Research Oil The Nanoparticles Cogulation And Dispersion In Shear Layers And The Related Experimental Methods

As a hot issue in fluids mechanics recently, sub-micron and nano-particles laden gas flow is a common phenomenon in nature, and widely applied in atmosphere research, advanced nanotechnology, engineering thermophysics, chemical synthesis, toxicology and medical treatment. As a significant branch of this subject, the nano-particles laden shear layer gas flow has gained great attention because of its important role in the engineering and science research. To our knowledge, the dynamic behaviors of nano-particles in the shear layers mainly include Brown coagulation/agglomeration and dispersion. In actually, ensembles of particles in shear layers experience coagulation and dispersion simultaneously in micro scale, although the multiphase system appear as only particles coagulation or dispersion occurred. This article focuses on the coagulation (or agglomeration) and dispersion of nano-particles in the shear layers.As to nano-particles coagulation, the field characteristics of particles size distribution in the wake of a circular cylinder at high Reynolds number was studied experimently, while numerical simulations coagulating and dispersing nanoparticles in an incompressible turbulent planar jet with constraints are performed by utilizing a moment method, and the evolution of nanoparticle field was obtained through the approximation of the particle’s general dynamic equation. Experiments to investigate the characteristic distribution of nanoparticle-laden gas flow around a circular cylinder were performed with a fast mobility particle spectrometer. With Reynolds number Re of 9000 and 16000, particle size and number density distribution were used to obtain flow field characteristics. Results show that the total particle concentration in the free stream is larger than that in the wake, which can be attributed to the particles capture around the cylinder and the particles coagulation in the wake. However, the geometric mean diameter of the particle in the free stream is smaller than that in the wake for different Re. The total particle concentration and geometric mean diameter in the free stream and the wake both change in the same way, but with an obvious lag that increases with Re. Despite particle deposition, the number density of particles with electrical-mobility-equivalent diameters in the range from 220.7 to 523.3 nm in the wake is still higher than that in the free stream. The distribution for total particle number concentration is affected by particle coagulation and dispersion as well as the flow entrainment, and decreases along stream directions in the incompressible turbulent planar jet with constraints. The total particle mass decreases continuously along stream direction, and is obviously affected by coherent vortices. The mean particle diameter increases along stream direction. The variation of mean particle diameter is dependent on particle coagulation, the largest particles are found within the center region of the jet.As to nano-particles dispersion, the realization of the continuous ENPs (engineered nanoparticles) dispersion by a vacuum generator, which is work as an eductor, and the analysis of its mechanism were carried out. A continuous "dry" ENPs disperser in which a precisely powder doser was used to supply ENPs into the eductor was built. That improves the existed method which is also based on the vacuum generator but cannot disperse ENPs continuously. A new device (called as pressure equilibrium unit, PEU) which is use to cutoff the passage between the doser and the vacuum generator to make sure both part avoiding suffer pressure fluctuation and as a transfer doser for ENPs simultaneously was designed. The dosing rate maintains linear with time when the doser and PEU are working corporately. A new fitted Multiple Charging Correction (MCC) based on Gumbel distribute function (He M. L. 2013) was adopted to correct the data measured by SMPS while the Hoppel Multiple Charging Correction widely used is violated because of the insufficient scanning range of DMA and the relative high number concentration around the largest scaning channel. The f-MCC code which was modified to meet our experiment was programming independently with Matlab. A smaller size distribution of TiO2 was attained through "wet" aerolization method, so SMPS scan range is surfficient to cover the largest particle of this aerosol just generated. Thus the f-MCC and the H-MCC were both applied to correct the raw singly charging signal. And the comparative analysis between them shows that except the slight large difference of number density around the the particles size distribution (PSD) mode with about 10% relative deviation between them, the PSD corrected by f-MCC compared well with that corrected by H-MCC with the relative deviations of other PSD characteristic parameters below 1%. If the PSD beyond the SMPS scan range, the error of PSD corrected by H-MCC is much larger than f-MCC and the result cannot be acceptable. The extent of the real PSD beyond the scan range greater, the error of the H-MCC larger, in contrast, the f-MCC has maitained efficient and relative accurate. For different ENPs, dosing rate and pressure inlet of vacuum generator, the PSDs were measured and corrected by f-MCC which had been verified. The results show that the primary diameters of ENPs has great influence on the PSD, the GMD, median diameter and mode diameter increase as the primary diameter decreasing. Most agglomerates of ENPs is in the transition regime because of their characteristic scale and the particle Stokes force decreased since the boundary layer slipping occured, while the effect of the Van der waals attractive force and electrostatic force become prominent with the decreasing primary diameter. Thus, it is more difficult to disperse the ENPs of smaller primary diameters with the aerodynamic force. As the dosing rate increasing, it is harder to disperse the ENPs, however, the larger pressure of vacuum generator inlet results in more excellent PSD which has lager total number concentration and smaller median diameter. It can be deduced from the characteristic relations between the PSD statistics and the experimental parameters, such as ENPs type, inlet pressure and dosing rate, that the aerodynamics and the paticle-wall collision mechanism coexit and both play an important role in the ENPs dispersion in the vacuum genterator.