Dynamic effects of occupant motion on indoor vertical thermal stratification in the displacement ventilation system

As an inevitable disturbance source, indoor occupant motion can weaken vertical thermal stratification and reduce the ventilation efficiency of the displacement ventilation (DV) system. However, the weakening process of vertical thermal stratification during occupant motion remains unclear. In order to clarify how occupant motion weakens the indoor vertical thermal stratification, occupant motion within a stratified environment in the DV system by numerical simulation was analysed. Initially, the Q criterion number, which represents the second invariant of the velocity gradient tensor, was used to visualise the vortexes in the wake of the moving occupant. Subsequently, the indoor vertical thermal profile variation during long-term occupant motions was analysed. Finally, the indoor vertical thermal profiles under different occupant moving velocities in the DV system were explored. Our research revealed that the reverse horizontal flow generated by the collision of the wake with the walls is the main reason for the weakening of the vertical thermal stratification. The vertical thermal profile undergoes four stages during the weakening process. In the first stage, the profile shape varies, followed by a monolithic translation. Thirdly, temperature variations only occur near the floor. Finally, indoor temperature stabilises even as occupants keep moving. When the human body moves at normal speeds ranging from 1.0 m/s to 1.6 m/s, the temperature gradient can decrease by approximately 45.0 % to 75.7 %. This study provides a more comprehensive understanding of how occupant motion affects the vertical thermal stratification of the DV systems to the designers, thereby offering robust support for research and practices in relevant fields.