Experimental study into the evolution of pulse atomization-generated aerosol systems in confined space

Basic characteristics of the powder aerosol generated by the shockwave method using a pyrotechnic atomizer were experimentally studied.


Introduction
The study of mechanisms and laws of the formation and propagation of aerosol media is among the fundamental problems of two-phase flow hydrodynamics, the solution of which is the basis for aerosol practical application in different industries.For instance, with developing chemical technologies and dangerous emissions of noxious and toxic agents resulting from the manufacturing process or man-caused emergency events, the fast and effective decontamination of toxicants is a hot topic.
The most commonly used method to neutralize toxic agents is their sorption by sorbentcontaining filters.With this, the contaminated air must be allowed to have completely passed through the said filters, which is not always technically feasible under conditions of a specific production.From this point of view, the most promising is the other technique of purifying the air from toxic gases -the adsorption by fine aerosols atomized in contaminated indoor space.The adsorption conceptually allows the extraction of any contaminant in a wide concentration range from the emissions.
The main indicator of the decontamination efficiency of a toxicant is the high dispersiveness of atomized aerosols because adsorption rate increases with increasing specific-mass surface of the sorbent particles.Under real conditions, the effective use of powders having high sorption activity to decontaminate confined indoor spaces or deactivate the surfaces depends on an atomization technique used and a device implementing the same.Today, the following techniques are commonly used: airstream atomization with an injector sand spray gun, pulsed dispersion with a shock-wave atomizer, pulsed dispersion with a pyrotechnic atomizer, and electrostatic atomization.
The pulsed dispersion techniques [1,2] (using shock-wave or pyrotechnic atomizers) has the advantage of generating aerosols at a high rate -for fractions of a second or for seconds-by the pulse action from the triggered pyrotechnic charge of an explosive or a high-energy material (HEM).Despite the similarity of these atomization techniques (the use of a pyrotechnic charge), there are differences between them.The shock-wave atomizer uses a more powerful charge with less gas formation.The combustion of a HEM produces a shock wave propagating through the layer of the atomized substance to break down agglomerates and additionally crush the particles.The atomization is due to the shock pulse from the triggered pyrotechnic charge.In the pyrotechnic atomizer, the HEM charge is a source of cold gas that pulls the powder out of the atomizer vessel.In this respect, the processes of formation and propagation of the aerosol cloud, when the said atomization techniques are used, take place in accordance with different regularities.
The present work reports the results of the experimental investigation into the formation and propagation laws of fine aerosols generated by pulsed atomization techniques.

Experimental test procedure for pulsed powder atomization
To study regularities of the dispersion process of powdery materials, experiments were run with shock-wave and pyrotechnic spray generators.The experiments with the shock-wave atomizer to generate fine aerosol were performed on a test bench rigged with an optical diagnostics system [3].A schematic of the test bench with a measurement box of 1 m 3 in volume is illustrated in Fig. 1.For the aerosol cloud to be uniformly distributed across the measurement box volume, the shock-wave atomizer was placed at the center of the measurement box (Fig. 1).
The initial dispersiveness of the powder was measured with a Pip9.0 optical particlesize analyzer in a measurement range of 0.5-3000 μm.The variation of the powder dispersiveness parameters in the flow was monitored by the laser radiation small-angle scattering method using a two-channel LID-2M laser instrument.The instrument allows continuous, remote, contactless measurements of particle size and concentration directly in the aerosol.
The high-rate generation of fine aerosol was carried out by the shock-wave atomizer.A conceptual diagram of the atomizer is displayed in Fig. 2.  To determine spatial-time parameters of the aerosol cloud, a VideoSprint G4/NG highspeed camera was used that is designed for recording high-speed processes at up to 250 000 fps.The high-speed videorecording of the aerosol cloud is able to identify basic parameters (geometry and velocity) and uncover peculiar features of aerosol formation and propagation.
The following requirements were followed to assure safety while experimenting: -the weight of the HEM used in a separate trial must be as little as 0.5 g; -the weight of the atomized powder should be no more than 7 g.The trials on examining the propagation of the aerosol cloud produced by the pyrotechnic atomizer were run in a measurement box of 4 m 3 in volume.With that, the atomizer was accommodated at the bottom of the box at 30° angle to the horizontal surface for the aerosol cloud to be uniformly distributed in the confined space.The appearance of the measurement box is depicted in Fig. 3.In the course of the experiments, a Canon XA20 video camera at a frame frequency of 25 fps was used.

Experimental results analysis
The experimental study into the aerosol cloud formation and propagation was conducted by the pulse atomization of inorganic powders: silicon oxide (SiO2), aluminum (ASD-6), calcium (chalk), and pseudoboehmite (AlO(OH)).Images of the original pseudoboehmite powder obtained on a JSM-840 scanning electron microscope are shown in Fig. 5. Figure 6 shows time profiles for the propagation velocity of the powder aerosol cloud generated by the shock-wave atomization in the confined space.Analysis of the experimental findings has revealed the following.
In the shock-wave atomization: the spray plume is conical and symmetric in shape; -the powder ejection from the atomizer is completed in 3.6 ms; -the travel velocity of the flow front is ~120 m/sec; -the aerosol cloud is formed within ~8 msec.
In the pyrotechnic atomization: -the propagation velocity of the powder aerosol is ~3.2 m/sec; -the aerosol cloud is formed for (1 ÷ 3) sec.

Conclusions
The findings from the experimental study have comparatively been analyzed and demonstrate that the shock-wave atomizer provides higher dispersiveness of the powder aerosol under the conditions examined, which can be explained by the destruction effect of the shock wave resulted from the HEM combustion.Herewith, it takes considerably less time for the aerosol cloud formation compared to the pyrotechnic atomization.However, it is more appropriate to use the pyrotechnic atomizer when it comes to large confined spaces requiring large amounts of the powder (~300 g) to generate fine aerosol cloud.
The work was accomplished with support from the RFBR grant No. 16-38-50168 (mol_nr).

Fig. 6 .Fig. 7 .
Figure6shows time profiles for the propagation velocity of the powder aerosol cloud generated by the shock-wave atomization in the confined space.Figure7displays video frames of the propagation of the aerosol cloud generated by atomizing the aluminum powder by the shock-wave dispersion method (each snapshot has a fixed moment of time).

Figure 8
Figure 8 depicts an image of the pseudoboehmite powder dispersed by the shock-wave atomizer.

Figure 9
Figure 9 illustrates video frames of the propagation of the powder aerosol generated by the pyrotechnic atomizer.

Table 1 .
Characteristics of the original and pulse-atomized powders (D 43 -s mass median particle diameter) are listed in Table1.Characteristics of pulse-atomized powders.