Dispersal of exhaled air and personal exposure in displacement ventilated rooms

The influence of the human exhalation on flow fields, contaminant distributions, and personal exposure in displacement ventilated rooms is studied together with the effects of physical movement. Experiments are conducted in full-scale test rooms with life-sized breathing thermal manikins. Numerical simulations support the experiments. Air exhaled through the mouth can lock in a thermally stratified layer, if the vertical temperature gradient in breathing zone height is sufficiently large. With exhalation through the nose, exhaled air flows to the upper part of the room. The exhalation flow from both nose and mouth is able to penetrate the breathing zone of another person standing nearby. The stratification of exhaled air breaks down if there is physical movement in the room. As movement increases, the concentration distribution in the room will move towards a fully mixed situation. The protective effect of the boundary layer flow around the body of a moving person disappears at low speed, and is reduced for a seated person placed nearby due to horizontal air movements, which can also cause rebreathing of exhaled air for the seated person. The results indicate that the effect of the exhalation flow is no acute problem in most normal ventilation applications. However, exhalation and local effects caused by movement may be worth considering if one wishes to contain contaminants in certain areas, as in the case of tobacco smoking, in hospitals and clinics, or in certain industries.     

Distribution of exhaled contaminants and personal exposure in a room using three different air distribution strategies

The level of exposure to human exhaled contaminants in a room depends not only on the air distribution system but also on people's different positions, the distance between them, people's activity level and height, direction of exhalation, and the surrounding temperature and temperature gradient. Human exhalation is studied in detail for different distribution systems: displacement and mixing ventilation as well as a system without mechanical ventilation. Two thermal manikins breathing through the mouth are used to simulate the exposure to human exhaled contaminants. The position and distance between the manikins are changed to study the influence on the level of exposure. The results show that the air exhaled by a manikin flows a longer distance with a higher concentration in case of displacement ventilation than in the other two cases, indicating a significant exposure to the contaminants for one person positioned in front of another. However, in all three cases, the exhalation flow of the source penetrates the thermal plume, causing an increase in the concentration of contaminants in front of the target person. The results are significantly dependent on the distance and position between the two manikins in all three cases. PRACTICAL IMPLICATIONS: Indoor environments are susceptible to contaminant exposure, as contaminants can easily spread in the air. Human breathing is one of the most important biological contaminant sources, as the exhaled air can contain different pathogens such as viruses and bacteria. This paper addresses the human exhalation flow and its behavior in connection with different ventilation strategies, as well as the interaction between two people in a room. This is a key factor for studying the airborne infection risk when the room is occupied by several persons. The paper only takes into account the airborne part of the infection risk.     

A simple method for differentiating direct and indirect exposure to exhaled contaminants in mechanically ventilated rooms

Many airborne infectious diseases can be transmitted via exhaled contaminants transported in the air. Direct exposure occurs when the exhaled jet from the infected person directly enters the breathing zone of the target person. Indirect exposure occurs when the contaminants disperse in the room and are inhaled by the target person. This paper presents a simple method for differentiating the direct and indirect exposure to exhaled contaminants in mechanically ventilated rooms. Experimental data for 191 cases were collected from the literature. After analyzing the data, a simple method was developed to differentiate direct and indirect exposure in mixing and displacement ventilated rooms. The proposed method correctly differentiated direct and indirect exposure for 120 out of the 133 mixing ventilation cases and 47 out of the 58 displacement ventilation cases. Therefore, the proposed method is suitable for use at the early design stage to quickly assess whether there will be direct exposure to exhaled contaminants in a mechanically ventilated room.     

Airborne transmission between room occupants during short‐term events: Measurement and evaluation

This study experimentally examines and compares the dynamics and short‐term events of airborne cross‐infection in a full‐scale room ventilated by stratum, mixing and displacement air distributions. Two breathing thermal manikins were employed to simulate a standing infected person and a standing exposed person. Four influential factors were examined, including separation distance between manikins, air change per hour, positioning of the two manikinsand air distribution. Tracer gas technique was used to simulate the exhaled droplet nuclei from the infected person and fast tracer gas concentration meters (FCM41) were used to monitor the concentrations. Real‐time and average exposure indices were proposed to evaluate the dynamics of airborne exposure. The time‐averaged exposure index depends on the duration of exposure time and can be considerably different during short‐term events and under steady‐state conditions. The exposure risk during short‐term events may not always decrease with increasing separation distance. It changes over time and may not always increase with time. These findings imply that the control measures formulated on the basis of steady‐state conditions are not necessarily appropriate for short‐term events.     

Comparison of gaseous contaminant diffusion under stratum ventilation and under displacement ventilation

The gaseous contaminant diffusion under stratum ventilation is investigated by numerical method which is validated by experiments carried out. The concentration of gaseous contaminants along the supply air jet is found to be lower than the other parts of the room. Compared with displacement ventilation, the formaldehyde concentration in breathing zone is lower when a contaminant source locates close to the occupant. The concentration is at the same level when the contaminant source locates up-steam to the occupant. The concentration in the occupied zone (<1.9 m from the floor) is also lower when the contaminant source locates on the floor. At supply air temperature optimized for displacement ventilation, the toluene concentration in breathing zone for stratum ventilation is higher than that for displacement ventilation when the area source locates on the four surrounding walls of the room.     

Vertical Temperature Profiles in Rooms Ventilated by Displacement: Full-Scale Measurement and Nodal Modelling

The vertical temperature profiles have been measured in a full-scale office room ventilated by displacement. Different wall radiative emissivities have been employed to study the effect of thermal radiation. The change of the vertical locations of the heat source does not affect the stationary front, but modifies the temperature profile. Two new nodal models, i.e. a four-node model and a multi-node model, are developed for predicting the temperature profile based on the flow and thermal characterization in the room. Agreement between the models and the experiments are very good. The calculated results are applied to show that the temperature profile is influenced considerably by the heat conduction through the walls and the thermal radiation between the wall surfaces. The models developed can be used for design purposes, as well as to supply the thermal boundary conditions in a CFD code.     

Human convection flow in spaces with and without ventilation: personal exposure to floor‐released particles and cough‐released droplets

The effects of the human convective boundary layer (CBL), room airflow patterns, and their velocities on personal exposure are examined. Two pollutants are studied which simulate particles released from the feet and generated at distances of 2 and 3 m by a human cough. A thermal manikin whose body shape, size, and surface temperatures correspond to those of an average person is used to simulate the CBL. The findings of the study reveal that for accurate predictions of personal exposure, the CBL needs to be considered, as it can transport the pollution around the human body. The best way to control and reduce personal exposure when the pollution originates at the feet is to employ transverse flow from in front and from the side, relative to the exposed occupant. The flow from the above opposing the CBL create the most unfavorable velocity field that can increase personal exposure by 85%, which demonstrates a nonlinear dependence between the supplied flow rate and personal exposure. In the current ventilation design, it is commonly accepted that an increased amount of air supplied to the rooms reduces the exposure. The results of this study suggest that the understanding of air patterns should be prioritized.     

Multiple steady states in stack ventilation

We investigate the natural ventilation of an occupied open-plan space, which is ventilated through two stacks connected to the roof of the space and a low-level opening or doorway located at the base of the space. The occupants at the floor level act as the source of heat and provide buoyancy driving the ventilation with a nearly uniform distribution of heat. These conditions can produce up to three steady state displacement ventilation regimes. In the first regime, colder air from the outside is drawn into the space through the shorter stack and bottom opening, while warm air inside the room leaves through the taller stack. In the second regime, colder outside air is drawn through the bottom opening and displaces warm air in the room upwards and out through both stacks. In the third regime, outside air is drawn through the taller stack and bottom opening, while warm interior air is displaced out through the shorter stack. A quantitative model is developed to describe these three steady state ventilation regimes. The model is successfully tested with laboratory experiments, and shows that the ventilation regime that actually develops in the room depends on the geometry of the room and the history of the flow. We discuss how these multiple ventilation regimes may be manipulated to bring about ventilation and thermal comfort to the occupants inside the ventilated space.     

Vertical profiles of temperature and contaminant concentration in rooms ventilated by displacement with heat loss through room envelopes

The purpose of this study is to examine the effect of heat loss through walls upon the gradients of temperature and contaminant concentration in room with displacement ventilation. It is known that conduction heat loss is governed by outside temperature, heat load inside the room, supply air temperature and overall heat transfer coefficient of walls. Experiments were conducted to measure the temperature gradient and the ventilation efficiency in the room ventilated by displacement ventilation with various combinations of heat load and temperature difference between supply air and outside air. In order to simulate the change of seasons, the supply air temperature was changed instead of the outside air temperature. The effect of supply air temperature and heat generation inside the room on the temperature gradient and the concentration of tracer gas were investigated through the experiments. As a result, it turned out that the higher the heat generation rate and the lower the supply temperature, the stronger the temperature stratification and the lower the concentration in the lower zone. Additionally, ventilation heat loss turned out to be a good index for assessing the concentration in the lower zone. Temperature differences of around 3 degrees C between supply air temperature and exhaust temperature are at least needed for displacement ventilation under the conditions of the experiment presented in this paper.     

Indoor air environment: more structures to see?

A convection transport visualization technique is suggested to analyze the indoor air environment (IAE). A two-dimensional and laminar displacement ventilated room with discrete heat and contaminant sources is numerically investigated. Based on the governing equations, three convection transport functions, i.e. streamfunction, heatfunction, and massfunction, are derived to describe the fluid, heat, and contaminant transport processes, respectively. Main attentions are focused on the effects of the strength of heat source (Gr), the strength of contaminant source (Br), the strength of external ventilation (Re), and the positions of inlet/outlet openings on IAE. Numerical results illustrated that the abstract transport behaviors of the fluid, heat, and contaminant indoors are clearly exhibited by the convection transport functions which provides a simple but practical means of assessing IAE.     

室内空气环境的数值研究方法

建筑室内空气环境(IAE)与人们的舒适、健康及工作效率密切相关，对其数值模拟与评价具有重要的实际意义。由于室内存在大量离散分布的热源与污染源，IAE表现为复杂的对流传热传质过程，如何有效地利用CFD模拟技术分析与评价IAE即是本文的研究内容。作者根据室内空气环境的特殊性提出了对流传输与自然模拟方法，简洁有效地分析了二维层流置换通风房间中离散热与污染源之间的相互作用及其对IAE的影响。     

Distributions of respiratory contaminants from a patient with different postures and exhaling modes in a single-bed inpatient room

This study investigated contaminant transport and evaluated the ventilation performance in a single-bed inpatient room. The study performed comparative experimental analysis on the distributions of respiratory contaminants breathed out and coughed out by a patient in a full-scale chamber, which simulated a single-bed inpatient room. The contaminant exhaled by the patient was simulated by an SF₆ tracer gas and 3-μm particles at steady-state conditions. The differences in the contaminant distribution between the coughing and breathing cases were insignificant for the mixing ventilation case, while for the displacement ventilation, the contaminant concentrations in the upper part of the room were higher for the coughing case. The contaminant concentrations in the inpatient room for the case with the patient sitting on the bed were lower than those for the patient supine on the bed for the displacement ventilation under the same supply airflow rate. The SF₆ tracer gas and 3-μm particles released at a notable initial velocity for simulating a cough could give similar contaminant distributions in the inpatient room. Therefore, the experimental data can be used to validate a CFD model, and the validated CFD model can be used to investigate transient coughing and breathing processes.     

Person to person droplets transmission characteristics in unidirectional ventilated protective isolation room: The impact of initial droplet size

Person to person droplets/particles or contaminant cross transmission is an important issue in ventilated environment, especially in the unidirectional ventilated protective isolation room (UVPIR) where the patient’s immune system is extremely low and easily infected. We simulated the dispersion process of the droplets with initial diameter of 100 μm, 10 μm and gaseous contaminant in unidirectional ventilated protective isolation room and studied the droplets dispersion and cross transmission with different sizes. The droplets with initial size of 100 μm settle out of the coughing jet quickly after coming out from mouth and cannot be carried by the coughing jet to the human thermal plume affecting (HTPA) zone of the susceptible manikin. Hence, the larger droplets disperse mainly in the HTPA zone of the source manikin, and the droplets cross transmission between source manikin and susceptible manikin is very small. The droplets with initial size of 10 μm and gaseous contaminant have similar dispersion but different removal process in the UVPIR. Part of the droplets with initial size of 10 μm and gaseous contaminant that are carried by the higher velocity coughing airflow can enter the HTPA zone of the susceptible manikin and disperse around it. The other part cannot spread to the susceptible manikin’s HTPA zone and mainly spread in the source manikin’s HTPA zone. The results from this study would be useful for UVPIR usage and operation in order to minimize the risk of cross infection.     

Performance of “ductless” personalized ventilation in conjunction with displacement ventilation: Impact of intake height

The importance of the intake positioning height above the floor level on the performance of “ductless” personalized ventilation (“ductless” PV) in conjunction with displacement ventilation (DV) was examined with regard to the quality of inhaled air and of the thermal comfort provided. A typical office room with two workstations positioned one behind the other was arranged in a full-scale room. Each workstation consisted of a table with an installed “ductless” PV system, PC, desk lamp and seated breathing thermal manikin. The “ductless” PV system sucked the clean and cool displacement air supplied over the floor at four different heights, i.e. 2, 5, 10 and 20 cm and transported it direct to the breathing level. Moreover, two displacement airflow rates were used with a supply temperature adjusted in order to maintain an exhaust air temperature of 26 °C. Two pollution sources, namely air exhaled by one of the manikins and passive pollution on the table in front of the same manikin were simulated by constant dosing of tracer gases. The results show that the positioning of a “ductless” PV intake height up to 0.2 m above the floor will not significantly influence the quality of inhaled air and thermal comfort.     

Removal of contaminants released from room surfaces by displacement and mixing ventilation: modeling and validation

This paper presents the experimental and numerical modeling of contaminant dispersion in a full-scale environmental chamber with different room air distribution systems. For the experimental modeling, an area source with uniform emissions of a hypothetical contaminant (SF6) from the entire floor surface is designed and constructed. Two different types of ventilation are studied: displacement and mixing ventilation. A computer model for predicting the contaminant dispersion in indoor spaces was validated with experimental data. The validated model is used to study the effects of airflow and the area-source location on contaminant dispersion. Results show that the global airflow pattern has a strong impact on the distribution of the contaminants. In general, the personal exposure could be estimated by analyzing the relative source positions in the airflow pattern. Accordingly, the location of an exhaust diffuser may not greatly affect the airflow pattern, but can significantly affect the exposure level in the room. PRACTICAL IMPLICATIONS: When designing ventilation in addition to bringing fresh air to occupants, it is important to consider the removal of contaminants released in the off-gassing of building materials. Typical indoor off-gassing examples are emissions of volatile organic compounds from building enclosure surfaces such as flooring and painted walls. In this study, we conducted experimental and numerical modeling of different area sources in a mock-up office setup, with displacement or mixing ventilation. Displacement ventilation was as successful as mixing ventilation in removing the contaminant source from the floor area. Actually, the most important consideration in the removal of these contaminants is the relative position of the area source to the main airflow pattern and the occupied zone.     

Influence of air supply parameters on indoor air diffusion

This paper presents the field distributions of air velocity, temperature, contaminant concentration, and thermal comfort in an office with displacement ventilation for different air supply parameters such as the effective area, shape, and dimension of the diffuser and the turbulence intensity, flow rate, and temperature of the air supplied. The research is conducted numerically by using an airflow computer program based on a low-Reynolds-number k-ε model of turbulence. It can be concluded that the effective area, shape, and dimension of the diffuser and the turbulence intensity of the air supplied have little effect on the room air diffusion except at floor level. The influence of the flow rate and temperature of the air supplied is very significant on the air diffusion as well as on the thermal comfort and indoor air quality.     

Concentration Distribution in the Centre Plane of a Ventilated Room Under Isothermal Conditions

This paper presents a series of fill-scale measurements of the concentration distribution in the centre plane of a room with isothermaI mixing ventilation. Vertical projiles of the concentration in the middle of the room have been measured under different conditions. With the contamination source in the middle of the room the vertical profiles were changed radically with an increase of the air change rate from n = 1.5h^?1 to n = 6h^?1 due to a change in the flow structure in the room. With a constant air change rate, the location of the contamination source in the room showed a great influence on the vertical profile. A high velocity around the contamination source resulted in a uniform contaminant distribution in the room, while a low velocity resulted in considerable differences. Contours of concentration in the centre plane of the room have been measured using different contaminant densities. The densities were low, neutral and high in relation to the density of air. The results showed that the contaminant distribution in the room with the chosen flow conditions depended strongly on the contaminant density, and that the high density case gave the highest concentrations in the occupied zone.     

CFD boundary conditions for contaminant dispersion,heat transfer and airflow simulations around human occupants in indoor environments

Indoor computational fluid dynamics (CFD) simulations can predict contaminant dispersion around human occupants and provide valuable information in resolving indoor air quality or homeland security problems. The accuracy of CFD simulations strongly depends on the appropriate setting of boundary conditions and numerical simulation parameters. The present study explores influence of the following three key boundary condition settings on the simulation accuracy: (1) contaminant source area size, (2) convective/radiative heat fluxes, and (3) shape/size of human simulators. For each of the boundary conditions, numerical simulations were validated with experimental data obtained in two different environmental chambers. In CFD simulations, a small release area of a contaminant point source causes locally high concentration gradients that require a very fine local grid system. This fine grid system can slow down the simulations substantially. The convergence speed of calculation is greatly increased by the source area enlargement. This method will not influence the simulation accuracy of passive point source within well-predicted airflow field. However, for active point source located within complicated airflow filed, such an enlargement should be carried out cautiously because simulation inaccuracy might be introduced. For setting thermal boundary conditions, convection to radiation heat flux ratio is critical for accurate CFD computations of temperature profiles around human simulators. The recommended convection to radiation (C:R) ratio is 30:70 for human simulators. Finally, simplified human simulators can provide accurate temperature profiles within the whole domain of interest. However, velocity and contaminant concentration simulations require further work in establishing the influence of simplifications on the simulation accuracy in the vicinity of the human simulator.     

A new approach on zonal modeling of indoor environment with mechanical ventilation

This paper reports the development of a new zoning approach based on room air age, a parameter that indicates the mixing condition of the air. Zoning criteria are developed based on the deviation ratio of air age as well as location of the key source that is of concern (e.g., temperature, air pollutant). By integrating the zonal model with other models such as dynamic models for heat and moisture transfer, and source/sink models for air pollutant, the dynamic characteristics of indoor parameters such as air temperature, humidity, and pollutant concentrations can be simulated. A case study was presented for a displacement ventilated room, and simulation results using the new zonal model were compared with those using a computational fluid dynamics (CFD) model and a conventional zonal model. Results demonstrated that the new zonal model is more accurate in calculating the zonal temperature distributions than the conventional zonal model. The model is suitable for dynamic simulations (e.g., whole year) of indoor environmental parameters.