Today, air quality is a major global health concern. Public transport is often presented as a greener alternative, as it reduces pollutant emissions per kilometer traveled. However, they are not totally pollution-free. The wear and tear of components used in rail operations, such as wheels, rails, ballast, pantographs, catenary and brakes, generates polluting particles.

These particles tend to accumulate more in underground rail enclosures due to the confinement effect. Unfortunately, studies on the impact of this pollution are still rare. That’s why Trapapart has launched a research project on the subject.

The problem of fine particles

Pollution levels in underground rail spaces are mainly caused by their confinement, which limits the air renewal needed to eliminate pollutants emitted by operating trains. As a result, older stations are more likely to accumulate pollution particles indoors.

The impact of outdoor air pollution on that of underground stations is not clearly defined; it largely depends on the specific architectural features of each underground transport system. Factors such as the type of ventilation (natural, forced, air-conditioned), the depth of the station (deeper stations are less sensitive to variations in outside air quality) and the number of entrances play an essential role.

Seasonal variations in the weather also seem to influence pollution levels on the docks. It is important to note that the materials used in the construction of stations, rail infrastructure and rolling stock, subject to wear and abrasion, can contribute to the variability of particulate pollutants.

Study of passive systems to capture fine particles

TrapAparT traps help reduce people’s exposure to harmful fine particles by equipping targeted areas, such as major urban thoroughfares and underground railway stations (metros), with pollution levels well above the thresholds recommended by the WHO for places with high human concentrations.

The heart of the device consists of
a fine-particle adsorbent media patented by TrapAparT
s patented fine particle adsorbing media, capable of trapping fine particles by bringing them into contact with the air using only natural air flows (wind and turbulence generated by vehicles). The media is regenerated by simply washing it with water at a frequency of around one month. The wash water is recovered and the pollutants removed.

The main aim of the study carried out by EOLIOS engineers is therefore to analyze air speeds and trajectories inside the station, in order to determine whether the devices installed on the platforms are effective in capturing the fine particles present in the atmosphere. This project is of great importance, as it aims to control the specific aeraulic phenomena that occur on the station platform. To this end, the study will focus on the application of CFD modeling to explore in detail the aeraulic principles inherent in the airflow generated by passing subways.

On-site audit

The aim of the audit is to carry out a series of measurements to study the air movements associated with the arrival of metros in the station. At the same time, it aims to assess the concentration of fine particles in the air. These surveys will be carried out exclusively on the quays and in the quayside technical area.

EOLIOS engineers have observed that air current velocities vary according to the direction of the train, with lower maximum amplitudes when the train is travelling on the opposite platform. These speeds are influenced by factors such as braking time and train power. What’s more, the reduction in air speed as the train passes depends on the direction in which the train is moving (arriving or leaving the platform) and the duration of the passage. It should be noted that the measurements, carried out close to the platform in the intervention zone, led to significant slowing down of trains for safety reasons, resulting in deviations from normal traffic conditions.

CFD simulation

CFD modeling of subway station and trains

Computational Fluid Dynamics (CFD) is a numerical method used to study fluid flows in given environments. It enables the complex equations governing these flows to be solved numerically, since they cannot be solved analytically. By applying CFD to buildings, we can obtain the following information air velocities, pressures and temperatures in and around construction spaces. This helps designers to optimize ventilation and air-conditioning, taking into account factors such as building structure, internal heat gains and air-conditioning systems, to ensure optimum comfort.

To solve the partial differential equations, we need to define the boundary conditions for the calculation. These are established on the basis of on-site measurements and information supplied by the prime contractor. Boundary conditions determine the type of walls, flows (unidirectional inlet or outlet), parameters such as velocity, flow rate or mean static pressure, as well as surface coefficients if required to simulate heat transfer.

The model mesh, composed of around 10 million orthogonal structured fluid elements with refinement in key areas, is essential for the accuracy of the study, but can lead to long computation times.

The 3D model of the station was drawn up using the plans supplied, using the simplified geometry of the site and its surroundings. To guarantee the accuracy of the measurements, the tunnels on either side of the station were included in the model with a sufficient length to avoid any influence from the model’s boundary conditions.

In addition, to study the impact of the metro’s passage on the station’s thermoaerodynamics, a specific 3D model of the metro train was created. This approach makes it possible to explore in depth the interactions between the train and the station environment, contributing to a better understanding of thermal and aeraulic phenomena in this space.

CFD study of air movements in the metro station

The train’s passage generates lasting disturbances in its wake. These disturbances show that the air velocity follows a trajectory tangent to the media, which can be advantageous given their characteristics.

In motion, the landing gear induces drag at the rear. When a train moves, it creates a zone of overpressure at the front and a zone of depression at the rear. This results in a flow of air from the sides of the undercarriage towards the rear to compensate for the negative pressure, thus increasing the speed of the air at the rear compared with static air.

The pressure planes illustrate the propagation of the pressure wave caused by the approaching train. The initial flow is from left to right, then reverses once the train is in the station, particularly at the head of the train. The pressure difference at the head of the train causes a flow of air through the media, although this pressure delta is short-lived.

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CFD modeling of air velocities for a train entering a station

Vorticity is a pseudo-vector field that describes the local rotational motion of a medium. It allows us to visually identify areas of intense turbulence. Vorticity patterns reveal that regions close to the media experience disturbances, particularly after the train has passed.

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CFD modeling of vorticity effects for a train entering a station

Additional studies have made it possible to precisely define the performance levels of the capture systems. Optimization solutions, such as the development of deflectors have improved fine particle capture performance.

EOLIOS is thus able to work on cases involving the release of fine particles, and to help manufacturers optimize their installations and design prototypes.

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CFD modeling of vorticity effects in the catchment area

EOLIOS is thus able to work on cases involving the release of fine particles, and to help manufacturers optimize their installations and design prototypes.