Entrainment, with forced

Velocity measurements showed an increase in mean flow with forcing implying a strong radial air entrainment caused by the coherent structures. 

For sufficiently large forcing amplitudes the oscillation becomes completely entrained, with a period exactly equal to one forcing period, whatever that value of a>/a>0. The entrainment may arise from a phase-locked response—as seen previously in Fig. 13.9—or from a quasi-periodic pattern. The boundary for full entrainment appears as an almost straight line with positive slope of oj/oj0 > 1 and negative slope for oj/oj0 < 1. 

In FFF systems, the separation along the flow axis is caused by the perpendicular field, whose crucial role is recognized by the word "field" 1n field-flow fractionation. The applied field Interacts with entrained particles, forcing them to accumulate at one wall (the accumulation wall) of the channel. Since the flow velocity near any wall is reduced by frictional drag, the downstream displacement of the particles 1s retarded. Retardation (or retention) is greatest for those particles forced most closely to the wall. Consequently, particles are separated according to the different forces exerted on them by the applied field. These forces normally depend on particle size, leading to a size-based separation. 

The vaporized glycerine stream passes through the entrainment separator [10] and condenses on the internal U-tube condenser [9]. Droplets of material entrained with the vapor stream impinge on the entrainment separator and flow back to the heated wall through centrifugal force of the rotating assembly. Distillate flows out the distillate outlet [11] and noncondensables flow out through the vacuum outlet [13]. 

The capsule membrane appeared to consist of an outer skin, a thin macropo-rous layer and a very thick dense membrane, with an overall (mean) thickness of 90 pm (Fig. 12). The capsule diameter was 900 pm after 7 days. Smaller capsules can be made with a variation of this method in which the needle is held stationary and the hexadecane pumped past the end of the needle the hexa-decane is recirculated and care must be exercised to ensure that capsules are not entrained with the recirculated hexadecane (Fig. 13). The effect of hexadecane flowrate on capsule size (as it leaves the needle) in an early prototype is shown in Fig. 14 capsules shrink to approximately half their initial diameter as solvent is extracted. Note that capsules as small as 300 pm can be produced. As a consequence of the thin skin it is possible to damage the capsules through mishandling forceps with serrated edges or forcing capsules through narrow bore needles are avoided as a result. 

Maximum Vapor Velocity. The maximum velocity of the vapor leaving a free-liquid surface from which liquid particles may be entrained by force balance has been investigated by Sanders and Brown (1934) in connection with distillation columns and by York and Popele (1963) in connection with mesh separators. 

Other Centrifugal Collectors. Cyclones and modified centrifugal collectors are often used to remove entrained Hquids from a gas stream. Cyclones for this purpose have been described (167—169). The rotary stream dust separator (170,171), a newer dry centrifugal collector with improved collection efficiency on particles down to 1—2 pm, is considered more expensive and hence has been found less attractive than cyclones unless improved collection in the 2—10-pm particle range is a necessity. A number of inertial centrifugal force devices as well as some others termed dynamic collectors have been described in the Hterature (170). 

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