Secondary flow effects

It is well known (15, 16, 17) that colled tube heat exchangers possess superior heat transfer characteristics because of secondary flow effects. At small curvatures, the Dean number, defin-N = N -V a... 

P 54] The mixing experiments were performed at relatively low Re. For this purpose, 85% glycerol-water solutions were used, with dynamic viscosity and density of about 100 mPas and 1.2 kg l-1, respectively [7]. Total flow rates in the range 0.2 1 h-1 Re = 0.22) and 2 1 h 1 Re = 2.2) were applied. For the given dimensions, CFD simulation showed that for Re above about 15, corresponding to a total flow rate of 13.5 1 h 1, secondary flow induced by inertial forces had a notable effect. Thus the applied flow rates were well below the critical value and pronounced secondary flow effects were not observed. 

The laminar flow data of Figure 3.39 have a higher slope (0.52 than predicted by theory (0.33)-probably because of "secondary flow effects". The data were taken in a spiral flow thin channel device. Whenever fluid passes through a curved tube or channel, centrifugal forces tend to throw fluid outward from the center of the channel. It then recirculates inward along the walls of the channel (see Figure 3.40). It is well known that coiled tube heat exchangers possesses superior heat transfer characteristics because of secondary flow effects. 

Qiu assumed uniform axial velocity and concentration profiles (with no secondary flow) at the curved tube entrance (0°), there is an entrance region ( 25°) where Sh essentially follows a L v que boundary layer development Eventually ( 50°), the secondary flow effects become manifest, and marked differences in transport rates between the inside wall (low transport Sh 2) and the outside wall (high transport Sh 55) develop. The regions of high and low transport in the curved vessel geometry cannot be associated with axial flow separation (as in the expansion, stenosis, and bifurcation) because flow separation does not occur at the modest curvature levels in the coronary artery simulation. 

The central element of the slit plate micromixer is a structured metallic plate that carries a number of slits [35]. These slits generate a multilamellar structure of the liquids to be mixed with slit structures of characteristic dimensions as low as 25 pm. Further mixing between the two lamellate liquids occurs by diffusional processes and secondary flow effects. These secondary flow effects were studied by computational fluid dynamics (CFD) simulations... 

Secondary flow effects are similar to the cone and plate. Turian (1972) reports that his eqs. 5.4.25a and 5.4.26 can be used for parallel disks with the substitution of h/R for fi. The experimental results of Greensmith and Rivlin described in connection with eq. 5.4.22 are for the parallel disks and agree widi the theory. 

The accuracy of rheological data obtained using parallel plate measnrements depends on a variety of factors. First, it is essential that sufficient T versns Q and F versus Q data be collected to properly use Equations 8.42 and 8.47. Second, a good temperature control system must be in place to ensure uniform temperatnre throngh-out the gap. Finally, experimental conditions should be chosen to rednce errors from edge fracture, sample expulsion from the gap, wall slip, secondary flow effects due to fluid elasticity and/or misalignment of the plates, and viscous heating. 

Many materials of practical interest (such as polymer solutions and melts, foodstuffs, and biological fluids) exhibit viscoelastic characteristics they have some ability to store and recover shear energy and therefore show some of the properties of both a solid and a liquid. Thus a solid may be subject to creep and a fluid may exhibit elastic properties. Several phenomena ascribed to fluid elasticity including die swell, rod climbing (Weissenberg effect), the tubeless siphon, bouncing of a sphere, and the development of secondary flow patterns at low Reynolds numbers, have recently been illustrated in an excellent photographic study(18). Two common and easily observable examples of viscoelastic behaviour in a liquid are ... 

Ide and White W studied the viscoelastic effects in agitating polystyrene solutions with a turbine. At concentrations below 50% PS, flow was normal. Abovfe 35%, the viscoelastic forces caused the flow to reverse, moving away from the impeller along the axis. At 30 to 35% PS, both occurred, causing a segregated secondary flow around the turbine. 

The density change on polymerization is typically about 20%, and this density gradient can cause significant secondary flows and natural convection effects. The experiments cited above for vinyl acetate polymerization were performed in a helical reactor. The centrifugal force in helical reactors induces secondary flows as well. The effects of helical flow have been analyzed, but were found to be less significant than the effects of natural convection [14]. 

This chapter presents a physical description of the interaction of flames with fluids in rotating vessels. It covers the interplay of the flame with viscous boundary layers, secondary flows, vorticity, and angular momentum and focuses on the changes in the flame speed and quenching. There is also a short discussion of issues requiring further studies, in particular Coriolis acceleration effects, which remain a totally unknown territory on the map of flame studies. 

A cascade control system can be designed to handle fuel gas disturbance more effectively (Fig. 10.1). In this case, a secondary loop (also called the slave loop) is used to adjust the regulating valve and thus manipulate the fuel gas flow rate. The temperature controller (the master or primary controller) sends its signal, in terms of the desired flow rate, to the secondary flow control loop—in essence, the signal is the set point of the secondary flow controller (FC). 

It has been demonstrated that radial dispersion contributes more significantly to the dilution of the sample in the flow than does axial dispersion. This type of fluid movement, termed secondary flow by Tijssen [43], results in a washout effect accounting for the low mutual contamination of samples successively injected into a carrier stream. TTiis advantageous feature is a result of the use of low flow rates and small tubing bores, and results in decreased peak-width and hence to increased sampling rate. 

In the reverse osmosis field, only one commerical design has exploited the effect of secondary flow and it is no longer available (Figure 41). 

The dispersion in geometrically deformed tubes (squeezed, twisted and coiled ) has been extensively studied by Halasz (6, 7 and 8), and the effect of radial convection (secondary flow) on the dispersion introduced in tightly... 

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