Research On The Effect Of Limited Space Air Stability On Human Breathing Microenvironment

In limited spaces,especially in indoor spaces where people spend most of their time,the air quality is closely related to human health.The metabolites released by the sick through breathing activities often contain a large number of bacteria or viruses.These substances would be carried by exhaled d roplets,which can travel in the air for a considerable time and distance.If they are inhaled by the healthy body,he or she may be in great risk of being infected by the pathogenic bacteria or viruses.Therefore,the breathing activity of the human body and the air quality in the breathing microenvironment are directly related to the health conditions of the people indoors.This study takes the breathing microenvironment as the objective,the main works include:(1)Atmospheric stability,limited space air stability and Rayleigh-Bénard instability were discussed and compared in this study.Furthermore,the corresponding dimensionless number of each concept,namely,_iR number,_cG number,_aR number were discussed.In the atmosphere,_iR number represents the ratio of temperature difference and turbulence shear stress,however,the potential temperature that used in the definition of _iR number was not suitable in the limited space where often the case the pressure can be regarded as a constant.Therefore,G_c number was put forward.G_c number uses the vertical inertia term from Navier-Stokes equation as its denominator,which was different from _iR number.In limited spaces,there is another buoyance-driven-convection-related dimensionless number,_aR number,which focuses on the same temperature gradients inside the limited spaces asG_c number does.However,the unstable condition,according to the de finition of Rayleigh-Bénard instability,is more about the formation of the convection inside the space,whereasG_cnumber deals with the effect of the temperature differences on the mass point's vertical inertia force.(2)Theoretical analysis of the effect of multiple buoyancy effect on buoyant jet's trajectory was carried out,and a new trajectory equation of the trajectory of buoyant jets was established.The results showed that the factors that affect jet trajectories are the temperature of the jet₀T,the temperature of the ambient _eT,the initial velocityu₀,the temperature gradients in the limited spaces d T dy,_rA number,_cG number,the dimension of the limited space.When _rA>0,jet moves upwards;when_rA<0,jet moves downwards.When_cG>0,jet moves along its mainstream direction;whenG_c<0,jet moves towards the conjugation direction of the mainstream.The travel distance of jet along x-axis was given by means of dimensionless analysis.The method to calculate two dimensionless constants C₁ and C₂ was presented by a case study.We found that the value of C₁ was the same in stable and unstable conditions,which amplified the conjugation effect of background temperature effect on the jet movement.(3)Full-scale experimental investigations and numerical simulations were carried out to study the mechanism of pollutant transmission in the human respiratory microenvironment under limited space air stability with normal gravity or microgravity conditions.By combining the limited space air stabilit y,i.e.,stable,neutral and unstable,with ventilation methods,the convection and diffusion of pollutants in the human breathing microenvironment were studied.An evaluation method was established to assess the impact of limited space air stability and v entilation on the pollutant transmission and air quality in the human breathing microenvironment.Results showed that CO₂ concentration of unstable conditions were 100-150 ppm lower than that of stable conditions in both single-person experiments and double-person experiments.At the same time,according to the experimental data,numerical models of single-person breathing in the microgravity environment and double-person interactive breathing in the normal gravity environment were established by means of Computational Fluid Dynamics(CFD).Reynolds Average Navier-Stokes(RANS)method was adopted in the numerical simulation to ca lculate the flow field around the human breathing microenvironment and the surrounding environment.CFD results were verified by experimental data.The biggest discrepancies between experimental results and numerical results were 7.61%for unstable condition,4.79%for neutral condition and 4.27%for stable condition.By comparing and discussing the temperature distribution,velocity distribution,and pollutant concentration distribution in limited spaces,the characteristics of the transmission and distribution of exhaled pollutants under different limited space air stability conditions were discovered.(4)Besides,this study experimentally studied the personal exposure of a standing person to the exhaled pollutant in the breathing microenvironment and the personal exposure of two standing persons in their interactive breathing microenvironment in a room equipped with a ventilation system(total volume ventilation)and different limited space air stability conditions.Results were used to evaluate the risk of the personal exposure to the indoor pollutants.The relationship between personal exposure levels and the combined effect of limited space air stability and ventilation was established.Personal exposure depends on the breathing activity.As the distance gets further away from the pollutant release area(mouth),the effect of limited space air stability and ventilation plays a more and more important role in person al exposure.In stable conditions,the severely polluted breathing microenvironment caused a high personal exposure level,whereas unstable conditions can greatly reduce the personal exposure to the pollutants in the breathing microenvironment.Exposure intensity depends on the time that people spend indoors and the number of occupants.Exposure intensity of stable condition was two times as large as that of unstable condition.The personal exposure intensity is far lower in unstable condition than in stable condition.The mechanism of the effect of limited space air stability on pollutants transmission in the breathing microenvironment can be an important factor for predicting pollutant transmission in the breathing microenvironment.The findings are of great theoretical and practical significances in contro lling pollutants transmission,reducing disease,and ensuring occupants'well-being.The modified equation of buoyance jet trajectories provides a more accurate prediction for the motion of buoyance jets.The proposed conception of limited space air stability in this study can provide technical support in optimizing the design and performance assessment of ventilation systems.The findings can help adopt the most effective ventilation and air-conditioning methods to design a more reasonable indoor environment control system and provide a theoretical basis for designing energy-saving green buildings.