Simulation Of Fine Particles Filtration Process Of Noncircular Fibers Using Lattice Boltzmann Method

Fine particulate air pollution(PM2.5)has caused serious impact on the environment and human health attracted more and more attention.Fibrous filter is a most widely used kind of air filters,which has the advantage of high overall collection efficiency.However,its classification efficiency of fine particles is not satisfying,which causes the fibrous filter difficultly to meet more strict emission standard.It has been found by researchers that noncircular fibers have larger specific surface area compared to the circular cross-section fiber with the same volume fraction,and can perform higher diffusional collection efficiency of fine particles.However,the increase of collection efficiency is limited and still cannot meet the strict requirements of some special occasions.Application of electrostatic forces can significantly augment the collection efficiency of a fibrous filter.To solve these problems,the following related researches were carried out.First,the lattice Boltzmann-cellular automata(LB-CA)probabilistic model is used to investigate the particle filtration processes of four kinds of noncircular fibers with rectangular,trilobal,quatrefoil and triangular cross-sections.The pressure drop and collection efficiency for the diffusion dominant regime are investigated.By normalizing the pressure drop and collection efficiency of noncircular fibers with those of the circular fiber,which can be calculated from the existing classical expressions developed in the past,the fitting expressions are proposed.The results show the pressure drop of noncircular fibers is dependent on the friction area and the windward area.The alterations in the flow field caused by the variation of orientation angle are smaller when the rotational symmetry of the noncircular fiber is better,which leads to a lower pressure drop.The diffusional collection efficiency ratio is almost independent of the orientation angle and Peclet number but proportional to the aspect ratio.The diffusional collection efficiency of noncircular fibers is higher than that of the circular fiber with the same volume fraction due to their larger specific surface area.Then,the LB-CA model is used to study the non-steady-state filtration of elliptical fibers for the diffusion dominant regime.The evolution of both pressure drop and capture efficiency are quantitatively analyzed.For the movement of fine particles dominated by diffusion mechanism,the deposited particles distribute relatively uniformly around elliptical fibers for an initial period of time.The particle dendrites grow up with filtration time,leading to the change of flow field and increase of capture area.Then more particles will deposit on the windward of the elliptical fibers at the dendrite capture stage.For the evolution of pressure drop,the rate of increase in normalized pressure drop is higher for smaller particle and smaller inlet flow velocity.The rates of increase in normalized pressure drop make little difference for different aspect ratios.The normalized capture efficiency is approximately a linear function of loaded particles in a unit fiber length when the rate of increase of normalized efficiency is stabilized and the increase rates decrease as the particle size,inlet flow velocity and aspect ratio increases.Last,the LB-CA model is used to simulate the steady-state particle filtration processes of unipolar charged fibers.The collection efficiency of circular electret fiber is studied to validate the simulation method.Later,the collection efficiency of elliptical electret fiber is simulated under different operation conditions.The results show that the collection efficiency of elliptical electret fiber is decided by the drag force,Coulomb force and Brownian force together.When the particle size increases,the collection efficiency of elliptical electret fiber firstly increases and then decreases.The collection efficiency is proportional to the particle charge and the fiber charge.The collection efficiency is higher for smaller inlet flow velocity and is almost independent of aspect ratio.