Air quality has become an important issue due to the health problems that degraded air can cause (1), notably in urban environments. One of the sources of air pollution is the presence of fine and ultrafine particles, especially in confined spaces where this type of particles can accumulate (2). Urban trains such as subways generate such particles during slowing down phases, that involve friction braking usage, and they end up trapped in the stations. To curb these emissions, the BREAQ project, short for "BRaking Emissions characterisation & mitigation for Air Quality," addresses the impact of railway equipment on its environment regarding braking particle emissions. This project is funded by the government as part of Programme d’Investissement d’Avenir, operated by ADEME and led by ALSTOM.

The work presented here focuses on the link between the emission of particles resulting from the disc/pad contact conditions, and the trajectories, velocities and concentration fields of particles. Experimentations are done with dynamic air conditions using a realistic geometry test bench integrated into a wind tunnel. These studies represent some of the first investigations connecting particle emission to the flow dynamics of these particles in the air.

The aim is to evaluate the velocity and concentration fields of particles emitted using laser visualization techniques. A laser sheet illuminates the plane in which the rubbing contact is located. Cameras record paired images synchronized with various braking parameters (sliding speed, disc and pad temperature, braking force, etc.) then processed using algorithms such as PIV (Particle Image Velocimetry) (3, 4, 5) and edge detection. These data are compared to an average velocity field measurement in 3 directions within a volume around the test bench conducted using a five-holes pressure probe.

First, the measurement methodology is presented, with its advantages, disadvantages, and limitations. Then the results show that the velocities and the concentration fields of the particles are correlated with the braking parameters and the incoming airflow conditions on the braking system. Finally, these analyses are compared with a particle counter along with.

The ultimate aim of the work is to establish the dispersion of particles following emission to design and size efficient recovery equipment. The data will also serve as a benchmark for a Computational Fluid Dynamics study in the BREAQ project. Preliminary analyses reveal the significant interaction between the disk-induced flow and the incoming airflow, and identify the impact on nano-particle trajectories in particular.

Air quality is a major public health issue over the world. Rail transportation is rightfully considered as one of the greener and more CO2-efficient mobility solution. However, many studies have evidenced bad air quality in confined railway stations leading to major concerns regarding particles from non-exhaust emissions (Querol et al., 2012; Martins et al., 2015). In addition to particles, Volatile Organic Compound (VOC) emissions are also expected to worsen air quality (Yang et al., 2023), although much less investigations have been carried out. The objectives of the presented works were (i) to characterize VOC emissions of a railway braking system from two braking benches: a 1:1-scale bench at Alstom Flertex facilities and a reduced scale bench at LaMcube laboratory; (ii) to evidence relationships between VOC emissions, brake materials, and braking conditions. Different braking conditions were tested like the initial speed and temperature and the force applied to the system. The two benches were instrumented to control braking parameters and to measure VOCs and particles (not reported here) emissions, both online and offline. VOC characterization instruments include an online Photo-Ionization Detector (PID, for total VOCs) and sampling on Tenax followed by gas chromatography mass spectrometry and DNPH (dinitrophenyl hydrazine) cartridges analyzed by liquid chromatography (for speciated VOCs and carbonyls, respectively). The methodology for VOCs measurements will be first presented, including the advantages and the drawbacks of the two setups and the associated limitations. The results pinpointed a number of identified and quantified VOCs. The potential origin of these compounds (brake pad, side emissions from lubricant or else, ambient air) was carefully looked at, with particular attention to the chemical composition of the organic brake pad. Possible relationships between VOC emissions, braking system and particles will be discussed. This work represents one of the first comprehensive analysis on VOC emissions from railway brake systems. Acknowledgements: The present work takes place within the BREAQ project funded by the French government as part of the PIA, Invest for the Future Plan, now integrated into France 2030, and operated by ADEME, the French ecological transition agency. References V. Martins, T. Moreno, M. C. Minguillón, F. Amato, E. de Miguel, M. Capdevila, X. Querol, Exposure to airborne particulate matter in the subway system, Sci. Tot. Environ. 511, 711–722 (2015) X. Querol, T. Moreno, A. Karanasiou, C. Reche, A. Alastuey, M. Viana, O. Font, J. Gil, E. De Miguel, M. Capdevila, Variability of levels and composition of PM 10 and PM 2.5 in the Barcelona metro system. Atmos. Chem. Phys. 12, 5055-5076 (2012) J. Yang, X. Fan, H. Zhang, W. Zheng, T. Ye, A review on characteristics and mitigation strategies of indoor air quality in underground subway stations, Sci. Tot. Environ. 869, 15 April 2023, 161781 (2023)

Railway disc brakes are safety components subjected to restrictive standards in terms of braking performance. Friction must remain the same regardless of use and environmental conditions. Recently, new constraints have stacked upon these requirements, related to the evolution of disc brake use, and raising new concerns about friction stability. Firstly, the development of complementary braking technologies leads to extended service life of disc brake components on rolling stock, exposing them to ageing issues. Secondly, in an effort to lighten train structures and increase service speeds, fewer braking systems must dissipate higher amounts of energy. And finally, the increasing traffic density leads to uneven rolling conditions, with pronounced acceleration and deceleration phases. All this may expose disc brakes to severe braking conditions and affect friction stability in a way that is currently misunderstood. Industrial tests performed by Alstom Flertex showed that a severe braking history can significantly affect friction. A full scale dynamometer, equipped with a steel disc and sintered copper-iron brake pads, was exposed to repeated emergency braking. In order to study friction evolution with braking history, service braking performance was characterized beforehand and afterwards. The results show that friction after the emergency braking series was reduced by about 10% as compared to before. The aim of this work is to establish a relationship between severe braking history and friction evolution. On a reduced scale pin-on-disc tribometer, similar disc and pad materials were exposed to either service or severe braking. Friction evolution was characterized beforehand and afterwards under service conditions. Infrared thermography and thermocouples embedded close to the pad and disc surfaces were used to monitor the heat dissipation and contact localization. The evolution of the disc surface was recorded by a high speed camera, and displacement sensors provided information about the pad wear and disc thermomechanical distortion. Surfaces rubbed during full scale and reduced scale experiments were analyzed using Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray spectroscopy (EDX), in order to characterize differences in sliding velocity accommodation and wear mechanisms between high and low friction circumstances. The oral presentation will focus on the analysis of the friction decrease process, from full scale observations to deeper reduced scale experiments. Similar friction behaviours and temperature levels are observed at both scales, under service and severe braking conditions. Reduced scale experiments produced a friction decrease similar to what was observed on the industrial dynamometer. Further analysis at reduced scale show that the friction deviation is correlated with smaller apparent contact area, attributed to uneven pad wear and disc thermomechanical distortion during severe braking. Service brake applications following the friction decrease show a slow increase in friction back to its former level. Possible causes of the reduced friction, and the recovery process, are discussed based on multi-scale surface analysis, regarding the evolution of load-bearing zones in terms of distribution, size and composition. Friction surfaces after full scale industrial tests are also analysed, to identify the mechanisms and processes observed during reduced scale analysis.