 m/s–0.22 m/s, the smoke could be controlled. However, the heat release rates in ventilation mode A could be larger than that under mode B by 33 %—99 % regarding the same pan size. Hence, in practice, the collaborative ventilation modes, in which the ventilation velocities in the branch and the main tunnel are close, should be avoided. This work could help to guide the emergency rescue in relevant fire accidents.

Simulation and risk assessment of hydrogen leakage in hydrogen production container

In this paper, the hydrogen leakage and diffusion characteristics analysis and risk assessment are carried out on the container where a 2 Nm3/h alkaline hydrogen production device is located. Firstly, the transient and steady process of hydrogen leakage from hydrogen production container is analyzed. Secondly, the dynamic balance of combustible hydrogen cloud is analyzed, the concept of critical ventilation flow is put forward. It was found that in order to reduce the flammable volume by 85%, the installed ventilation can only cover the leakage flow of 1.0 Nm3/min. Finally, TNT equivalent method is used to evaluate the hazard degree of hydrogen leakage. It is found that the existed hydrogen production container ventilation device can only exhaust the hydrogen-air cloud with small flow leakage, while the accumulation of gas cloud still exists in large flow leakage. Under the critical ventilation flow, the minor injury radius can be reduced from 4.8 m to 2.78 m. The effect of critical ventilation flow was verified.

Variations of smoldering spread characteristics of Shanxi anthracite during forward smoldering propagation: Effect of ventilation rate

In view of the increasing risk of smoldering fire in coal mines, and to study the effect of ventilation rate on smoldering propagation deeply, the forward smoldering propagation process of Shanxi anthracite (SXA) is studied for nine different ventilation rates (10.0–30.0 L/min). Both simulations and micro-structure testing were performed. The results show that there are five distinct combustion stages (t0-t5) for the coal samples in the temperature range between 25.0 and 800.0℃. With increasing ventilation rates (33.3 to 100.0 mL/min), both the characteristic temperature and the activation energy of the studied coal at each stage decrease first before they increase. The heat release, however, occurs in reverse order (all samples reach a turning point at 75.0 mL/min). The critical ventilation rate range for a unit mass of SXA coal to sustain smoldering propagation was 0.18–0.27 m3/h kg−1. With increasing ventilation (between 10.0 and 30.0 L/min), key parameters such as average peak smoldering-temperature, average peak smoldering-temperature rise rate, spread rate of the average moisture-evaporation front, and average peak-temperature front also increase before they decrease, reaching their maxima at 22.5 L/min. The smolder reaction occurs in the following sequence: The moisture evaporation occurs first, which is followed by the pyrolysis reaction and the char-oxidation reaction. Then, the temperature gradually decreases after reaching its maximum.

Numerical investigation of critical velocity in curved tunnels: Parametric study and establishment of new model

Curved tunnels are more likely to have an accident and a fire source due to their physical characteristics. Using a 3D computational fluid dynamics tool with body-fitted grids, three different fire locations were investigated numerically in a series of curved tunnels with a turning radius of 50–1000 m and a heat release rate of 5–15 MW. Results showed that the critical velocity increased with increasing tunnel turning radius, and the straight tunnel had the highest critical velocity in all scenarios. For a tunnel with a 10 MW heat release rate, the critical velocity increased by 19 % from R = 50 to 1000 m. Furthermore, it was shown that the critical velocity was proportional to one-third power of the heat release rate. A gradual increase in critical velocity was also observed as the fire source was displaced from the tunnel's center to the walls. Furthermore, this increment increased as the fire source approached the internal curvature. During the displacement of the fire to the inner and outer walls of a tunnel with R = 100 m and a 10 MW heat release rate, the critical velocity increased by 7.7 % and 5.6 %, respectively. Therefore, the worst fire scenario occurs when a fire is close to a tunnel's internal curvature and has the largest turning radius and the maximum heat release rate. Tunnel curvature and fire location are not considered in normal dimensionless analysis (commonly used for tunnels). Thus, the numerical results were unified in a non-dimensional form, and a modified formula was developed to calculate critical velocities in curved tunnels. The proposed formula includes tunnel curvature, heat release rate, and fire location with a coefficient of determination of 0.98. Numerical simulations confirmed the predictions of the proposed formula. Compared to other models in straight and curved tunnels, the predicted correlation developed in the present study can accurately predict critical ventilation velocity in curved tunnels.

Numerical investigation of airborne transmission in low-ceiling rooms under displacement ventilation

This study employs computational fluid dynamics (CFD) simulations to evaluate the risk of airborne transmission of COVID-19 in low-ceiling rooms, such as elevator cabins, under mechanical displacement ventilation. The simulations take into account the effects of the human body’s thermal environment and respiratory jet dynamics on the transmission of pathogens. The results of the study are used to propose a potential mitigation strategy based on ventilation thermal control to reduce the risk of airborne transmission in these types of enclosed indoor spaces. Our findings demonstrate that as the ventilation rate (Qv) increases, the efficiency of removing airborne particles (εp) initially increases rapidly, reaches a plateau (εp,c) at a critical ventilation rate (Qc), and subsequently increases at a slower rate beyond Qc. The Qc for low-ceiling rooms is lower compared to high-ceiling rooms due to the increased interaction between the thermal plume generated by the occupants or infectors and the ventilation. Further analysis of the flow and temperature fields reveals that εp is closely linked to the thermal stratification fields, as characterized by the thermal interface height and temperature gradient. When Qv &amp;lt; Qc, hT,20.7 &amp;lt; him (him is the height of infector’s mouth) and aerosol particles are injected into the upper warm layer. As Qv increases, the hti also increases following the 3/5 law, which helps displace the particles out of the room, resulting in a rapid increase of εp. However, when Qv &amp;gt; Qc, hT,20.7 &amp;gt; him and aerosol particles are injected into the lower cool layer. The hti deviates from 3/5 law and increases at a much slower rate, causing an aerosol particle lockup effect and the εp to plateau. In addition, as the Qc increases, the local flow recirculation above the infector head is also enhanced, which leads to the trapping of more particles in that area, contributing to the slower increase in εp. The simulations also indicate that the location of infector relative to ventilation inlet/outlet affects Qc and εp,c with higher Qc and lower εp,c observed when infector is in a corner due to potential formation of a local hot spot of high infection risk when infector is near the ventilation inlet. In conclusion, based on the simulations, we propose a potential ventilation thermal control strategy, by adjusting the ventilation temperature, to reduce the risk of airborne transmission in low-ceiling rooms. Our findings indicate that the thermal environment plays a critical role in the transmission of airborne diseases in confined spaces.

Comparative analysis on temperature characteristics of hydrogen-powered and traditional fossil-fueled vehicle fires in the tunnel under longitudinal ventilations

Vehicle fires in the tunnel are a great threat to the safe operation of the tunnel. Due to the rapid development of the hydrogen economy, the fire due to the hydrogen leakage could not be avoided and may bring great damage to the passengers and infrastructure. Due to the large difference between pool fires of traditional fossil-fueled and jet fires of hydrogen-powered vehicles, it is in doubt whether the existing longitudinal ventilation design could still be effective for the safety issue of hydrogen powered vehicles. To solve this problem, it is necessary to compare temperature characteristics of hydrogen-powered and traditional vehicle fires with and without longitudinal ventilations. In present work, we conducted a numerical investigation to discuss the different temperature distributions of traditional and hydrogen-fueled vehicle fires. Results indicate that the high temperature zone of the pool fire only exists above the ceiling of the vehicle. For hydrogen-powered vehicle fire, the high-speed hydrogen jet with the strong inertial force could push the hot smoke flows back to the ground. The ceiling temperature of hydrogen-powered vehicle fire is larger since hydrogen-powered vehicle has a larger heat release rate and the fire hazard of jet fires bring more danger compared with the pool fire. Although the temperature stratification is also obvious for the hydrogen-powered vehicle fire, the air temperature in the lower region could be heated and still high enough to bring a great damage to the passengers’ lives. This is quite different with the traditional pool fire. In addition, the critical ventilation velocity is also discussed. The theoretical equation could well predicted the critical ventilation velocity of traditional vehicle fires. For hydrogen-powered vehicle fires, the critical ventilation velocity could reach up to 6 m/s. The theoretical equation could not well predict the critical ventilation velocity of hydrogen-powered vehicle fires due to exist of hydrogen jet fires.

Numerical study on fire-induced smoke temperature characteristics in small curvature radius UTLT-like tunnels under emergency state

Extensive studies have usually focused on an ordinary single straight or large curvature radius tunnel fire. However, the curvature effect of small curvature radius curved tunnel on the fire-induced smoke temperature characteristics still needs to clarify. Therefore, a three-dimensional transient computational fluid dynamics (CFD) fire model for small curvature radius urban traffic link tunnel (UTLT)-like tunnel was established to understand the scope and extent of curvature effect on the fire-induced smoke under emergency state. The results show that, in the curved tunnel area, small curvature radius or large critical ventilation velocity results in stronger centrifugal force of the fire smoke, readily destabilizing the fire smoke downstream of the fire source. Compared with large radius of curvature and straight tunnels, due to the greater resistance of curved walls in small curvature radius tunnel, the length of smoke back-layering is longer. Simultaneously, the temperature of the high-temperature smoke on the outer wall of the ceiling cross section is higher than the inner wall. Prediction mathematical models for the maximum temperature and longitudinal temperature profiles were further developed by taking the curvature and ventilation velocity effect, which is basically consistent with experimental and simulated results.

Theoretical and numerical study on critical velocity and driving force for preventing smoke backlayering in a connection roadway fire of coal mines

H-shape tunnel is a common roadway structure in the coal mine. When a fire occurs in the connection roadway, the characteristics of smoke movement and control are different from those in the traditional single-hole tunnel. This paper studies the critical ventilation velocity and driving force for preventing smoke backlayering in a mine connection roadway fire by numerical modeling. Results indicate that when the dimensionless heat release rate (Q̇*) is less than 0.28, the dimensionless critical ventilation velocity (Vc*) varies as the 1/8 power of the dimensionless heat release rate. Beyond 0.28, Vc* remains almost constant, independent of the heat release rate. For a certain heat release rate, the critical velocity in the connection roadway is lower than that in the single-hole tunnel, which is probably attributed to the smaller ventilation resistance or the effect of the downstream flow field on the smoke backlayering front. The current study prefers to use the ventilation resistance for further analysis. Besides, a calculation model of driving force for preventing smoke backlayering in the connection roadway fire is put forward by theoretical analysis. The predictions by the proposed model are found to comply well with the numerical simulation data. The outcomings of the current study are of guiding significance for the fire prevention and smoke control in the tunnel with a similar structure.