It is essential to investigate person-to-person contaminant transport in enclosed environments to
improve air distribution design and reduce the infection risk from airborne infectious diseases.
This study aims to develop various models for quickly predicting person-to-person contaminant
transport in enclosed environments and investigate the effect of influencing factors on the
contaminant transport.

Firstly, this study introduces a hybrid DES-Lagrangian and RANS-Eulerian model for simulating
transient particle transport in enclosed environments; this hybrid model can ensure the accuracy
and reduce the computing cost. Our investigation estimated two key time constants for the model
that are important parameters for reducing the computing costs. The two time constants
estimated were verified by airflow data from both an office and an aircraft cabin case. This study
also conducted experiments in the first-class cabin of an MD-82 commercial airliner with heated
manikins to validate the hybrid model. A pulse particle source was applied at the mouth of an
index manikin to simulate a cough. The particle concentrations versus time were measured at the
breathing zone of the other manikins. The trend of particle concentrations versus time predicted
by the hybrid model agrees with the experimental data. The proposed hybrid DES-Lagrangian
and RANS-Eulerian model can be used for investigating transient particle transport in enclosed
environments with relatively high accuracy, while the computing time can be reduced by 80%.

This study further developed a combined Computational Fluid Dynamics (CFD) and Markov
chain method for quickly predicting transient particle transport in enclosed environments. The
method first calculated a transition probability matrix using CFD simulations. Next, the Markov
chain technique was applied to calculate the transient particle concentration distributions. This
investigation used three cases, particle transport in an isothermal clean room, an office with an
Under-Floor Air-Distribution (UFAD) system, and the first-class cabin of an MD-82 airliner, to
validate the combined CFD and Markov chain method. The general trends of the particle
concentrations versus time predicted by the Markov chain method agreed with the CFD
simulations for these cases. The proposed Markov chain method can provide faster-than-realtime
information about particle transport in enclosed environments. Furthermore, for a fixed
airflow field, when the source location is changed, the Markov chain method can be used to
avoid recalculation of the particle transport equation and thus reduce computing costs.

In addition, this study investigated the effect of influencing factors on person-to-person
contaminant transport in enclosed environments. This study first collected available cases of
person-to-person contaminant transport from the literature to create a database. Then this
investigation identified the limitations of the existing data and added a number of cases to
complete the database. The additional cases were generated by using a RANS-Eulerian model
that was validated by experimental data from an occupied office with under-floor air-distribution
(UFAD) systems. The database shows that the overall performance of displacement ventilation
and the UFAD systems was better than that of mixing ventilation. A higher ventilation rate was
beneficial in reducing person-to-person contaminant transport to some extent. Person-to-person
contaminant exposure increased rapidly with a decrease in person-to-person distance when the
distance was smaller than 1.1 m. Generally speaking, person-to-person distance is an important
parameter when compared with ventilation mode and ventilation rate.

Finally, this study developed simplified models for predicting the airflow by a cough with a
mouth covering. This investigation firstly conducted smoke tests to visualize exhaled airflow by
a cough with a mouth covering by 16 human subjects. Then this study developed and verified
simplified models for predicting the airflow by a cough with a mouth covering based on the
experimental data. It was found that covering a cough with a tissue, a cupped hand and an elbow
can significantly reduce the horizontal velocity and make the droplets move upward with the
human thermal plume. Furthermore, covering a cough or turning head can avoid the receptor's
direct exposure at the first stage. In term of inhaled dose, it is reasonable to use the zero
momentum assumption for covering a cough with a mouth covering. In addition, based on the
available emission data in the literature and theoretical analysis, the mouth coverings can remove
about 60% of total number of coughed droplets.

The models developed in this study can be used for quickly predicting person-to-person
contaminant transport in enclosed environments. The correlations between the influencing
factors and person-to-person contaminant transport can be used as general guidance on how to
design ventilation systems to minimize inter-personal infection in a mechanically ventilated
space.

ASHRAE Research Reports