Back-layering Length Research Articles

Effect of the Metro Train on the Smoke Back-Layering Length under Different Tunnel Cross-Sections

Smoke back-layering length is an important aspect of tunnel fire research, and the influence of metro train blockage cannot be ignored. In previous studies, less attention has been paid to the phenomenon of back-layering caused by the same train blocked in tunnels with different cross-sections. This paper investigates this point through dimensionless analysis and fire dynamic simulator (FDS) numerical simulation. Several full-scale model tunnels (300 m in length), with different tunnel cross-sections, were constructed in FDS. According to the FDS simulation result, the smoke back-layering length was compared and analyzed. To investigate the effect of the tunnel cross-sections on smoke back-layering length under metro train blocking, the headroom ratio ε was proposed. Then, the influence of the tunnel cross-section on the smoke back-layering length was discussed in detail. Based on the dimensional analysis and FDS simulation results, a new model for predicting the smoke back-layering length was proposed. The prediction model obtained in this paper was compared with the FDS simulation results and the prediction model proposed by our predecessors. It is proven that the proposed model can better predict the length of smoke back-layering when metro trains are blocked in tunnels with different cross-sections. The outcomes of this work are expected to provide theoretical guidance for fire smoke control in metro tunnels with different cross-sections.

Theoretical and Numerical Study of Smoke Back-Layering Length for an Inclined Tunnel Under Longitudinal Ventilation

Theoretical and Numerical Study of Smoke Back-Layering Length for an Inclined Tunnel Under Longitudinal Ventilation

A simple dimensionless model for the confinement velocity of the lateral concentrated smoke exhaust in tunnel fire

Taking Wuhan Yellow Crane Tower tunnel as the prototype, a series of reduced-scale experiments and numerical simulations were carried out to study the tunnel fires’ confinement velocity. Using theoretical analysis, model experiment, and numerical simulation, the formula for calculating the dimensionless confinement velocity of fire with the lateral concentrated smoke extraction mode was derived. The smoke back-layering length was calculated by the smoke’s temperature distribution under the tunnel ceiling, and later redefined the confinement velocity. Results showed that the smoke back-layering length is insensitive to the velocity of the lateral exhaust vent at different heat release rates. When the distance between the fire source and the lateral exhaust vent is less than one meter, the Richardson number (Ri) of 2 can be used to determine the phenomenon of reaching the confinement velocity. When the distance between the fire source and the lateral smoke outlet is greater than one meter, Ri of 0.8 can be used to determine the confinement velocity at all heat release rates.

Study on real-time heat release rate inversion for dynamic reconstruction and visualization of tunnel fire scenarios

This study proposes a framework integrating tunnel fire knowledge with the machine learning approach to provide key fire parameters and reconstruct tunnel fire scenarios. The machine learning approach establishes mapping rules between tunnel ceiling temperatures measured by Fiber Bragg Grating (FBG) system and the real-time heat release rate (HRR) based on the tunnel fire database. The database employed in training and testing algorithms was generated by numerical simulations and full-scale tunnel fire tests, respectively. In addition, the HRR database establishment procedures, hyperparameter selection of machine learning algorithms and test dataset selection are systematically analysed. The real-time HRR inversion performance of four machine learning methods (support vector regression (SVR), random forests (RF), multilayer perceptron (MLP), long short-term memory network (LSTM)) and two tunnel fire knowledge methods (maximum ceiling gas temperature, smoke back-layering length) are evaluated. It is shown that the LSTM method provides the best prediction accuracy on full-scale tunnel fire test datasets, and the RF method is found to be most resilient despite the loss of partial temperature data due to the failure of temperature sensors. The corresponding models have been verified by the full-scale tunnel fire tests, and can provide the scientific decision support for intelligent fire-fighting.

Influence of current direction in longitudinal ventilated road tunnels on the backflow of combustion Produits

The results of numerical modeling in longitudinal ventilated sloping road tunnels are given. The slope of the tunnels varies in the range of 0-6%. The geometry of the tunnel is as follows: length: 100 m; width: 8 m; height: 6 m; area of the seat of fire: 16 m2. The seat of fire sized: 2.75x5.8x1.5 m is in the central part of the tunnel. The scenarios of development of 5, 10, 20, 30, 50 MW fires are studied in the case of positive and negative directional ventilation flows. The time of modelling was 120 seconds. The numerical problems were modelled with a volumetric grid method. The grid cell dimensions were: 0.5*0.5*0.5 m. Virtual point and volumetric measuring equipment was used to record the modeling results. The modelling used 4 groups of measuring devices that measured and recorded air velocity, temperature, and air and smoke densities. The paper discusses cases of algebraically summarizing the ventilation and fire-induced flows. Based on the results of numerical modeling, we can point out that the widely accepted indices of critical velocity and back-layering length in inclined road tunnels often give erroneous results. Therefore, in strategies for emergency ventilation, indicators such are critical velocity and back-separation should be used with caution.

Characteristics of a buoyant plume in a channel with cross-flow

We present data from direct numerical simulation (DNS) of a buoyant plume in a channel with cross-flow. The plume originates from a line heat source at the bottom channel wall, impinging the top wall and propagates downstream and/or upstream based on the strength of the cross-flow. We study the critical velocity, defined as the minimum cross-flow velocity which prevents negative backlayering length from occurring. For cross-flow velocities higher than the critical velocities, all the plume generated from the source would be forced downstream and the location of the backlayering front would move downstream. We also visualise the structure of the plume for different source Reynolds number, which indicates that the backlayering is thicker at lower source Reynolds numbers. The thicker backlayering acts like as a blockage, which influences the cross-flow. In addition, an investigation into the relationship between critical velocity, source Reynolds number and the heat release rate has been carried out. A one-third power between the heat release rate critical is found, which is in good agreement with previous studies. For a fixed heat release rate, the critical velocity increases with increasing source Reynolds number. However, the critical velocity approaches an asymptotic value when the source Reynolds number is sufficiently high.

Experimental investigation of the thermal back-layering length in a branched tunnel fire under longitudinal ventilation

The smoke propagation in branched tunnel is more complicated because of the special bifurcation structure that has not been clarified, despite the fact that the scenario exists in practice. In this paper, the thermal smoke movement behavior in branched tunnel fire were experimentally investigated under the combined effect of bifurcation angle and ventilation velocity. The theoretical analysis was conducted to develop the smoke back-layering length equation based on the applied force of smoke propagation upstream of the fire source, and the smoke back-layering length under various longitudinal ventilation velocities was quantified. The results indicate that the smoke back-layering length decreases with the increasing of longitudinal ventilation velocity for a given heat release rate. The stronger heat release rate results in the longer smoke backflow distance under same ventilation velocity. The smoke back-layering length varied with bifurcation angle due to the bifurcation structure weakens the inertial force of longitudinal ventilation, but the affected degree varies with bifurcation angle. The experimental smoke back-layering length can be well correlated using the deduced logarithmic function. The empirical model taking the bifurcation angle into consideration is proposed to predict the smoke back-layering length in branched tunnel fire. Finally, the correlation for critical ventilation velocity of branched tunnel fire under varied bifurcation angles was also proposed based on the boundary condition of none smoke back-layering.

Rapid prediction of mine tunnel fire smoke movement with machine learning and supercomputing techniques

A method and a thorough demonstration combining supercomputing and machine learning (ML) were proposed to help quickly predict the spread of fire smoke in mine tunnels. 1000 cases of a three-dimensional mine tunnel fire problem under different ventilation, thermal and geometric conditions were numerically simulated using supercomputing. Then four ML models were trained and applied to predict fire dynamics in the tunnel. It is revealed that all ML models performed well in predicting the occurrence of backflow and the back-layering length of the smoke. In particular, the random forest (RF) and support vector machine (SVM) models have the best performance for predicting whether backflow of fire smoke will occur, while the artificial neural networks (ANN) model shows the best performance in predicting the back-layering length. In addition, the ML models were used to evaluate the factors that affect the fire dynamics in the tunnel. The results show that the ventilation velocity and tunnel inclination angle are the most critical factors under the investigated ranges of ventilation, thermal and geometric conditions. Owing to the high performance in numerical simulations and prediction, the proposed method combining supercomputing and ML may provide a novel and efficient way for rapid prediction of mine tunnel fire dynamics.

Experimental study of back-layering length and critical velocity in longitudinally ventilated tunnel fire with various rectangular cross-sections

Back-layering length and critical velocity are important parameters in longitudinally ventilated tunnel fires. The present study explored the effects of tunnel aspect ratios on smoke back flow by scale model tests and theoretical analyses. Results show that the increase of the tunnel width will enhance the heat transfer between the smoke and the tunnel boundaries and decrease the smoke layer thickness, reducing the back-layering length and critical velocity. New correlations to predict back-layering length and critical velocity with more comprehensive physical significance were proposed by improving the aspect ratio correction coefficient. The prediction results of the model are in good agreement with the experimental data. The new model indicates that under small fire conditions, non-dimensional critical velocity gradually decreases as the aspect ratio increases and eventually approaches a limiting value, which is the critical velocity without sidewall constraints. The determination of the fire scale relies on the aspect ratio when non-dimensional heat release rate Q* is lower than 0.44. In contrast, the fire is always a large one when Q* is larger than 0.44.

Effect of ceiling extraction on the smoke spreading characteristics and temperature profiles in a tunnel with one closed end

A series of fire tests in a 1/15 scaled-model tunnel with one closed end have been conducted. Analysis was carried out to explore temperature distribution and smoke propagation under the influence of ceiling extraction system. Five different heat release rates, three dimensions of exhaust outlet, and numerous extraction rates were considered. Experimental results led to some interesting findings about the relationships between smoke extraction rate, fuel mass burning rate, and ceiling temperature. Two distinctive ceiling temperature regions were identified according to their different responses to smoke extraction rate, i.e., ceiling temperature decay between the fire and outlet was almost independent of smoke extraction rate while temperature upstream of the outlet decreased sharply with the increase of smoke extraction rate. Analysis was also conducted about smoke back-layering length, revealing its strong dependence on heat release rate and induced air velocity. Based on the experimental results and dimensional analysis, three empirical formulas were proposed to capture ceiling temperature decay and smoke back-layering length for tunnels with one closed end utilizing ceiling smoke extraction.

Experimental investigation on the fire and thermal smoke spread in tunnels: Effects of fire elevation

Previously, studies related to tunnel fires predominately concentrated on the fire at ground level while the risk of elevated fires caused by different vehicle heights was mostly ignored. In the current research, pool fires elevated at seven levels were experimentally studied in two reduced-scale model tunnels with both natural and longitudinal ventilation. Fuel burning rate, temperature profile, and thermal smoke stratification affected by the fire elevation were successively explored. Experimental results declared the major findings as 1) Fuel burning rate of the 8 cm × 8 cm pool fire was indistinctly affected by the fire elevation while it increased rapidly with the elevated height as pool size increased; 2) In the natural-ventilated tunnel, the maximum temperature significantly increased with the fire elevation and was found to be well predicted by the modified models with a linear correlation versus Q2/5Hd¯ or QD∗2/5·DfHd¯ and a constant value equal to 2.9. When longitudinal ventilation was activated, flame deflection, shortened back-layering length, and decreased maximum temperature were respectively observed. 3) Interface of the smoke-air layer was lifted, and thermal smoke stratification was promoted by the fire elevation. Good agreement was achieved between the present measurements and temperature quotient proposed by Newman. Correlation by using modified Froude number denoted different characteristics of smoke stratification of ground fire and elevated fire.

Experimental investigation on the smoke back-layering length in a branched tunnel fire considering different longitudinal ventilations and fire locations

Most previous works concentrated on the smoke back-layering length in traditional single tunnel fires, whereas very few studies have been performed related to the smoke back-layering length(L) in a branched tunnel fire. In the present paper, a series of fire tests were performed in a 1/10 scale branched tunnel to investigate L under various longitudinal ventilation velocities (v), heat source locations(d) (the distance between the heat source centerline and junction) and heat release rate (HRR). The smoke back-layering length and smoke temperature for various conditions are obtained and analyzed. The results are summarized as follows: With longitudinal ventilation velocity increasing to large enough, the smoke back-layering phenomenon will be observed and the maximum smoke temperature tends to deflect downstream. The larger the v and the farther the heat source is to the intersection, the smaller the L is. The dimensionless smoke back-layering length L/H is proportional to gHQ˙/ρ0cpT0v3A, which is consistent with Thomas's work. Whereas, due to that the smoke will split in intersection of branched tunnel, the proportional coefficient increases from 0.467 to 0.754, which is less than that in traditional single tunnels in previous work. Moreover, a revised relation is proposed to describe the smoke back-layering length in branched tunnel fires considering the longitudinal ventilation and fire location.

Influence of the competitive effect of V-shaped slope tunnel on smoke characteristics

Numerical simulations are conducted to study the influence of the heat release rate, the fire source location, and the tunnel slope on the smoke spread characteristics of V-shaped slope tunnels. Theoretical analysis and simulation results show that the smoke movement in the V-shaped slope tunnel is affected by the competition effect on both sides, which leads to the fact that the smoke only overflows from the tunnel portal closer to the fire source, and a large amount of air flows into the tunnel from the other portal. Due to the competition effect, the downstream smoke back-layering length first decreases and then increases slightly with the increasing distance between the fire source and the grade change point (refering to the intersection point of two adjacent slope lines). As the tunnel slope increases, both the back-layering length and the influence of the fire source location on the smoke spread decrease gradually. Besides, the heat release rate has little effect on the smoke movement characteristics. Based on the back-layering length and the mass flow rate distribution, the smoke movement behaviors in the V-shaped tunnel can be divided into the double-slope control zone, the transition zone, and the single-slope control zone. When the vertical distance between the fire source and the grade change point is more than 2.1 m, the smoke movement can be classified in the single-slope control zone. The mass flow rate reaches the peak value when the fire source is located in the transition zone, and the dimensionless mass flow rate is proportional to the tunnel slope and the 0.3 power of the dimensionless heat release rate.

Temperature distribution and characteristics induced by fire smoke in L-shaped utility tunnels with small curvature radii

A computational fluid dynamics model is established to investigate the behavior of fire smoke spreading in L-shaped utility tunnels with small curvature radii (R). The fire evolution process for tunnels with R = 0–25 m and critical ventilation velocities (V) of 0.5–3.0 m/s is analyzed, and the Richardson number (Ri) is introduced to determine the critical value for the development of fire smoke back-layering. The results show that R and V strongly influence the fire-induced spatiotemporal evolution characteristics in such utility tunnels. Smaller R or higher V are associated with stronger centrifugal forces, which causes the smoke to move downstream of the fire source and increases the instability and risk. The non-dimensional smoke back-layering length shows a linear relationship with ln(Q*/V*3), where Q* is the dimensionless fire source power. Integrated with the V and smoke force in a curved tunnel, the critical Ri value is found to be 6.34 for R = 10 m. This means that smoke back-layering will appear when Ri > 6.34 and disappear when Ri < 6.34. The present work positively provides a tool for the assessment of thermal environment of L-shaped utility tunnel's fire safety in engineering practice.

Experimental study on smoke movement characteristics in tunnel fires with different canyon cross wind yaw angles

High-speed canyon cross wind with random directions is common in mountain areas and induces complex three-dimensional flow fields inside the tunnel while flowing across the tunnel portal, which changes smoke temperature distribution and movement characteristics in tunnel fires. The effect of canyon cross wind yaw angle on maximum ceiling gas temperature, position of maximum ceiling gas temperature, smoke back-layering length and downstream smoke spread length of tunnel fires was investigated by experiments. Results showed that the maximum ceiling gas temperature increased with the increasing wind yaw angle in the range of 45°-90°, but remained steady when the angle was in 90°-135°. The maximum ceiling gas temperature was higher than other fire locations when the fire was close to the upstream portal because of the airflow barrier. Additionally, a normalized maximum ceiling gas temperature linear correlation was established based on dimensional analysis. The position of maximum ceiling gas temperature was different in both horizontal and longitudinal directions due to the canyon wind. The transverse smoke distribution in the tunnel was uneven, which resulted in a double peak value phenomenon of ceiling gas temperature. Besides, when the wind yaw angle was in 45°-90°, the smoke back-layering was resisted and the downstream smoke spread was promoted by the canyon wind. However, the smoke back-layering was promoted and the downstream smoke spread was resisted due to the negative pressure induction when the angle was in 90°-135°.

A numerical study on critical velocity and back-layering length with trains’ blockage in longitudinally ventilated tunnel fires

Longitudinal ventilation is commonly adopted to prevent toxic smoke from spreading in tunnel fires. The understanding of the parameters, such as the critical velocity and the smoke back-layering, should be of significant for the ventilation system design in tunnels. In this paper, numerical simulations are carried out to investigate the critical velocity and back-layering length in an arched tunnel with trains’ blockage. A total of 51 simulated cases with varied heat release rates and longitudinal ventilated velocities are considered. The result shows that the smoke layering can be divided into two parts according to the velocity direction of the smoke movement. Based on previous theories, a formula taking the effect of tunnel cross-section and blockage into account for predicting the critical velocity is proposed, which shows good consistency with these results. According to the distribution of longitudinal velocity, it is inferred that different HRR have little effect on the reduction rate of velocity. It is also observed that the variation of dimensionless back-layering length can be divided into three regions as the dimensionless control velocity varies. Finally, the correlation between dimensionless smoke back-layering length and dimensionless control velocity in longitudinally ventilated tunnels is also proposed.

ANALYSIS OF THE PARAMETERS OF THE FIRE MODELED IN A ROAD TUNNEL

The essence of the problem lies in the experimental study of the laws governing the changes in the aerodynamics of air flow and the most important ventilation parameters, taking into account the effect of fire. During the experiments, the aerodynamics of the tunnel is complicated by the presence of additional resistance, and the variables will be: the slope of the tunnel, the heat release rate, the cross section of the tunnel, the ratio of the width of the tunnel to the height, the fill factor of the tunnel by transport. The solution to the problem is an improved ventilation technology in case of fires to save lives. The aim of the research is to study the critical velocity, the backlayering length and the gradient-factor using numerical and physical models, as well as mathematical analysis for the downward movement of fresh air, when the fresh air inlet is above the fire level. The scale of the physical models is 1:40 and 1:60. Numerical models are full scale using of modern engineering technique Pyrosim and Fluent. The generalization of the results is carried out using piece-wise constant functions. The obtained results are also be compared with similar results known from scientific literature.

A SYSTEM OF TRANSFORMABLE CROSSPIECES TO BLOCK HARMFUL COMBUSTION PRODUCT PROPAGATION IN TUNNELS

Controlling the events and processes caused by fires is one of the key issues of all projects dedicated to the fire safety of tunnels. These processes are characterized by the dynamics of the propagation of high temperature, smoke and toxic combustion products around the seat of fire and in tunnels. With longitudinal ventilation, two main parameters are to be considered: the critical velocity and the backlayering length. An important impact on both parameters is exerted by the proposed system of flexible crosspieces, which, by increasing the aerodynamic resistance of a tunnel, makes it possible to reduce the speed of propagation of harmful factors of fire through the tunnel. Moreover, with certain limitations, the given crosspieces can be used to divide the tunnel into small sections what, among other things, will hinder the propagation of fire for a certain time. Thorough theoretical and experimental study of the mentioned transformable crosspieces, as well as the development of their various structures and operating principles is necessary to ensure the safety of traffic tunnels. The present article proposes a novel technology of light transformable crosspieces, which can be used in both, the existing road tunnels and the ones planned to design.

Experimental investigation of smoke temperature and movement characteristics in tunnel fires with canyon cross wind

A series of tests were conducted in a model-scale tunnel to investigate the influence of canyon cross wind on smoke temperature and movement characteristics of tunnel fires, including temperature distribution below ceiling, smoke back-layering length, and smoke stratification characteristics. Canyon cross wind speed, pool size, and fire location were considered. Experimental results show that the temperature along tunnel ceiling decays exponentially, but the variation laws of the upstream and downstream temperature decay coefficients are different with the increasing canyon cross wind speed. The dimensionless confinement velocity is defined as the ratio of the canyon cross wind speed to the critical velocity, and the relationship between the dimensionless smoke back-layering length and the dimensionless confinement velocity is obtained. The state of the smoke stratification is related to the canyon cross wind speed and the fire source location. When the fire source is subject to a relatively strong canyon cross wind or closer to the upstream tunnel portal, the smoke stratification will become more unstable. The results can provide engineering reference for smoke control and safe evacuation of mountain tunnels.

Numerical simulation Simultaneous use of longitudinal ventilation and smoke extraction from the ceiling in fires inside tunnels

Ventilation is essential to provide a smoke-free path for safe passenger evacuation and effective rescue services in case of a tunnel fire, because the closure tunnels increase consequences of accidents significantly. In the present study, the simultaneous use of longitudinal ventilation and smoke extraction from the ceiling in fires inside tunnels and physical phenomena has been investigated. Fire dynamics simulator will be used as a CFD tool. This simulation was performed to investigate the effect of the longitudinal distance of the smoke extraction system from the fire source on the smoke back-layering length and the maximum temperature in the two operating conditions used by this system downstream and upstream the fire source. In the present work, the smoke extraction system is located on the ceiling of the tunnel. The results show that using a smoke extraction system upstream the fire source will increase the maximum temperature, but using the same system downstream will reduce the temperature throughout the tunnel and prevent smoke back-layering. However, attention to the smoke extraction velocity for prevent the plug-holding phenomenon. The results also show that the simultaneous use of two smoke extraction systems at the upstream or downstream of the fire source will have a better result and The maximum temperature is reduced by 10%.