IDENTIFICATION OF VENTILATION PROBLEMS IN AN UNDERGROUND BUS TERMINAL IN KOREA H Han 1* and Y-I Kwon 2 1 Dept. Of Mechanical Engineering, Kookmin University, Korea 2 Dept. Of Building Mechanical Engineering, Shinheung College, Korea ABSTRACT It is the objective of the present study to identify ventilation problems and to suggest design modifications of the ventilation system to improve IAQ in the terminal. Numerical results on air velocity and concentration distributions are presented along with some of field experimental data on indoor concentrations of NO x, CO and suspended particles. Results show many interesting aspects of airflow patterns affecting the ventilation characteristics of the building. System modifications on airflow rates and grille locations have been suggested from the numerical results in order to prevent direct short-circuiting of outdoor air and recirculation of contaminated indoor air. The overall exhaust performances of the modified conditions are compared quantitatively with existing ventilation condition using the concept of the mean residual-life-time of contaminated air. INDEX TERMS Ventilation effectiveness, Underground bus terminal, Numerical, Indoor air quality INTRODUCTION Bus terminals are usually built on a ground level, which are completely open or partially enclosed for weather protection. Engine emissions are exhausted mostly by natural ventilation with supplementary mechanical ventilation equipments for platform areas. For an enclosed underground terminal, however, a proper design of mechanical air distribution system is very important to control terminal environment since the natural ventilation is limited (ASHRAE, 2001). A large bus terminal complex with ground area of 27308 square meters was constructed in a newly built city of Sung-Nam; located approximately 30km south of Seoul. In order to prevent engine emissions from being dispersed out of the terminal to nearby residential areas, the underground level was assigned for a bus terminal. As there was a great concern on indoor air quality in the terminal, however, bus companies and shop renters asked for clarifications of the indoor air quality issue before they started businesses in the building. A research project has been undertaken for the assessment of the indoor environment and the ventilation systems of the terminal building, and for the suggestions of ventilation system modifications (SAREK, 2001). Field experiments were conducted to visualize airflow patterns by the existing ventilation system and to measure indoor concentrations of CO, NO x, and suspended particles with 19 buses idling at the platform. The actual airflow rates through supply and exhaust ducts and grilles were measured and compared with the design values. Computational fluid dynamic calculations were also conducted to simulate airflow and concentration distributions not only for the current ventilation condition but also for suggested modified conditions. * Contact author email: hhan@kookmin.ac.kr 337
The objective of the present paper is to report some of the engineering findings from the investigation. The distributions of local mean residual-life-time of contaminated air are obtained from the calculated airflow velocity distributions for various configurations of ventilation systems; i.e. exhaust grille locations and ventilation airflow rates. CFD results show that ventilation effectiveness can be improved significantly by relocating exhaust grilles and supply ducts to prevent a stagnation of indoor air and a direct short-circuiting of outdoor air to exhausts. VENTILATION SYSTEM The terminal complex consists of a 11-story shopping center building and a multilevel (7 levels above ground and 3 stories underground) garage building. The basement of the shopping building is used for ticket counters, waiting rooms, and some retail stores. The underground level of the garage building is used for passenger platforms and other facilities for bus operations. The platform has a sawtooth gate configuration as is shown in Fig. 1. There are two busway ramps to ground level, A and B, and two ramps to B2 level, C and D. Supply grilles are located above the platform in two rows, and exhaust grilles are located near ramps on the other side of the platform. There are two fan rooms for exhaust. Exhaust grille α is connected to Fan room 1, and grilles β, γ are to Fan room 2. The principal design concept is to displace contaminated indoor air in one direction, from the platform areas to the other side of the building. Tangential long-fans are installed on the ceiling to help creating uniform flow in the direction. The locations of long-fans are also shown in Fig. 1. Figure 1. Plan view of the terminal building and calculation domain. The design airflow rate is 180,000CMH, which corresponds to 8ACH approximately. 100% outside air is supplied with no recirculation. There is no IAQ standard for underground bus terminals. As a pertinent standard, there is an Act on Air Quality in Underground Spaces, 338
which states PM10 be less than 150µg/m 3, CO 10ppm, CO 2 1000ppm, NO 2 0.15ppm. There is a regulation on outdoor parking lots, which states CO should be below 50ppm on 8hr basis. SIMULATION METHODS The airflow rates under actual operations of the existing ventilation systems have been measured lower compared to the design values, because of duct leakages and fan performances. Numerical simulations are conducted for four difference conditions as shown in Table 1. Case 1 is the design condition, and Case 2 is the measured condition. Case 3 and 4 are modified conditions with increased airflow rates and modified grille locations and configurations. Table 1. Airflow conditions for numerical simulation Design T.A.B. Modified Condition Condition Condition 1 (Case 1) (Case 2) (Case 3) Supply airflow rate Exhaust airflow rate Modified condition 2 (Case 4) 180,000 103,000 230,000 230,000 180,000 140,000 265,000 265,000 Exhaust at α 90,000 61,400 80,000 130,000 Exhaust at β+γ 90,000 78,600 185,000 130,000 Remarks -Ramps D closed. -Additional supply diffusers at corners. -Ramps D closed. -Additional supply diffusers at corners. -Supply air in the downward direction. -Long-fan locations shifted. A commercial CFD code based on the finite element method has been utilized. A continuity equation and momentum equations are solved simultaneously along with temperature and concentration equations. A standard k-e model is used to calculate turbulent quantities. The dimensions of the simulation domain are 110mx70mx5m, and the numbers of grids are 204x99x50. A body-fitted coordinate system is used to account for complex boundaries as shown in Fig. 1 with bold lines. Boundary conditions used are no slip conditions for velocity, and adiabatic conditions for heat and mass transfer along the boundaries except bus ramps and supply/exhaust grilles. Pressure boundary conditions are applied for the ramp openings. The indoor air temperature is assumed to be isothermal at 20 o C, except the temperature along the boundaries of Ramps A and B at 30 o C. Nineteen buses with diesel engines are simulated, idling at the platform area. Since the concentration of emission gas varies significantly from one engine to another, the concentration equation is non-dimensionalized with a tailpipe emission concentration. Local mean residual-life-time (LMR) is defined as the time for an air particle at a point P to reach an exhaust. It indicates how quickly contaminated air can be removed from the system. LMR can be calculated from a transient concentration decay, C ex p (t), measured at the exhaust by a step-up generation at point P using the following equation (Etheridge and Sandberg, 1996). LMR p Cex ( t) p = 1 dt (1) 0 p Cex ( ) where t= means a steady state condition. The mean value of LMR shows overall ventilation performance of the exhaust system with the given emission locations. The mean values can be also calculated from the concentration at the exhaust (Han, 1996). The higher the 339
concentration is at the exhaust, the higher the exhaust effectiveness is for the given emission source locations. The overall exhaust effectiveness is defined as the ratio of the nominal time constant to the mean value of LMR. RESULTS AND DISCUSSION Concentration distributions and air quality standards The dimensionless concentration contours are shown in Fig. 2a superimposed with some of measured CO and NO x concentrations. Measured CO varies from 4.1 to 13.1ppm. Even though the actual TAB airflow rate is lower than the design value, measured CO is below the parking lot standard, but is slightly over indoor air quality standard of 10ppm. NO x is measured between 1.0 and 2.9ppm, and PM10 is measured between 297 and 359 µg/m 3, which is approximately twice of the IAQ standard. Generally speaking, there is a good agreement between calculated and measured concentrations in a relative sense. Figure 2b shows the concentration distribution for the modified condition 2 (Case 4). The concentration is significantly lowered especially at platform areas not only because of the increased flow rate but also because of the unidirectional airflow pattern created in the terminal. Details of the system modifications will be shown subsequently in the followings based on numerical observations. (a) TAB condition (Case 2) (b) Modified condition 2 (Case 4) Figure 2. Concentration distributions calculated for Case 2 and Case 4 (Y=0.3m) Stagnation of contaminated air at platform corners As the supply grilles have not been distributed uniformly along the platform, contaminated air is stagnant at corners and is not pushed toward exhaust grilles. As is shown in Fig. 2, relatively high concentrations are obtained near buses 1-2 and 18-19. Additional supply diffusers need to be installed at the corners of the platform. The concentrations are significantly decreased especially at the corners as is shown in Fig. 2b for the modified condition 2. Short-circuiting of outdoor air to exhaust Since the exhausts are located near large ramp openings, fresh air from outside is shortcircuited to the exhaust grilles directly without attracting contaminated indoor air as was originally designed. For winter conditions, significant amount of cold air is expected to be down-drafted from the ground level due to the temperature difference. The entrained outdoor air would lower the concentration level in the vicinity of the entrance, but deteriorate the overall exhaust performance seriously. This is not the case for summer conditions. However, a short-circuiting at the exhaust α is serious for summer conditions. Flow visualization also 340
exhibits the fact and the entrainment takes place mostly through the upper part of Ramp D. A concentration distribution normal to the exhaust grille is plotted in Fig. 3. Because of the entrainment, the concentration is decreased in the range of 1-3m away from the exhaust especially at Y=4m. It is suggested to close the ramps to B2 level completely to minimize short-circuiting of entrained air from below. Figure 3. Concentration distribution in the normal direction from the center of Exhaust α Induction by high-speed air jets It is desirable to pressurize the platform area slightly so that contaminated air should displace away from passengers. The horizontal supply air jets of high velocity generate entrainment from surroundings. In Fig. 4, it is interesting to notice a region of negative air velocity for Z=16m. Because of the high speed discharges from the jets, the air between the jets can flow in the opposite direction towards the platform area to satisfy flow continuity. As the air jets move toward the exhausts (as Z increases), two jets are merged together eventually. Some of the horizontal jet are impacted on the crossing beams and deflected. They do not contribute to push the contaminated air to exhaust area effectively. It is suggested to discharge supply air in the downward vertical direction with increased grille areas. Decreased face velocity would help to reduce thermal discomfort by draft also. Figure 4. Velocity profile in the x-direction between two horizontal supply jets (Y=1.0m). Distraction of thermal plume by long fans Hot exhaust gases from a tail pipe are discharged horizontally, but have slight upward buoyancy. One of the trajectories of the exhaust plumes is shown in Fig. 5 for bus #19 idling at the platform. The dispersion of an exhaust gas depends on the surrounding airflow conditions. However, it is found, on the average, that the exhaust plumes reach the ceiling between 8-13m from the bus rears. The existing long fans distract the upcoming plumes. The locations of the long fans are required to shift in the z-direction by 6-7m approximately to push effectively the emission plumes. With the shifted long fans, emission plumes no longer 341
rise to the ceiling but move horizontally to exhausts, except the plumes from bus #14-#19. Ceiling exhausts, if needed, are suggested to install at approximately 10m from the rear of the buses to capture the rising plume effectively. Figure 5. An emission plume from a bus tail pipe Intrusion of contaminated air to waiting areas The original design intends to neutralize the pressure in the underground terminal by balancing supply and exhaust airflow rates. In winter, the pressure is negative in the basement of the shopping building due to the stack effect. The contaminated terminal air can be entrained into the waiting area through the cracks between doors. The underground terminal should be depressurized. Moreover, it is suggested to use swing doors to minimize the airflow exchange between the terminal building and the shopping building. Overall exhaust effectiveness Results on overall exhaust performances are compared in Table 2. The current TAB condition shows the worst effectiveness. The design condition and the modified condition 1 show similar effectiveness. The effectiveness is greatly improved up to 128.5% for the modified condition 2. Note the exhaust effectiveness is 100% for a completely mixed condition. Table 2. Overall exhaust effectiveness for the conditions considered. Design Condition T.A.B. Condition Modified Condition 1 Modified Condition 2 Mean concentration at exhaust 1.06x10-3 2.17x10-3 1.80x10-4 2.62x10-4 Space average concentration 2.30x10-3 4.57x10-3 1.82x10-3 1.72x10-3 Mean residual-life-time of exhaust (sec) 992 3075 746 355 Nominal time constant (sec) 582 1018 456 456 Exhaust effectiveness (%) 63.1 33.1 61.1 128.5 ACKNOWLEDGEMENTS This paper is an extension of the project funded by the Korea Real Estate Trust. The authors also would like to express thanks to the persons participated in the project, especially Dr. H.J. Shin, Prof. J.O. Yoo, and Prof S.K. Sung. REFERENCES ASHRAE. 2001. Fundamentals-Handbook, Atlanta: American Society of Heating, Refrigerating, Airconditioning Engineers, Inc. Etheridge DW, and Sandberg M. 1996. Building Ventilation: Theory and Measurement. Chichester: John Wiley and Sons. Han H. 1996. Evaluating Exhaust Effectiveness using Inverse-Flow Calculation Method, Proceedings of 5 th Int. Conference on Air Distribution in Rooms Room Vent 96, pp 47-52. Yokohama: Room Vent 96. SAREK. 2001. Problem Identifications and Remediation Suggestions on Ventilation Systems of Sung- Nam Passenger Bus Terminal Complex Underground Platforms. Final Report: Seoul: The Society of Air-conditioning, Refrigerating Engineers of Korea. 342