Dynamic interaction of a downward plane jet and a cough jet with respect to particle transmission: An analytical and experimental study

ABSTRACT

A cough jet can travel beyond the breathing zone of the source person, and thus, infectious viral- and bacterial-laden particles can be transported from the source person to others in close proximity. To reduce the interpersonal transmission of coughed particles, the objective of this study was to analytically and experimentally investigate the performance of downward plane jets with various discharge velocities. Chamber measurements were conducted to examine the interaction between a transient cough jet (discharge velocities of 12 m/sec and 16 m/sec) and a steady downward plane jet (discharge velocities from 1.0–8.5 m/sec) with respect to the transport of and human exposure to coughed particles. The results show that a relatively high-speed cough can easily penetrate a downward plane jet with a discharge velocity of less than 6 m/sec. A downward plane jet with a discharge velocity of 8.5 m/sec can bend the cough jet to a certain extent. In this study, momentum comparison of the cough jet and the downward plane jet shows that the value of personal exposure to coughed particles depends on the ratio of jet momentums. The results show that when the two momentums are equivalent or if the downward plane jet has a greater momentum, the cough jet is deflected downward and does not reach the breathing zone of the target thermal dummy. Using the ratio of the two momentums, it may be estimated whether the transmission of a cough jet can be controlled. A trajectory model was developed based on the ratio of the two momentums of a cough jet and a downward jet and was validated using the experimental data. In addition, the predicted trajectory of the cough jet agreed well with the results from smoke visualization experiments. This model can be used to guide the design of downward plane jet systems for protection of occupants from coughed particles.

KEYWORDS:

• Airflow distribution
• cough jet
• downward plane jet
• dynamic interaction
• personal exposure

Funding

The authors wish to express their thanks for financial support from the Academy of Finland through the postdoctoral project POWER-PAD (NO. 259678) and financial support from Luonnontieteiden ja Tekniikan Tutkimuksen Toimikunta (252708).

Nomenclature

A	=	a constant in the equation of cough trajectory
Ar	=	Archimedes number
a0	=	the area of the “mouth” to expel a cough, (m2)
B	=	a constant in the equation of cough trajectory
Cexp	=	particle number concentration measured in the breathing zone of the target dummy, count of particles/cm3
Ccough	=	particle number concentration measured in the cough generator, count of particles/cm3
d	=	the nozzle diameter of the cough generator (m)
g	=	gravitational acceleration (m/s2)
h	=	the slot width (m)
K	=	a dimensionless constant of the downward plane jet, 2.4 is used in this study
k	=	a dimensionless constant of a cough jet, 6.2 is used in this study, which is obtained from measured cough velocities
PE	=	the personal exposure to coughed particles (%)
PEbkg	=	the personal exposure to background particles (%)
r	=	effective velocity ratio
U0	=	the initial velocity of a downward jet (m/s)
Ucm	=	the local maximum jet velocity at a distance of x (m) downstream from the cough generator (m/s)
Uc0	=	the cough discharge velocity (m/s)
Um	=	the local maximum centreline velocity of a downward jet (m/s)
x	=	the horizontal distance downstream from the cough generator (m)
y	=	the vertical distance of downstream from the slot (m)
β	=	the coefficients of thermal expansion 3.4 × 10−3 (1/K)
ρcj	=	the air density of a cough jet (kg/m3)
ρdf	=	the air density of a downward jet (kg/m3)
ΔT0	=	the temperature difference between a cough jet and the background air in the chamber (°C)