Investigation of thermodynamic parameters of the air environment in subway lines with single-track and double-track tunnels

The study of thermodynamic parameters in the air environment of subway lines is of particular relevance due to the substantial differences in air temperatures between Russian cities and those abroad. These temperature variations influence the formation of aerothermodynamic characteristics, which must be considered when selecting methods to ensure compliance with standard climatic parameters in subway systems. The objective of the study presented in this article was to identify, based on experimental data, the patterns governing aerothermodynamic processes, with the aim of providing recommendations for the standardization of air environment parameters in subway tunnels (both single- and double-track) located at different depths. The primary tasks of the research involved identifying the factors influencing the distribution and variation of temperature and humidity, conducting instrumental measurements of temperature and humidity distribution along the length of the transit tunnel sections under investigation, both in the absence of trains and during varying intensities of train movement. Proposals were then developed to apply the identified patterns in maintaining standard air parameters. The article posits that the selection of technical solutions for improving subway ventilation systems should be based on the unique features of aerothermodynamic processes, which depend on the structural characteristics of the transit tunnels and their depth. Experimental studies revealed the patterns governing the formation of ventilation and thermal regimes in single-track and double-track tunnels, as well as at junction sections, and provided recommendations for optimizing ventilation and thermal regimes to ensure compliance with climatic standards. Specifically, the study found that in deep single-track tunnels, the thermal regime is influenced by the presence of circulation loops between adjacent stations, created by the piston effect and the heat emissions from moving trains. Circulation loops in shallow single-track tunnels, by contrast, are characterized by strong aerodynamic connections with the surface, as surface air enters the stations and tunnels via pedestrian walkways and inclined passages. In double-track underground structures, in the absence of train movement, the variation in air temperature along the length of the transit tunnel is determined by the amount of heat accumulated in the surrounding ground during periods of train operation. When trains are in motion, the heat emitted by trains moving in opposite directions is evenly distributed along the tunnel, due to the near absence of a piston effect, resulting in a stable air temperature throughout the tunnel. However, the sections adjacent to stations experience localized increases in air temperature due to the maximum heat generated during braking and train stops, with tunnel air temperatures in these sections rising by 2–3 °C compared to those in the transit sections. In sections where both single-track and double-track tunnels are present, a potential rise in air temperature at the station adjacent to single-track tunnels is associated with the formation of circulation loops between the junction of different tunnel types and the station itself. Recommendations for normalizing the aerothermodynamic regime in the various tunnel types studied include provisions for mitigating potential summer air temperature increases above the standard levels by either increasing air flow or cooling the air in cross passages adjacent to stations. Methods for increasing air temperature may include organizational, aerodynamic, and heat engineering techniques.