Tunnel Fire Experiments Research Articles

Repeatability Assessment in Tunnel Fire Experiments

Repeatability Assessment in Tunnel Fire Experiments

Optimised prediction of tunnel fire heat release rate using the ResNet18_2CLSTM model with bagging for multimodal data

Accurate predictions of HRR will improve preparedness and response strategies, enhance safety, and minimise damage in tunnel fires. In this study, a deep learning prediction model for HRR under multimodal data fusion is proposed. A multimodal dataset is first established to obtain flame images and flue gas time series data through model-scale tunnel fire experiments. During the model training process, ResNet18 was used to extract features from the flame image, and 2CLSTM was employed to understand the time series of the flame image features and flue gas features to establish the correlation with the HRR. It was evaluated that the error analyses of the measured and predicted values of the validation set yielded R2 greater than 0.85, with errors and standard deviations less than 4 kW. And the model predicted better in the flame growth and decay phases. However, there is some deviation in the predictions near the peak HRR. To address this issue, the Bagging algorithm was introduced to optimise the model. The results show that the ResNet18_2CLSTM model with Bagging reduces the RMSE by 20.47 % and increases the R2 by 4.64 % compared to the original model, and the accuracy is greatly improved.

Study on the Configuration and Fire-Resistant Property of Cable Tunnel Fireproof Clapboard Based on Equivalent Fire Condition Testing

At present, the selection criteria and configuration methods for fireproof clapboards in cable tunnels are not yet perfect, making it difficult to achieve effective fire protection. Therefore, an equivalent fire condition testing method is proposed to analyze the fire-resistant property of fireproof clapboards of different materials. Firstly, a tunnel fire experiment platform was built to carry out the combustion experiment of the high-voltage cable intermediate joint. The cable combustion equivalent fire source device is developed based on the temperature rise characteristics under different combustion conditions. However, the temperature rise characteristics of the equivalent fire source and the actual cable combustion error are within 10%. Then, four typical fireproof clapboards were tested under equivalent fire sources. The results indicate that the organic molded board has the best performance. In addition, factors such as the thickness, side panel height, and installation method of the fireproof clapboards were tested and analyzed. The results indicate that a minimum thickness of 5 mm for the fireproof clapboard and a height of 200 mm for the side panel of the clapboard are necessary to ensure effective protection. The installation method of hoisting fireproof clapboards can effectively extend the protection time by about 30% compared to the flat method.

Modelling the maximum ceiling temperature with bifurcated plume in tunnel fires

Evaluation of the temperature profile induced by tunnel fire is of great significance for fire detection and structural damage assessment. In this work, a special plume model, namely, the “plume bifurcation” is introduced, and its impact on the temperature distribution is analysed theoretically. Full-scale tunnel fire experiments and numerical simulations are conducted to investigate the plume evolution under various heat release rates and ventilation conditions. Results show that the plume will sperate into two sub-streams under strong ventilation condition, resulting in the multi-dimensional plume movement pattern when rising. The sub-plume impingement point location, quantified by the longitudinal vertical deflection angle θv and horizontal bifurcation angle θh, is correlated with the fire heat release rate and ventilation condition. The plume bifurcation angle is 0 when the dimensionless ventilation velocity V′≤0.33, marking a single-stream plume; and rapidly increase and then slowly decrease with V′ when V′>0.33, representing the sudden occurrence and decay trend of plume bifurcation with the ventilation velocity. Finally, a novel bifurcated plume-based model to estimate the maximum ceiling temperature is proposed. This study reveals the role of multi-dimensional smoke movement in reshaping the temperature field, providing new perspectives for a deeper understanding of the smoke transport behaviour in longitudinally ventilated tunnel fires.

Particle swarm optimization inverse tracing method for mine tunnel fire based on quasi-steady-state heat transfer mechanism

Rapidly locating and assessing fires in coal mines is essential for effective response and minimizing damage. Traditional methods often struggle to swiftly and accurately pinpoint fire locations and statuses. Hence, this study introduces a particle swarm inverse tracing approach based on the quasi-steady-state heat transfer between high-temperature flue gases and mine tunnel walls. This method aims to remotely monitor smoke flow temperatures and gas concentrations to determine fire locations and combustion trends. Validation of the fire source tracing model was conducted through comprehensive tunnel fire experiments and numerical simulations. The particle swarm optimization inverse tracing method yielded a maximum 7.65 % error in fire location determination compared to actual measurements, while accurately matching the measured fire source heat release rate curve. These findings underscore the method’s high precision and reliability, particularly evident during the developmental and stable stages of fire burning.

Burning characteristic and ceiling temperature of moving fires in a tunnel: A comparative study

Moving fire is a very important fire scenario in highway tunnel and subway tunnel. Compared to fixed fire sources, there is still limited research on moving fires in tunnels. In order to reveal the burning characteristics and evolutionary mechanisms of moving fires in tunnels, this study conducted reduced scale tunnel fire experiments on moving fires under natural ventilation conditions (Movefire) at travelling speeds ranging from 0.5 m/s to 2.0 m/s, as well as fixed fires under longitudinal ventilation (Ventfire) velocities ranging from 0.5 m/s to 2.0 m/s. The results demonstrate that it is unreasonable to solely use Ventfire experiments as a substitute for Movefire experiments to study fire development and ceiling smoke temperature distributions. When the travelling speed of fire and the ventilation velocity were equal, the HRRs of Movefires were less than those of Ventfires. At the same relative moving velocity of 0.5 m/s, Ventfire received more fresh air to increase burning intensity, while Movefire experienced decreases in thermal feedback and burning intensity. The Movefire smoke temperature rise was small and short in duration, while the Ventfire smoke temperature rise was large and long in duration. Furthermore, the dimensionless maximum ceiling temperature rise of the Movefire experiment was lower than that of the Ventfire experiment. And then a modified ceiling temperature model applicable to the Movefire experiment was proposed by introducing a speed correction factor.

Studying the Synergetic Effect of Point-Extraction and Longitudinal Ventilation on the Maximum Smoke Temperature and Back-Layering Length in Tunnel Fires

This study investigates the impact of combining longitudinal and point-extraction ventilated systems on temperature distribution and back-layering length in tunnel fires. Numerical simulations are conducted using a fire dynamic simulator (FDS), and reduced-scale tunnel fire experiments with a scale of 1/10 are introduced to provide supplementary data. Results indicate that the longitudinal velocity is more critical than other factors in reducing the highest temperature and casualties. Lowering the temperature below the tunnel ceiling is not caused by increasing the ceiling extraction velocity. Additionally, the study reveals that the fire source-ceiling distance and their relative positions play a crucial role in temperature distribution and plug-holing phenomenon in the tunnel. By using the Taguchi method, it is determined that a fire at a height of 0.125 m has a maximum ceiling temperature of 1.8, 1.3, and 1.1 times when the fire source happens on the floor with longitudinal velocities of 0.133, 0.265, and 0.53 m/s, respectively. The extraction point has diverse effects provided that the longitudinal ventilation velocity is set by critical velocity. The study’s objective is to provide tunnel engineering managers with a correlation to predict the highest temperature, which is a vital parameter for emergency evacuation. In conclusion, this study highlights the importance of considering longitudinal and point-extraction ventilated systems and their relative speeds in reducing the severity of tunnel fires.

Experimental field study on smoke characteristics and CO concentration of high-altitude tunnel fires induced using gasoline and diesel

High-altitude environments exhibit reduced atmospheric pressure, low oxygen density, and low air temperature conditions, which considerably affect tunnel fire characteristics. In this study, small-scale tunnel fire field experiments were performed at high altitudes to investigate the heat release rate (HRR), smoke temperature, and CO concentration of gasoline and diesel pool fires. The results revealed that with the increase in altitude, the HRR declined but the combustion duration increased. Furthermore, the maximum ceiling temperature decreased with the increase in altitude. The longitudinal ceiling temperature declined slowly in the high-altitude tunnel. By contrast, the smoke temperature spread rapidly among the cross-section in the high-altitude tunnel. The ceiling CO concentration behaviors for gasoline and diesel pool fires were categorized into four and three stages, respectively. Furthermore, the steady ceiling CO concentration and the maximum ceiling CO concentration decreased slightly with the increase in the altitude for gasoline pool fires, whereas the differences were greater for diesel pool fires. However, the steady ceiling CO concentration per unit mass burning rate increased linearly with the altitude for both fuels. The results revealed that the steady CO concentration at the height of 110 mm in the cross-section was nearly identical for various altitudes in the case of gasoline pool fires. However, for diesel pool fires, the CO concentration was higher at higher altitudes.

A dimensionless model and ant colony optimization fusion temperature prediction in tunnel fires

Prior studies have noted the importance of ceiling temperature distribution prediction of tunnel fires. To overcome the limitations of precise modeling difficulties for model-driven methods and biased toward the convergence for the data-driven methods due to loss of physical interpretations, a hybrid model-driven and data-driven fusion algorithm is established to predict ceiling temperature distribution in tunnel fires based on dimensional analysis method and ant colony optimization algorithm. Thus, full-scale tunnel fire experiments with different ventilation speeds are conducted to support the algorithm. In contrast to the experimental data as well as the known theoretical model, data-driven back propagation neural network algorithm and hybrid ant colony optimization and back propagation neural network algorithm, the effectiveness and ability of the algorithm are verified, which can be used for the tunnel fire prediction.

The use of multi-zone modelling for tunnel fires

This paper introduces the use of a multi-zone modelling approach for analysing smoke spread in tunnels. The approach suggested in this paper is based on an existing model, i.e., the Multi-Zone Fire model developed for large spaces. The Multi-Zone Fire model has been adapted and modified for tunnel fire scenarios by including features that consider longitudinal ventilation flow, tunnel gradient and tunnel section representations. An evaluation of the model has been conducted through benchmarking against experimental data from the BeNeLux tunnel experiments and the Runehamar tunnel fire experiments. The results from the Multi-Zone Fire model results were also compared against results from the Fire Dynamics Simulator. The results of the benchmarking exercise indicate that the multi-zone approach can be a time-efficient and useful tool for studying tunnel fire dynamics. The Multi-Zone Fire model performs well 50–200 m from the fire for heat release rates of 5–20 MW and moderate longitudinal ventilation flows. The model results are more conservative for the studied scenario with a higher heat release rate.

BP neural network-based adaptive spatial-temporal data generation technology for predicting ceiling temperature in tunnel fire and full-scale experimental verification

This study proposes a novel adaptive spatial-temporal data generation method to achieve precise ceiling temperature perception in tunnel fires based on the modified BP neural network. In the method, an automatic update of training data set procedure is coupled in the BP neural network, which establishes an effective coarse-to-fine data generation method from coarse monitored data to fine ceiling temperature prediction in spatial-temporal scale. The method belongs to a kind of data-driven algorithms for ceiling temperature prediction in tunnel fires, which is not limited to the special fire scene. Additionally, full-scale fire experiment was conducted to verify the effectiveness of the method in China's largest tunnel fire experimental platform. Twenty-three thermocouples measured the ceiling temperature of the tunnel fire experiment. In contrast to the experimental results and the results obtained from the traditional BP neural network, the method can be used an effective numerical tool to predict the precise fine ceiling temperature spatial-temporal distribution, and the prediction precision is higher than the traditional BP neural network algorithm.

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

SummaryBased 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.

A generic flame shape model and analytical models for geometric view factor calculation on the fire exposure surface

Fires have attracted special attention in recent years due to catastrophic accidents. Flame radiation is the dominant heat transfer mode in fires and it plays an important role in the structure damage. Therefore, for accurate theoretical assessment, it is critical to calculate the distribution of radiant heat flux on the fire exposure surface, which is closely related to the geometric view factor. Based on the previous experiment observations, a ‘back-to-back conical frustum’ is introduced to describe the visible flame shape of gas fire in cross wind. The calculation of geometric view factor on the ground level, a typical fire exposure surface, is deduced in detail. The expressions of horizontal view factor, vertical view factor and maximum view factor and the applicable scopes are given. For the same gas flow rate with different cross air velocities, the visible flame envelopes are described by the proposed shape model and traditional tilted cylinder model. By comparing the view factor predictions from the two models, the accuracy and applicability of the proposed flame shape model are discussed. The proposed model is validated using measurements from a reduced-scale tunnel fire experiment and a large-scale jet fire experiment from literature. The predicted radiant heat flux using the proposed flame shape model is in good agreement with the measured value, with an approximate 20% discrepancy.

Determination of smoke layer thickness using vertical temperature distribution in tunnel fires under natural ventilation

A thick smoke layer propagating in a tunnel or corridor decreases the illuminance and visibility near the road or floor surfaces, leading to delays in finding exit and subsequent evacuation. Hence, smoke layer thickness is as important as the attenuation properties of temperature and velocity of a ceiling-jet in fire safety science. In this study, fire tests were conducted to select an appropriate method for determining the smoke layer interface height based on the temperature distribution inside the smoke layer. Two types of rectangular tunnels with different cross-sectional aspect ratios are employed for small-scale tunnel fire experiments. In this paper, well-established methods for determining the smoke layer thickness, such as the N-percentage rule, are reviewed and evaluated by comparing their results with visual observations of the smoke layer. Based on this comparison, the inflexion point method is proposed. The smoke layer thicknesses calculated from each method were compared with the visual observation results. Consequently, it was found that the buoyant frequency method is the most suitable for a tunnel having a vertically longer section shape and the inflexion point method for a tunnel having a transversely longer section shape. We also confirmed heat release rate have no significant effect on the thickness of the smoke layer originating from the weak plume in the quasi-steady state under natural ventilation. A new characteristic length for normalisation is discussed to derive a simple and easy-to-use ceiling-jet thickness formula in the tranquil flow region.

Numerical investigation of prediction performance of design fire curves for a tunnel fire

The prediction performance of design fire curves is numerically investigated for tunnel fire using the fire dynamics simulator (FDS). A large eddy simulation (LES) was adopted in the simulation of a previous 750 kW tunnel fire experiment. Based on the experimental heat release rate, t2-fire growth, quadratic and exponential design fire curves (DFCs) are mathematically constructed and adopted in the FDS simulation. The predictions of each DFCs are compared against the experimentally measured smoke temperature, smoke travel time, and carbon monoxide (CO) concentration. In addition, the prediction performance of the mixture fraction (MF) and mixing controlled fast chemistry (MCFC) combustion models, is compared. The simulation results of the MF and MCFC models are similar except for the CO concentration features. For the performance of the DFCs, t2-fire growth curve with the MF combustion model is the most effective combination, which demonstrated the most reasonable agreement with the experimental data.

Scaling nonreactive cross flow over a heated plate to simulate forest fires

The paper reports visualization of the flow of smoke over a flat surface inside of a low-speed wind tunnel. A heating plate flush mounted on the wind tunnel floor simulated a spreading line fire that produces uniform heat flux under constant wind speed condition. A paper-thin cloth was soaked with commercially available Vaseline and placed on top of the heating plate; when it is heated, it produced thick white smoke, ideal for flow visualization. Two sides and top of the wind tunnel were made of a transparent acrylic sheet that enabled LED and laser sheet light illumination of the flow. A still camera with a full-frame CMOS sensor was used to record time-series images of illuminated smoke flow patterns from different angles. From these images, the following four flow structures were identified: organized horizontal vortex flows, weak vortex flow interactions, strong vortex flow interactions (also described as the ‘transition regime’), and, turbulent flows. Previously developed scaling laws on forest fires were applied to find similarity in flow structures created by the current small-scale convective heat-transfer experiments and the USDA's mid-scale wind tunnel fire experiments.

Interpretation of flow fields induced by water spray systems in reduced-scale tunnel fire experiments by means of CFD simulations

Computational Fluid Dynamics (CFD) simulation results, obtained with FDS 6.0.1 (McGrattan et al., 2013), are presented of reduced-scale tunnel fire tests. In (Sun et al., 2016), an extensive data set has been discussed in terms of temperature measurements in a reduced-scale tunnel, involving longitudinal ventilation and a variety of water supply through nozzles, in the context of potential smoke blockage. In (Sun et al., 2016), 10 different scenarios have been discussed, for different numbers of nozzles and different nozzle configurations. Given the limitation of the experimental instrumentation, as a series of thermocouple trees, a full interpretation of the flow field was impossible. Nevertheless, a detailed characterization and interpretation of this turbulent flow field under different circumstances is essential in the discussion of potential smoke blockage. To that purpose, CFD can be a very useful tool. In this paper, as a first step, results are presented for 2 cases, without mechanical longitudinal ventilation, in order to illustrate the validity and potential of the CFD simulations, with and without the water system (4 nozzles) activated. The validity of the CFD results, using the default FDS settings for turbulence and combustion modeling, is illustrated first through comparison of the temperature profiles with the experimental data. A comprehensive sensitivity study on the computational mesh and model settings for the water sprays is included. Subsequently, the mean flow and temperature fields are analyzed, providing significant additional insight into the impact of the water system. The entrainment, induced by the water sprays, is illustrated. This causes downward motion of the smoke in the sprays. By bumping onto each other, the impinging flows onto the floor in their turn create an upward flow in between the water spray envelopes that impinges onto the ceiling in the absence of longitudinal ventilation. The global effect is smoke blockage by the water system.

Experimental study on smoke temperature distribution under different power conditions in utility tunnel

The longitudinal and cross-sectional temperature distributions are measured in reduced-scale tunnel fire experiments with different fire power conditions. Field tests are carried out in a utility tunnel with pool fires. The experiments studied the ceiling temperature along the tunnel centerline, the vertical temperature below the ceiling and the temperature in the direction of radian in the same section with the fire source. The results from the burning experiments are analyzed. The longitudinal temperature distribution along the tunnel can be fairly well fitted by exponential equations. And the thermal radiation of the smoke layer has a significant effect on the temperature below the ceiling. In addition, in the direction of tunnel radian, the temperature growth rate of the region above 45° are obviously faster and the temperature is higher. Therefore, to shorten the response time of the detector to the fire, it is desirable to place the heat and smoke detectors at the top of the utility tunnel.

Critical velocity and backlayering distance in tunnel fires with longitudinal ventilation taking thermal properties of wall materials into consideration

A set of tunnel fire experiments using a 1:20 scale model tunnel was conducted taking into account the scaling of heat conduction through the tunnel walls. The critical velocity and backlayering distance measured in the tunnel fire experiments were compared with other reference data to examine the effect of the thermal properties of the tunnel walls on these two parameters. In this paper, we present new correlations for estimating the critical velocity and backlayering distance, and discuss the difference between our correlations and the previous ones from the viewpoint of the thermal properties of the tunnel wall materials.

CFD Simulations of the Interaction of the Water Mist Zone and Tunnel Fire Smoke in Reduced-scale Experiments

Abstract Computational Fluid Dynamics (CFD) is a useful tool in tunnel fire safety, providing reliable data as a supplement and extension of experiments. In this work, simulations of reduced-scale tunnel fire experiments, concerning in a concrete tunnel model with a scale of 1/3, are presented. It mainly focuses on evaluating the quality of the CFD results. The results show that the simulations consist with the experiments with natural ventilation and longitudinal ventilation. Extension tests are also presented, considering the length and the working pressure of the water mist zone under longitudinal ventilation. It concludes that the downstream temperatures decrease with the length of water mist zone, and no pronouncedly change is obtained as water pressure increases under longitudinal ventilation.