The lock-up phenomenon of exhaled flow in a stable thermally-stratified indoor environment

Expiratory flows of an infected individual can spread diseases in indoor environment. This probably explains why there have been many studies of how exhaled flow disperses and interacts with ventilation. A number of experimental studies have found that the exhaled flows can be locked up in the stratified layer in a room ventilated by displacement; however, its mechanism has not been explored so far. A non-dimensional theoretical buoyant jet dispersion model was developed here to compare the dispersion characteristics of the exhaled flow in a thermally stratified environment with those in a thermally uniform environment. Our results show that the exhaled flow can freely float upward in a uniform environment while it exhibits an oscillatory trajectory at a certain height in a thermally stratified environment. The terminal temperature of exhaled jet flow is equal to the ambient temperature. The latter is the so-called lock-up phenomenon as observed in some early studies. In displacement ventilation, the location of the lock-up layer is lower than the stratification height. A smaller Ar number and a steeper temperature gradient lead to a lower lock-up height in a thermally stratified environment. The variation of the lock-up height with different temperature gradients is found to follow a power law relation.