An analytical model to predict the temperature in subway-tunnels by coupling thermal mass and ventilation

Abstract

There is an increasing incidence of overheating in subway tunnels in recent years especially in old subways without air-conditioning e.g., London Underground. There is still lack of a clear understanding how tunnel-air temperature is determined by complex thermal processes in subway tunnels. In this study, a mathematical model that describes the thermal processes in deeply buried subway tunnels was developed. Analytical solution was derived by separating the solution into time-averaged component and periodic component. The results show that the time-averaged component of tunnel-air temperature will approach steady state as the time tends to infinity, which has a positive linear relation with internal heat-source and average ambient temperature. Active cooling or heat-recovery systems could soon become a necessity in subway tunnels due to both global warming and increasing internal heat generation. Compared with outdoor air, the amplitude of the tunnel-air temperature shows a significant reduction in the day period but not in the year period. The surrounding soil temperature will keep changing for thousands of years. This study offers a new physical insight to analyse and mitigate overheating in subway tunnels.