Xiangdong Li

Source and trajectories of inhaled particles from a surrounding environment and its deposition in the respiratory airway

Abstract The inhalation exposure to airborne particles is investigated using a newly developed computational model that integrates the human respiratory airway with a human mannequin and at an enclosed room environment. Three free-stream air flow velocities (0.05, 0.20, and 0.35 m s−1) that are in the range of occupational environments are used. Particles are released from different upstream locations and their trajectories are shown, which revealed that the trajectory paths of 80 μm particles that are inhaled are the same from the three different upstream planes evaluated. Smaller particles, 1 and 10 μm, exhibited different inhalation paths when released from different upstream distances. The free-stream velocity also has an effect on the particle trajectory particularly for larger particles. The aspiration efficiency for an extended range of particle sizes was evaluated. Reverse particle tracking matches the deposition in the respiratory airway with its initial particle source location. This can allow better risk assessments, and dosimetry determination due to inhalation exposure to contaminant sources.

Numerical investigation of micron particle inhalation by standing thermal manikins in horizontal airflows

Computational fluid dynamics computations were conducted to investigate the particle inhalation characteristics of a thermal manikin standing in a horizontal airflow with different orientations, leg postures, wind speeds and particle sizes. The computations revealed that only when the manikin’s thermal plume moves into the breathing zone (namely, the manikin is facing the lee side) could the body heat affect the characteristics of particle inhalation. Further computations demonstrated that, when facing the lee side, the manikin’s particle inhalation is highly sensitive to its leg posture. When the legs are separated, air can flow through the gap, causing more particle entrainment into the breathing zone from the lower level. Although the thermal effect of body heat is gradually suppressed with increasing wind speed or particle size, different leg postures have different environmental sensitivities.

Effects of passenger thermal plume on the transport and distribution characteristics of airborne particles in an airliner cabin section

Computational fluid dynamics simulations were conducted in this study to investigate the effects of the buoyancy-driven thermal plume on the airflow pattern and transport characteristics of airborne particles in airliner cabins. A cabin section containing three seats and three passengers was built and numerical simulations were conducted using thermal and isothermal conditions, respectively. Airborne particles were assumed to be released by the passengers through coughing. The predicted airflow field was validated using experimental data available in the literature. Comparison of the computational results revealed that the thermal plume significantly changed both the airflow filed and the trajectories of particle transport. In addition, the spatial distribution characteristics of the particles and their residence time in the passengers’ breathing zones were highly sensitive to the location of released particles. Comparatively, the particles released by the passenger seated close to the window may have the highest health risk to other two passengers.

A computational fluid dynamics study on the effects of computer fan on indoor airflow and indoor air quality in breathing zone

Although desktop computers have been considered in many computational fluid dynamics (CFD) indoor simulations, they were usually simplified as hot boxes. A real desktop computer, however, is not only a heat source but also a momentum source due to the jet airflow generated by the computer case fan. The airflow induced by a desktop case fan could have the same order of magnitude as the ventilation rate of the whole room, and thus is expected to have significant influence upon the indoor airflow field and distribution of contaminants. Obviously, the simplification of desktop computer into a hot box does not sufficiently stand for the actual situation. In this study, a CFD model of a typical office room was built and simulations were conducted to investigate the airflow field and volatile organic compounds (VOC) distribution under two scenarios: (1) computer as a ‘hot box’ without a fan and (2) a more realistic computer model with an operating case fan. The comparison of CFD results yielded from these two scenarios demonstrated that the role of computer fan is very important in terms of airflow field and contaminant distribution and should be included in CFD office simulations for the purpose of improved reliability.

Effects of manikin model simplification on CFD predictions of thermal flow field around human bodies

Simplified computational thermal manikins are beneficial to the computational efficiency of computational fluid dynamics simulations. However, the criterion of how to simplify a computational thermal manikin is still absent. In this study, three simplified computational thermal manikins (CTMs 2, 3 and 4) were rebuilt based on a detailed 3D scanned manikin (CTM 1) using different simplification approaches. Computational fluid dynamics computations of the human thermal plume in a quiescent indoor environment were conducted. The predicted airflow field using CTM 1 agreed well with the experimental observations from the literature. Although the simplified computational thermal manikins did not significantly affect the airflow predictions in the bulk regions, they strongly influenced the predicted airflow patterns near the computational thermal manikins. The predictive error of the computational thermal manikin was strongly related to the simplification approach. The computational thermal manikins generated from the surface-smoothing approach (CTM 2) was very close to CTM 1, while the required mesh elements for a stable numerical solution dropped by over 75%. Comparatively, the predictive errors of CTMs 3 and 4 were considerable in the near-body regions. This study has illustrated the importance of keeping the key body features when simplifying a computational thermal manikin. The surface-smoothing-based simplification method was shown to be a promising approach.

An Energy Saving Ventilation Strategy for Short-Term Occupied Rooms based on the Time-Dependent Concentration of CO2

Abstract Most HVAC systems are designed to supply air based on assumed (usually maximal) rather than actual occupancy, therefore often resulting in over-ventilation. The concept and theories of demand-controlled ventilation (DCV), which are to find better ventilation strategies according to actual occupancy, have been developed for more than two decades and have been applied to many situations. However, a certain type of room (i.e. short-term occupied room) seems to have been neglected in the literature of DCV. The aim of this study is to work out an energy saving strategy for such types of rooms based on time-dependent concentration of CO2. This study investigated the time-dependent nature of the concentration of gaseous contaminants by both theoretical and computational fluid dynamics (CFD) approaches, and then applied this model to CO2 cases. Compared with existing models in the literature, the new theoretical model developed in this study has taken ventilation effectiveness into account to avoid the “instant perfect mixing” assumption thus better fitting actual situations. The theoretical predictions by this model fit CFD simulation results very well. Based on this model, an energy saving strategy for short-term occupied rooms was then proposed. Two practical examples in this study show that ventilation rates could be reduced to 50%~80% of the standard rates.

Modelling of evaporation of cough droplets in inhomogeneous humidity fields using the multi-component Eulerian-Lagrangian approach

Abstract This study employed a multi-component Eulerian-Lagrangian approach to model the evaporation and dispersion of cough droplets in quiescent air. The approach is featured with a continuity equation being explicitly solved for water vapor, which allows comprehensively considering the effects of inhomogeneous humidity field on droplets evaporation and movement. The computational fluid dynamics (CFD) computations based on the approach achieved a satisfactory agreement with the theoretical models reported in the literature. The results demonstrated that the evaporation-generated vapor and super-saturated wet air exhaled from the respiratory tracks forms a “vapor plume” in front of the respiratory track opening, which, despite the short life time, significantly impedes the evaporation of the droplets captured in it. The study also revealed that due to the droplet size reduction induced by evaporation, both the number density of airborne droplets and mass concentration of inhalable pathogens remarkably increased, which can result in a higher risk of infection. Parametric studies were finally conducted to evaluate the factors affecting droplet evaporation. Summary The study demonstrated the importance of considering inhomogeneous humidity field when modelling the evaporation and dispersion of cough droplets. The multi-component Eulerian-Lagrangian model presented in this study provides a comprehensive approach to address different influential factors in a wide parametric range, which will enhance the assessment of the health risks associated with droplet exposure.

Effects of cough-jet on airflow and contaminant transport in an airliner cabin section

The goals of this study were to investigate the effect of cough-jet on local airflow and contaminant transport in a typical cabin environment by using computational fluid dynamics. A fully occupied...