Flow profile distortion

Pipe fittings such as elbows, tee-pieces, reducers, expanders, valves, etc. can all alter the symmetry of the flow profile. MlLLER(2) has listed the following effects  

Two or more of these conditions can occur at the same time, resulting in asymmetric axial, radial and tangential velocity vectors. Some flowmeters are more sensitive than others to particular types of flow distortion, e.g. orifice meters are affected by pure swirl more than venturi meters are magnetic flowmeters are unaffected by changes in the radial velocity component whereas ultrasonic time-of-flight meters are highly susceptible thereto swirl and asymmetry have the least effect on positive displacement meters and the greatest effect on variable area meters. 

Swirl is produced by the flow being made to change direction twice—each time in a different plane (Fig. 6.8b). This can be due to a combination of two successive 90° elbows or of a 90° elbow closely followed by a valve. The strongest swirl occurs when the two elbow planes are at 60° to each other(2). 

To avoid swirl, elbows should be well separated and have large radii of curvature. If this is not possible then the flowmeter should be sited at least 40 pipe diameters downstream of fittings causing asymmetric flow only and a minimum of 100 pipe diameters downstream when swirl is likely to occur0 S). There should also be at least 10 pipe diameters allowed downstream of the meter free of any obstruction or fitting. If the flow is laminar then these distances should be doubled. 

Flow profile distortion (a) velocity distribution after a single elbow (b) velocity distribution and swirl following two 90° elbows in different planes 

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FIGURE 11.32 Flow profiles in microchannels, (a) A pressure gradient, - AP, along a channel generates a parabolic or Poiseuille flow profile in the channel. The velocity of the flow varies across the entire cross-sectional area of the channel. On the right is an experimental measurement of the distortion of a volume of fluid in a Poiseuille flow. The frames show the state of the volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule, (b) In electroosmotic flow in a channel, motion is induced by an applied electric field E. The flow speed only varies within the so-called Debye screening layer, of thickness D. On the right is an experimental measurement of the distortion of a volume of fluid in an electroosmotic flow. The frames show the state of the fluorescent volume of fluid 0, 66, and 165 ms after the creation of a fluorescent molecule [165], Source http //www.niherst.gov.tt/scipop/sci-bits/microfluidics.htm (see Plate 12 for color version). 

The plug flow profile would only be distorted in very narrow bore capillaries with a diameter smaller than the thickness of two double-layers that then overlap. To achieve an undisturbed flow, Knox suggested that the diameter should be 10-40 times larger than 6 [15]. This can easily be achieved in open capillaries. However, once the capillary is packed with a stationary phase, typically small modified silica beads that carry on their own charged functionalities, the distance between adjacent double-layers is only a fraction of the capillary diameter. However, several studies demonstrated that beads with a submicrometer size can be used safely as packings for CEC columns run in dilute buffer solutions [15,35]. 

The classical FEE retention equation (see Equation 12.11) does not apply to ThEEE since relevant physicochemical parameters—affecting both flow profile and analyte concentration distribution in the channel cross section—are temperature dependent and thus not constant in the channel cross-sectional area. Inside the channel, the flow of solvent carrier follows a distorted, parabolic flow profile because of the changing values of the carrier properties along the channel thickness (density, viscosity, and thermal conductivity). Under these conditions, the concentration profile differs from the exponential profile since the velocity profile is strongly distorted with respect to the parabolic profile. By taking into account these effects, the ThEEE retention equation (see Equation 12.11) becomes ... 

If the spherical particle were not present in Figure 2.3, the volume elements of the flowing fluid would move upward in straight lines. In the presence of the particle, however, the flow profile is distorted around the sphere in the manner suggested by Figure 2.3. It is apparent that the velocity of any volume element passing the sphere is a function of both time and location and must be described as such in any quantitative treatment. The trajectory of such a volume element is called the flow streamline function. For spherical particles, this was analyzed by G. G. Stokes in 1850. 

In Fig. 13.8, Sample 1 shows that the outline (shape) of the front is circular in the deep section and becomes somewhat distorted upon entering the thinner region. The flow is split by the T insert and reunites past the insert, forming a weld line. The location and the shape of the weld line are determined by the flow profile around the insert. The insert strongly affects the direction of the advancing front which, as we see later, determines the direction of molecular orientation. We would expect, therefore, a highly nonuniform orientation distribution in such a mold. 

The static stability of the air stream usually changes as it moves into and out of the urban area, typically becoming less and more stable, respectively. However it should not be assumed that the boundary layer profiles over the urban area and downwind are identical to the equilibrium states found in neutral, stable and unstable boundary layers over flat terrain. In fact as the flow adjusts characteristic distortions of the air flow profiles occur on these scales, such as blocked flow, unsteady slope flows, gravity currents and boundary layer jets especially near hills, coasts and urban/rural boundaries. These distorted profiles (which are ignored in most mesoscale atmospheric models) significantly affect dispersion (e.g. Hogstrom and Smedman, [274] Owinoh et al., [477]). 

The solution of this simplest model just obtained brings the credible flow profiles for a duct with symmetric EPRs shown in Fig. 3.2. It can be seen that the velocity distribution varies from a parabolic shape (taking place in the absence of the EPR, A = 0 or <5 = 0) to a very distorted one which depends on the dimensionless density of the penetrable obstruction layer A. The shear stress keeps linear outside the obstruction layer but significantly bends within it. Both kinds of profiles quantitatively correspond to the experimental distributions measured in, for example, laboratory water flumes [231], Phenomenon of drag discontinuity (Chapter 6) can be observed at the top of the EPR that means that the profile r(z) is continuous but not differentiable. 

The scaling-up of packed beds is subject to the difficulties of maintaining even flow distributions. Removal of solution through screens on side walls is not recommended and the flow of resin from one section into another of much greater area could distort the resin flow profile. 

A variation of the truncated cone pin is the stepped spiral pin (Fig. 2.13), a design developed for high-temperature materials (Ref 41, 47, 48, 66, 68, 87). During the friction stir processing (FSP) of Ni-Al bronze, a threaded profile distorted, and threadless tools did not produce sufficient material flow to obtain 6 mm (0.25 in.) deep deformation regions. Thus, the stepped spiral tool was designed with robust... 

Distortion in the flow profile around bends reduces dispersion. 

Inviscid Parallel Flow Stability with Mean Profile Distortion, Journal of Hydronautics, July 1980... 

The original clamshell device was first introduced in 1989. It consisted of a double umbrella composed of four stainless steel arms covered with a polyester (Dacron ) meshwork. Each arm had a single springed hinge (see Fig. IS.S). - At the time of its introduction, double-umbrella devices were attractive because they offered the potential for minimal blood-flow pattern distortion due to their low profile. Additionally, since they possessed low alloy content, their corrosive potential was minimal. This device was deployed using an IIF sheath, which was smaller than the sheaths used by other devices at the time, but larger than those employed by the current Amplatzer Septal Occluder. ... 

Piping arrangement at the ship s manifold will often distort the flow profile. The flow sensor, when operated under the piping and flow conditions at the ship s manifold, must meet the accuracy criteria in 12.2. 

Thermostating is required in any non-equilibrium MFC simulation, where there is viscous heating. A basic requirement of any thermostat is that it does not violate local momentum conservation, smear out local flow profiles, or distort the velocity distribution too much. When there is homogeneous heating, the simplest way to maintain a constant temperature is to just rescale velocity components by a scale factor 5, = Sva, which adjusts the total kinetic energy to the desired value. 

Enough space must be available to properly service the flow meter and to install any straight lengths of upstream and downstream pipe recommended by the manufacturer for use with the meter. Close-coupled fittings such as elbows or reducers tend to distort the velocity profile and can cause errors in a manner similar to those introduced by laminar flow. The amount of straight pipe required depends on the flow meter type. For the typical case of an orifice plate, piping requirements are normally Hsted in terms of the P or orifice/pipe bore ratio as shown in Table 1 (1) (see Piping systems). 

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