⚡ electrical engineering

Effect of lateral mechanical smoke exhaust on smoke backflow behavior and temperature profile in a mountain tunnel: A numerical investigation and analysis

This study employs numerical simulations to analyze the effects of lateral mechanical smoke exhaust on smoke backflow and temperature profiles in mountain tunnels, revealing asymmetric flow characteristics and developing empirical models to predict longitudinal temperature decay and back-layering length for improved ventilation system design.

Original authors: Mintao Jia, Shaoxuan Wang, Li Chen, Xudui Liu, Yupeng Hao, Mingxuan Yang, Jiaxian Cheng, Wenxin Qiu, Zhisheng Li

Published 2026-07-06

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CC BY 4.0

Original authors: Mintao Jia, Shaoxuan Wang, Li Chen, Xudui Liu, Yupeng Hao, Mingxuan Yang, Jiaxian Cheng, Wenxin Qiu, Zhisheng Li

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

Imagine a mountain tunnel as a long, narrow hallway. If a fire starts inside, hot smoke acts like a giant, invisible river flowing along the ceiling. The goal of safety systems is to stop this "smoke river" from flowing backward toward people trying to escape.

This paper investigates a specific type of safety system: a side-opening shaft. Instead of a vent right in the middle of the ceiling (like a skylight), this system uses a tunnel that runs sideways from the main road to a vertical shaft going up the mountain. The researchers used powerful computer simulations to see how this "side door" affects the smoke compared to a standard "ceiling door."

Here is what they found, explained simply:

1. The "Crooked" Smoke Flow

When smoke is sucked out of a ceiling vent, it behaves like water pouring straight down a drain; the flow is symmetrical and balanced on both sides.

However, when the vent is on the side (connected by a sideways tunnel), the smoke flow gets "crooked."

The Analogy: Imagine blowing air through a straw that is attached to the side of a cup. The air doesn't swirl evenly; it hits one side of the cup harder than the other.
The Result: The smoke piles up unevenly. The side of the tunnel near the sideways connection gets much hotter and more turbulent than the far side. The smoke also tends to get "stuck" or separated from the walls in the sideways tunnel, creating a lopsided flow pattern.

2. The "Vacuum Cleaner" Effect (Temperature)

The researchers tested how fast the exhaust fan should spin (the exhaust velocity) and how big the fire was (Heat Release Rate).

The Finding: The size of the fire matters less than you might think for how quickly the smoke cools down upstream (before it reaches the vent). The most important factor is how hard the fan blows.
The Analogy: Think of the exhaust fan as a vacuum cleaner. If you turn the vacuum up to "High," it sucks up the smoke and cools the air behind it very quickly, regardless of whether you are vacuuming a small pile of dust or a large rug. The speed of the fan is the main driver for cooling the smoke.
The Math: The team created a "recipe" (a mathematical model) to predict exactly how fast the temperature drops as you move away from the fire, based entirely on how fast that fan is blowing.

3. The "Backflow" Barrier

"Smoke back-layering" is the distance the smoke travels backward against the wind before it stops. This is critical because if the smoke goes too far back, it traps people.

The Finding: The stronger the exhaust fan, the shorter the distance the smoke travels backward. A bigger fire makes the smoke travel further back.
The Analogy: Imagine the smoke trying to swim upstream in a river. The exhaust fan creates a "current" pushing against the smoke.
- If the fan is weak, the smoke (the swimmer) can push back a long way.
- If the fan is strong, it acts like a powerful dam, stopping the smoke much closer to the fire.
The Result: They developed a new formula to predict exactly how far the smoke will travel back in this specific "side-door" setup. They found that because the side-door setup is less efficient than a ceiling vent, the smoke tends to travel further back unless the fan is very powerful.

Summary of the "Recipe"

The paper concludes with two main tools for engineers:

A Temperature Recipe: To predict how hot the ceiling will be at any point, you mostly just need to know how fast the side-fan is blowing.
A Distance Recipe: To predict how far the smoke will back up, you need to balance the fire's size against the "push" of the side-fan.

The Bottom Line:
Using a side-opening shaft to clear smoke is different from using a ceiling vent. It creates an uneven, "crooked" flow of heat and smoke. To make it safe, engineers need to use these new formulas to ensure the fans are strong enough to stop the smoke from backing up too far and to manage the uneven heat distribution.

Technical Summary: Effect of Lateral Mechanical Smoke Exhaust on Smoke Backflow Behavior and Temperature Profile in a Mountain Tunnel

Problem Statement
Mountain tunnels often utilize lateral opening shafts connected via transverse channels to facilitate ventilation and smoke extraction, particularly when geological constraints or existing structures prevent the construction of direct vertical shafts. While extensive research exists on smoke movement in conventional single-tube tunnels and those with ceiling-mounted centralized exhaust systems, there is a significant knowledge gap regarding the specific behavior of smoke in tunnels with lateral open shafts. Previous studies have largely focused on longitudinal ventilation or ceiling extraction, leaving the asymmetric flow fields, temperature distribution, and smoke back-layering characteristics unique to lateral shaft configurations insufficiently understood. This study addresses the need to quantify how mechanical smoke exhaust in a lateral opening shaft influences smoke migration, temperature attenuation, and backflow length compared to traditional ceiling extraction methods.

Methodology
The research employed a numerical investigation using the Fire Dynamics Simulator (FDS) developed by the National Institute of Standards and Technology (NIST). The computational model was based on a survey of a tunnel in Shenzhen, featuring a main tunnel (165 m long, 12 m wide, 6.6 m high) connected to a transverse channel (12 m × 7.5 m × 4.2 m) and a vertical shaft (42 m high).

Key simulation parameters included:

Fire Scenarios: Five Heat Release Rates (HRR) were simulated (3, 5, 10, 15, and 20 MW) to represent different vehicle fire sizes.
Exhaust Conditions: Mechanical smoke exhaust velocities were varied between 3 m/s and 5 m/s.
Grid Independence: A grid sensitivity analysis was conducted, determining a grid size of 0.25 m to balance computational efficiency with temperature prediction accuracy.
Comparative Analysis: The study compared the performance of the lateral opening shaft against a hypothetical ceiling opening shaft with identical geometric dimensions to isolate the effects of the lateral connection.

Key Contributions

Identification of Asymmetry: The study elucidates the distinct asymmetry in smoke flow behavior caused by lateral opening shafts, contrasting it with the axisymmetric flow observed in ceiling extraction systems.
Quantitative Modeling: Two empirical models were developed to predict:
- The longitudinal temperature decay upstream of the fire source.
- The smoke back-layering length under lateral mechanical exhaust conditions.
Mechanism Elucidation: The research clarifies the phenomenon of boundary layer separation within the transverse channel and its impact on temperature distribution and smoke accumulation.

Results and Analysis

Smoke Flow and Temperature Asymmetry: Unlike ceiling extraction, which produces symmetric temperature profiles, the lateral opening shaft creates a highly asymmetric flow field. The temperature gradient near the shaft side is significantly steeper than on the opposite side due to higher turbulent kinetic energy. Within the transverse channel, boundary layer separation occurs, causing smoke to accumulate on one side of the channel while the other remains a low-temperature zone. The specific side of accumulation depends on the interplay between combustion intensity and exhaust velocity; high exhaust velocities with low heat release rates tend to concentrate high temperatures on the left side of the channel, whereas high heat release rates with low exhaust velocities shift the high-temperature zone to the right.
Longitudinal Temperature Decay: The study found that the longitudinal temperature decay is primarily governed by the mechanical smoke exhaust velocity rather than the heat release rate. The dimensionless longitudinal temperature follows an exponential decay function. The decay coefficients were correlated with the dimensionless mechanical exhaust velocity, revealing that the exhaust velocity has a linear relationship with the attenuation parameters. The order of significance for the decay coefficients was determined to be $t > A > y_0$ . An empirical model was formulated to predict the temperature distribution based on these findings.
Smoke Back-layering Length: The length of the smoke back-layering (the distance smoke travels upstream against the longitudinal wind) decreases as the mechanical exhaust velocity increases. This is attributed to the increased dynamic pressure generated by the higher exhaust velocity, which resists the buoyancy-driven smoke front. Conversely, higher heat release rates increase the thermal buoyancy, leading to a longer back-layering length. The study introduced an induced longitudinal velocity to quantify the impact of the lateral shaft and derived a modified predictive correlation for the smoke back-layering length, incorporating a correction coefficient ( $\lambda = 3.14$ ) to account for the specific dynamics of lateral extraction.

Significance and Claims
The authors assert that this work provides a critical reference for the design of ventilation and smoke extraction systems in mountain tunnels featuring lateral open shafts. By demonstrating the disparity in smoke migration between lateral and ceiling extraction modes, the study highlights that lateral systems may require higher mechanical exhaust capacities to overcome increased resistance from horizontal passages and manage the resulting asymmetric temperature fields. The developed empirical models for temperature decay and back-layering length offer practical tools for engineers to assess fire hazards and optimize safety designs in such complex tunnel geometries. The study concludes that understanding these specific flow behaviors is essential for effective fire prevention and control in modern mountain tunnel infrastructure.

1. The "Crooked" Smoke Flow

2. The "Vacuum Cleaner" Effect (Temperature)

3. The "Backflow" Barrier

Summary of the "Recipe"

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