Sampling anisokinetic

It should be noted that, due to the different inertia of different sized particles, an anisokinetic sampling of polydispersed particle suspension affects not only the mass of particles in the sample but also the particle size distribution. Sub-sucking 

It is worth pointing out that the isokinetic sampling is not required when very small particles are sampled. A correlation for a better approximation of the aspiration efficiency was proposed by Belyaev and Levin (1972, 1974) as 

FIGURE 2.10 Particle mass flux in anisokinetic sampling (Zhu etal., 1997). 

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For sampling particulate matter, one is dealing with pollutants that have very different inertial and other characteristics from the carrying gas stream. It becomes important, therefore, to sample so that the same velocity is maintained in the probe tip as exists in the adjacent gas stream. Such sampling is called isokinetic. Isokinetic sampling, as well as anisokinetic sampling, is illustrated in Fig. 13-3. 

Error from anisokinetic sampling can be investigated theoretically (Davies, 1966). Suppose an aerosol of concentration c0 flowing with a velocity of u0 is drawn with a velocity u into a sampling tube of radius R (assume infinitely thin tube walls, sample collected parallel to flow). Gas streamlines entering the tube at some distance away are confined within a cylinder of radius R0 such that... 

The effect of anisokinetic sampling is plotted in Fig. 8.4. This analysis indicates that isokinetic sampling is not necessary when very small particles are sampled. Sometimes, in an effort to be precise,... 

Figure 8.4 Effect of anisokinetic sampling on measured sample concentration.

As indicated before, previous detailed studies of particulate formation and emission processes in automobile engines and exhaust systems have been done using probes which extracted a (hopefully) representative sample of the exhaust gas that was subsequently analyzed for various particulate properties (5). Similar techniques have been used in the most recently developed systems to measure the amounts of particles emitted from auto exhaust. Such sampling techniques, however, suflFer from inherent disadvantages in that they interfere with the hot flow particulate processes and have an unknown effect on the particulates as they flow through the system to an exterior point of analysis. This problem can also be compounded by several phenomena anisokinetic sampling, deposition of particulates in the sampling system, or condensation caused by temperature drops. 

Figure 2. Isokinetic and anisokinetic sampling. (Reproduced with permission from reference 23. Copyright 1989 Gulf Publishing Company.)...

Sampling. Sampling practices play an important role in particle size characterization using cascade impactors. Short-duration sampling may result in unrepresentative size estimates [74]. This is also the case for anisokinetic sampling [19]. 

The error obtained when anisokinetic sampling conditions are chosen can be caused by two factors misalignment of the sampling probe and different velocity In the sample tube than In the bulk stream. The magnitudes of both types of error are dependent on the particle diameter through Stokes number. The Stokes number for the sampling tube Inlet Is defined by ... 

Figure 12. Effect of Anisokinetic Sampling on Particle Concentration.

Particle counts are also Increased by anisokinetic sampling except under turbulent conditions, when the sampling velocity Is greater than the main gas velocity for particles larger than about 0.4 pm diameter. Therefore, for particles with diameters less than or equal to 1.0 pm, turbulent flow conditions and anisokinetic sampling are preferred to yield the highest counts. 

In a particle sampling process, numerous mechanisms contribute to the error in the particle mass flux measurements. These mechanisms include gravitational sedimentation, impaction on the wall or at the tube bends, wall deposition due to the diffusion of small particles, flow turbulence, surface drag, agglomeration of fine particles, electrostatic charge, stickability of particles to the wall, and flow disturbance by the insertion of the probe, in addition to anisokinetic sampling discussed in 2.2. 

Badzioch, S., "Correction for Anisokinetic sampling of Gas-borne Dust Particles", J. Inst. Fuel, 33, 106 (1960). 

Vitols, J. H., "Theoretical Limits of Errors due to Anisokinetic Sampling of Particulate Matter", JAPCA, 16, 79 (1966). 

Zhu, C., Yu, T., and Huang, D., "Numerical Study of Effect of Velocity Slip on Isokinetic/Anisokinetic Sampling of Gas-Solid Flows", Int. Symp. on Multiphase Fluid, Non-Newtonian Fluid and Physicochemical Fluid Flows 97 Beijing, Oct. 7-9, Beijing, China, 1997. 

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