Impinging jet duct diffuser ventilation effectiveness benchmarking with mixing ventilation in a mock-up classroom

The benefits of effective classroom ventilation for health and learning outcomes are well known. However, studies continue to report inadequate air quality, insufficient air change rates, and too high air velocities. This study investigated classroom air distribution solutions in the ventilation air distribution laboratory at Tallinn University of Technology. Nozzle diffusers and duct diffusers mounted in the ceiling were compared with two impinging duct diffuser configurations installed on the shorter sides or the single longer wall of the mock-up classroom. Draught risk was assessed using air velocity probes in a mock-up classroom setup with a dedicated room-based air handling unit and thermal dummies simulating student heat loads. Contaminant removal effectiveness was evaluated using the continuous tracer gas injection method with multiple point source locations. Results showed that close to fully mixed ventilation effectiveness was achieved both with ceiling and perimeter impinging ventilation. Measured contaminant removal effectiveness varied across 0.85 to 1.16. The 10° nozzle duct diffuser configuration demonstrated the best performance of impinging ventilation in terms of ventilation effectiveness and draught control. When positioned to the longer side of the room with the extract close to the opposite wall, this configuration further improved overall performance, though it resulted in elevated air velocities at floor level. To avoid draught, the airflow rate needed to be reduced with these perimeter nozzle duct diffusers to 3.3 L/s m2 while the ceiling nozzle and ceiling duct diffusers were capable of 4.5 L/s m2 without draught. The results demonstrate the potential and also a challenge of floor level air velocity of perimeter impinging jet ventilation worth to be further studied for practical applications.