Numerical investigation of impinging jet ventilation in ICUs: Is thermal stratification a problem?

Intensive care units (ICUs) are the high incidence sites of hospital-acquired infections, where impinging jet ventilation (IJV) shows great potential. Thermal stratification of IJV and its effect on contaminants distribution were systematically investigated in this study. By changing the setting of heat source or the air change rates, the main driving force of supply airflow can be transformed between thermal buoyancy and inertial force, which can be quantitatively described by the dimensionless buoyant jet length scale ((l) over bar (m)). For the investigated air change rates, namely 2 ACH to 12 ACH, (l) over bar (m) varies between 0.20 and 2.80. The thermal buoyancy plays a dominant role in the movement of the horizontally exhaled airflow by the infector under low air change rate, where the temperature gradient is up to 2.45 degrees C/m. The flow center remains close to the breathing zone of the susceptible ahead, resulting into the highest exposure risk (6.6 parts per thousand for 10-mu m particles). With higher heat flux of four PC monitors (from 0 W to 125.85 W for each monitor), the temperature gradient in ICU rises from 0.22 degrees C/m to 1.02 degrees C/m; however, the average normalized concentration of gaseous contaminants in the occupied zone is reduced from 0.81 to 0.37, because their thermal plumes are also able to carry containments around them to the ceiling-level readily. As the air change rate was increased to 8 ACH ((l) over bar (m) = 1.56), high momentum weakened the thermal stratification by reducing the temperature gradient to 0.37 degrees C/m and exhaled flow readily rose above the breathing zone; the intake fraction of susceptible patient located in front of the infector for 10-mu m particles reduces to 0.8%. This study proved the potential application of IJV in ICUs and provides theoretical guidance for its appropriate design.