Scale Model Tunnel Research Articles

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.

Experimental study on mechanical behavior and countermeasures of mountain tunnels under strike-slip fault movement

In the seismic mountainous regions such as western China, it is usuallly inevitable to construct tunnels near active fault zones. Those fault-crossing tunnel structures can be extremely vulnerable during earthquakes. Extensive experimental studies have been conducted on the response of continuous mountain tunnels under reverse and normal fault movements, limited experimental investigations are available in the literatures on mountain tunnels with special structural measures crossing strike-slip faults. In this study, a new experimental facility for simulating the movement of strike-slip fault was developed, accounting for the spatial deformation characteristics of large active fault zones. Two groups of sandbox experiment were performed on the scaled tunnel models to investigate the evolution of ground deformation and surface rupture subjected to strike-slip fault motion and its impact on a water conveyance tunnel. The nonlinear response and damage mechanism of continuous tunnels and tunnels incorporated with specially designed articulated system were examined. The test results show that most of slip between stationary block and moving block occurred within the fault core, and significant surface ruptures are observed along the fault strike direction at the fault damage zone. The continuous tunnel undergoes significant shrinkage deformation and diagonal-shear failure near the slip surface and resulted in localized collapse of tunnel lining. The segments of articulated system tunnel suffer a significant horizontal deflection of about 5°, which results in opening and misalignment at the flexible joint. The width of the damaged zone of the articulated system tunnel is about 0.44 to 0.57 times that of the continuous tunnel. Compared to continuous tunnels, the articulated design significantly reduces the axial strain response of the tunnel lining, but increases the circumferential tensile strain at the tunnel crown and invert. It is concluded that articulated design provides an effective measure to reduce the extent of damage in mountain tunnel.

Numerical simulation of the impact of rainfall on tunnel fire

An improved understanding of tunnel fire dynamics is crucial for fire and life safety. This work highlights the significance of Computational Fluid Dynamics (CFD) techniques in addressing the interaction between tunnel fires and rainfall. The discrete phase model based on the Lagrangian approach is applied to simulate raindrops, while the species transport model is used to simulate fuel combustion. A numerical model is established to investigate the impact of rainfall on tunnel fires, and the correctness of the model is verified by comparing the results to model-scale tunnel experiments. Results show that the raindrops increase the local pressure in the rainfall area, creating a pressure difference between the rainfall tunnel portal and the no-rain portal, which leads to longitudinal airflow inside the tunnel. This rain-induced airflow prevents smoke from moving toward the rainfall tunnel portal and decreases the smoke height near the no-rainfall portal. Correlations between the local increased pressure, induced-airflow velocity, and rainfall parameters are proposed. Besides, the model is scaled up to full-size, and real-scale tunnel fires under the influence of rainfall are evaluated. Findings draw attention to tunnel fire dynamics under extreme weather conditions for improved fire safety and evacuation strategies.

The effect of soil conditions on the dynamic response of shield tunnels under train‐induced vibration loads

In this article, the influence of soil condition on the dynamic response of a tunnel and the surrounding soil was studied by both experimental model tests and numerical simulations. We tested a 1/20‐scale tunnel model with three different soil conditions: upper soft soil and lower hard soil, homogeneous soft soil and homogeneous hard soil. We also applied dynamic loads, sweep loads and train loads on the model tunnel for time domain and frequency domain analysis. The experimental and numerical results revealed that the interface between the soft and hard soil strata has an obvious amplification effect on the vibration wave. With the propagation of the vibration wave to the surface, the damping effect of the soil above the tunnel becomes the main factor affecting the dynamic response of soil. The internal force response of the tunnel structure is for the most part concentrated in the section under the excitation load, which is mainly affected by the soil properties beneath the tunnel.

Centrifuge modelling of the dynamic response of twin tunnels under train-induced vibration load

The impact of train-induced vibrations originating from underground tunnels has emerged as a significant environmental issue within urban areas. A multitude of studies, encompassing both numerical analyses and experimental investigations, have been undertaken to explore and address this concern. In the experimental tests, the dynamic characteristics of tunnels and soil has generally been studied using physical models with a single tunnel under a 1 g (gravity) environment. However, twin tunnels are commonly constructed in the field, and the stress of the soil is unrealistically simulated in 1 g models. In this paper, the dynamic characteristics of twin tunnels were studied by centrifuge testing. In this paper, results are presented from scaled model tunnel tests conducted at 50 g for four different conditions: a single tunnel, vertical twin tunnels with 60 mm spacing, horizontal twin tunnels with 60 mm and 120 mm spacing. Train load and sweep load were applied by a parallel pre-stressed actuator at the tunnel invert. Accelerometers were employed to gauge the dynamic response of both the tunnels and the surrounding soil. The experimental results illustrate that the interaction between twin tunnels has a clear amplification effect on the dynamic response of the loading tunnel, with the maximum peak particle acceleration (PPA) increasing by up to 43 %. This amplification phenomenon is reduced as the space between the twin tunnels is increased. The experimental results also indicate that the interaction between twin tunnels has a comparatively lower impact on the dynamic response of the surrounding soil when contrasted with the dynamic response of the tunnels themselves.

Experimental study and newly proposed correlations for maximum ceiling gas temperature rise in tunnel fires with natural ventilation

As a critical factor in tunnel fire safety, previous research on model predictions of the maximum ceiling gas temperature rise under natural ventilation has not demonstrated consistent results. These discrepancies are commonly attributed to variations in tunnel dimensions and materials. To address this issue, an experimental investigation utilizing heptane pool fires with different dimensions was conducted in a model-scale tunnel. The results show that the burning rate increases with an increasing aspect ratio of the fire source for the selected diameter range of 0.078–0.169 m. The flame length was characterized using a modified diameter of the fire source considering the air entrainment, and found that the non-dimensional flame length is 2.4 times the 2/5th power of the non-dimensional heat release rate. Finally, incorporating the concept of flame virtual origin, two different forms of correlations were developed to quantify the maximum ceiling gas temperature rise under different impingement conditions of the fire plume, and the coefficients were determined as 0.065 and 0.215 by numerous experimental results from different studies. The newly proposed models demonstrate good applicability to the fire sources with different dimensions, especially for the rectangular fire source with a large aspect ratio, thus solving the problem of insufficient accuracy of classical models under this condition.

Structural equation modeling of negative emotion and walking behavior by smoke-filled model-scale tunnel experiments

This study investigated the relationship between emotion and evacuation behavior in smoke for future modeling of irrational behavior by conducting experiments using a model-scale smoke-filled tunnel. Furthermore, the causal relationship between emotions and walking behavior was investigated through structural equation modeling. The results show that emotions influence slow walking behavior. The increase in heart rate due to the walking exercise was smaller than that caused by emotions. Moreover, the heart rate change degree with emotion corresponded to Proulx’s proposal order, from low-stress to anxiety.

Fire control and self-extinguishment by blocking smoke flow with water spray in a tunnel fire

This study clarified the effect of water spray on fire control and extinguishment in a tunnel fire. A series of fire experiments were conducted using a 1:10 scale model tunnel to investigate the performance of water spray for suppressing the fire and blocking the smoke produced by the fire. The water spray trapped the smoke in the fire zone and prevented the inflow of fresh air from the tunnel portal. That is, the water spray sealed off the smoke and compartmentalized it in the tunnel. A calorimetry method was developed to estimate the chemical heat release rate (HRR) in the tunnel with the sealed-off descended smoke. Within the test conditions considered in this study, the following results were obtained. First, self-extinguishment occurred in 86% of the cases tested in the fire zone sealed off by the water spray. Second, the maximum HRR of the vitiated fire induced by the water spray was reduced to one-fifth compared to that during free burning. Finally, a ghosting flame and self-extinguishment by suffocation occurred when the smoke blocking rate exceeded about 60% and 80%, respectively.

Study on the Effect of Blockage Ratio on Maximum Smoke Temperature Rise in the Underground Interconnected Tunnel

The model-scale tunnel is used in this investigation to analyze the maximum smoke temperature rise of the interconnected tunnel for various longitudinal ventilation velocities, blockage ratios, and heat release rates where the fire is at the confluence of the underground interconnected tunnel. The results showed that the longitudinal ventilation velocities of both the ramp upstream of the fire source and the adjacent ramp influenced the maximum temperature rise under the underground interconnected tunnel, and the ventilation of both ramps jointly affected the maximum temperature rise. The change in the maximum temperature rise depends on who is more affected by the longitudinal ventilation velocity or the vehicle blockage ratio. As the longitudinal ventilation velocity in the interconnected tunnel increases, the convective heat transfer near the fire source increases, resulting in a decrease in the maximum temperature rise, and the effect of the blockage ratio on the maximum temperature rise is reduced. In this paper, a maximum temperature rise prediction model suitable for the case of blockage in the interconnected tunnel is proposed.

Study on smoke blocking and thermal radiation attenuation by water curtain in tunnel fire

A 1:10 scale model tunnel with a length, height and width of 9 m, 0.6 m and 0.8 m, respectively, was set up in this paper. A water curtain system was installed in the model to investigate the effect of water curtain systems on smoke flow and heat propagation. A reduced-scale experimental and theoretical study was carried out by varying the heat release rate of the fire source, the water curtain pressure, and the number of water curtain rows. A series of tests were carried out for various setups to quantify each mechanism of interaction between the water mist and hot smoke, to propose a method for qualitatively analysing water curtain systems blocking the propagation of heat radiation and the flow of smoke from combustion, and to propose a method for predicting heat fluxes. The study found that the pressure of the water curtain, the number of rows, and the heat release rate of the fire source all had an effect on the smoke blocking effect of the water curtain system. This effect decreased as the heat release rate of the fire source increased and increased significantly with the pressure of the water curtain and the number of rows. The smoke blocking effect was quantified using conservation of momentum by establishing a dimensionless parameter R to represent the ratio of water curtain momentum to smoke momentum, as well as the ratio of heat flux before and after the water curtain to represent the smoke blocking capacity delta of the water curtain. The smoke blockage rate delta ranges between 40 and 75%, and the smoke blockage rate increases as the momentum R increases. Finally, in tunnel fires, a predictive model for the attenuation of heat radiation by water curtains has been developed, providing theoretical support for the quantitative study of the smoke and thermal blockage effects of water curtains, which is beneficial to the protection of human life in confined spaces.

An Improved Method for Determining The Interface Height of Smoke Layer in A Tunnel Fire

Smoke layer interface height is an important parameter in fire safety science. In this paper, two methods are proposed on the basis of integral ratio method and the least squares method. a series of experiments were conducted in a 1:10 scale model tunnel for determining the smoke layer interface height in tunnel fire. The interface height of the smoke layer determined by the new methods have been compared with other existing approaches, including integral ratio method, least squares method, N-percentage rule. The comparison results show that the results of the least squares method based on the transition layer can be relatively reasonable and agree well with the visual observations for all experimental condition. In addition, the usage scenarios and considerations of the N-percentage rule were also discussed.

The characteristics of ventilated pool fires of Daegok-Sosa subway lines using Model-Scale tunnels and stations

The characteristics of ventilated pool fires for Daegok-Sosa subway lines with a length of 15.12 km in Seoul, South Korea were studied. The model-scale tunnels and stations were built in scale 1:50 and temperature distribution, smoke (Carbon Monoxide (CO)) spreading, back-layering phenomenon and longitudinal ventilated flow velocity were systematically investigated. Present subscale model was based on the Froude number similitude and a heptane pool fire was used as the heat source. Heat release rate of pool fire in model-scale tunnel was 1.13 kW corresponding to an equivalent 20 MW in full scale. The occurrence of smoke back-layering phenomenon could be observed by whether the upstream temperature rose at a certain position during the whole process of the fire test. In the single line parallel tunnel, the smoke was restricted to downstream of fire source, however, in the double track tunnel the smoke back layering slightly occurred in upstream region of fire source and then it soon disappeared. The smoke propagated far downstream side and the tunnel safety measures should be taken for passenger’s safe evacuation from the CO toxic gas. The longitudinal air velocities in upstream side near the fire source were accelerated and were about 11% higher than the downstream velocities in single line parallel tunnel and more than 20% higher in double track tunnel caused by the blocking effect of fire source. A generalized quasi one-dimensional flow network solver was employed to calculate the longitudinal air velocity in tunnels using the finite volume Ghost Junction Method (GJM) which were compared with the present experimental data. The required minimum longitudinal air velocities (2.0 m/sec for single line parallel tunnel and 2.5 m/sec for double track tunnel) specified in the Korean railway facility technical standards were well satisfied in this experiment.

Large eddy simulation of smoke blocking by water sprays in a tunnel fire

This study presents a large eddy simulation (LES) of smoke blocking by water sprays in tunnel fires to investigate the effect of water sprays on the characteristics of the smoke flow. This study uses an LES solver – FireFOAM. In addition, fire experiments using a 1/10 scale model tunnel with two water spray nozzles are conducted to validate the LES results. The length of the model tunnel is 12 m, the height and width are 0.5 m, respectively, and the heat release rate of the fire is 7.38 kW (2.33 MW in full scale). Each spray nozzle operates at a pressure of 0.3 MPa, and the volume median diameter of spray droplets is 226 μm. First, a grid sensitivity analysis on the gas temperatures under free-burn conditions is performed. A 1.25 cm uniform mesh is chosen in the tunnel domain for all subsequent simulations. Then, FireFOAM predictions are validated against experimental data on the distribution of water spray mass flux under conditions without the fire and the gas temperatures measured under conditions with and without the water sprays. The water spray mass flux and gas temperatures predicted by FireFOAM show good agreement with the measurements. Next, the characteristics of the smoke flow are investigated in the presence of the water sprays. It is found that the water sprays block part of the smoke flowing under the ceiling and force it to descend to the ground. The descended smoke flows back in the lower part of the fire zone, and a vitiated air layer is formed. The vitiated air layer is composed of the smoke flowing back and fresh air drawn from the portal. On the other hand, some smoke leaks through the water spray zone and flows along the ceiling on the portal side. It is also found that the spray cooling delays the propagation of the leaked smoke toward the tunnel portal. Furthermore, it is quantitatively shown that the buoyant flow stratification destroyed by the water sprays becomes thermally and chemically stabilized again on the portal side, for the specific case under this study. Last, the smoke blocking performance is quantified based on the mass flow rates of the smoke flowing under the ceiling in the fire zone and the smoke flowing back toward the fire. A comparison of the smoke blocking rates between the predictions (time dependent: 40–50%) and measurements (steady: 39.4%) shows good agreement.

Performance validation of the heat extraction performance by the natural ventilation with a board-coupled shaft in a deep buried tunnel

Experiments were conducted in a 1/20 scale model tunnel, a higher shaft (5 times the tunnel height) was applied to model the shaft in a deep buried tunnel. Researches have validated that the board-coupled shaft can efficiently eliminate the plug-holing phenomenon and improve the smoke and heat extraction performance when the board-coupled shaft is suitably applied in a shallow buried tunnel. The purpose of this paper is to explore whether the board-coupled shaft (BCS) is effective and has stable heat extraction performance in deep buried tunnels. The influence of heat release rate (HRR) and board location on the heat extraction performance was investigated. The experimental results show that the BCS is effective and has stable heat extraction performance for the deep buried tunnels applied in this study. Also, an analytical solution for predicting the dimensionless smoke layer thickness in the one-dimensional flow region was developed, and it can be found that the dimensionless smoke layer thickness in the one-dimensional flow region depends only on the longitudinal position and doesn’t rely on the HRR. Furthermore, based on the general condition that the first shaft cannot extract the entirety of the smoke, two empirical correlations were derived for predicting the dimensionless temperature difference (ΔTs∗) and the dimensionless heat extraction rate (qs∗) in the BCS under different HRRs and board locations. These studies may contribute positive significance for promoting the application of natural ventilation with vertical shafts in deep buried tunnels.

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 studies on the smoke extraction performance by different types of ventilation shafts in extra-long road tunnel fires

Ventilation shafts are widely applied in extra-long road tunnels. Experiments were conducted in a 1/20 scale model tunnel to investigate the smoke extraction performance with a ventilation shaft. Four types of shafts were compared under the normal ventilation mode and the smoke extraction mode. The influence of shaft types, the operation frequency of the jet fan and board locations were investigated. Results indicate that the smoke extraction capacity of the mechanical ventilation shaft (MTS) is almost the same as the operation frequency increases, but the smoke extraction efficiency declines as the operation frequency increases. Besides, it can be found that the smoke extraction capacity and the smoke extraction efficiency of the mechanical board-coupled shaft (MBCS) decrease as the operation frequency increases. Moreover, it was found that the smoke extraction performance of the MBCS is highly affected by the distance between the board and the shaft. And the optimal board location for the MBCS changes with the operation frequency. Furthermore, the coupling-effect of the momentum induced by the mechanical ventilation and the buoyancy induced by the hot smoke in the ventilation shaft during a fire was explored. And the dimensionless smoke extraction Froude number (Frs), was developed for characterizing the buoyancy-momentum transition during the smoke extraction process with the mechanical ventilation shaft. These studies may contribute positive significance in the smoke control and extraction design with extra-long road tunnels.

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.

Temperature distribution in a blocked tunnel with one closed portal under natural ventilation

A new tunnel with closed portal has been constructed in the real project, most of current research on fire smoke movement are focus on traditional tunnel with two opening portals. In this paper, 44 numerical simulation tests were conducted in a 1:10 small scale model tunnel with different sealed situations, blockages and cross-sections. More specifically, two blockage ratios, width-blockage ratio β1 and height-blockage ratio β2 were introduced. Moreover, the maximum temperature under tunnel ceiling, as well as temperature distribution in downstream direction along the tunnel centerline were especially investigated. The results indicated that: 1) The smoke accumulated and temperature increased in the tunnel caused by the closed portal. 2) The stability of smoke layer in the tunnel destroyed and the height of smoke layer interface reduced by the blockage of metro train. 3) As the width-blockage ratio β1 decreased, the vertical temperature and temperature in downstream direction decreased. Meanwhile, as the height-blockage ratio β2 decreased, the vertical temperature decreased but temperature in downstream direction increased. 4) The maximum temperature beneath tunnel ceiling was positively related to the height-blockage ratio β2, but negatively related to the width-blockage ratio β1. Based on theoretical analysis and simulation results, a revised prediction model for the maximum temperature beneath tunnel ceiling was proposed, which taking into account the blockage, tunnel cross-section and heat release rate.

The maximum gas temperature rises beneath the ceiling in a longitudinal ventilated tunnel fire

This paper studies the maximum gas temperature rises beneath the ceiling in tunnel fires. The fire scenarios with higher longitudinal ventilation velocities are of key concern. This work was done in a 1/10 model-scale tunnel and propane gas burner was used as fire source. The variables include longitudinal ventilation velocity and heat release rate. In this study, the dimensionless longitudinal ventilation velocity varies from 0 to 0.84 and the dimensionless heat release rate is less than 0.15 in all tests, corresponding to a scenario of small fire. It is assumed that the maximum gas temperature rises in the longitudinal ventilated tunnel fires almost equals that in tunnels without longitudinal ventilation by use of the length of inclined path of plume as the effective tunnel height. By analyzing the length of inclined path of smoke plume that correlates the plume inclination angle, a piecewise model for predicting maximum gas temperature rises was developed, using the dimensionless longitudinal ventilation velocity of 0.42 as the dividing point. The comparisons between the calculations from this new model and the experimental results in this study as well as some data collected from other tests show a good agreement.

Experimental studies on the smoke extraction and smoke control performance by the ventilation shaft in extra-long road tunnels

Ventilation shaft is widely employed in the extra-long road tunnels in China. For deeper understandings of the smoke extraction and smoke control characteristics of the ventilation shaft, a total of 98 experiments were conducted in a 1/20 model scale tunnel. The effects of heat release rate, board location, and operation frequency were investigated. The results indicate that the mechanical board-coupled shaft (MBCS) can achieve better smoke extraction and smoke control performances than the traditional mechanical ventilation shaft (MTS). Though further research on the MBCS, it can be found that smoke extraction and smoke control performance of the MBCS are highly affected by the board location and operation frequency. Besides, as the power of the fire source increases, the smoke extraction performance of the MBCS increases, but the corresponding smoke control will be more difficult. Moreover, it was found that the optimal operation conditions for the MBCS to achieve the best smoke extraction performance and smoke control performance are different. Therefore, the uniform parameter for characterizing both the smoke extraction performance and the smoke control performance was established. And based on this parameter, the overall performance of the MBCS under different operation conditions can be compared, which may contribute positive significance in the smoke control and extraction design with extra-long road tunnels.