Case 4 Help: Emergency Ventilation

Case 4 simulates pressures and velocities when a train stops in a tunnel and an exhaust fan is switched on to ensure that an adequate rate of airflow is maintained past the stationary train. The fan is located in a shaft in the tunnel. This case is directly relevant to some long tunnels and it is a reasonable approximation to a tunnel between adjacent stations in some underground railways.

In this example, you specify the entry speed of the train and the distance that you want it to travel before it stops. ThermoTun-Online then chooses the distance to be travelled at constant speed before decelerating to rest at -1 m/s2. In the full version of ThermoTun, you can choose whatever speed history you wish between train-entry and train stopping and you can also tell the stationary train to re-start, either forwards or in reverse.

In this example, you specify the instant at which the fan is to be switched on and a target flow rate through the fan when it is rotating at full speed. ThermoTun-Online chooses simple fan performance characteristics and the time needed for the fan to run up to speed.

In this example, you can monitor the direction and amplitude of the velocity past the train. In the full version of ThermoTun, you can tell the train to emit heat and smoke and you can then track the consequences of these emissions throughout the tunnel system.


red dot Location of pressure and/or velocity output

Output Locations

The principal use of Case-4 simulations is expected to be the prediction of air flow rates alongside a stationary train, including the rate at which these develop. pressures experienced by passengers on trains. However, some output is also provided at selected tunnel locations.

Graphical output is provided as follows:

Box-1: Pressure histories in the tunnel
(1) at the middle of Tun-1 (upstream of the airshaft)
(2) at the middle of Tun-2 (downstream of the airshaft)

Box-2: Velocity histories local to the nose and tail of the train
(1) in the tunnel ahead of (or behind) the train
(2) in the annular region alongside the train
NB: When the whole of the train is in the tunnel on the same side of the shaft, the long-term velocities at the nose and tail are similar (in the absence of thermal effects such as fire). When the train straddles the shaft, the velocities at the nose and tail will normally be in opposite directions.

Box-3: Mean air velocity histories in the tunnel
(1) at the middle of Tun-1 (upstream of the airshaft)
(2) at the middle of Tun-2 (downstream of the airshaft)
(3) at the middle of the air shaft
NB: The first two graphs are the same as those in Box-2, but the scale will be different unless the chosen flowrate through the fan is small.

Box-4: Train speed

Text output is provided in the V01.txt file as follows:

LTUN1, m: Distance between entrance portal and shaft

For most purposes, the meanings of distances between locations in the tunnel is self-evident. If you want to be more precise, give the distance between (i) the centroid of the portal cross-section and (ii) the centroid of the shaft cross-section at its junction with the tunnel. This is not a formal definition, but it is probably good enough.

In ThermoTun-Online, the distance between the entrance portal and the shaft must be at least 200m. The full version of ThermoTun allows experienced users to choose any length greater than zero. ThermoTun-Online is intended only for applications in which one-dimensional analyses can be used with reasonable confidence.

LTUN2, m: Distance between shaft and exit portal

For most purposes, the meanings of distances between locations in the tunnel is self-evident. If you want to be more precise, give the distance between (i) the centroid of the shaft cross-section at its junction with the tunnel and (ii) the centroid of the portal cross-section. This is not a formal definition, but it is probably good enough.

In ThermoTun-Online, the distance between the shaft and the entrance portal must be at least 200m. The full version of ThermoTun allows experienced users to choose any length greater than zero. ThermoTun-Online is intended only for applications in which one-dimensional analyses can be used with reasonable confidence.

ATUN, m2: Cross-sectional area of tunnel

For most purposes, the meaning of the cross-sectional area of the tunnel is self-evident. If you want to be more precise, calculate the total volume of air in the tunnel and divide this by the total tunnel length. This is not a formal definition, but it is probably good enough.

In ThermoTun-Online, the area of the tunnel must be at least 10m2. The full version of ThermoTun allows experienced users to choose any area greater than zero. ThermoTun-Online is intended only for conventional train:tunnel applications.

With large cross-sectional areas, the assumptions of one-dimensional flow can become suspect. It is recommended that the effective diameter of the tunnel should not exceed 10-20% of the length of any region of tunnel between boundaries. (NB: The “effective diameter” is the diameter of a circle with the same cross-sectional area as the tunnel).

FTUN (f): Tunnel friction coefficient (Fanning, not Darcy-Weisbach)

The tunnel skin friction coefficient determines the resistance to airflows along the tunnel. There are two definitions in common use:

Fanning (f ): Wall shear stress = f x ρU 2/2

Darcy-Weisbach (λ ): Wall shear stress = λ x ρU 2/8

where ρ is the density of the air and U is the mean velocity along the tunnel.

It follows from the definitions that λ=4f so it is important to know which one is used in any particular software. ThermoTun uses f, not λ.

The Fanning friction coefficient for real tunnels will exceed 0.002. It will usually be within the range 0.005 < f < 0.010.

DXW0, m: Numerical grid length

Values of pressure and velocity are calculated at closely spaced points throughout the tunnel system. In Case 4 of ThermoTun-Online, the distance between adjacent calculation points (i.e. the numerical grid length) is approximately 0.5% of the total length of the tunnel. The full version of ThermoTun allows experienced users to choose any grid length greater than zero,

LSHA, m: Length of shaft

For most purposes, the meanings of distances between locations in the tunnel system is self-evident. If you want to be more precise, give the distance along the shaft axis between (i) the centroid of its portal cross-section and (ii) the centroid of the tunnel cross-section at its junction with the shaft. This is not a formal definition, but it is probably good enough.

For numerical accuracy, the length of the shaft must be at least one grid length and preferably at least 2 grid lengths. . The full version of ThermoTun allows experienced users to choose any length greater than zero. ThermoTun-Online is intended only for applications in which one-dimensional analyses can be used with reasonable confidence.

ASHA, m2: Cross-sectional area of shaft

For most purposes, the meaning of the cross-sectional area of the shaft is self-evident. If you want to be more precise, calculate the total volume of air in the shaft and divide this by the shaft length. This is not a formal definition, but it is probably good enough.

In ThermoTun-Online, the area of the shaft must be at least 2% of the area of the tunnel. The full version of ThermoTun allows experienced users to choose any area greater than zero. ThermoTun-Online is intended only for conventional train:tunnel applications.

KINSHA: Loss coefficient, inflow to tunnel through shaft

Because shafts are usually quite short, the resistance to flow along them is often influenced more strongly by end conditions that by friction on their internal surfaces. ThermoTun-Online assumes that the overall resistance to inflow through the shaft is equivalent to a stagnation pressure loss of KINSHA x ρU 2/2 . The full version of ThermoTun allows experienced users to specify local losses at the ends of shafts and skin fiction along them.

The prescribed values of the shaft loss coefficients must be in the range 0 to 5. This range is sufficient for conventional shafts. The full version of ThermoTun allows experienced users to choose any value greater than zero.

KOUTSHA: Loss coefficient, outflow from tunnel through shaft

Because shafts are usually quite short, the resistance to flow along them is often influenced more strongly by end conditions that by friction on their internal surfaces. ThermoTun-Online assumes that the overall resistance to outflow through the shaft is equivalent to a stagnation pressure loss of KOUTSHA x ρU 2/2 . The full version of ThermoTun allows experienced users to specify local losses at the ends of shafts and skin fiction along them.

The prescribed values of the shaft loss coefficients must be in the range 0 to 5. This range is sufficient for conventional shafts. The full version of ThermoTun allows experienced users to choose any value greater than zero.

QFAN, m3/s: Target flow rate through the fan

QFAN is the target flow rate through the fan when it is rotating at full speed. ThermoTun-Online chooses simple fan performance characteristics and the time needed for the fan to run up to speed. It increases your target flow rate by up to 25% when there is low resistance and decreases it when there is high resistance. It assumes that the maximum pressure that the fan can provide is 3 kPa. In the full version of ThermoTun, you can choose whatever fan performance characteristics you wish and you can choose the time needed to run up to speed.

In ThermoTun-Online, the minimum allowable target flowrate is equivalent to a velocity of 2 m/s in the shaft and the maximum is equivalent to a velocity of 100 m/s. These restrictions do not exist in the full version of ThermoTun.

TFAN, s: Time when the fan is switched on

TFAN is the instant when the fan is switched on. In an ideal response to an emergency, this will usually be before the train has come to a halt. In many real scenarios, however, the fan will not be activated until after that train has been at rest for some time. This parameter enables you to explore the consequences of the delay.

LZUG, m: Length of train

For most purposes, the meaning of train length is self-evident. If you want to be more precise and the train ends are strongly tapered, give the distance between the most extreme points. This is not a formal definition, but it is simple and it is probably good enough. The full version of ThermoTun allows experienced users to simulate trains comprising multiple coaches, each with any length greater than zero.

The prescribed length of the train must be at least 50m. The full version of ThermoTun allows experienced users to simulate trains comprising multiple coaches, each with any length greater than zero. ThermoTun-Online is intended only for applications in which one-dimensional analyses can be used with reasonable confidence. In reality, phenomena such as nose-entry and tail-entry to a tunnel exhibit three-dimensional behaviour that extends the period of development of pressure changes.

It is recommended that the train length should be equivalent to at least 10 tunnel diameters. This is because ThermoTun-Online is intended only for applications in which one-dimensional analyses can be used with reasonable confidence. The full version of ThermoTun allows experienced users to simulate trains comprising multiple coaches, each with any length greater than zero.

For numerical accuracy, it is preferable for the train length to be at least 1% of the total tunnel length. This is because the numerical grid length used in ThermoTun-online is approximately ½% of the tunnel length. The full version of ThermoTun allows experienced users to vary the numerical grid size within wide limits.

AZUG, m2: Cross-sectional area of train

For many purposes, the meaning of the cross-sectional area of a train is self-evident. For the calculation of airflows in tunnels, a rather special meaning is implied (irrespective of which software you use). The key need is to simulate the blockage effect of the train. For a passenger coach, for example, the effective aerodynamic area is bigger than the cross-section of the passenger compartment, but smaller than the overall structural envelope including the underbody and bogies, etc.

The area of the train must be at least 5m2 and the blockage ratio in the tunnel must not exceed 50%. The full version of ThermoTun allows experienced users to choose any area greater than zero. ThermoTun-Online is intended only for conventional train:tunnel applications.

FZUG (f) Train friction coefficient (Fanning, not Darcy-Weisbach)

The train skin friction coefficient determines the main component of resistance to airflows in the annulus between the sides of the train and the tunnel. There are two definitions in common use:

Fanning (f ): Wall shear stress = f x ρU 2/2

Darcy-Weisbach (λ ): Wall shear stress = λ x ρU 2/8

where ρ is the density of the air and U is the mean velocity (relative to the train in this instance).

It follows from the definitions that λ=4f so it is important to know which one is used in any particular software. ThermoTun uses f, not λ.

If you have no experience of estimating train friction coefficients, the following values are indicative of some typical trains (but only indicative):

modern streamlined passenger train: f ≈ 0.003

conventional regional passenger train: f ≈ 0.005

container-like freight train: f ≈ 0.007

The Fanning friction coefficient for real trains will exceed 0.002. It will usually be within the range 0.003 < f < 0.010.

KNOSE: Loss coefficient, flow from open tunnel into annulus around train

In addition to skin friction on train and tunnel surfaces in the annulus around a train, account is usually taken of local losses at the nose and tail. These influence the amplitudes of pressure waves as well as the overall resistance to flow. When the direction of airflow relative to the train is from the open tunnel into the annulus, ThermoTun allows for a stagnation pressure loss given by KNOSE x ρU 2/2, where ρ denotes the air density and U is the air velocity relative to the train in the annulus.

The prescribed value of the nose loss coefficient must be in the range 0 to 0.5. This range is sufficient for all conventional trains. The full version of ThermoTun allows experienced users to choose any value greater than zero.

STAIL: Shape factor, flow from annulus around train into open tunnel

When the direction of airflow relative to the train is from the annulus into the open tunnel, ThermoTun allows for a stagnation pressure loss given by KTAIL x ρU 2/2, where ρ denotes the air density and U is the air velocity relative to the train in the annulus. The most suitable value of KTAIL varies strongly with the train:tunnel blockage ratio. Instead of asking users to prescribe different values for each tunnel area, ThermoTun calculates the expected loss for a blunt-ended train and uses a shape coefficient STAIL to define the actual loss as: STAIL x KBLUNT x ρU 2/2.

The prescribed values of the tail shape coefficients must be in the range 0 to 1. This range is sufficient for conventional trains. The full version of ThermoTun allows experienced users to choose any value greater than zero.

VZUG, km/h: Speed of train

In ThermoTun-Online, the train speed is constant and is defined in km/h. The full version of ThermoTun allows accelerating, decelerating, stopping, starting and reversing. It also allows speeds to be defined in other units if required.

The prescribed value of the train speed must be in the range 50 to 400 km/h. This range is sufficient for most practical purposes.

XSTOP, m: Distance moved before stopping

XSTOP is the distance to be travelled by the train before it comes to a halt. ThermoTun-Online ensures that the train begins to decelerate at the correct instant to reach this stopping position. Because the deceleration rate is pre-defined as -1 m/s2, the minimum possible distance in metres is (VZUG)2/2 where VZUG is expressed in m/s or approximately (VZUG)2/26 where VZUG is expressed in km/h.