Investigations On Particle Distribution In Rooms With Impinging Jet Ventilation

The ventilation mode is an important part in the air conditioning system,which has a direct impact on the indoor thermal environment,the distribution characteristics of pollutants and the energy consumption of buildings.Therefore,it is particularly significant to choose a reasonable ventilation mode in the design of air conditioning system.The displacement ventilation(DV)can achieve the goal of energy saving and good air quality,but it can not be used for heating in winter.Mixing ventilation(MV)can give consideration to the demand for cooling and heating,but it is needed to find possibilities for further improvements in energy utilization.Impinging jet ventilation(IJV)is a new ventilation strategy for using in public and industrial buildings.It combines advantages of both DV and MV in theory.So IJV is considered to be a more efficient,energy saving,and new air conditioning ventilation strategy,which would be a new ventilation strategy for air conditioning and replace traditional ventilation.However,there are few researches on the indoor thermal environment and indoor air quality in the IJV room,especially the problems such as particle resuspension in the chamber and the diffusion characteristics of the suspended particles.Compared with traditional underfloor air distribution system(for example DV and underfloor air condition(UFAC)),the fresh air is sent out from the lower part of the room,there is a potential risk associated with the IJV system,i.e.,particle resuspension from the floor and then spread to indoor activity area due to the supply air,by which the high momentum jet is directed towards the floor.And the high momentum jet also causes different air flow distribution in IJV room compared with other rooms.Thus it is necessary to understand the advantages and disadvantages of the IJV system relative to others as a potential substitute for traditional ventilation system.The indoor thermal environment and air quality were comparative studied in IJV,UFAC,DV and MV rooms,and a detailed study of the IJV room associated with particle resuspension is involved to discuss the way to avoid it.In the current paper,experiment in an artificial climate room and numerical simulation were carried out to study the indoor tempreture,air flow,particle concentration distributions in the rooms equipped with four different ventilation mode.The results show that it is easier to form a stronger feeling of blowing in the human foot in the IJV room than other rooms due to the higher speed and cold air directly into the occupied zone.But the average flow velocities in other positions are less than 0.3 m/s,which meets the requirement of indoor thermal comfort.By analyzing indoor temperature distribution,it is found that similar to DV,there is a obvious temperature gradient along the height direction in the IJV room,and the thermal stratification may be one of the solutions to get better energy saving for IJV compared to that for MV.In order to know whether the supply airflow will cause particle resuspension and its effect on indoor air particle concentration distribution in an IJV room,particle concentrations under different supply airflow conditions were measured continuously in a model IJV room with indoor pollutant sources.The effect of supply airflow on the particle resuspension was analyzed in an IJV room by using the measured data.The results show that the potential risk of particle resuspension in an IJV room is higher than that in both the DV and MV rooms,but it can be controlled or avoided by reducing the supply velocity.For this purpose,the critical supply velocities uc causing particle resuspension for different supply heights and particle sizes were analyzed by using the measured data.The experimental results show that supply velocity us,particle size dp and supply height h have substantial effects on particle resuspension in IJV system.In detail,higher supply velocity us,larger particle size dp and lower supply height h result in higher risk of resuspension.For the supply heights of h = 0.6 and 0.3 m,in order to avoid the available particle resuspenion by the supply airflows,the supply velocities should be less than 2.6 and 2.2 m/s,respectively.While for the supply heights greater than 0.9 m,there are little risk of particle resuspension caused by supply airflow in an IJV room with the supply velocity in an appropriate range,which meet the needs of thermal comfort.In addition,in practical applications,there is little risk of the significantly degrading IAQ induced by particle resuspension via supply airflow in an IJV room,as the risk of particle resuspension decreases with decreasing particle diameter and the upward diffusion momentum of large particle is limited by the inertial force.Moreover,comparing the resuspension amount caused by the supply airflow of an IJV system with that caused by an occupant walking past,a sharp rise of supply velocity in a short time span in an IJV room will not result in significantly worse IAQ.People walking and other behaviors in the room will cause particle resuspension.Because of the different distribution characteristics of airflow and temperature in the rooms equipped with different ventilation systems,the distribution of the particles in the room and the bad influence on people are also different.In the current study,indoor particle distribution with people walking was studied in IJV room in summer by employing experiment and numerical simulation.The concentration field in the IJV room was analyzed and compared with the cases of the other ventilation modes(MV,DV and UFAC).The influences of particle size on the concentration distribution in breathing plane and other typical functional regions in the IJV room were also analyzed.The results show that the particle concentration distribution is not uniform obviously,and the higher particle concentrations are found in some places such as indoor walking area,human body around and exterior wall of building,which is similar with that in a room equipped with DV system.Because the air-supply ducts are arranged on one of the walls in IJV and DV rooms,the mass concentrations of particule around the staffs are influenced by spatial position.In detail,the farther the position is from the air-supply duct,the greater the concentrations of particles are.And the average mass concentration of particles in breathing area is lower in IJV room compared to that in MV room.Moreover,the airflow pattern is only influenced by inertia force in the MV room,while it is affected by inertial force and thermal buoyancy in IJV room.Thus there is only one concentration peak around people in the former one,and there are two peak concentrations in the latter one.And the peak concentration near the ground caused by the inertial force in IJV room is much higher than that in the rooms with other 3 ventilation modes.