Longitudinal Ventilation Velocity Research Articles

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

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

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.

Effect of Semi-Transverse Ventilation Velocity on Combustion Characteristics of Pool Fire Sources in a Scaled Tunnel

Compared to longitudinal ventilation, there are few studies on fire source development under semi-transverse ventilation. This work studied the influence of semi-transverse ventilation on the combustion characteristics of fire sources in a scaled tunnel. The burning rate and heat transfer feedback during pool fire combustion were revealed under different longitudinal and transverse ventilation velocities. The results showed that transverse ventilation had little influence on combustion characteristics, and the burning rate was more obviously affected by longitudinal ventilation. The heat convection feedback increased monotonically with the increase of the longitudinal ventilation, which led to the increase of the total heat feedback on the fuel. The heat radiation feedback changed little, and the heat conduction feedback decreased monotonically with the increase of the longitudinal ventilation velocity. By aid of a Fire Dynamics Simulator, it was found that the flame tilted downstream and was in the flow line of the lower cold air flow coming from upstream and the upper hot smoke flow outgoing in the downstream direction. The transverse ventilation of 2 m/s or lower hardly affected the combustion field of the fire source. Therefore, semi-transverse ventilation is preferable to longitudinal ventilation from the point of view of limiting fire expansion.

Study on the smoke evolution mechanism of a subway tunnel with a multi‐door carriage fire under longitudinal ventilation

AbstractThis paper has analyzed the longitudinal ventilation on the effect of the efficiency of the smoke evolution mechanism in a metro tunnel of multi‐window carriage fires. These were simulated by Large Eddy Simulation (LES) with Fire Dynamics Simulator (FDS). In the past, analyses of smoke temperature under the tunnel ceiling and smoke overflow characteristics have been conducted. However, longitudinal ventilation has a different impact on temperature than natural ventilation, especially in a subway tunnel with a multi‐door carriage fire. Consequently, several simulations were run in a subway tunnel (360‐m long, 6.0‐m wide, and 4.8‐m high). The longitudinal ventilation velocity is set by 0–10 m/s with the heat release rate of 1–10 MW. The results show that there is a linear relationship between the maximum temperature and the longitudinal ventilation velocity. An empirical model considering various longitudinal ventilation velocities was developed to predict the maximum smoke temperature underneath the subway tunnel ceiling. The effects of the longitudinal ventilation velocity, the heat release rate, and the distance of the fire source on the characteristics of longitudinal temperature distribution were analyzed. What's more, smoke overflow characteristics under different longitudinal ventilation velocities have been described. The findings and results can also provide a reference for the fire risk assessment of a metro tunnel of multi‐window carriage fires.

Experimental Investigation on Fire Smoke Temperature under Forced Ventilation Conditions in a Bifurcated Tunnel with Fires Situated in a Branch Tunnel

In this work, a number of experiments were conducted in a reduced scale bifurcation tunnel with a ratio of 1:10 to explore the influence of the position of longitudinal fires (placed in branch tunnel) on smoke temperature profile under forced ventilation. Three heat release rates, six ventilation velocities, and three fire locations were considered. The main findings are summarized below, as follows: The temperature of smoke downstream of the main tunnel decreases with the rate of ventilation and longitudinal fire location. In contrast, the smoke temperature downstream of the fire source inside the branch tunnel drops with the ventilation velocity; the maximum temperature of the flame under the ceiling of the tunnel rises with longitudinal fire location. The dimensionless longitudinal smoke temperatures downstream of the main tunnel decrease exponentially with longitudinal distance, and the same observation is found in the branch tunnel. The attenuation coefficient k in the main tunnel increases with longitudinal ventilation velocity according to a power law but does not change significantly with longitudinal fire locations. However, the exponential coefficient k′ in the branch tunnel decreases linearly with ventilation velocity, whereas it increases with longitudinal fire location inside the branch tunnel. Lastly, modified models are established for estimating the longitudinal profile of temperatures downstream of the main tunnel and branch tunnel, where the influence of the rate of ventilation and location of the fire are taken into account.

Experimental evaluation and real-time forecast of smoke propagation in tunnels under intervention of sub-critical longitudinal ventilation

Sub-critical longitudinal ventilation can restrict the back-layering length in the upstream region and maintain suitable smoke stratification in the downstream region of a tunnel fire. In this study, experiments using a 1/15 scale, 80-m long, 1.2-m wide, and 0.5-m high model tunnel were conducted to investigate the effects of sub-critical longitudinal ventilation velocity and heat release rate (HRR) on the key parameters describing smoke propagation. The smoke front temperature near the fire source decreased significantly as the longitudinal ventilation velocity increased. The longitudinal ventilation velocity and HRR were incorporated into a regression model describing the smoke front velocity, which was applied to the evaluated fire scenarios using an ensemble Kalman filter to account for intervention of sub-critical longitudinal ventilation. The smoke front temperatures at discrete locations were assimilated to estimate the heat exchange correction coefficient and air entrainment ratio of smoke front in real-time and dynamically correct the forecasted smoke front temperature and velocity accordingly. When sub-critical longitudinal ventilation intervened, the smoke front parameters could be forecasted with a maximum lead time of 60 s and an average error within 5%. Therefore, the proposed forecasting method can inform the selection of disposal measures in early tunnel fire emergencies.

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.

Brine-water experimental study on the propagation of stratified smoke flow in tunnel fires under subcritical longitudinal ventilation

In the early stage of tunnel fires, longitudinal ventilation velocity is often smaller than the critical velocity to preserve the smoke stratification and therefore facilitate the evacuation. A series of brine-water experiments are conducted to investigate the unsymmetrical propagation of the stratified smoke flow under subcritical ventilation. A light attenuation technique is used to measure the distribution of the reduced gravity in the tunnel. The ventilation velocity V and the source buoyancy flux per unit width B0 are two dominant parameters for the flow. The reduced gravities at both sides of the source are quantitatively compared, and then a model is proposed to estimate the reduced gravities under different B0 and V. In terms of the propagation velocity, two flow regimes are observed. When V/B01/3 is smaller than 0.56, both the propagation velocity of the downstream smoke front, ud, and that of the backlayering smoke front, ub, are independent of time; moreover, ud is smaller than the propagation velocity of the smoke following the downstream smoke front. However, when V/B01/3 is larger than 0.56, ud is still independent of time, but ub decreases with time; furthermore, ud is approximately equal to the propagation velocity of the smoke following the front. Prediction models regarding the propagation velocities at both sides of the fire are established. As the backlayering flow advances forward, its thickness approximately linearly decreases. The maximum thickness of the backlayering flow is correlated with V/B01/3. The study might be useful for determining the ventilation and evacuation strategies in tunnel fires.

Smoke movement and control in longitudinal ventilated tunnel fires with cross-passages

This paper studies the characteristics of smoke movement and control in longitudinal ventilated tunnel fires with cross-passages through numerical modeling. Previous reports mainly focused on fires in single-hole tunnel, however, tunnels with cross-passages have been widely applied in real life, the cross-passages between main tunnels provide additional path for air and fire smoke, and significantly affect smoke temperature and back-layering length, which has not been studied in the past. Hence, full-scale simulations were conducted considering different heat release rates, fire locations, longitudinal ventilation velocities, and cross-passage intervals in this study. It shows that the smoke temperature distribution with or without cross-passages is almost the same under natural ventilation condition, while the temperature gradually increases with the decrease of interval of cross-passages under longitudinal ventilation condition. Due to the more air leakage, the smoke back-layering length increases with decreasing interval of cross-passage. The air leakage mass through the cross-passage was then characterized by the interval of cross-passages and fire location. Considering fire position and interval of cross-passages, the actual ventilation velocity is used to establish a new smoke back-layering length model. A basic understanding was provided by new findings in this study for smoke movement and control in tunnel fires with cross-passages.

Numerical Study on Coupled Smoke Control Using Longitudinal Ventilation and Naturally Ventilated Shafts during Fires in a Road Tunnel

Longitudinal ventilation and smoke extraction by shaft are common smoke control methods in road tunnel fires. Tunnels often adopt one of these methods in practical engineering. However, it may have a better effect to adopt the method of mixing the two smoke exhaust methods together, which has not been revealed in the previous literature. Hence, the coupled effects of longitudinal ventilation and natural ventilation with shafts on the smoke control in tunnel fires were studied in this work. Numerical simulation was carried out considering different longitudinal ventilation velocities (0–4 m/s) and 4 kinds of typical shaft arrangements (shaft lengths range of 3–12 m, shaft intervals range of 27–60 m). The smoke spread length and smoke exhaust efficiency were analyzed systematically. Results show that (1) with the increase in longitudinal ventilation velocity, the total smoke spread length firstly decreases (V < 1 m/s) and then keeps almost constant (1 m/s < V < 2 m/s), finally increasing significantly (V > 2 m/s). (2) The length of the dangerous area (over 60 °C) at human height is basically 0 for all cases (except for Scenario 4 of shaft arrangement) when the longitudinal ventilation velocity is less than 2 m/s. (3) The CO smoke flow rate through the shaft is relatively high when the longitudinal ventilation velocity is within the range of 1–2 m/s for 4 kinds of shaft arrangement scenarios. Factors such as smoke spread and smoke exhausted through the shaft are comprehensively considered to judge smoke exhaust performance. The following conclusions can be drawn: when the ventilation velocity ranges from 1–2 m/s, it has a positive impact on the smoke control in tunnel fires with natural ventilation with shafts. When the ventilation velocity exceeds 2 m/s, the total smoke spread length and the length of the danger area increase, and the smoke stratification becomes worse, which brings inconvenience to rescue work. The results can provide reference for the design of fire protection in tunnels.

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.

Effects of ambient pressure on characteristics of smoke movement in tunnel fires

This paper systematically studies the effect of ambient pressure on smoke movement in tunnel fires by numerical investigations. Because of the correlation between ambient pressure and air density, the influence of pressure on smoke movement can be considered as the effect of ambient air density in this study. The results of numerical simulation show that under a certain fire HRR, the average smoke layer thickness downstream of fire increases with the increase of both ambient pressure and longitudinal ventilation velocity. Compared with ambient pressure, the ventilation velocity has a greater impact on smoke layer thickness. The concept of turning velocity VT was put forward for the first time when analyzing the effect of ambient pressure on smoke backlayering length. When V<VT, the smoke backlayering length will decrease with the decreasing pressure. When VT<V<VC, the smoke backlaying length will increase with the decreasing pressure. Such results can be attributed to the two-sides effect of ambient pressure on the thermal buoyancy of smoke plume at the smoke backlayering front. Furthermore, the critical velocity has a negative correlation with the ambient pressure. According to the results of dimensional analysis, the prediction models of the smoke backlayering length and critical velocity considering the effect of ambient pressure were established. These findings will provide some basic references for the design of ventilation system in high-altitude tunnel.

Safety Assessment of Hydrogen Jet Fire Scenarios within Semi-Confined Spaces

Hydrogen fuel cell vehicle (HFCV) technology poses great promise as an alternative to significantly reduce the environmental impact of the transport sector’s emissions. However, hydrogen fuel cell technology is relatively new, therefore, confirmation of the reliability and safety analysis is still required, particularly for fire scenarios within confined spaces such as tunnels. This study applied the computational fluid dynamics (CFD) simulations in conjunction with probabilistic calculation methods to determine the associated thermal risk of a hydrogen jet fire in a tunnel and its dependency on scenarios with different tunnel slopes, longitudinal and transverse ventilation velocities, and fire positions. A large-scale model of 102 m in which the effects of outlined parameter variations on the severity of the fire incident were analysed. It is found that both tunnel ventilation techniques and slope were critical for the effective ejection of accumulated heat. With ventilation playing a primary role in the ejection of heat and gas and slope ensuring the stability of the ejected heat, probabilities of thermal burns were found to be reduced by up to approximately 35% with a strong suggestion of critical combinations to further reduce the dangers of hydrogen tunnel fires.

Numerical study on plume bifurcation in longitudinally ventilated tunnel fires

The buoyant plume theory is one of the essential fundamental theories of tunnel fire dynamics. In this work, a special phenomenon, “plume bifurcation,” was studied numerically. The multi-dimensional characteristics of the buoyant plume under different ventilation conditions and aspect ratios are analyzed. The plume bifurcation is formed by the density-gradient counter-rotating vortex (CVP). Results show that the plume bifurcation only occurs when the longitudinal ventilation velocity reaches a certain value. The bifurcation angle increases rapidly at first and decreases slowly as the ventilation velocity increases. Meanwhile, the bifurcation angle increases with tunnel width when the tunnel aspect ratio is small; and is insensitive to the tunnel width under a large tunnel aspect ratio. A modified Richardson number Ri’ is proposed to determine the plume morphology. When Ri’ > 3.8, the plume is dominated by buoyancy and keeps single; when Ri’ < 2.7, the entrainment intensity is large enough to tear the plume into two sub-streams and results in plume bifurcation; and when 2.7 ≤ Ri’ ≤ 3.8, there is a transition region, where the plume has the trend to extend along the transverse direction, but no obvious bifurcation can be found. Finally, the relationship between the “plume bifurcation” and “smoke layer bifurcation” is analyzed and discussed. This research reveals the multi-dimensional plume characteristics and improves the classical plume theory under tunnel fire scenarios.

Application of the high-pressure water mist system in a railway tunnel rescue station

• Full-scale fire suppression experiments and numerical simulations in a rescue station were performed. • Suppression effect inside and outside train increased with increasing droplet size. • Convective cooling was the main controlling mechanism on suppressing a train fire. • Radiation heat flux received by the floor exponentially decayed with the distance from fire. • Longitudinal ventilation could effectively improve the suppression effect of water mist system. The fire safety of the rescue station as an important issue has not been well concerned. This study investigated the suppression performance of the high-pressure water mist system in a railway tunnel rescue station through CFD (Computational Fluid Dynamics) simulation and full-scale experiments. Eleven groups of numerical simulations and full-scale experiments were designed with various working pressures (5 and 8 MPa), mist droplet diameters (173.2, 178, and 192.9 μm), and longitudinal ventilation velocities (0, 1.5, and 2.5 m/s), in which the simulations were carried out by employing SIMTEC (Simulation of Thermal Engineering Complex) software. Results show that the temperature control effect both in the carriage and above the platform increases along with a bigger mist droplet diameter, in which convective cooling is found to be the main controlling mechanism. However, with increasing mist droplet diameter, smoke concentration increases, and oxygen concentration decreases. Moreover, the radiant heat flux received by the floor presents an increasing first and then decreasing tendency with the increase of distance away from the fire source (denoted by l ). Furthermore, in the region of l ≥ 2.5 m, radiant heat flux has an exponential attenuation with l . In addition, it is also found that the longitudinal ventilation system can effectively improve the suppression performance of the water mist system on a shielded fire. With increasing ventilation velocity, smoke temperature and radiant heat flux in the region of l ≥ 2.5 m decrease. In all cases, numerical results match well with experimental results, which validates the effectiveness of the CFD simulation. This study provides a significant reference for the fire safety design of the railway tunnel rescue station.

Reduced pressure effects on smoke temperature, CO concentration and smoke extraction in tunnel fires with longitudinal ventilation and vertical shaft

Nowadays, high-altitude tunnels at reduced pressure are emerging in the world's road network. A better understanding of the temperature and CO concentration distribution as well as the shaft extraction performance during tunnel fires with combined longitudinal ventilation and shaft exhaust under reduced ambient pressure is particularly crucial for the tunnel fire protection design. Series of tunnel fire simulations were carried out with ambient pressures of 50 kPa–101 kPa, fire heat release rates (HRRs) of 3 MW and 10 MW, and longitudinal ventilation velocities of 0 m/s to 2 m/s. The results show that the reduced ambient pressure enhances the difference in vertical and longitudinal distribution of CO concentration and smoke temperature, while the longitudinal ventilation gradually weakens this difference in longitudinal distribution. The natural exhaust shaft reduces the difference in vertical distribution but increases the difference in longitudinal distribution. As the reduction of ambient pressure, the smoke extraction efficiency of the shaft will increase slowly. Longitudinal ventilation can enhance the shaft's smoke extraction performance, but the general smoke extraction efficiency of the shaft deteriorates with the increasing longitudinal ventilation velocity. This study provides a fundamental reference for tunnel natural ventilation system under different ambient pressures, particularly in high-altitude tunnel fires.

Study on the variation rule of smoke back‐layering length of tunnel ceiling jet induced by strong fire plume

AbstractTunnel fire is a major research topic in tunnel safety. Because of the tunnel's narrow and enclosed structure, smoke movement plays an important role in the investigation of the tunnel fire. In this paper, the Froude similarity principle is used to study the variation rule of the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume with different heat release rates, longitudinal ventilation velocities, and effective heights of fire source analyzed. It is found that the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume increases with the increase of effective height of fire source and decreases with the increase of longitudinal ventilation velocity. When the heat release rate is relatively small (), the smoke back‐layering length of the tunnel ceiling jet induced by strong fire plume increases as the heat release rate increases. However, for the larger heat release rate (), the smoke back‐layering length of the tunnel ceiling jet induced by strong fire plume is mainly related to the longitudinal ventilation velocity. Based on Thomas's model and Li's model, by introducing the correction coefficient, correction formulas of the smoke back‐layering length of tunnel ceiling jet induced by strong fire plume changing with the effective height of fire source are proposed.

Effects of blockage on the flame morphologic characteristics in a ventilated tunnel

As a practical but seldom concerned issue, the morphologic characteristics of a turbulence diffusion flame under the effects of upstream blockage in a longitudinally ventilated tunnel was studied based on a series of model-scale experiments. Different blockage sizes, blockage-fire distances, longitudinal ventilation velocities, and fire sizes were considered for research. Numerical simulations were also carried out to calculate the flow field for analyzing the causes of the variety in the flame morphology and determining the length of the near wake. Experimental results showed that the flame morphology can be divided into two stages according to the dimensionless blockage-fire distance normalized by the length of the near wake, namely flame bifurcation and flame tilt. By comparing with previous literature, the critical blockage-fire distance corresponding to the symmetrical bifurcation was achieved which follows a linear relationship with the length of the near wake. On the basic of tilt angle and centerline trajectory, dimensionless models were developed to predict the flame tilt under the effects of blocking ratio and blockage-fire distance. The research outcomes can provide a reference and tool to assess the fire risks of tunnels with blockage effects.

An experimental study on the intermittent flame ejecting behavior and critical excess heat release rate of carriage fires in tunnels with longitudinal ventilation

This paper describes the quantitative study on the effects of tunnel longitudinal ventilation velocities on the critical excess heat release rate and intermittent flame ejecting behaviors of carriage fires. Experiments were carried out in a 1/8 model tunnel with the dimensions of 22 m × 1.2 m × 0.8 m (length × width × height). 442 repeatable test conditions contained gas fuel (propane) and solid fuel (corrugated paperboard), including variations on fuel supply rates, tunnel opening sizes, and longitudinal ventilation velocities. The intermittent flame ejecting behaviors were expressed as the flame ejection probability in carriage fires, which was characterized by image statistical processing. The results showed that the flame ejection probability was positively correlated with the fire heat release rate, but it was negatively correlated with the ventilation factors of carriage opening and longitudinal ventilation velocities. New non-dimensional correlations between the critical excess heat release rate and the longitudinal ventilation velocities are proposed. Moreover, a piecewise equation is proposed to describe flame ejection probability that is affected by longitudinal ventilation velocities for the first time. All the proposed correlations are found to fit well with the experimental data, which established an essential foundation to quantify the critical heat release rate and intermittent flame ejecting behaviors of carriage fire in various longitudinal ventilation velocities. Finally, the some limitations of propane gas fuel are analyzed. Moreover, this paper depicted the flame ejecting behaviors induced by gas fuel and solid fuel, and reveal the difference and relationship between them.

An improved theoretical model for the maximum smoke temperature rise in a tunnel based on equivalent fire source

The maximum smoke temperature rise underneath tunnel ceiling is of great importance for the assessment of heat exposure, estimation of fire detection time and possibility analysis of fire spread. Previous studies mainly focused on the scenario with a single fire source, but the existence of two fire sources is common due to the increase traffic volume. The maximum smoke temperature rise induced by two fire sources is studied through theoretical analysis in this study. A virtual equivalent fire source is proposed, which is mainly affected by the total fire heat release rate (HRR) of two fires, longitudinal ventilation velocity, effective tunnel height and separating distance between two fires. The HRR of the equivalent fire source decreases with a longer separating distance, which indicates that the influences of the downstream fire on the maximum ceiling temperature weakens gradually as the separating distance increases. An improved model on the maximum ceiling smoke temperature rise caused by two fire sources was developed, and the predictions based on the revised model agree well with the experimental data.