Measured influence of supply airflow rate and supply air temperature on air mixing time in a room with overhead mixed system

¹ Indoor Environment Group, Sustainable Energy and Environmental Systems Department, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
² Center for the Built Environment, University of California, Berkeley, CA 94720, USA
³ Department of Architecture, University of California, Berkeley, CA 94720, USA
⁴ Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, USA

Abstract

Air mixing and movement are often driven by the design and operating conditions of the heating, ventilation, and air conditioning (HVAC) system, and are influenced by occupants and thermal gradients at windows and walls. Air mixing affects the indoor-generated pollutants dispersion and thus influences the effectiveness of ventilation and within-room air cleaning systems, including upper-room germicidal ultraviolet disinfection (GUV). In rooms with a ceiling exhaust and/or upper-room GUV, upward airflow from occupants can enable faster pollutant removal compared to well-mixed conditions. We used pulsed ethanol as a tracer and measured concentration at 2 s time resolution using fast-response metal oxide sensors at three levels: near the floor at 0.1-0.4 m, mid-height at 1.1-1.4 m, and at 2.4 m, 0.3 m from the 2.74 m ceiling. Forty experiments were conducted under the following conditions: HVAC off; supply air at ~380 (low) or ~1200 m³ h⁻¹ (high) at neutral, cooling, or heating temperatures with 20% or 100% outdoor air; and added mixing fans. Air mixing times were determined from the start of ethanol release until the relative standard deviation of the concentrations fell below 20%, indicating an approximately well-mixed condition. We found that the air mixing time was longest with the HVAC off (15.1-15.4 min), followed by conditions with a low total supply airflow rate at all temperatures (6.6-11.4 min under heating; 5.6-10.1 min under neutral/cooling), and was fastest under high airflow at neutral/cooling temperatures (2.9-5.6 min) or when mixing fans were added (1.9-1.8 min). The outdoor airflow rate (20% or 100%) did not significantly affect spatial heterogeneity or air mixing time. Long air mixing times resulted in high spatial variability of absolute integrated concentrations and relative exposures. Under slow mixing conditions (HVAC off or low airflow rate) and with releases associated with heaters simulating occupants, several upper or mid-level sensors peaked after the release, and mixing into the upper room and occupied space was slow. Under fast mixing conditions (high airflow rate with neutral/cooling settings or added fan), air reached and mixed in the upper room more quickly and spread faster in the occupied zone.