Carrier flow regime

As already mentioned, some instruments do not show big sensitivity changes with the carrier flow regime (see Fig. 16). With constant-flow-rate regime reproducibility is much more preserved (Fig. 17). 

FFF process can be theoretically described because (a) the flow regime inside the channel of well-defined geometry can be mathematically represented (see Equation 12.1), and (b) the tractability of the various force fields employed in the different techniques allows one to describe the analyte concentration profile (see Equation 12.3). The retention ratio expresses the retardation of an analyte zone caused by its interaction with the field, and it is given by the ratio of the average velocity of the analyte zone and the average velocity of the carrier liquid (n) ... 

In this context, Griffiths reported in 1911 the interactions of an aqueous plug with a chemically inert carrier stream flowing through a narrow, straight tube [53]. He carried out the first experimental work demonstrating the essence of the dispersion process and concluded (without a mathematical treatment) that "a tracer injected into a water stream spreads out in a symmetrical manner about a plane in the cross section that moves with the mean flow velocity" [54], He also pointed out the establishment of a fully developed laminar flow regime. 

Sample dispersion is altered when the flowing sample merges with a confluent stream [27]. At the vicinity of the confluence site, convective mass transport is strongly altered by the sudden change in concentrations caused by the convergence of the confluent and sample carrier streams. The laminar flow regime tends to be maintained therefore, interactions between the sample carrier stream and the confluent stream are mainly dictated by radial diffusion. 

For longer residence times, the axial convection inherent to the laminar flow regime starts to contribute more significantly to the dispersion process, especially when higher carrier stream flow rates are used, and a tendency of the sample zone towards a skewed Gaussian distribution due to the combined influences of convection and diffusion manifests itself in a more pronounced manner. 

Most conditions and features of the conceptual process are chosen to assure high throughputs (small equipment) and hence relatively low capital and fixed costs. These include the choice of flow regimes, heat carriers (density and heat capacity) and the solids-to-gas weight ratios. Attendant features of the process, such as baffle design and gas routing, are chosen to achieve operability and optimum operation. 

The Oroskar-Turian s correlation and previous ones were developed to determine the critical deposit velocity of Newtonian carrier fluids with various particle sizes and concentrations. Shah and Lord (7) generalized equation 2 to extend its capability to correlate the critical deposit velocity for non-Newtonian carrier fluids (power law). The parameter X was eliminated from equation 2 because of its insignificant contribution to the correlation results and because it would be undefined for the laminar flow regime of non-Newtonian fluids. The generalized form of equation 2, which can be applied to either critical deposit (VD) or resuspension velocity (Vs), is as follows ... 

Figure 25.5 Finite fluid volume (FV) for an inner element of spray modeling, with an overlap between a continuous steady-state flow regime of the carrier medium and a non-stationa7 modeling of evaporating fluid elements by moving a second finite fluid volume.

Transmission mode FT-IR imaging of microfluidic devices remains attractive because it is important to be able to spectroscopically access the full flow of the system. Certain flow regimes, droplets in oil for instance, can produce results that are obscured by the ATR approach as the depth of penetration may not reach the aqueous droplet and could instead image only the carrier fluid, providing insufficient information about the desired system. 

FIA is a chemical analysis method based plug-wise injection into a carrier stream [114—116]. These plug samples can be further manipulated, e.g. by reaction to compounds better detectable by a detector. The sample consumption can be reduced via miniaturization however, then usually laminar-flow conditions are given so that micro mixers are needed which are efficient in that regime. 

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