Velocity flow improvements with

Vanes may be used to improve velocity distribution and reduce frictional loss in bends, when the ratio of bend turning radius to pipe diameter is less than 1.0. For a miter bend with low-velocity flows, simple circular arcs (Fig. 6-37) can be used, and with high-velocity flows, vanes of special airfoil shapes are required. For additional details and references, see Ower and Pankhurst The Mea.surement of Air Flow, Pergamon, New York, 1977, p. 102) Pankhurst and Holder Wind-Tunnel Technique, Pitman, London, 1952, pp. 92-93) Rouse Engineering Hydraulics, Wiley, New York, 1950, pp. 399 01) and Joreensen Fan Engineerinp, 7th ed., Buffalo Forge Co., Buffalo, 1970, pp. Ill, 117, 118). 

While some of these disadvantages can be overcome by devising improved algorithms, the problem of level of description of the RANS turbulence model remains as the principal shortcoming of composition PDF code. One thus has the option of resorting to an LES description of the flow combined with a composition PDF code, or a less-expensive second-order RANS model using a velocity, composition PDF code. 

Typical flux data with two interpromoter spacings (AL) are shown in Figure 28 as a function of the cross-flow rate. The flux Increased by a factor of 3 for the best case. Though Probstein did not plot his data in this way, it is Interesting to note that the empty channel flux has a predictable 0.33 power dependence on tangential velocity. With the turbulence promoters, the slope shifts closer to the 0.7-0.8 power dependence normally observed in turbulent flow. Unfortunately, data are not available in Probstein s paper on the increased pressure drop associated with the turbulence promoters, but it would appear that the flux to power ratio is greatly improved with turbulence promoters. 

The results of Fig. 19.8 for a swirl number of 1.35 show that the attenuation increased to 10 dB with the velocity of the axial jet up to 42 m/s, and further increase to 47 m/s caused the amplitude to fall from around 6 kPa to less than 1.5 kPa and the attenuation to decrease from 10 dB to almost zero. Similar results were observed with the swirl number of 0.6 the attenuation improved with axial jet velocity up to 60 m/s, after which the amplitude and attenuation decreased. The decline in the amplitude of oscillation and its attenuation by active control was due to the interaction between the axial jet with a large velocity and the central recirculation zone, which caused the flame to move further downstream of the swirler and heat release to occur further from the pressure antinode. The consequent increase in the distance between the point of entry of the oscillated fuel and the active burning zone reduced the effectiveness of the oscillated input due to increased fluid dynamic damping and development of a large difference in phase between different parts of the oscillated flow, especially with swirl surrounding the oscillated axial jet. 

An important advantage of the use of EOF to pump liquids in a micro-channel network is that the velocity over the microchannel cross section is constant, in contrast to pressure-driven (Poisseuille) flow, which exhibits a parabolic velocity profile. EOF-based microreactors therefore are nearly ideal plug-flow reactors, with corresponding narrow residence time distribution, which improves reaction selectivity. 

The technique of continuous flow was further improved and expanded with the introduction of pulsed flow, which uses short pulses of constant-velocity flow and cuts the reagent consumption to 3-4 mL per determination [6]. The combination of large flow velocities and short times was experimentally achieved by use of a special syringe ram assembly. 

Instead of using a furrowed or dimpled membrane support plate, Sobey [41] observed that a single flow deflector in a flat membrane channel could produce many vortices under oscillatory flow conditions, an effect named the vortex wave. An important feature of the vortex wave is that it could occur under low crossflow velocity conditions or with laminar flow so that it can be used for shear-sensitive fluids. Millward et al. [42] tested the effect of vortex waves on plasma filtration with waves produced by flow deflectors with cross-sectional area of 1 x 1 mm in a 2.25 mm high channel as shown in Figure 8.22. The aim was to improve membrane applications for the separation of plasma from cellular blood components for both donor... 

Because of the larger ID when operating a standard diameter SEC column at a flow rate of 1 ml/minute, the linear velocity is 2.5 times lower than when the same flow rate is used on a 4.6 mm ID column. Thus, an SEC column is operated closer to the velocity at which the column performs at optimal efficiency. As discussed, however, at least a 10-fold drop in flow rate is required for the column to perform near its optimum for most proteins. This effect is illustrated in Figure 3, in which a protein test mixture is separated at various flow rates on a 25 cm x 4.1 mm ID column packed with 10 pm, 250 A, amidebonded silica (52). Clearly, resolution improves with decreasing flow rate the optimum efficiency had not yet been reached at a flow rate of 65 pl/minute or a linear velocity at 0.13 mm/s. 

CFD analysis computes local fluid velocity, pressure, and temperature throughout the region of interest for problems with complex geometries and boundary conditions. By coupling the CFD-predicted fluid flow behavior with the electrochemistry and accompanying thermodynamics, detailed predictions are possible. Improved knowledge of temperature and flow conditions at all points in the fuel cell lead to improved design and performance of the unit. 

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