Flow irregularities

Deviation from laminar shear flow [88,89],by calculating the material functions r =f( y),x12=f( Y),x11-x22=f( y),is assumed to be of a laminar type and this assumption is applied to Newtonian as well as viscoelastic fluids. Deviations from laminar flow conditions are often described as turbulent, as flow irregularities or flow instabilities. However, deviation from laminar flow conditions in cone-and-plate geometries have been observed and analysed for Newtonian and viscoelastic liquids in numerous investigations [90-95]. Theories have been derived for predicting the onset of the deviation of laminar flow between a cone and plate for Newtonian liquids [91-93] and in experiments reasonable agreements were found [95]. 

Flow irregularities at gap angles of 30° were observed in viscoelastic liquids [94]. It has been indicated in theoretical treatments that the possibility of secondary flows [96,97] in rotational devices is to be expected if the gap angle is much greater than 5°. For viscoelastic fluids deviations from laminar flow have only been reported in cone-and-plate geometries with gap angles above 10°. 

In a previous paper (16) the hardware design of the SEC/Viscometer system used in this work has been described. The effects of operational variables, e.g. pump pulsations, flow rate and flow irregularities on the performance of the viscometer and the enhancement of the signal-to-noise ratio by using mechanical dampers also has been described. Our current work has focused on the development, implementation and application of automated data... 

Figure 20 presents mesophase rods produced by extrusion alone and by a light draw after extrusion. As Jenkins and Jenkins observed (34), the strong preferred orientation induced by extrusion was easily disturbed by pyrolysis bubbles or even by small flow irregularities. However, modest draws (e.g., draw ratio = 2) after extrusion produced fibrous morphologies with good uniformity. At these draw levels, the nodes and crosses characteristic of wedge disclinations could be resolved on transverse sections. 

The steady-state flow properties of block copolymers are often hard to measure. In steady shear, the shear stress often does not reach a clear steady-state value (Lyngaae-Jorgensen 1985). In cone-and-plate rheometers, steady shearing of an ordered block copolymer can result in edge fracture and flow irregularities, as might be expected when one forces a quasi-solid structure to flow (Winey et al. 1993a). 

When the sample is introduced into the column, usually in the form of a zone of vapor, it takes the form of a narrow band. During transit through the column, various factors influence the width of this band, which is continuously increased due to various dispersion processes. These include diffusion of the solute, resistance to mass transfer between and within phases, and the influence of flow irregularities and pertur-bations.f A simple concept, the theoretical plate, carried over from distillation processes, has been used to compare columns and account for the degree of dispersion that influences bandwidth. A chromatographic column may be considered to consist of numerous theoretical plates where the distribution of sample components between the stationary and mobile phase occurs. Hence, a measure of the efficiency of a GC column may be obtained by calculating the number of theoretical plates, N, in the column from ... 

Solutes flow out in the order of increasing distribution coefficients, and thus can be separated. Although a sample is applied as a narrow band, the effects of finite mass transfer rate and flow irregularities in practical chromatography result in band broadening, which may make the separation among eluted bands of solutes... 

The simplest gas distributor is a perforated pipe. Since the pipe creates a significant amount of flow irregularity in the distributor region, the packed material is usually placed a distance away from the gas inlet to ensure the gas flow becomes more uniformly distributed upon entering the packed region. The separation distance is... 

The mass transfer performance under slug annular and churn flow regime is depicted in Table 7.5. From the reported values of mass transfer coefficients, it is observed that k a is higher in churn flow compared to Taylor flow. However, the flow irregularity and low stability of these regimes limit their use for mass transfer. 

As any other liquid chromatographic technique, SEC analysis can be influenced by peak broadening due to axial dispersion and flow irregularities. A description of this phenomenon is beyond the scope of this article. For more information, see the brief summary presented by Styring and Hamielec and the related references cited by these authors (33). 

The fourth term is Eq. 15 is the sum of the instrumental contributions from injection, detection, dead volume of the system, and flow irregularities. This term is not considered in the present work. 

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