Numerical study of the effects of human body heat on particle transport and inhalation in indoor environment
Highlights
► CFD technique presented to integrate gas-particle two-phase flows and heat transfer. ► Body heat causes a significant uprising airflow on the downstream side of the human body. ► The importance of body heat on particle inhalation is associated with the occupant orientation relative to the free stream. ► Body heat has a significant effect on particle inhalation when the occupant is back to the wind.
Introduction
The transport characteristics of aerosol particles and their inhalation characteristics by human occupants in indoor environments have been attached a great importance as people spend approximately 90% of their time indoors and a number of health problems have been found to be associated with particle inhalation [1]. During the past decades, numerous experimental and numerical investigations [2], [3], [4], [5], [6], [7] have been conducted under various conditions. It is generally accepted nowadays that the particle inhalability by a human occupant is subjected to many factors such as the particle size, ambient wind speed, occupant movement, airflow pattern, inhalation rate, inhalation pattern (mouth or nasal inhalation), and even the human facial features.
However, most of the previous investigations on particle inhalation failed to take into account the effects of the metabolic heat released from a human body. In fact, a human body is continually exchanging energy with its environment. The average thermal energy generated by a human body with an ordinary activity level and at a moderate room temperature was found to be up to 100 W [8]. Due to this heat, a temperature gradient is formed and drives a buoyant convection in the vicinity of the human body, which is known as the human thermal plume. Homma and Yakiyama [9] measured the human thermal plume around a person standing in quiescent air using smoke wire photography and hot-wire anemometry. It was found that the thermal boundary layer was approximately 50 mm thick at the face level and its velocity was upto 0.25 m/s. Using a laser Doppler anemometer, Johnson et al. [10] measured the airflow around a human body standing with its back towards the free stream (0.2 m/s). A significant upward airflow was observed in the downstream side of the human body and the upward velocity was found to be approximately 0.19 m/s at the nose level. A synthetic literature survey demonstrated that the human thermal plume can produce vertical air velocities of 0.1–0.25 m/s in the breathing zone [9], [10], [11], [12]. This vertical velocity induced by body heat is roughly equal to the average wind speed in most indoor environments (0.05–0.25 m/s according to Baldwin et al. [13] and Schmees et al. [14]), it is therefore reasonable to expect that the buoyancy-driven convection may change significantly the airflow pattern and play an important role in transporting aerosol particles in the vicinity of a human body. This is especially true for fine and ultrafine particles as their movement is mostly controlled by the indoor flow field [15].
During the past years, the effects of human thermal plume on aerosol particle transport and inhalation in quiescent air have been investigated by few researchers. For example, using a sedentary thermal manikin, Rim and Novoselac [12] experimentally investigated the effects of human thermal plume on the inhalability of fine/ultrafine particles in stratified indoor air. It was found that when the particle source was at floor level and in near proximity to an occupant, the inhaled particle concentration by the manikin was upto 4 times higher than the ambient concentration. This proved that the thermal plume plays an important role in transporting pollutants and particles from the floor level to the breathing zone. However, the conclusions based on quiescent or quasi-quiescent indoor air conditions may not be quantificationally applicable to realistic situations where ventilation is operating and the orientation of an occupant relative to the free stream may be random. Therefore, for the purpose of accurate description of the effects of human metabolic heat on particle transport and inhalation, some important factors including the wind speed and the occupant orientation relative to the wind have to be taken into account.
Therefore in this study, in order that the role of human body heat in particle transport and inhalation could be better understood, a series of CFD computations were conducted using a standing thermal manikin. Various factors including the wind speed (0.05–0.25 m/s), manikin orientation relative to the free stream (back-to-the-wind or facing-the-wind) and their combinations were investigated. For the purpose of comparison, computations with isothermal conditions were also performed. The predicted airflow field was in good agreement with the experimental data available in the literature. The trajectories of inhaled particles under various conditions were also presented and discussed. The outcome yielded from this study can help to reduce contaminant exposure through appropriate orientation arrangement in indoor environments.
Section snippets
Geometric model and boundary conditions
The computational domain of this study was a rectangular wind tunnel containing a 1.6 m tall standing human manikin facing the +X direction and with its nose tip in the plane of X = 0 m, as illustrated in Fig. 1. The dimensions of the computational wind tunnel (4 m-width × 7 m-depth × 3 m-height) were created large enough so that the flow field near the manikin was free from the effects of the no-slip condition of the stationary surrounding walls. In order that the detailed airflow pattern and
Airflow field
The predicted airflow field was compared against Heist et al.'s [24] experimental data. In their experiments, Heist et al. [24] investigated the airflow pattern around a child-size manikin (80 cm in height, without breathing) standing with its back towards the coming free stream in a wind tunnel using laser Doppler anemometry. Particle transport was not included in the experiments. Heist et al.'s [24] experiments were conducted under isothermal and thermal conditions, respectively. In the
Conclusion
The effect of body heat on particle inhalation by a human manikin in a ventilated indoor environment was numerically investigated using CFD. Computations were conducted under both the facing-the-wind and back-to-the-wind conditions, as well as the isothermal and thermal conditions, respectively. Satisfactory agreement was obtained between the predicted airflow fields and the experimental observations available in the literature. The main conclusions arising from this study are as follows:
- (1)
For
Acknowledgements
The financial supports provided by the National Basic Research Program (973) of China (Grant No. 2012CB720100) and by the Australian Research Council (project ID LP110100140) are gratefully acknowledged.
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