Graded ventilation with primary supply air confined in critical demand zone by overcoming negative and positive thermal buoyancies

Developing advanced air distribution systems is a crucial approach for achieving both energy efficiency and carbon neutrality. While supply-momentum-driven air distribution strategies enhance ventilation performance by delivering conditioned air directly to the head-level critical demand zone, their effectiveness is often compromised by thermal buoyancy forces that divert airflow away from the target region. To address this challenge, this study proposes a novel graded ventilation strategy designed to confine primary supply air within the critical demand zone. This strategy employs two distinct mechanisms to ensure the optimal utilization of fresh, cooled primary air. During the initial stage of the supply air jet development, a secondary air supply positioned beneath the primary airflow generates a lifting force, preventing the primary airflow from sinking prematurely into the lower zone due to negative thermal buoyancy. During the later stage of the supply air jet development, the return airflow exerts a downward suction on the underutilized primary air, preventing it from being entrained upward by thermal plumes driven by positive thermal buoyancy. Case studies compare the proposed graded ventilation with two representative supply-momentum-driven air distribution strategies (i.e., stratum ventilation and interactive cascade ventilation). Flow field and dimensionless analysis demonstrate that graded ventilation more effectively confines conditioned fresh air within the critical demand zone. Under different thermal comfort levels, the proposed graded ventilation improves indoor air quality by 30%–81% and 22%–66% and reduces energy consumption by 14.9%–17.7% and 7.5%–12.2%, compared to stratum ventilation and interactive cascade ventilation, respectively.