Ensure Low Carbon Monoxide Levels in Parking Structure
Computer simulation helped ensure low carbon monoxide levels in a new parking structure by making it possible to evaluate the performance of different ventilation system designs without the expense of actually building and testing them. The main concern in the design was ensuring that carbon monoxide would remain below specified levels even when 125 cars were waiting to exit the garage with their engines running for a long period of time.
Engineers evaluated the performance of the ventilation system diffuser with an easy-to-use computational fluid dynamics (CFD) tool that lets the user accurately model airflow, heat transfer, contaminant transport and thermal comfort for internal as well as external building flows. Five different diffuser configurations were evaluated while the total capacity of the ventilation system was maintained at a constant level.
Carbon monoxide concerns
The operation of automobiles indoors presents many concerns, such as the emission of carbon monoxide, nitrous oxides, and oil and gasoline fumes. However, it is generally accepted that the ventilation required to dilute carbon monoxide to acceptable levels can control the other contaminants satisfactorily. A secondary concern is maintaining sufficient draft on each underground floor to avoid mold growth.
The traditional approach to designing the ventilation system would be to use hand calculations, whose accuracy is reduced by several factors. First, these calculations don't take the geometry of the structure into account. Second, they determine only average carbon monoxide content but not the spatial distribution or gradients in the distribution, which can have an important impact. The result is that engineers are unable to be certain about the performance of the design until the ventilation system is installed and tested. The possibility exists that expensive changes will have to be made after testing is performed.
A ventilation tool
Dunham engineers selected Airpak, a CFD software package from Fluent Incorporated, Lebanon, NH, that is specially designed to simplify the process of modeling ventilation systems. Airpak is an accurate, quick, and easy-to-use design tool which simplifies the application of state-of-the-art CFD technology to the design and analysis of ventilation systems, which are required to deliver indoor air quality (IAQ), thermal comfort, health and safety, air conditioning, and contamination control solutions.
The underground parking structure for which the ventilation system was designed is fully enclosed and has seven levels. Its total size is 614 feet by 224 feet by 98 feet. The ground surrounds the structure on four sides, and the top is covered by a residential facility. The seven levels are labeled from A (the highest) to G (the lowest) and each has a different floor plan. Level A is on the ground level. There is also an excavation area along the parking levels that creates a vertical flow channel. There are three entrance and exit areas in this structure, on levels A, C and D. The 125 cars were simulated in seven groups.
One criteria for the design was to minimize the areas with air velocity above 200 fpm in order to avoid draft discomfort and to reduce the area below 20 fpm to avoid possible contamination buildup. At this moment, there is no clear understanding of the relation between the speed of the air and the microbial growth. But it has been widely accepted that low velocities provide a more comfortable environment for microbial buildup. For this reason, special efforts were made to avoid stagnant air in the excavation area. Five different diffuser configurations were evaluated in the analysis. Each of these cases maintains the same total supplied air volume and the same exhaust system.
Only the locations and size of diffusers were modified in each case. Case 1 has two columns of diffusers on the north and south side. Case 2 has split diffusers (each diffuser split into two smaller ones) on both the north and south sides. Case 3 has a split diffuser oriented in the 6:00 and 7:30 directions on the north side. Case 4 also has split diffusers that are oriented in the 4:00, 6:00 and 10:30 directions. The idea in Case 4 is to reduce the high draft area by directing the flow into an excavation area. Case 5 maintains the split configuration but diffusers are laid side by side and flow directions are 10:30 and 7:30.
After a thorough comparison of all of the results, engineers selected the best design in terms of carbon monoxide distribution and airflow. The ventilation system was built to this design and testing showed that it met all the design requirements without any modifications.
Heejin Park Ph D., P.E., is Senior Technical Specialist with Dunham Associates in Minneapolis, MN.
For more information, contact
Fluent Inc. (www.fluent.com).