Air Flow and Heat Transfer in a Temperature-Controlled Open Top Enclosure

A large 12-meter-diameter open top enclosure (OTE) equipped with two unique belowground and above ground heating systems was built and intensively tested in Oak Ridge, TN, USA. The OTE is a prototype for use within an environmental change experiment, in which replica units will be built in Minnesota to assess the response of northern peatland ecosystems to increases in temperature and elevated atmospheric CO₂. For several months, temperatures, energy, wind speed and relative humidity were monitored throughout the enclosure space to assess the enclosure performance and efficiency. In parallel, Computational Fluid Dynamics (CFD) simulations were performed with ANSYS-CFX to investigate the impacts of external wind, buoyancy, and OTE design on the temperatures achieved within the enclosure. The addition of a frustum that partially reduced the top opening was also investigated experimentally and numerically. The OTE is capable of achieving a temperature differential of at least +6°C for air using a combination of 8 electrical heaters. Differential temperatures were sustained for several months. The experimental data and the numerical results showed that the addition of a frustum dramatically decreases the operating cost of the OTE and leads to better control over the differential air temperature in the enclosure. Buoyancy forces and winds heavily impacted enclosure performance. It was also found that the heating efficiency of the OTE depends mainly on the wind speed, and that there exists a critical wind speed at which the heating efficiency is the highest.