According to the magazine Travel + Leisure, New York City’s Grand Central Terminal is the sixth most visited tourist attraction in the world. Built in 1913, the terminal is the principal hub of the Metro-North Railway, the second busiest commuter railroad in the country (after the Long Island Rail Road).
As the result of a 1997 renovation, Grand Central’s concourses lead to the terminal’s train shed. With 30 platforms on two levels, the train shed occupies 2.5 million square feet, making it one of the largest underground structures in Manhattan.
Except for a few areas, such as near the extract ducts or sidewalk grilles, the train shed had little air movement and a number of heat sources, including waste heat from air conditioning. Summer temperatures in the train shed were typically 15° Fahrenheit higher than the ambient temperature, creating uncomfortable conditions for rail employees and for passengers on platforms, and decreasing the service life of the trains.
Mott MacDonald was retained by Metro-North Railroad in 2000 to study thermal mechanisms in the train shed and determine the effect and relative cost/benefit of various design measures that might be used to improve the environment.
Using site visits and surveys of the train shed, we identified heat sources including train air conditioning, braking, radiation, and lighting.
Mott MacDonald used global lumped-parameter and 3D computerized fluid dynamics (CFD) models to understand current ventilation conditions and the impact of changes made in recent years. Lumped-parameter models were used to evaluate the main variables over long time periods, while CFD models were used for shorter time periods and steady-state calculations.
To reduce the computational requirements, we create separate models for the upper and lower levels of the train shed, and used the results of each one as a boundary condition for the other. Although they were only three feet across, the sidewalk grilles were found to have a relatively large impact on available ventilation.
We then validated the model by comparing its predictions to temperature and humidity measurements in the terminal. Air currents produced by the model were compared with observations of the smoke from small trash fires that sometimes occur in the train shed.
Measurements correlated so well with the results of our study that Mott MacDonald was awarded an additional contract to design a new ventilation system for Grand Central Terminal.
We determined that the solution of the problem required a combination of additional grille space and increased air supply through active ventilation. Air needed to be moved from the south end of the train shed to the north end so it could flow out the sidewalk grilles.