Interaction characteristic of dual supply jets of graded ventilation: thermal barrier effect of secondary supply jet on primary supply jet for high ventilation performance

Graded ventilation, featured by its primary and secondary supply jets, is a promising and high-performance air distribution strategy for ensuring comfortable and healthy indoor environments efficiently. To clarify the mechanisms driving the high performance of graded ventilation, the characteristics of the interaction between its two supply jets are investigated. The bi-level contour analysis method is employed to map trajectories of the supply jets, utilizing the highest velocity and lowest temperature as defining criteria. The interaction of the two mechanical force asymmetry and non-isothermal supply jets is characterized by two subprocesses of converging and merging, and lacks subprocesses of recirculation and combination. The absence of the subprocesses of recirculation and combination allows the secondary supply jet to function as a thermal barrier on the primary supply jet that effectively inhibits the entrainment from the unoccupied zone. The suppression significantly reduces the velocity attenuation and temperature attenuation of the primary supply jet by 1.7%−9.7% and 3.0%−8.1%, respectively, and increases the supply efficiency into the occupied zone, boosting the thermal utilization effectiveness from 0.17 to 0.56, leading to an energy-saving of 16.1% and an improvement of 68.8% in inhaled air quality at thermal neutrality. Supply jet trajectory models of graded ventilation are also developed with high accuracy and robustness. This study contributes to providing theoretical support for the development of advanced air distribution.