User:P Brinckerhoff/Subway Environmental Simulation (SES) Program

Subway Environment Simulation

Subway Environment Simulation (SES) is a computer program used by engineers to model various aspects of train movement through a subway line and its effects on the surrounding environment.

Background and History

The SES Program is a designer-oriented tool which provides estimates of airflows, temperatures, and humidity, as well as air conditioning requirements, for both operating and proposed multiple-track subway systems. The capabilities of the SES program are comprehensive, permitting the user to simulate a variety of train propulsion and braking systems; various systems of environmental control (including forced air ventilation, station air conditioning, and trackway exhaust); airflows in any given network of interconnected tunnels, stations and underground walkways; any desired sequence of train operation (including the mixing of trains with different operating characteristics and schedules); various steady-state and non-steady-state heat sources; emergency situations with trains stopped in tunnels and air movement solely by mechanical ventilation and buoyant forces; and a special feature to simulate the long-range thermal impact of the possible reduction in the heat-absorbing capacity of tunnel walls after many years of system operation.

Notable Projects

The SES program was developed by Parsons Brinckerhoff under the aegis of the Transportation Systems Center of the United States Department of Transportation. The SES program was field validated in Montreal, Pittsburgh, San Francisco, Toronto, Washington and the Memorial Tunnel and has been applied to transit systems in Atlanta, Boston, Bucherest, Buenos Aires, Caracas, Frankfurt (Airport), Hiroshima, Hong Kong, Istanbul, London, Los Angeles, New York City, Philadelphia, San Francisco (BART, Caltrain, MUNI), Singapore, Toronto, and Washington, the Flathead, Hong Kong West Rail, Lotschberg, Moffatt, Mount Shaughnessy, Mount MacDonald/Rogers Pass and Stampede rail tunnels, Amtrak's New York rail tunnels, and a proof-check of the English Channel Tunnel ventilation concept.

Design Applications

As indicated above, the SES program has been validated in model tests and in actual practice. It is applicable to a variety of subway operating and design configurations and has been demonstrated to be a cost-effective tool for evaluating the performance of most types of environmental control strategies. Examples of situations in which the program can provide important design information include the following engineering questions:

• What is the most effective size, configuration, spacing and location for ventilation shafts and/or fan shafts in the system in terms of overall system environmental conditions (temperatures, humidities, air velocities and the movement of smoke and gases during a fire emergency) and power requirements for environmental control?

• What are the impacts of various operating schedules, vehicle headways, vehicle speeds, and train sizes on vehicle power demand and system temperatures, air velocities, and pressure transients?

• What is the impact of vehicle air conditioning on overall heat rejection in the system and on the temperatures and humidities in stations and tunnels?

• What are the comparative impacts of various vehicle propulsion and braking systems on overall system temperature?

• What is the effect of track vertical alignment on system temperatures and power consumption? What are the long-term trade-offs between lowering track sections between stations and the costs of power for propulsion and environmental control?

• What are the energy consumption implications of vehicle air conditioning alone versus air conditioning the entire system?

• What are the long-term and short-term effects of heat-sink? How much heat can be absorbed by the tunnel walls? What rate of heat absorption can be sustained for 30 years or longer?

• What effect does evaporation from wetted walls have on the overall system temperatures and humidities?

• What effects does operating the mechanical ventilation fans have during normal scheduled train operation?

• What are the acceleration profiles of vehicles at various parts of the system? How much time is required for a vehicle to traverse the length of the system?

• What are the effects of emergency control procedures on the subway environment? (For example, what are the purge times for smoke in the system?)

• What effect does the heat release from an emergency fire in the system have on the overall environmental conditions?

• What are the dynamic temperature and airflow conditions that prevail during a fire emergency?

• What ventilation system capacity is adequate to control the spread of smoke and heat during a fire emergency?

The above noted examples of program applications are by no means exhaustive. Indeed, the program is capable of estimating the environmental effects and implications of varying many of the design parameters of a multi-track subway system.

Subprogram Computation Sequences

Train Performance

The operation of trains provides a forcing function for the air movement in an underground transit system, and the heat dissipation from transit vehicles may account for as much as 90 percent of the heat released to the system. Consequently, a knowledge of the location, speed and acceleration of the trains within the subway system is essential to determine the rate and location of subway heat release as well as the system airflow regime.

The SES train performance subprogram provides the engineer with several options for simulating the operation of trains within a subway of which the most comprehensive is the implicit train performance. This option provides the engineer with a complete simulation of all aspects of train operation in the system. This option represents both the highest level of sophistication in the SES train performance computations and the greatest flexibility in evaluating trade-offs between train operations (headway, speed, etc.) and subway environment. The SES implicit train performance option differs from most conventional train performance computations in two important respects: (a) the SES subprogram has been designed specifically to accommodate accurate, continuous computation of the total heat released by trains, passengers and ancillary equipment such as air conditioning, and (b) the SES program permits the direct computation of the aerodynamic drag acting on each of the trains in the system, using continuously computed aerodynamic parameters. Conventional train performance programs are not ordinarily concerned with the continuous evaluation of vehicle heat release, and in evaluating vehicle aerodynamic drag these programs ordinarily settle for a semi-empirical relationship based on train velocity and blockage ratio (the ratio of the train frontal area to that of the tunnel cross section). The actual aerodynamic drag on a train fluctuates continuously as it encounters variable annular airflows resulting from changes in tunnel diameter, ventilation shaft location, mechanical ventilation, and the pressure caused by other trains.

Aerodynamics

The airflow through a subway system affects the comfort of subway patrons both directly and indirectly. Air movement is directly responsible for the convective transfer of heat and humidity through the system, and the cooling effects of moving air can directly influence the comfort of persons in non air-conditioned vehicles and in station areas. Furthermore, the buildup of excessive air pressures in stations from train piston effect has been known to constitute a separate operating problem, sometimes causing doors at entranceways to swing hazardously or become difficult to open. Airflow indirectly influences the heat content of subway air in two respects: (a) the aerodynamic drag on vehicles resulting from air motion relative to the train affects the power consumption (and heat rejection) of the vehicle motors, and (b) the rate of heat transfer into the surrounding deep-heat sink is dependent upon the air velocity at the air-wall interface.

Airflow in a subway is generated by two primary sources: the piston effect of trains moving through confined tunnels and mechanical ventilation by fans. The mathematical model which has been developed to describe this flow for a subway assumes the flow to be unsteady, turbulent, incompressible, and effectively one-dimensional. The unsteady nature of airflow in subways precludes the use of approximate analyses based on the assumption of steady-state flow, because the air velocities generated by trains with arbitrary operating schedules moving through tunnels of varying shape and size must fluctuate continuously. Steady-state flows may only develop in the absence of train movement.

Temperature/Humidity

The temperature and humidity of the air throughout a subway system reflect the heat added or removed by underground equipment, trains, and patrons, as well as by the rate of heat exchange across the system walls and by mixing with external ambient air. An analytical treatment of this dynamic heat regime must provide a means to describe these phenomena mathematically in an operating system. The acceleration and braking of trains produces the main source of sensible heat in an operating subway system, but sensible and latent heat are also added by electrical equipment, patrons, and in certain instances, the surrounding earth. Heat is removed from the system mainly by the expulsion of warm system air through ventilation shafts and by heat conduction across the tunnel walls into the surrounding heat sink. Heat may also be added or removed by mechanical means such as heating and air conditioning.

In developing an analytical description of the heat regime, it was concluded that the system could be treated as one-dimensional, meaning that the air temperature and humidity can be considered uniform over any cross section. Axial conduction heat transfer in the system air was assumed to be negligibly small in comparison with the heat convected by moving air. The heat contributed by viscous dissipation resulting from air friction against the system walls, while usually small, can optionally be considered as a variable heat source.

Heat Sink/Environmental Control

There are three key independent factors which influence subway air temperature: system ventilation as determined by geometrical configuration, train operations and mechanical systems; system heat load, which relates directly to utilization of the subway; and outside ambient temperature. A fourth factor affecting subway air temperature is the heat transfer between the air and the surrounding structure and earth. In contrast with the first three factors, an interdependence exists between this heat transfer (commonly referred to as a "heat sink" effect) and the air temperature: the subway air temperatures directly influence the heat conduction history of the surrounding earth, since the rate of heat flux between the subway air and the walls is dependent on the convective heat transfer coefficient and the temperature difference between the air and the wall surfaces. One purpose of the heat sink/environmental control subprogram is the evaluation of this interdependent behavior.

During the relatively short-term simulation periods of the SES aerodynamic and temperature/humidity subprograms, the surface temperature of the subway structures is essentially constant. However, subway wall temperatures ordinarily experience daily and annual fluctuations because of variations in outside conditions and subway operating schedules. There may also occur a gradual increase in the average wall surface temperature over a period of years either as a result of prolonged internal temperatures above outside ambient conditions or because of increases in system utilization. Thus, to accomplish its purpose, the heat sink/environmental control subprogram must address not only the air-wall temperature interdependence, but also the conduction of heat in the earth as influenced by the daily, annual, and long-term variations in the subway air temperature. Whereas the short-term simulation evaluates subway airflows and temperature on a second-by-second basis, the heat/sink environmental control subprogram evaluates a phenomenon which is measured in terms of hours, days and years. Thus, this subprogram involves a shift in time scales and the link with the short-term simulation is accomplished through a process involving the averaging of short-term simulation results.

References

Parsons Brinckerhoff, Quade & Douglas, Inc. Subway Environmental Design Handbook. 3rd ed. Vol. 2. New York, NY: Parsons Brinckerhoff, Quade & Douglas, 1975. Print. Subway Environmental Simulation Computer Program (SES)

External Links

http://www.fta.dot.gov/assistance/technology/research_4507.html