Effect of occupant age and activity level on the performance of a displacement ventilation system

Displacement ventilation (DV) systems rely on occupant-generated thermal plumes to drive upward airflow and maintain vertical thermal stratification. As heat output strongly influences DV performance, we investigate system behavior with pre-pubertal child occupants producing substantially less heat than adults and at lower elevations. We use a child-specific heat balance model to tabulate sensible and latent heat generation values for pre-pubertal children, pertinent for HVAC design of elementary classroom environments, currently unavailable in literature. A computational fluid dynamics model of a DV-conditioned room with two heated cylinders representing 8-year-old occupants is developed and experimentally validated. Using the validated model, we compare system performance for 8-year-old children and adult females under low, medium, and high metabolic rates, while maintaining identical DV design parameters intended for adult occupancy. Results show child occupants substantially weaken vertical thermal stratification by 1.9 °C and modify airflow patterns, particularly at lower activity levels. This shift modified the diffuser flow, producing a largely isothermal supply-air trajectory in the low-activity child case. With child occupants, indoor air quality indicators showed reduced performance compared to adults; age of air increased by 11% and ventilation efficiency within the occupied zone decreased by 10%. Overall, results demonstrate that DV system behavior changes notably when children occupy spaces designed for adults, particularly at low-activity conditions, affecting airflow, stratification, and air quality. These findings emphasize the need to reassess DV performance in child-dominated environments such as classrooms. The presented sensible and latent heat tables also provide practical reference for future HVAC simulations and thermal comfort evaluations with child-occupants.