Transport Of Droplets With Contaminant In Air-Conditioned Room And Thermal Calculation Of Buildings

Nowadays people spend most of their time at indoor environment. With the improvement of the quality of the people's life, more and more buildings are installed air conditioner and under a controlled air exchange system. The air exchange system should be well designed to keep a safe and comfort indoor environment for people inside. However, there are many airborne pollutants inside the building, some are from the human's body, such as the droplets expelled through breathing, coughing and sneezing. The conditioned indoor air flow may dilute the concentration of the airborne particles/droplets and expel them out of the building, it may also transport them to the breathing zone of other people to cause the health problem, especially in the hospital ward where the patient could expel droplets containing high infectious pathogens, which may result in cross infection inside the hospital. In previous studies of the transport of droplets containing contaminant in the air flow, the droplets are simply treated as airborne particles without evaporation. Taking consideration of the evaporation process, this paper is aimed to understand the characteristics of the transport of the droplets, to find out the effectiveness of different ventilation system in removing contaminated droplets, and the appropriate ventilation set-up to prevent cross infection.The numerical simulation with computational fluid dynamics (CFD) is the main approach in this study to investigate the transport and dispersal of droplets and solid remains of droplets that is the solid component of droplets left after the volatile component evaporated. To study the motion of droplets, both Eulerian model and Lagrangian model are used for the droplets expelled from stationary source and coughing, respectively. The drift-flux approach of Eulerian model is widely used in the study of the transport of solid particles in ventilated room. In order to reflect the characteristics of the evaporating of droplets, we proposed to remodel the calculation of drift velocity, to set up a method to construct the algebraic relationship of the drift velocity and evaporation rate with the changing diameters of droplets. The remodeled drift-flux model thus can simulate transport of evaporating droplets in the air flow.Droplets expelled from coughing are primarily carried by the air jet pulse produced by coughing during the first 10 seconds from mouth. Because the air jet is highly transient and turbulent, the Lagrangian model was adopted in tracking the evaporating droplets transported in the ventilated room. Since the coughing air jet provides the initial impetus for the traveling of droplets, it is important to know the initial momentum of the coughing jet as it determines the penetrating distance of coughing droplets/particles. Laser Doppler Anemometry was used to measure the initial velocity of the coughing air. According to the experimental data, the initial velocity of the coughing air is the range of 8-30 m/s, the duration of the coughing impulse is 150-600 ms.The numerical simulation results indicate that the diameter of the droplets has to be equal or less than 100μm to disperse in the air, otherwise the droplets will fall to the ground or the bed. The humidity of the air can highly affect the evaporating rate of droplets. The time for droplets to dry up in the air of relative humidity of 75 % can be two times of that in the air of relative humidity of 55 %. The simulation results show that the gravitational effect is negligible on droplets smaller that 50μm, while larger ones show more or less settling feature. Droplets with initial diameter of 80μm drop to the lower part of room in the beginning, and then become airborne particles at low position while water dry up, which may later be brought up to the breathing zone by the air flow.It has been investigated in this paper the characteristics of transport of droplets of different initial sizes in three types of ventilation system as mixing, displacement and downward ventilation. It is found out that the posture of source manikin and the coughing direction affect the effectiveness of ventilated air in removing droplets and particles. The displacement ventilation shows obvious advantage in removing small airborne particles/droplets, but difficulty to carry larger droplets and their solid remains out of the room. The simulation results also show that the air flow mixed the droplets and dilute the concentration in the mixing and downward ventilated room as time goes on. With comparison, the downward ventilation set-up is recommended as it has equal ability in removing passive particles and lager droplets, moreover in this type ventilation set-up, the number of droplets attached less on the body of health care worker.PV-Trombe wall is a novel building design in utilization of solar radiation to provide both electricity and passive heating. This study has investigated the system efficiency in solar radiation utilization and the flow and temperature field inside the room with both experimental and numerical method. The numerical simulation results indicate that not only the electricity output is in the linear relationship with the coverage rate of the PV cells on the glazing of the system, but also the passive heating efficiency does. Both the experimental and numerical simulation results show that during day time the temperate of the room air is in stratified distribution along the height, with the highest temperature on the top of the room, except the air just above the floor area with the incident solar radiation through the window, which may be of the highest temperature.Inside a room with a south-faced window, there is a solar light spot moving with time during daytime in winter. The solar light sport not only brings in the solar energy inside the room, but also increases the unbalance of the room temperature, which results in the further complication of the flow and temperature field. CFD modelling of the flow field of such kind building has been studied. Because the ambient condition and the indoor field are all change with time, we proposed to set heat absorption rate or heat generation rate in the solid wall region, the system transient factors can thus be added into a computational domain with the steady calculation method. With this approach, the steady simulation method can be used to simulate the indoor flow field at certain time. The simulation results are in good consistent with experimental data.