Isokinetic flow aerosol sampling

Aerosol monitor(s), automatic concentration range problems, 64 concepts, 63 data storage, 62 filter-paper tape, 66-67 flow conditions, varying, 65 isokinetic flow, 64 sample manipulation, 66 sampling time, 65-66... 

Figure 1. Representation of an aircraft aerosol sampling inlet showing flow streamlines for subisokinetic, superisokinetic, and isokinetic sampling. Under subisokinetic conditions large particles will be oversampled under superisokinetic conditions small particles will be oversampled and under isokinetic conditions, the particle size distribution will be preserved.

Isokinetic flow can be achieved if the sampling system monitors the ambient wind velocity and adjusts the pumping speed as appropriate. Unfortunately, this system exacerbates the dynamic-range problem because the system samples faster in high winds, when the aerosol concentration tends to be higher. 

Figure 2. The effects of sa7npling under isokinetic (A), low-flow (B), and high-flow (C) conditions. The cross-hatched regions represent the sampling orifice large and small symbols represent large and small aerosol particles, respectively solid lines are air streamlines ar dashed lines are heavy particle trajectories.

Work with slurries requires that the slurries are first nebulized and behave just as solutions with respect to the sample introduction into the aerosol. From electron probe micrographs of aerosol particles sampled on Nuclepore filters under isokinetic conditions, it was found that at nebulizer gas flows of 3 L/min, being typical of plasma spectrometry but far below those for flame atomic absorption, particles with a diameter of up to 15 pm can be found in the aerosol (Fig. 44) [117]. This would imply that powders with a grain size of up to about 15 pm could still be nebulized as could a solution. This, however, is not true as the mass distribution in the case of powders may be quite different in the slurry and in the aerosol, as shown for the case of SiC (Fig. 45) [118]. The nebulization limitations for the case of slurry nebulization thus must be investigated from case to case and leads to certain types of restrictions. 

In the applications of gas-solid flows, measurements of particle mass fluxes, particle concentrations, gas and particle velocities, and particle aerodynamic size distributions are of utmost interest. The local particle mass flux is typically determined using the isokinetic sampling method as the first principle. With the particle velocity determined, the isokinetic sampling can also be used to directly measure the concentrations of airborne particles. For flows with extremely tiny particles such as aerosols, the particle velocity can be approximated as the same as the flow velocity. Otherwise, the particle velocity needs to be measured independently due to the slip effect between phases. In most applications of gas-solid flows, particles are polydispersed. Determination of particle size distribution hence becomes important. One typical instrument for the measurement of particle aerodynamic size distribution of particles is cascade impactor or cascade sampler. In this chapter, basic principles, applications, design and operation considerations of isokinetic sampling and cascade impaction are introduced. 

The applications of isokinetic sampling cover but are not limited to the sampling of aerosols such as flu gas in chimney, soots (unbumed carbons) from diesel engine exhaust, dusts suspended in the atmosphere, and fumes from various sprayers measurements of particulate mass fluxes in pneumatic transport pipelines and other particulate pipe flows solid fuel (also some liquid fuels) distributions in furnaces, engines, and other types of combustors and calibrations of instruments for the measurements of particle mass concentrations. Isokinetic sampling can also be applied to flows with liquid droplets. In this case, the droplet sample is usually collected by an immiscible liquid (Koo et al., 1992 Zhang and Ishii, 1995). 

Isokinetic sampling is for the direct measurements of particulate mass fluxes or concentrations of aerosols (assuming no slip between aerosols and the gas flow) in gas-solid suspension flows. This is accomplished by inserting a thin-wall tube into the particulate suspension flow to draw samples at the isokinetic condition and by passing the collected particles into a sampling train. Typical... 

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