Study On Thermophoresis Effect Of Indoor Fine Particles

As the indoor is the main place which the people live,indoor air quality problem has already become the focus of people's attention.Fine particles can enter human body inside and trigger a range of diseases.The presence of particle deposition onto the surface of industrial products and electronic equipment which will cause the surface corrosion and the damage of the equipment.Thus,the study of dynamic behaviors of fine particles is significant.Presently,the relevant studies have made considerable advances,including fine particle dynamics,particle motion characteristics under different indoor environment,and the influence of thermophoresis on particles in room condition.However,the research on the effect of thermophoresis of different temperature gradient circumstances on different diameters particles and the thermophoresis effect on particles sedimentation is still rare.In this paper,a set of theoretical analysis and numerical are carried out to probe the mechanism of particle deposition influenced by the high temperature gradient from near-wall.The Gravity,Drag,Thermophoresis and Saffman force were mainly analyzed.Meanwhile,the force on and deposition velocity of particles with variable diameters in different temperature gradient were discussed.The main research contents are as follows:Firstly,the theoretical analyses were conducted according to volume fraction and relaxation time.Then we found that the two phase flow indoor belong to the dilute.This provided the appropriate condition of numerical simulation of the particles motion affected by the thermophoresis.Significantly,the temperature gradient will result in the inhomogeneous distribution of particle,the collision and coagulation of submicrometer particles have the close relationship with the concentrations distributions.Thus,in order to perfect the particle motion model,the collision model of indoor particle and the collision result were discussed.The aggregate possibility of particles will decrease by the increase of the velocity of the particles,and the finer particles tents toaggregate after collision.The experiment was conducted to determine the particle size range indoor,and the result showed that the particles were mainly distributed in the range from 5 nm to 500 nm.Thus,the particle less than 1 ?m were chosen to conduct the simulation.Secondly,the numerical simulation of indoor flow was conducted.The typical mechanical ventilation meeting room was chosen as the numerical model,and a wall that the temperature is higher than the air flow was set in the room.Subsequently,the temperature gradient and velocity gradient near the hot wall were calculated.As a result,the obvious disturbance,the large temperature gradient and velocity gradient of indoor flow was found near the hot wall.Thirdly,based on the numerical simulation result of indoor flow,the force on single particle near the hot wall was calculated and compared:the particle that?0.1?m will receive stronger thermophoretic force,and the amount of submicron particles that deposit on the hot wall will reduced;for the larger particles which are subjected to stronger gravity,tends to deposit easier than smaller particles;Fourthly,the movement of particles of different temperature situations was analyzed based on numerical model,The result showed that: under the influence of thermophoresis effect,the particle concentration is low in the high temperature gradient zone near the hot wall,at a certain area away from the wall,the particles present a high concentration;at the high temperature gradient zone near the cold wall,the particles exhibit a higher concentration distribution,away from the wall,the particles exhibit a low concentration distribution;Finally,the deposition velocity model of different diameter particles were built.And consider the high concentration particles aggregation phenomenon caused by thermophoresis effect,put forward the method of adding the collision factors in the model,The model is verified by the study of particles deposition on the upward horizontial wall.The result showed that: Regardless of the thermophoresis force effect,turbulence and diffusion play the leading role in the particle ?0.1?m deposition;for particles ?0.1?m,the gravity is the leading function,with the increase of particle size,the corresponding deposition velocity will increas;whileconsidering the thermophoresis force effect,the submicrometer particles less than 0.3?m would receive stronger thermophoretic force,and the deposition of particle on horizontal hot wall was decreased obviously due to the thermophoretic.When the difference of temperature gets to 20?,the deposition velocity reduces to 10-10,the particles of this range were rarely deposition on the horizontal hot wall surface,larger particles are more likely to deposition for the gravity force is stronger.