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

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Introduction
In recent times, the growth of road tunnel networks around the world has resulted in a rise in the possibility of tunnel fres, increasing the risk of death and destruction of assets and posing excessive hazards to human life. Te study of a fre in a tunnel is an important subject regarding the safety of the tunnel [1][2][3]; therefore, considering a development of the smoke temperature close to the ceiling is an important aspect that must be considered. Due to limited space and inadequate exits, fre normally occurs in tunnels when a signifcant amount of hot smoke is accumulated in the tunnel as a result of the fre [4,5]. Because of the high temperatures and the limited visibility, it can be a dangerous place for people. Tunnel disasters have caused difculties for emergency evacuation, and performant smoke control is an elemental need in a tunnel fre. Designing a practical smoke control program is essential to preclude possible fres in a tunnel. Ventilation systems and smoke control policies must guarantee that people in the tunnel can escape harmlessly under smoke peril [6][7][8]. Assorted smoke control systems are grown for smoke extraction when a fre disaster starts. Longitudinal extraction system and point-extraction system (PES) are the utmost frequently approved procedures in road tunnels [9]. For longitudinal extraction strategies, a jet fan on the top of the tunnel generates the longitudinal fow and is capable of managing the fow towards the exit.
Many preceding numerical and experimental investigations on the ventilation of the tunnel, in the case of fre incidence, have been carried out [10][11][12]. Song et al. [13] studied the various smoke extraction accomplishment in a tunnel with 100 MW heat release rate. Tey concluded that extracting smoke with a single point is more benefcial compared to devising several extracting points under celling. A computational fuid dynamics simulation means to explore that the optimum rate of exhaust fow was employed by Lin and Chuah [14], and Yoon et al. [15] studied numerically the practicability of the point-extraction system policies in a large cross-section tunnel. Te smoke back-layer length and its width in tunnel fres with the single-point and multipoint extractions were obtained by Zhu et al. [16]. Mei et al. [17] carried out several computer simulations; they analyzed the smoke temperature and the area of visibility to assess the efcacy of the point-extraction system (PES). Tey found that the evacuation can be slightly afected by the transfer location fre. In addition, a model was developed by Yan and Zhang [18] to quantify the smoke back-layering length in tunnel fres using longitudinal velocity and point-extraction system.
Studies by Wang et al. [19], Jiang et al. [20], and Tang et al. [21] have focused on the fow patterns, back-layering length of smoke, and temperature distribution in tunnel fres with various extraction systems. Te point-extraction system is considered a practical solution to manage tunnel fres, and research has been conducted on the extreme temperatures under diferent situations, including natural smoke exhaustion, inclined tunnels, blockage infuence, and various fre locations [22][23][24][25][26][27][28][29]. Recently, the use of accompanying ventilation schemes, combining longitudinal ventilation with a single-vent system, has become more common in road tunnels [30]. However, limited research has been conducted on the temperature distribution when a conventional ventilation system is supported by a vent system. Tis research aims to explore the smoke temperature, thermal back-layering length, and plug-holing phenomenon under the infuence of point-extraction system and longitudinal ventilation, using the Taguchi method to manipulate essential factors such as ceiling extraction velocity or longitudinal ventilation velocity.
Te primary innovation of this study is to reduce the highest temperature of the tunnel during a fre and improve the understanding of smoke performance to fnd an appropriate smoke control approach for safe evacuation under various fre situations. By combining longitudinal and pointextraction ventilated systems, this study aims to provide insight into the impact of diferent ventilation velocities and fre source-ceiling distances on the severity of tunnel fres. Te results of this study can be used by tunnel engineering managers to predict the highest temperature and plan emergency evacuation procedures accordingly. Ultimately, this research aims to contribute to the development of more efcient and efective smoke control strategies for tunnel fres.

Problem Statement and Physical Description
Tis paper analyses a numerical simulation based on Navier-Stokes equations achieved by using the Fire Dynamics Simulator (FDS 6.6). Tis software ofers the capability of solving a mathematical approximation of the Navier-Stokes equations numerically for a low Mach number, thermally driven fow with an emphasis on the transport of smoke and heat from fres. It is necessary to use the large eddy simulation (LES) method in order to predict the turbulence and buoyancy of a fuid fow. As a result of the use of this method, a number of studies have been conducted in the felds of fre safety and fre prevention. In order to visualize the dispersion of smoke particles in the tunnel fres as well as the velocity feld in the tunnel fres during construction, we used Smokeview and Tecplot.

Physical Description of the Tunnel Model.
Te numerical modeling is performed in a tunnel model with dimensions of 0.25 × 0.25 × 12 m. In the context of a tunnel fre, an intentional ignition source in the form of a burner is employed to initiate and sustain the fre within the tunnel. Te dimension of the squared burner is 0.1 m, acting as a fre, to release heat continuously, and the vent is placed 0, 1, and 3 m away from the upstream and downstream of the burner. Te size of the vent is 0.1 × 0.1 mounted at the central of the ceiling. Te burner is centrally positioned within the tunnel, and the length of backlayering is restrained through the synergistic interaction of the ventilation system and the upstream and downstream vent systems adjacent to the fre source. All simulations are conducted for 5 kW heat release rate with the four levels of vent vertical velocities: 0 m/s, 1 m/s, 2 m/s, and 3 m/s, and three levels of longitudinal ventilation velocities: 0.133 m/s, 0.265 m/ s, and 0.53 m/s. Te ambient temperature is considered 20°C.

Numerical Model
Te computing power maturity and the extension of numerical modeling have directed investigators to the CFD applied to a fre modeling. Te numerical model is constructed by Fire Dynamic Simulator. Te Fire Dynamics Simulator (FDS) has been advanced at NIST to explore fre behavior and to analyze the efciency of fre protection systems. Simulation of fredriven fow can be conducted in FDS by employing the LES turbulence model. FDS has been extensively utilized in the exploration of smoke behavior, and it is validated broadly. Te numerical solution of the governing equations, specifcally the Navier-Stokes equations and the energy equations, is performed within the Fire Dynamics Simulator software package, employing advanced numerical methods for accurate and efcient computations. It is worth mentioning that the FDS code numerically solves a special form of Navier-Stokes equations for low Mach fows. Current derivatives in the equations of conservation of mass, momentum, and energy are discretized using the fnite diference method with second-order accuracy and are solved explicitly in time. However, the phenomenon of radiation is calculated using the control volume method.
To have reasonable accuracy, mesh refning is performed. In the FDS code, the following equations will be solved numerically: Equation (2) is related to the mass constancy of the species. Fick's law states that in a two-component mixture, the mass fux of α type is obtained from the following equation: _ m α ″ � Y α _ m tot ″ − ρD αβ ∇Y α . Terefore, assuming that Fick's law is correct in multicomponent mixtures, the general form of the equation of the mass of the species will be equation (2). In this regard, term _ m ‴ α is the rate of production or consumption of species α.
By applying Newton's second law on a diferential element of fuid, the equation of conservation of momentum will be obtained as equation (3). In this equation, u is the velocity vector, f b is the external force, and τ ij is the stress tensor for Newtonian fuids.
Equations of energy conservation based on sensible enthalpy will be obtained by applying the frst law of thermodynamics to a diferential element of fuid in the form of equation (4), where h s represents the sensible enthalpy of the fuid, which is a function of temperature, and ε represents the decay term in the energy equation. _ q ‴ is the rate of heat released per unit volume by chemical reaction and _ q ″ shows the rate of heat transfer through radiation and conduction. Te FDS user guide proposes a nondimensional expression of D * � (Q/ρTC p � � g √ ) 0.4 for assessing a mesh resolution with D * . Te recommended value of D * /δx is in the range of 4-16. In this investigation, fre grid numbers are studied to confrm that the results are grid independent. In the context of FDS 6.6 fre numerical simulations in a tunnel, grid independence assumes paramount importance to ascertain the credibility and precision of the results. In this work, the investigation into the results' independence on the grid number substantiates that the mesh resolution does not signifcantly impinge upon the reliability of the outcomes. Figure 1 shows the meshing used in the FDS 6.7 code. Along with the numerical simulations described in the next section, other simulations are conducted with the precise target of verifying the model validity, by examining the agreement between experimental results and model predictions. Figure 2 illustrates the variation of critical velocity with heat release rate (HRR) and a remarkable agreement is observed, and the simulation is compared with the study by Li et al. [31] and Wu and Bakar [32]. Presented in this study are simulations of reduced-scale tunnel fre experiments, utilizing a scale of 1/10. It is worth noting that previous research conducted by Wu and Bakar has confrmed the reliability of reduced-scale simulations, making them a viable option for further experimentation [33]. As a result, this study employs the reduced scale for the remaining simulations, allowing for signifcant computational time savings while maintaining accuracy.

Results and Discussion
Once a fre occurs, an exhaust fan close to the fre source should be turned on to preserve the smoke at the top of the tunnel; then, the exhaust fan can extract the smoke. Simultaneous operation of the longitudinal ventilation and point-extraction system has apparent benefts in the case of trafc jams and fre situations; thus, activating both the systems should be considered. A fan, placed at the entrance of tunnel, is capable of generating velocity longitudinally, while exiting from opposite side. Te vertical extraction fow is generated with the aid of fan fxed on the ceiling. Adjustment to various speeds is possible to provide the necessary fow rate. In the numerical simulation, there are three diferent longitudinal ventilation velocities: 0.133, 0.265, and 0.53 m/s, and four ceiling extraction velocities: 0, 1, 2, and 3 m/s. A huge temperature can be a tremendous hazard to the people in an incidence of fre when the ventilation system is not designed correctly. All emergency evacuation plans  should secure the domain in the upstream area of the tunnel at a bearable temperature. To research the temperature distribution in the reduced-scale tunnel, the ceiling temperature is determined to preserve an acceptable condition in the tunnel area from the fre incident; it is vital to consider the required minimum longitudinal fow velocity, thereby preventing smoke back-layering. For the present work with HRR = 5 kW, the critical velocity is 0.53 m/s when ceiling extraction is 0 m/s. Figure 3 displays the smoke temperature distribution under the ceiling with varied ceiling smoke extraction velocities and various longitudinal velocities. From Figure 3, it is obvious that for E.V. = 0, the maximum smoke temperature is 215, 230, and 170 when longitudinal ventilation velocity varies from 0.133, 0.256, and 0.53 m/s, respectively, when the fre is on the foor. As fre occurs at the height of 0.125 m, the temperature increases rapidly. Figure 3 illustrates that the temperature of smoke near the tunnel ceiling predominantly increases with an increase in ceiling extraction velocity and subsequently reduces with the increment of mass fow rate. It is worth mentioning that the ceiling smoke extraction sucks in heat and smoke. Terefore, the stack efect occurs when the velocity of extraction is relatively low during the growth period. It can be seen that when the extraction point is placed 1 m upstream side of the fre source, the extraction velocity has diferent efects on the temperature distribution. For a critical longitudinal velocity (L.V. = 0.53 and E.V. = 0), the extraction has diverse efects, thereby being responsible for experiencing the higher temperature distribution. Moreover, the fre source-ceiling height has remarkable efects on a maximum temperature. Tese fndings emphasize the crucial role of fre source height in determining maximum temperature, which has signifcant implications for smoke layer thickness and plugholing occurrence. In light of these relationships, understanding the interplay between fre source height and these variables is critical for efective tunnel fre management and the development of future safety measures. Figure 4 displays the importance of longitudinal fow rate to control the thermal back-layering length. In the condition of longitudinal ventilation, the hot smoke generated by the fre is mainly blown downstream of the fre source; when the velocity of longitudinal ventilation is not enough to control the fow of the hot smoke, some smoke will spread upstream to form the smoke lack-layering. It is observed that increasing the longitudinal velocity results in a reduction of the thermal back-layering length. From Figure 4, it is clear that the longest thermal back-layering is corresponded to L.V. � 0.131 m/s. Vent system is regularly utilized to govern the smoke dispersion in a ventilation system in the tunnel, and the evacuation process will be afected by a thick smoke layer, and smoke control is challenging. Terefore, employing point-extraction system associates with the smoke layer thickness reduction. Considering Figures 4(a) and 4(b), it is observed that for critical velocity (L.V. � 0.53), the lowest smoke back-layering length occurs when there is no extraction point. Considering Figures 4(a) and 4(b), when L.V. � 0.53 m/s and E.V. � 3 m/s, it is observed that the point-extraction system does not operate properly and the extraction point drags the smoke towards itself, and its adverse impact is obvious. Figure 5 displays the vertical temperature 1 m away from the fre source. It may be perceived that the vertical temperature contour is strongly swayed by both the longitudinal ventilation and ceiling extraction. It is obvious that employing the ceiling extraction corresponds to a decrease in vertical temperature profles. It is worth mentioning as the longitudinal ventilation becomes stronger, the operating of ceiling extraction induces adverse efects. From Figure 5, it is obvious that occurring fre at the height of 0.125 m results in undergoing the higher temperature distribution for E.V. � 0, and combining longitudinal ventilation and pointextraction system associates with enduring lower temperature distribution compared with fre on the foor.
Observing the distribution of the smoke temperature through the tunnel for various extraction velocities is interesting. Figure 6 represents the efects of an extraction velocity on the temperature distribution in the tunnel for L.V. � 0.265 m/s. Te plug-holing phenomenon easily occurs when the ceiling mass fow rate increases; subsequently, the lower air layers, fresh air, are drawn into the vent. Apparently, point-extraction system performance will lessen meaningfully when the plug-holing takes place. From Figure 6, it is clear that an increase in ceiling extraction velocity is responsible for the occurrence of plugholing. From Figure 6(e), it is found that the plug-holing is responsible for the division of the back-layering length into two parts. Figure 7 illustrates the ceiling temperature distribution when the extraction point is positioned precisely on the fre source. It is evident that elevating the longitudinal ventilation velocity leads to a reduction in the efect of the exhaust vent. Comparison between Figures 3 and 7 reveals that placing the extraction point at 0 meters away from the fre source results in a lower temperature distribution for various extraction velocities. As a consequence, the extraction point at 1 meter away from the fre source does not signifcantly afect the ceiling temperature distribution compared to the ceiling-fre source distance of 0 meters.   Journal of Engineering Figure 8 presents the reversed smoke fow length at the upstream of the fre for fres on the foor. It is evident that the extraction velocity and longitudinal ventilation velocity have a noteworthy infuence on the back-layering length. As the ceiling extraction velocity reaches 3 meters per second, the back-layering length diminishes to 0 meters. For extraction velocity of 3 m/s and longitudinal velocity of 0.53 m/s, the plug-holing phenomenon occurs, causing a weakening in longitudinal velocity, and consequently, smoke moves towards the upstream side. Tus, the extraction has an unfavorable impact, indicating that the ceiling extraction vent on the fre source drags fresh air instead of smoke. Figure 9 describes the vertical temperature 0 m away from the fre source. Terefore, the ceiling extraction is not responsible for a decrease in vertical temperature profles. Te development of smoke and plug-holing occurrence in the tunnel are investigated when the longitudinal fre source-extraction distance is 0 m. Figure 10 presents the efects of extraction velocity on the temperature distribution in the tunnel for L.V. � 0.265 m/s. Due to the position of the extraction point, the plug-holing phenomenon does not occur. It is important to mention that abridging the distance between the fre source and vent (1 m to 0 m) benefts shortening the back-layering length.
In a fundamental sense, it is also meaningful to investigate the temperature distribution when the extraction point is settled at the downstream side of the fre source. Figure 11 displays the temperature variations through the tunnel when the extraction opening is placed downstream, and the distance of extraction point and fre source is 1 m. Te selection of plans of extraction point position will modify the performance of smoke extraction. When combined longitudinal ventilation and point-extraction system is activated, smoke accumulation will take place just in the vicinity of fre location. Employing the extraction point downstream of the fre is a typical policy to pull smoke of the space beneath the tunnel. It is observed that extraction velocity only infuences the ceiling temperature at the downstream side of the fre. Figure 12 displays the stretch of the inverted smoke fow upstream of the fre source, while the extraction point is placed at 1 m away of the fre, concluding that the extraction velocity has no efects on the back-layering length. Figure 13 describes the variation of ceiling temperature through the tunnel when the distance of extraction point and the fre source is 3 m and the extraction point is placed upstream side (Figure 13(a)) and downstream side (Figure 13(b)) of the fre source. From Figure 13(b), it is clear that extraction velocity does not have an infuence on the maximum temperature, but the downside of the fre source experiences lower temperature when the extraction velocity increases. As the distance of fre source and extraction point increases, the infuence of extraction on ceiling temperature distribution decreases. Figure 13(a) shows that placing the extraction point 3 m upstream side corresponds to drag the smoke to the upstream side, which has a catastrophic efect on passenger evacuation in the upstream side.
In order to gain a deeper understanding, it is benefcial to determine which factors play more important roles in controlling the smoke temperature. Te Taguchi method is recognized as one of the useful tools for engineers, who can determine the best choices of causes infuenced on a phenomenon. In the current work, a L 18 orthogonal array is applied, transforming the results into a signal-to-noise ratio (SNR). Signal and noise represent controllable and uncontrollable features in a physical phenomenon, respectively. Te frst column of Table 1 is dedicated to signals, where the noise is a maximum temperature in the tunnel. With respect to the Taguchi method, the optimal condition links to a situation where the noise factors indicate the least variation of the system performance. Indeed, the situation of the highest SNR introduces the best confguration of the system. Currently, the purpose of optimization is to minimize the maximum temperature of tunnel during a fre. Te maximum temperature is considered to be minimized (smaller-better) by the following equation [34,35]: Implementing the Taguchi analysis, Table 2 and Figure 14 show the response for signal-to-noise ratios (SNRs) in the situation assumed. Te factor with the higher diference between maximum and minimum values of SNR should attach more signifcance due to its dominant efect. It is obvious that the longitudinal velocity is of paramount which can decrease the maximum temperature; thus, Analysis of variance would be the statistical method employed to clarify data and assist in making the required decisions. In this study, the efectiveness of fre position, longitudinal ventilation system, and exhaust vent system on the extreme temperature in the tunnel can be measured by the exploitation of ANOVA. Te P values achieved from the ANOVA indicate that longitudinal ventilation velocity is a more efective parameter in minimizing temperature owing to its minimum P value (see Table 3).
Having implemented ANOVA analysis, a new model for predicting the maximum temperature is proposed. Considering Table 4, each parameter and its coefcient value represent their weight in our suggested model. Equation (6) is a useful tool for predicting the highest temperature for diferent scenarios, thereby promoting the deeper understanding of this complex problem.
In Figure 15(a), the highest temperature as a function of longitudinal ventilation velocity and ceiling extraction velocity is illustrated as a contour plot. It is obvious that without applying correct longitudinal ventilation velocity, using the point-extraction system cannot help decrease the highest temperature efectively. Considering Figure 15(b), moving the extraction point from the downstream side of the fre source to upstream should be accompanied by an increase in ceiling extraction velocity to guarantee that the lower temperatures can be experienced in the tunnel. Te strong dependence of temperature on the vertical position of fre is depicted in Figure 15(c); meanwhile, longitudinal ventilation velocity is still an applicable tool to decrease the temperature. Indeed, the higher longitudinal ventilation velocity is, the lower the temperature can exist in the tunnel, thereby diminishing the infuence of the extraction point position (Figure 15(d)).

Future Work
In the case of fre incidents, this study can further explore the use of combined ventilated systems to control temperature distribution and back-layering length, which are critical factors in ensuring the safety of tunnel occupants during fres. Meanwhile, crowd counting and localization can utilize computer vision to analyze crowd scenes in real-time, making it an essential tool for security and public safety personnel [36][37][38]. Both felds will be employed to ensure people safety to address complex problems in diferent settings.  Signal-to-noise: smaller is better

Conclusion
During the course of this investigation, we describe a method for determining the quantity and quality of the temperature distribution when a longitudinal ventilation system is combined with a point-extraction system. In terms of longitudinal ventilation speed, the values of 0.133, 0.265, and 0.53 m/s are set, while the vertical exhaust speed is set at 0, 1, 2, and 3 m/s when the value of heat release rate is 5 kW. A smoke back-layering is generated upstream of the fre source and is controlled by a combination of longitudinal ventilation and point-extraction systems when extraction settles at the upstream side of the fre. Since the fre smoke will be expelled by the vent and blown downstream by the longitudinal ventilation system, it is controlled by the mingled infuence of these systems. Te foremost outcomes are abridged as follows: (1) Te extraction point has diverse efects provided that the longitudinal ventilation velocity is set by critical velocity (2) Increment of ceiling extraction velocity and longitudinal ventilation velocity is associated with a reduction in smoke back-layering length (3) For a fre at a height of 0.125 m, the value of maximum ceiling temperature is 1.8, 1.3, and 1.1 times when the fre source happens on the foor with longitudinal velocities 0.133, 0.265, and 0.53 m/s, respectively (4) As the fre source-ceiling extraction distance decreases, the possibility of plug-holing decreases (5) Te ceiling extraction is not responsible for decreasing vertical temperature profles when the longitudinal fre source-ceiling extraction distance is 0 m By utilizing the Taguchi method, this innovative research aims to investigate the efects of point-extraction systems and longitudinal ventilation on smoke temperature, thermal back-layering length, and plugholing phenomenon. Te study seeks to determine the optimal combination of longitudinal velocity and pointextraction system for ensuring the safe evacuation of people. Furthermore, the proposed equation, which is generated through the Taguchi method, serves as a useful tool for predicting the highest temperature in various scenarios. Trough careful examination of essential factors, including ceiling extraction velocity or longitudinal ventilation velocity, the study endeavors to provide valuable insights into these variables and contribute to the current understanding of these phenomena.

Data Availability
Te data used in this study are available on request from the author.

Conflicts of Interest
Te authors declare that they have no conficts of interest.