INTRODUCTION
A tunnel ventilation system is an engineering system designed to manage and control the airflow and air quality within tunnels, ensuring the safety and comfort of people inside and maintaining the structural integrity of the tunnel itself.
Fires in tunnels pose major safety issues and challenges to the designer, especially with the increase in the number of tunnels, their length, and number of people using them. Life can be threatened in a number of ways: the inhalation of combustion products such as carbon monoxide and carbon dioxide, and the exposure to high temperatures and heat fluxes. Furthermore, evacuation can be significantly hindered by poor visibility, power failure, blocked exits due to traffic jams or crashed vehicles, or obstruction resulting from a collapse or explosion in the tunnel. For safe evacuation, tolerable temperatures, acceptable visibility, and adequate air quality must be maintained by providing an appropriate ventilation system.
TYPES OF TUNNEL VENTILATION SYSTEMS
All tunnels require ventilation to maintain accept-able levels of contaminants produced by vehicle engines during normal and congested traffic operation, and to remove and control smoke and hot gases during a fire emergency (emergency ventilation). The ultimate goals of smoke management systems are to:
- Provide an environment sufficiently clear of smoke and hot gases to permit safe evacuation
- Allow relatively safe access for rescue services as a function of the fire scenario
A. Natural Ventilation:
The consideration of natural smoke venting in the design of new tunnels is gaining more importance with the continued drive toward environmentally sustainable infrastructures to reduce energy consumption and save costs. Natural ventilation is achieved by using:
- Air pressure difference within and outside the tunnel,
- Tunnel slope which is commonly known as tunnel gradient. By using difference in the elevation to produce air flow.
- Piston effect of moving vehicle in to tunnels.
Natural ventilation is dependent upon the meteorological conditions like External wind speed, temperature etc. hence uncertainty is always there. This approach can be useful for shorter tunnels but it is not advisable to adopt in long tunnels because above mentioned factors should be large enough to support smoke movement from fire location to the tunnel portal. Which is not possible in most practical scenarios.
B. Mechanical Ventilation system
Mechanical ventilation system can be divided into two strategies.
1. Transverse Ventilation
Transverse ventilation is a approach where supply and extraction points are provided along the entire length of the tunnel connected to supply and extract fans to obtain uniform collection of smoke. This approach is adopted in extremely long tunnel (more than 2000m) or tunnels with bi-directional traffic flow. Transverse system can be further divided into three categories. Semi transverse supply, semi transverse exhaust and fully transverse. Below figure is for illustration. Due to its high cost of installation and maintenance this system is not cost effective. Alternative is Longitudinal ventilation system. Which is the focus of this article. We will discuss in detail below.
2. LONGITUDINAL TUNNEL VENTILATION SYSTEM
Longitudinal ventilation system creates a longitudinal air flow into the tunnel from one portal to other. The ventilation is provided either by injecting the fresh air, by extracting the exhaust air by jet fans or combination of both. It is the most effective method to control the smoke in unidirectional tunnels. In the event of fire this is usually assumed that the traffic ahead of fire will proceed for the exit portal while traffic behind the fire will stop. This strategy is adopted in tunnels to provide tenable environment to the occupants who stuck behind the fire.
Critical Velocity:
Critical velocity is the minimum velocity of the air in side the tunnel which is required to prevent backlayering. backlayering is defined as the length of the reversed smoke flow upstream of the fire when the ventilation velocity is lower than that of the critical velocity. Below figure will illustrate the phenomena of backlayering in longitudinal tunnel ventilation system.
As shown above critical velocity is one of the most important aspects in design of tunnel longitudinal ventilation system. To push the smoke in unoccupied direction, air velocity must be greater than critical velocity otherwise desired results cannot be achieved. Critical velocity depends upon the fire Heat release rate (HRR), slope and tunnel section geometry. Critical velocity is used to determine the fan requirement for smoke control and can be determined from simultaneous solution of below equations. (Note: Normal Range for Vc is 2 m/s to 3.5m/s)
FIRE LOAD APPROXIMATION FOR TUNNELS
It is clear from the above discussion critical velocity is most important aspect of tunnel ventilation system and in depends on the fire load. Hence it is necessary to consider correct fire load for design of performance-based system. Over designed and under designed conditions can negatively affect the performance of the system. Fire load is defined as Heat release rate in Megawatts as a function of time. It provides a fire characteristic that are used to establish sizing of equipment in tunnels.
A perspective approach generally been adopted in which the specific fire size is chosen as a basis tunnel fire and life safety design. These have been peak release of order 5MW to 30MW depending upon the type of traffic allowed within the tunnel. Some authorities have adopted larger fire sizes into the codes and standards. Permanent International Association of Road Congresses (PIARC) has provided some guidelines for selecting fire load to design of life safety system in tunnels.
CFD STUDY ON BEHAVIOUR OF SMOKE IN DIFFERENT TUNNEL AIR VELOCITIES
A study has performed using CFD to visualize the smoke behaviour in 400m long tunnel with different air longitudinal velocities. Three different simulations have performed with different tunnel air velocities and movement of smoke has observed in fire Scenario. Some other parameters like smoke layer height, temperatures, Visibility and Pollutant levels were also observed and conclusion has drawn out of it. Fire Load of 15MW has considered in the middle of the tunnel.
Scenario – I
A fire load has placed in the middle of the tunnel and behavior of smoke has observed without any ventilation system. Smoke has propagated in both the direction and filled the complete tunnel in few seconds making environment untenable. Results are as below:
Smoke layer reached near the portals (air velocity less than critical velocity)
Scenario – II
A fire load has placed in the middle of the tunnel and behavior of smoke has observed with any oversized ventilation system. Ventilation inside the tunnel is provided by set of jet fans. Where air velocity inside the tunnel increased extremely high due to high thrust of jet fans. No Major Backlayering was observed but higher air velocities increase the fire parameter and spread up the fire on larger area with large turbulence in smoke layer which is not desirable.
Scenario – III
A fire load has placed in the middle of the tunnel and behavior of smoke has observed with correctly sized ventilation system. Ventilation inside the tunnel is provided by set of jet fans. where air velocity inside the tunnel reaches to just above the critical velocity (around 3.8m/s). backlayering has observed in the initial stage but as the air flow crossed above critical velocity backlayering will start diminishing. Lower Air velocities help keep the fire intact to its origin with uniform smoke layer.
Conclusion:
The results of the study on tunnel ventilation systems have revealed a critical finding: the over-sizing of tunnel ventilation systems can inadvertently exacerbate the parameters of a fire within the tunnel. This counterintuitive outcome underscores the need for a balanced approach to ventilation system design in tunnel environments. While it may seem logical to provide larger ventilation systems for enhanced safety, our study has demonstrated that an excess of airflow can lead to unintended consequences during fire scenarios. This insight highlights the importance of tailoring ventilation systems to the specific needs of the tunnel, carefully considering factors such as fire loads. By doing so, we can mitigate the potential dangers associated with over-sized systems and work towards optimizing both normal operating conditions and emergency scenarios, ultimately enhancing tunnel safety and the well-being of its users.
Engr. Faizan CFPS®️ (NFPA) – CFI®️ (NFPA)
He Lead and manage fire and smoke simulation projects from initiation to completion, ensuring adherence to timelines, budgetary constraints, and quality standards. He has in depth knowledge of Fire and life safety with vast experience in the field of Computational Fluid Dynamics. Expertise include Pyrosim-FDS, Pathfinder, Ansys etc
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