A. Renoux

PENETRATION AND PRESSURE DROP OF A HEPA FILTER DURING LOADING WITH SUBMICRON LIQUID PARTICLES

Abstract Experiments and a model have been made of the effects of mass loading of a HEPA fiber filter during filtration of submicron liquid aerosol particles. The measurements reveal that penetration of the test medium increased during clogging by a liquid aerosol, irrespective of particle size within the chosen range (0.02–0.5 μm). The physicochemical properties of the test aerosol did not seem to affect this phenomenon. Application of a non-stationary filtration model by Payet [1991, These, Universite Paris, 150 p] (based on the correlation of Liu and Rubow [1990, 5th World Filt. Congress, Nice 3, 112] showed that the increase in penetration can be explained in part by an increase in interstitial velocity and in part by a decrease in the number of fibers available for capture of particles.

Experimental study of thermophoretic particle deposition in laminar tube flow

Abstract The deposition of aerosol particles due to the presence of a temperature gradient (called thermophoresis deposition) was studied experimentally in a laminar tube flow, for the special case of uniform distribution of particle concentration and gas temperature at the inlet of a deposition tube with cooled walls. The experimental results clearly indicated a decrease in thermophoretic velocity as particle diameter increased. They also demonstrated that the limit deposition efficiency for an infinitely long tube was independent of the flow rate. On the basis of these results two correlations were established which can be used to predict thermophoretic deposition of submicron or micronsized particles in a laminar tube flow.

Electrical charging of radioactive aerosols comparison of the Clement-Harrison models with new experiments

The electrical charge of beta-active aerosols was determined using a radial flow DMA and a CNC. The radioactive aerosol could evolve and sampled within different geometrical volumes. Our results which are compared with the Clement and Harrison models, show an increase in the mean charge of the radioactive aerosol when the intensity of ionisation is reduced by the sampling geometry. In this situation, considering the ion recombination process the models slightly underestimate the electrical charge, while it overestimates the results when ion removal is determined by the diffusion to the walls of the sampling volume. Our results therefore confirm the theoretical studies carried out by Clement and Harrison, in particular the possibility of observing localised enhancement of the electrical charging of radioactive aerosols.

A new experimental wind tunnel facility for aerosol sampling investigations

Abstract A new experimental wind tunnel facility for aerosol sampling investigations has been built and its performance evaluated. Subsequently, an experimental methodology using a polydisperse test aerosol of glass beads to measure entry, transmission and overall sampling efficiencies has been developed and tested. The new facility is composed of a horizontal cylindrical pipe of 5 m long and 30 cm in diameter. The measurement zone is located just at the exit, allowing to take benefit of the whole cross-sectional area inside a stabilised aerosol flow. The working air velocity range is 0.5–4.5 m s−1 Air velocity and turbulence profiles are uniform within 10%. Turbulence in the working section is controlled with a square mesh grid. The test aerosol is generated by a fluidized-bed generator and dispersed into the clean air flow upstream of the horizontal part. Generated particles are within a size interval extending from a few μm to about 80 μm in aerodynamic diameter. Tests of time and space stability of the test aerosol in the working section were carried out. They have shown a reasonably uniform spatial distribution and time stability considering the size range of generated particles. The experimental method allows to obtain, simultaneously with the same technique entry, transmission, and overall sampling efficiencies of samplers from several μm up to 70 μm in particle aerodynamic diameter with a good accuracy. It is based on the measurement of the distribution of particle number concentration vs particle aerodynamic diameter of deposited and sampled aerosols in a reference probe and in the test sampler. To evaluate both the new wind tunnel facility and the methodology, measurements of the different efficiencies were achieved using a cylindrical sharp-edged thin-walled probe as a test sampler. This evaluation was performed in three steps. At first, the reproducibility of transmission efficiency measurements of the probe working in isokinetic conditions was determined. It appears fairly good between 10 and 70 μm in particle aerodynamic diameter. Then, the methodology was applied to the assessment of the aspiration efficiency of a probe working in subisokinetic conditions. Finally, a consistency test of the data was proposed and applied to our data; it consists in comparing the mass fractions of collected samples (deposited on the internal sampler walls, collected onto filters) calculated from the efficiency data and the distributions of particle concentrations, with those which are directly recovered after each experiment and weighed. This test yields an indicator of the quality of the whole efficiency data set.

DEPOSITION OF NANOSIZED PARTICLES IN CYLINDRICAL TUBES UNDER LAMINAR AND TURBULENT FLOW CONDITIONS

Abstract There are few studies of deposition of nanoparticles due to difficulties in their generation and size measurement. These difficulties can be overcome by the use of radioactive nanoparticles ( 218 Po ). This aerosol is obtained with a constant production rate from radon decay, and its concentration is determined using nuclear measurement techniques. We present an experimental and numerical study of 218 Po deposition in laminar and turbulent tube flows, in order to validate the appropriated theories. For laminar flows, we observe a better correlation between our experimental and numerical results, as these take into account, unlike analytical theories, all the phenomena occurring. For turbulent flows, the theories which are the most suitable are those of Wells and Chamberlain (1967, Brit. J. Appl. Phys. 18, 1793–1799) and Friedlander (1977, Smoke, Dust and Haze, Wiley, New York). Our experimental results show that an unexpected surface effect on nanoparticles deposition can be observed, even for a hydraulically smooth surface. We also assessed the influence of surface roughness on nanoparticle deposition experimentally. Unexpectedly, an influence of surface texture on the deposition rate was observed, although all tube surfaces were hydraulically smooth. This could be explained by a diffusion process.

Experimental determination of the dynamic shape factor of the primary sodium peroxide aerosol

Abstract A hypothetical accident in a fast breeder reactor could cause aerosols to be generated in a sodium fire. The computer codes relating to the modelization of these accidents make it necessary to use various input parameters, among which is the dynamic shape factor κ of the aerosols produced. This study concerns the shape factor of sodium peroxide; the divergence of the values of this parameter given in the literature justifies the usefulness of our work. We have tried to use the simplest method possible for this experiment. The dynamic shape factor is proportional to the ratio of the equivalent volume diameter to the aerodynamic diameter for a given particle. Therefore, these two quantities must be determined. A centrifuge enables classification of the particles as a function of their aerodynamic diameter; the equivalent volume diameter of the particle thus selected can then be determined by assessing the mass (neutron activation) and the number (electron microscope) on the same sample of particles. Our results show that the dynamic shape factor of the sodium peroxide submicronic particles generated by a fire is nearly 1 and that the values of this parameter increase with the size of the particles.

Recent advances in nanoparticle size measurement with a particle growth system combined with an optical particle counter : A feasibility study

Abstract A particle growth system (PGS) of original design is presented here, which, when combined with an opticle particle counter has the same function as a condensation nucleus counter (CNC). After an experimental study validated by a numerical model of transfers in a continuous-flow condenser, a relationship of unambiguous dependence is established between the droplet sizes and the size of the primary particles when the latter are between 4 and 20 nm. Furthermore, when aerosol sample is made up of an ultrafine fraction and a fine fraction (about 50 nm), the size distribution of the droplets obtained from the PGS is shown to be bimodal too. Finally, the activation efficiency of our PGS is found to be of the order of 100% for particles as small as 4 nm.

Behavior of Radon and Its Daughters in a Basement: Model–Experiment Comparison

ABSTRACT On the basis of Jacobis model (1972), we have developed the PRADDO model, which describes the behavior of radon and its daughters in a system of one or more chambers. In this article we compare theory and practice in the case of a one-chamber system. The principal equations of the classical room model, describing one single enclosure, are given and the assumptions made in modeling are discussed. Field measurements in the basement of a four-story house in Brittany are described. Radon and its daughters, the ambient aerosol, and ventilation were measured in order to characterize the system accurately and to define the models input parameters. The measurement protocol used allowed definition of the extent to which the assumptions of the model were respected in the real system. Results of the theory-experiment comparison show satisfactory agreement between calculation and measurement of the radon daughters activities, the potential alpha energy concentration and the equilibrium factor; the model re...

Production of monodispersed aerosol by evaporation and condensation of vapor under control process

Abstract A new aerosol generator for simple practical use, called PGS (Particle Growth System) has been developed. Its working principle, based on the controlled growth of a nucleus is presented. Its performance is given by the size range, standard deviation and concentration of produced aerosol.

An experimental facility for powder reentrainment studies

Particle reentrainment from surfaces to turbulent air flow is an important subject in many different fields like nuclear safety, environmental air pollution, sediment transport by wind, surface contamination in semiconductor operations. Theoretical and experimental studies have been numerous and cover different aspects of the phenomena (Ziskind et al., 1995). Although a number of theoretical works have been devoted for describing the mechanisms of detachment of primary spherical particles from flat smooth surfaces in a turbulent flow, experimental data are still needed in order to comparison. Moreover, the knowledge of the effect of parameters related to the deposit (monolayer, multilayer, conelike pile), the powder particles (particle-size distribution, adhesive properties), the surface (roughness . . . . ), the airflow (velocity, acceleration, turbulence) or the environment (humidity . . . . ) is still in an elementary stage. The main objective of our work is to contribute to the understanding and quantification of the relevant parameters that govern the particle reentrainment from a powder deposit (particle size in the range I to approximately 120 I.tm) in an horizontal airflow (air velocity up to 10 m/s). Therefore, a new experimental facility has been designed and built in our laboratory.