Vibrating Orifice Generator Research Articles

The effect of the generation and handling in the acquired electrostatic charge in airborne particles

The measurement of the charge distribution in laboratory generated aerosols particles was carried out. Four cases of electrostatic charge acquisition by aerosol particles were evaluated. In two of these cases, the charges acquired by the particles were naturally derived from the aerosol generation procedure itself, without using any additional charging method. In the other two cases, a corona charger and an impact charger were utilized as supplementary methods for charge generation. Two types of aerosol generators were used in the dispersion of particles in the gas stream: the vibrating orifice generator TSI model 3450 and the rotating plate generator TSI model 3433. In the vibrating orifice generator, a solution of methylene blue was used and the generated particles were mono-dispersed. Different mono-aerosols were generated with particle diameters varying from 6.0 × 10 − 6 m to 1.4 × 10 − 5 m. In the rotating plate generator, a poly-dispersed phosphate rock concentrate with Stokes mean diameter of 1.30 × 10 − 6 m and size range between 1.5 × 10 − 7 m and 8.0 × 10 − 6 m was utilized as powder material in all tests. In the tests performed with the mono-dispersed particles, the median charges of the particles varied between − 3.0 × 10 − 16 C and − 5.0 × 10 − 18 °C and a weak dependence between particle size and charge was observed. The particles were predominantly negatively charged. In the tests with the poly-dispersed particles the median charges varied fairly linearly with the particle diameter and were negative. The order of magnitude of the results obtained is in accordance with data reported in the literature. The charge distribution, in this case, was wider, so that an appreciable amount of particles were positively charged. The relative spread of the distribution varied with the charging method. It was also noticed that the corona charger acted very effectively in charging the particles.

Particle formation in spray drying

Theoretical and experimental investigations of the particle formation process during spray drying are presented. A novel experimental method allows observation of individual, free flowing droplets during drying in a laminar gas flow and subsequent analysis of the resulting monodisperse, monomorphic dry particles. A second method combines a vibrating orifice generator and a bench top spray drier, which allows production and sampling of monodisperse particles at different drying stages. The experimental results are compared to a full numerical model and a simplified analytical model. Two dimensionless parameters are identified that influence particle formation: the Peclet number, which is the ratio of the diffusion coefficient of the solute and the evaporation rate, and the initial saturation of the excipients. In an application example, particle design is shown to improve the aerosol properties of powders intended for pulmonary drug delivery.

Measurement of the electrostatic charge in airborne particles: II - particle charge distribution of different aerosols

This work gives sequence to the study on the measurement of the electrostatic charges in aerosols. The particle charge classifier developed for this purpose and presented in the previous paper (Marra and Coury, 2000) has been used here to measure the particle charge distribution of a number of different aerosols. The charges acquired by the particles were naturally derived from the aerosol generation procedure itself. Two types of aerosol generators were used: the vibrating orifice generator and turntable Venturi plate generator. In the vibrating orifice generator, mono-dispersed particles were generated by a solution of water/ethanol/methylene blue, while in the rotating plate generator, six different materials were utilized. The results showed no clear dependence between electric charge and particle diameter for the mono-dispersed aerosol. However, for the poly-dispersed aerosols, a linear dependence between particle size and charge could be noticed.

Laboratory Characterization of Modified Tapered Element Oscillating Microbalance Samplers

Laboratory tests with generated aerosols were conducted to test the efficacy of two recent design modifications to the well-established tapered element oscillating microbalance (TEOM) continuous particulate matter (PM) mass monitor. The two systems tested were the sample equilibration system-equipped TEOM monitor operating at 30 °C, which uses a Nafion dryer as part of the sample inlet, and the differential TEOM monitor, which adds a switched electrostatic precipitator and uses a self-referencing algorithm to determine “true PM mass.” Test aerosols included ammonium sulfate, ammonium nitrate, sodium chloride, copper (II) sulfate, and mixed aerosols. Aerosols were generated with an atomizer or a vibrating orifice generator and were equilibrated in a 450-L slow flow chamber before being sampled. Relative humidity in the chamber was varied between 10 and 90%, and step changes in humidity were executed while generating aerosol to test the response of the instruments. The sample equilibration system-equipped TEOM monitor does reduce, but not totally eliminate, the sensitivity of the TEOM mass monitor to changes in humidity. The differential TEOM monitor gives every indication of being a very robust technique for the continuous real-time measurement of ambient aerosol mass, even in the presence of semi-volatile particles and condensable gases.

Drop formation from a vibrating orifice generator driven by modulated electrical signals

The recently introduced technique of amplitude-modulated (AM) excitation of laminar liquid jets to produce drops by controlled Rayleigh-type jet disintegration is extended to frequency modulation (FM) and investigated in detail to quantify the available ranges of excitation wavenumbers and modulation depths for various jet velocities and liquids. The AM and FM techniques are compared regarding the achievable ratios of carrier and modulation frequencies. The modulated excitation is applied to a vibrating orifice drop generator (TSI Inc.). Four different drop formation processes are observed for different wavenumber domains. Experiments with three different Newtonian liquids and a dimensional analysis yield a generalized representation of the parameter windows of the technique in terms of excitation wavenumber and jet velocity for a wide range of liquid viscosity and surface tension.

Linear Raman Spectroscopy on Droplet Chains: A New Experimental Method for the Analysis of Fast Transport Processes and Reactions on Microparticles

A new experimental method for the analysis of mass and energy transport and reactions on microparticles is presented. A chain of microdroplets from a vibrating orifice generator was injected into a quiescent gas phase. Linear Raman spectra from the microparticles and the surrounding gas were taken at different distances from the generator. Concentration changes were measured as a function of droplet lifetime. A period of time of up to 20 ms could be studied with a resolution of 10 μs. An argon-ion laser in a 90° scattering geometry was used for excitation. Spectra were taken through a modified double monochromator with a two-dimensional charge-coupled device (CCD) detector, one axis of which was used for spatial resolution. Profiles of gaseous components near the droplets could be measured with a resolution of 50 μm. The method has been applied to analysis of absorption, dissociation, and isomerization in the SO2–H2O system and to the investigation of the desorption process of CO2 from water droplets. Chemical components in gas and liquid phase could be separated. The detection limit in aqueous media was 1 mmol/L.

Droplet-wall collisions: Experimental studies of the deformation and breakup process

Experimental studies of liquid spray droplets impinging on a flat surface have been performed with the aim of formulating an empirical model describing the deposition and the splashing process. Monodisperse droplets with a known viscosity and surface tension, produced by a vibrating orifice generator, were directed towards a rotating disc and the impingement was visualized using an illumination synchronized with the droplet frequency. A rubber lip was used on the rotating disk to remove any film from previous depositions. The test matrix involved different initial droplet diameters (60 < d 0 < 150 μm), velocities (12 < w < 18 m/s), impingement angles (4° < α < 65°), viscosities (1.0 < μ < 2.9 mPas) and surface tensions (22 < σ < 72 mN/m). The liquids used to establish the different viscosities and surface tensions were ethanol, water and a mixture of water-sucrose-ethanol. One major result from the visualization is a correlation of the deposition-splashing boundary in terms of Reynolds number and Ohnesorge number. Noteworthy is that a distinct correlation between the Re and Oh number, K = Oh · Re 1.25, is only achieved if the normal velocity component of the impinging droplets is used in these dimensionless numbers. For the case of a splashing droplet, a two-component phase Doppler anemometer was used to characterize the size and velocity of the secondary droplets. The obtained droplet size distributions and correlations between droplet size and velocity around the point of impingement constitute the basis of an empirical numerical model.

Experimental Study of Particle Deposition on Semiconductor Wafers

A sensitive method for detecting particle deposition on semiconductor wafers has been developed. The method consisted of generating a monodisperse fluorescent aerosol, depositing the known-size monodisperse aerosol on a wafer in a laminar flow chamber, and analyzing the deposited particles using a fluorometric technique. For aerosol particles in the size range of 0.1–1.0 μm, the mobility classification-inertial impaction technique developed by Romay-Novas and Pui (1988) was used to generate the monodisperse test aerosols. Above a particle diameter of 1.0 μm, monodisperse uranine-tagged oleic acid aerosols were generated by a vibrating-orifice generator. The test wafer was a 3.8-cm diameter silicon wafer placed horizontally in a vertical laminar flow chamber which was maintained at a free stream velocity of 20 cm/s. A condensation nucleus counter and an optical particle counter were used to obtain the particle concentration profile in the test cross section and to monitor the stability of aerosol concentra...

Continuous monitoring of droplet production of a vibrating orifice generator by laser light extinction

To monitore droplet production of the vibrating orifice aerosol generator (VOAG) continously an optical droplet control has been developed on the bases of laser light extinction. The monodispersity of the droplets, the presence of satellites as well as bi - or multi - modal structures of the size distribution can be identified on the display of a oscilloscope.

Drag coefficients of single droplets moving in an infinite droplet chain on the axis of a tube

The drag of monodisperse droplets moving in an infinite droplet chain is studied numerically and experimentally. The droplet chain produced by a vibrating orifice generator is composed of an infinite number of droplets with the same diameter σ P and a constant spacing d P. The droplets move with a velocity u 0 along the axis of a cylindrical tube of radius R. In the experiments the velocity decrease of the droplets is measured by a laser Doppler velocimeter. It is shown that near the wall the droplet chain produces the same flow field as a cylindrical rod with a diameter of σ P exp(−ƒ) , where ƒ is a function of the velocity u 0, the droplet diameter σ P, the spacing of the droplets d P and the radius of the tube R. With these assumptions the Navier-Stokes equations have been solved numerically. Drag coefficients c D have been computed for Reynolds numbers from 20 to 100. The Reynolds number is based in this case on the droplet velocity u 0 and the droplet diameter σ P. The dimensionless spacing d P σ P was varied between 2 and 12 and the dimensionless tube diameter 2 R/ σ P ranged from 20 to 1000. In addition, numerical results are presented for the radial coordinate δ of the boundary, at which the flow becomes essentially parallel to the tube, for the axial velocity at this boundary u z ( z, δ) and for the maximum value of the velocity along the axis of symmetry u z ( z, 0) max. The results of the numerical calculations are compared with the experiments.

Size Separation of Particles from Aerosol Samples using Impactors and Cyclones

AbstractInvestigations are presented on particle separation in impactors, sampling cyclones and other precollectors. These devices are increasingly used for the characterization of particulate matter. Size selective samplers must be characterized by their grade efficiency curves. A new measuring procedure is presented, which permits the determination of the grade efficiency curves of various sampling devices. An optical particle counter and a vibrating orifice generator are used in these measurements. This calibration technique is fast and has a high resolution. Investigations were carried out on the influence of various parameters on the classification efficiency of impactors and sampling cyclones. Using the extensive data measured a better understanding and quantification of impactor and cyclone behaviour is developed which may improve their applications in particle sampling.

Counting efficiencies of six commercial particle counters

Counting efficiencies of a condensation nuclei counter (TSI 3020), a white-light optical particle counter (Climet CI-8060) and four laser instruments (PMS LAS-X, LAS-250X, LPC 525 and HP-LAS) were determined relative to the LAS-X. Measurements were made in the geometric diameter range of 0.1–4 μm using latex spheres as well as monodisperse organic and inorganic particles produced by a vibrating orifice generator. The high-pressure in-line counter (HP-LAS) shows a nonlinear response to gas velocity which can be taken into account by a calibration. The lower detection limits (50% points) of the conventional laser counters (LAS-X and LAS-250X) agree within 0.05 μm with their nominal specifications; for the white-light counter (CI-8060) the actual lower limit is at about 0.4 μm. For all counters, the degree of inlet losses for larger particles varies greatly with inlet design and flow velocity of each counter.

Laser Settling Velocimeter: Aerodynamic Size Measurements on Large Particles

A Laser Settling Velocimeter (LSV) has been developed for the measurement of large particle aerodynamic diameters and has been applied in the range 10—20 μm. A vertical settling tube is illuminated along the axis by a 5 mW He—Ne laser beam to permit manual timing of falling particles. Measurements on DOP particles from a vibrating orifice generator agree with diameters calculated from the generator operating parameters to within 0.3% when the slip correction is included. The LSV is a simple instrument which is particularly useful for the direct determination of the aerodynamic diameters of solid particles, irrespective of density or shape.

Aerosol testing techniques for size-selective samplers

Methods are presented for the use of liquid particles to measure sampling efficiencies and wall losses in size-selective samplers and for the use of solid particles to assess bounce and reentrainment. By testing a dichotomous sampler, it is demonstrated that the liquid particle methods are accurate; solid particle methods are less accurate and are still under development. Two methods are presented; in the first, particles are produced by spray-drying droplets from the vibrating orifice generator. Aerodynamic diameters are determined directly with a new Laser Settling Velocimeter or, indirectly, based on optical microscopy and the properties of the vibrating orifice generator. Alternatively, solid particles, such as pre-sized glass beads, are produced from bulk materials by a novel Sonic Fluidized Bed. The solid particle techniques are illustrated by testing the Size-Selective Inlet.

Measurement of the size distribution of liquid and solid aerosols by doppler shift spectroscopy

Particle Doppler shift spectroscopy (PDSS) is being used at NBS for measuring the size distribution of particles produced by aerosol generators, calibrating other types of particle sizing instruments, and studying the agglomeration, coalescence, evaporation, and condensation of liquid and solid aerosols. Size distributions of particles in the size range 0.5–50 μm can be measured in minutes or seconds. Number densities in the scattering volume from single particle to 10 5/cm 3 can be handled. The technique at present has a precision of 0.05 μm. Measured sizes are accurate to 0.08 μm. Calculations of the Mie scattering function for materials of known refractive index serve to normalize the size distributions and provide instrument calibration from first principles. Light scattered out of a horizontally propagating laser beam by falling particles is collected at one angle in the vertical scattering plane. Beat frequencies in the photocurrent of the detector due to the Doppler shift of the radiation scattered by the settling aerosol are analyzed to determine particle velocities. The slip-corrected Stokes law settling velocity gives the particle's size for a known particle density, while the amplitude of the beat frequency component contains information on the number of particles of that size. Dioctylphthalate (DOP) aerosol produced by a Berglund-Liu vibrating orifice generator, polystyrene latex spheres, and powder samples have been studied thus far by the PDSS technique at NBS.

On unipolar diffusion charging of aerosols in the continuum regime

Some recently completed unipolar diffusion charging experiments in the continuum regime are described. Monodisperse acrosols generated by the vibrating orifice generator and the mobility classification method were exposed to unipolar positive ions produced by a corona discharge under conditions approximating pure ionic diffusion, and the resulting particle charge was measured as a function of the charging parameter, N 0t , where N 0 is the unipolar ion concentration and t is the charging time. The particle diameter was varied between 0.1 and 5.04 μm and the N 0 t product, between 2.56 × 10 6 and 5.1 × 10 7 (ions/cm 3) (sec). A comparison of the data with the available theories shows that the best agreement is obtained with the theory of Fuchs and Bricard, whereas the agreement with the theory of White is less satisfactory. It is concluded that for unipolar charging in the continuum regime, the theory of Fuchs and Bricard should be used.

Experimental studies of optical particle counters

Abstract The response characteristics of several commercially available optical particle counters have been studied experimentally using monodisperse aerosols generated by the vibrating-orifice generator. The particle diameters ranged from 0.5 to 10 μm and the particle refractive index from 1.4 to 1.6. Aerosol material used in the study included DOP (di-octyl phthalate) of 1.49 refractive index and Cargille index-of-refraction liquids of 1.4, 1.5 and 1.6 refractive index. The response of the forward scattering counters (Royco 245 and Bausch & Lomb 40-1) was found to be independent of the particle refractive index above 1 μm in particle diameter. The right-angle scattering counter (Royco 220) showed a more pronounced particle refractive index effect. The experimental results are in good agreement with theoretical predictions of counter response based on Mie-theory calculations. The resolution of the counter had been studied as a function of the particle size. The relative standard deviation of the pulse height distribution was found to increase with decreasing particle size due to the decreasing signal-to-noise ratio of the counter. A sheath-air inlet and a conical inlet are described. These new optical counter inlets improved significantly the resolution and the sampling efficiency of the counters. The use of the vibrating-orifice monodisperse aerosol generator as an aerosol concentration standard for evaluating sampling efficiencies of optical counters is described.