By Galen Burrell and Michael J. Holtz, Architectural Energy Corporation, Boulder, CO

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Architect's rendering of the building exterior

The National Museum for the Marine Corps will soon be under construction in Quantico, Virginia. A project 10 years in the making, the Museum is dedicated to sharing the heritage of the Marines with the American public. Visitors will enter the museum through an entry courtyard and lobby into a dramatic, glass-enclosed atrium called the Central Gallery. Symbolizing the Marine's World War II victory at Iwo Jima, the Central Gallery features a steel mast rising 180 feet from the floor to the top of a conical glass skylight. The base of the gallery is underground and measures 150 feet in diameter and 45 feet high. The glass skylight structure rises an additional 110 feet above the gallery walls. Several full-size airplanes will be suspended from the skylight structure, while kiosks and displays will be located throughout the Central Gallery and adjacent exhibit halls.

Given the large skylight glazing area, concerns arose about the influence of solar heat gain in the summer and condensation on the glass during the winter. Fentress Bradburn Architects, the museum designers, asked Architectural Energy Corporation, the project's energy, daylighting and sustainable design consultants, to explore these issues using Airpak. Architectural Energy Corporation has been performing sophisticated energy and daylighting modeling for many years, and has added CFD to their portfolio of modeling tools. Another purpose of the CFD analysis was to overcome the limitations of traditional HVAC design tools, which are constrained by a fully mixed (i.e. uniform temperature) zone model, and tend to overpredict the anticipated cooling load. In this tall glass structure, it was obvious that thermal stratification would occur during the summer, thus requiring only the lower occupied zone to be conditioned.

Using the CAD import tool in Airpak, a simplified model was created with 700,000 cells. Conditioned air was supplied by nozzle type diffusers located around the perimeter of the Central Gallery drum at a height of 18 feet. The nozzles were modeled using an effective area calculated by the Airpak diffuser macro to achieve the correct throw specified in the manufacturer's performance data. Air was returned or exhausted in three locations: a fan in the top of the skylight, for venting to the outside, and return grilles in the entrance ceiling and along the floor, for returning the remaining air to the air handling unit.

Several indoor and outdoor design conditions were simulated to determine the extent of thermal stratification that would occur, and what supply air flow rate was necessary to maintain comfort in the occupied zone. A 90,000 CFM supply scenario was suggested by a mixed-zone model to satisfy the cooling load. Airpak showed that this scenario significantly overcooled the occupied zone during summer design conditions.

Reducing the supply air flow to 60,000 CFM achieved near ideal comfort conditions in the occupied zone (75°F), while allowing the skylight to stratify between 80°F and 115°F. A 40,000 CFM scenario was also analyzed, and showed overheating occurring in the occupied zone during peak summer design conditions. Using these results, Architectural Energy Corporation recommended lowering the cooling supply air flow from 90,000 CFM to 60,000 CFM, allowing the design team to downsize their HVAC equipment (and budget). All three scenarios required that at least 15,000 CFM be exhausted out of the top of the skylight to keep temperature extremes below 120°F.

Temperature contours on a slice through the gallery

Nighttime banquet conditions during winter were also modeled to determine the potential of condensation forming on the glazing and framing system of the skylight. Using Airpak and THERM, a glazing system heat transfer program developed by Lawrence Berkeley National Laboratory, it was found that with 500 people eating hot meals on a cold winter night, condensation would not occur. Taken together, the results allowed the architects and clients to feel confident in the proposed Central Gallery HVAC and skylight design.

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