Rail Tunnel Temperature Stratification and Implications for Train and Tunnel Ventilation Design

For an air-conditioned transit vehicle delayed in a tunnel, a key tunnel ventilation design requirement is the provision of sufficiently ‘cool’ air to the air conditioner condensers.
This paper investigates the implications that tunnel temperature stratification has on ventilation equipment requirements. Many metro systems run rolling stock with above-car air conditioning condensers. These have the disadvantage that hot condenser exhaust may remain stratified in the tunnel obvert affecting the performance of units on downstream cars.
The issue can drive the entire ventilation design in tight tunnels with rooftop condensers. In environments with very high dry bulb temperature, the issue could prevent successful longitudinal ventilation using unconditioned outside air alone.

This paper discusses:

• The level of stratification that occurs during congestion, accounting for train AC heat rejection, ventilation flowrate and thermal effects of the tunnel lining. This is determined by a transient computational fluid dynamic (CFD) simulation.
• The effect on equipment sizing. The increase in ventilation capacity to account for the stratification effect is defined. This is determined by SES simulations of practical conditions to evaluate flows and pressure head required.
• An assessment of typical underground conditions given various ambient conditions.