The exhaust-return air decoupled strategy using high-temperature and low-temperature air as exhaust and return air, respectively, can effectively reduce the cooling load of the thermal buoyancy-driven stratified thermal environment. However, there is no exhaust-return air decoupled strategy for the other type of stratified thermal environments, i.e., the supply momentum-driven stratified thermal environment represented by stratum ventilation. This study uses experimentally validated CFD simulations to investigate the mechanisms behind the energy efficiency of the proposed decoupled strategy for the supply momentum-driven stratified thermal environment. According to the thermal stratification characteristics of stratum ventilation, the proposed decoupled strategy positions the exhaust and return vents above and below the supply vent, respectively. The mechanisms of energy efficiency due to the decoupled strategy are revealed and compared with two typical coupled strategies of stratum ventilation (the upper coupled strategy and the lower coupled strategy with the shared vents of the exhaust and return air located above and below the supply vent, respectively). The results show that the decoupled strategy has two major mechanisms impacting energy efficiency, i.e., 1) the positive mechanism lowers the return air temperature relative to the exhaust air temperature to reduce the cooling load of the return air; and 2) the negative mechanism with the upper vent drags the supply air up to the unoccupied zone, lowering air supply efficiency. With mechanism manipulation, the decoupled strategy saves energy by 30.7% and 21.3% compared with the upper and lower coupled strategies, respectively.
