⚡ electrical engineering

Smoke Temperature Characteristics in Longitudinally ventilated mountain Tunnels featuring on Lateral-opening Vertical Shaft

This study experimentally investigates the smoke temperature distribution in longitudinally ventilated mountain tunnels with lateral-opening vertical shafts under varying fire sizes and ventilation conditions, ultimately developing empirical correlations to predict maximum excess and longitudinal temperatures for improved fire safety system design.

Original authors: Zhisheng Li, Fengliang Wu

Published 2026-09-08

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Zhisheng Li, Fengliang Wu

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

When a fire breaks out deep inside a mountain tunnel, the greatest danger often comes not from the flames themselves, but from the thick, superheated smoke that follows. This smoke rises, hits the ceiling, and spreads rapidly along the tunnel's length, turning a manageable incident into a deadly trap for anyone trying to escape or for rescue teams trying to enter. To keep these tunnels safe, engineers rely on ventilation systems that push fresh air in from one end and pull the smoke out from the other. However, many modern mountain tunnels are too long to be ventilated by simple fans at the ends. Instead, they are built with tall vertical shafts that reach up to the surface, connected to the main tunnel by a horizontal duct. These shafts act like giant chimneys, using the natural rise of hot air to help pull smoke out. But when a fire starts near such a shaft, the interaction between the smoke being sucked up the shaft and the air being pushed along the tunnel creates a complex and unpredictable flow. Understanding exactly how hot the smoke gets in these specific conditions is critical, because the temperature determines how quickly detectors will trigger alarms and how long the structure can withstand the heat before failing.

Researchers from universities in China set out to map this hidden behavior by building a precise, one-tenth scale model of a mountain tunnel equipped with a vertical shaft. In their laboratory, they constructed a tunnel that was 16.5 meters long and installed a vertical shaft that rose 4.2 meters, connected by a side duct. They placed a fire source upstream of this connection and then ignited diesel fuel in square trays of varying sizes to simulate different fire intensities. To mimic real-world conditions, they used powerful fans to create a steady stream of air moving through the tunnel, while simultaneously using a mechanical system to pull air up the vertical shaft at different speeds. They placed dozens of tiny temperature sensors just beneath the tunnel ceiling to record exactly how the heat moved and changed as they adjusted the speed of the fans and the size of the fire.

The experiments revealed a clear pattern in how the smoke behaves when these two ventilation forces work together. When the mechanical fans pulling smoke up the shaft were turned on, the temperature of the smoke directly under the ceiling dropped. This cooling effect happened quickly at first as the fan speed increased, but then the rate of cooling slowed down, suggesting that there is a limit to how much faster ventilation can cool the smoke. The size of the fire also played a major role, but only in specific areas. Near the fire itself, a larger fire caused a dramatic spike in temperature. However, as the smoke traveled further away from the flames, the difference in temperature between a small fire and a large fire became much less noticeable. This happens because the powerful buoyancy of the hot smoke is strongest right above the fire, but as the smoke travels down the tunnel, it mixes with the cooler air and loses that intense heat energy.

The researchers found that the speed of the air moving along the tunnel was the most dominant factor in controlling the smoke's temperature. When the air moved slowly, the smoke tended to linger and stay hot. As the air speed increased, the smoke was pushed away more effectively, and the temperature dropped significantly. However, if the air moved too fast, the smoke could be pushed past the exhaust shaft entirely, escaping the control of the ventilation system and spreading further down the tunnel. This finding highlights a delicate balance: the ventilation system must be strong enough to clear the smoke, but not so strong that it blows the smoke past the exit point.

To make these findings useful for real-world safety, the team developed a new way to predict the maximum temperature a fire could reach in this specific type of tunnel. They combined the data from their experiments with the physical principles of how heat and air move to create a formula that accounts for the size of the fire, the speed of the air moving along the tunnel, and the speed of the air being pulled up the shaft. This formula allows engineers to estimate the hottest point a fire could reach without needing to build a new model for every single tunnel design. The results suggest that for tunnels with these vertical shafts, the placement of fire detectors and the design of the ventilation system must be carefully tuned to the specific speeds of the fans and the expected size of a potential fire. By understanding exactly how the heat decays as it moves away from the fire, safety systems can be designed to warn people earlier and keep the escape routes clear of lethal heat.

Technical Summary: Smoke Temperature Characteristics in Longitudinally Ventilated Mountain Tunnels Featuring Lateral-Opening Vertical Shafts

Problem Statement
Mountain tunnels frequently require vertical shafts connected via lateral smoke exhaust ducts to facilitate ventilation and emergency egress. In the event of a fire, the interaction between longitudinal ventilation and mechanical smoke extraction through these lateral openings creates complex smoke migration behaviors that differ significantly from standard horizontal point extraction or purely longitudinal systems. While extensive research exists on smoke temperature distribution in longitudinally ventilated tunnels, there is a notable gap in understanding the specific thermal characteristics and temperature decay mechanisms in mountain tunnels featuring lateral-opening vertical shafts, particularly when the fire source is located upstream of the exhaust duct. This complexity poses challenges for the effective design of smoke control systems and the placement of fire detection and early warning systems.

Methodology
The study employed a 1/10 scale reduced-model experimental approach based on Froude similarity rules to investigate smoke temperature profiles. The experimental setup consisted of a 16.5 m long tunnel with a vertical shaft (4.2 m high) connected via a 2 m lateral smoke exhaust channel.

Experimental Conditions: A total of 54 burning experiments were conducted using diesel pool fires with three different source sizes (0.15 m, 0.20 m, and 0.23 m side lengths), corresponding to full-scale heat release rates (HRR) ranging from 2.29 MW to 9.89 MW.
Variables: The study varied three key parameters:
1. Fire Source Size: Three distinct pool dimensions.
2. Mechanical Smoke Exhaust Velocity ( $V_s$ ): Three levels (2.03, 2.79, and 3.53 m/s in the model; 6.42–11.17 m/s full-scale).
3. Longitudinal Ventilation Velocity ( $V_l$ ): Seven levels ranging from 0.18 m/s to 1.23 m/s (model scale).
Instrumentation: Temperature data was collected using 71 K-type thermocouples positioned 0.1 m below the tunnel ceiling, with varying spacing densities near the shaft intersection and further downstream. Air velocities were measured using hot-wire anemometers.

Key Results

Longitudinal Temperature Distribution:
- As mechanical ventilation velocity increases, the smoke temperature beneath the ceiling exhibits an initial rapid decline followed by a more gradual decrease.
- At lower longitudinal velocities, ventilation conditions are the dominant factor influencing smoke development. At higher longitudinal velocities, the fuel source properties become the dominant factor, and the temperature gradient reduces significantly.
- The temperature deviation caused by varying fire source sizes is substantial near the fire source but becomes negligible in the far field, indicating that buoyancy effects weaken with distance.
- The dimensionless longitudinal temperature distribution is minimally affected by the magnitude of the fire source, suggesting that backflow smoke decay behavior is independent of fire size in this context.
Temperature Decay Coefficient:
- The longitudinal smoke temperature decay coefficient ( $k$ ) was found to decrease as the longitudinal velocity increases.
- A linear relationship was established between the decay coefficient and a modified Froude number ( $Fr'$ ), which accounts for the combined effects of mechanical exhaust and longitudinal ventilation: $k = 0.394 Fr' + 0.08$ .
Maximum Excess Temperature:
- The maximum excess temperature beneath the ceiling increases rapidly with the size of the fire source.
- A global empirical correlation was developed to predict the maximum excess temperature. The dimensionless maximum excess temperature ( $\Delta T_{max}/T_{\infty}$ ) correlates with the dimensionless heat release rate ( $Q^*$ ) and the modified Froude number ( $Fr'$ ) according to the relationship: $\frac{\Delta T_{max}}{T_{\infty}} = a(Q^*)^{2/3} + b(Fr')^{-1/3} + c$ .
- The fitted coefficients for this model were determined to be $a = -2.34$ , $b = 0.53$ , and $c = 1.37$ .

Significance and Claims
The authors claim that this research provides critical insights into the thermal behavior of smoke in mountain tunnels with lateral-opening vertical shafts, a scenario previously under-investigated. The primary contributions include:

Empirical Correlations: The development of specific correlations to predict both the maximum excess temperature and the longitudinal temperature decay under the combined influence of lateral concentrated smoke extraction and longitudinal ventilation.
Design Guidance: The results offer valuable data for optimizing the arrangement of fire detectors and early warning systems in similar mountain tunnel configurations.
Mechanism Clarification: The study clarifies the transition in dominant factors (from ventilation to fuel properties) as longitudinal wind speeds increase and quantifies the impact of the "chimney effect" inherent in tall vertical shafts on smoke control.

The paper concludes that understanding these specific temperature characteristics is essential for improving smoke management strategies and ensuring the safety of emergency evacuation and fire rescue operations in complex mountain tunnel environments.

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