Separation, flow

There are two main reasons why a pump should not operate below its MCSF (/) the radial force (radial thmst) is increased as a pump operates at reduced flow (44,45). Depending on the specific speed of a pump, this radial force can be as much as 10 times greater near the shut off, as compared to that near the BEP and (2) the low flow operation results in increased turbulence and internal flow separation from impeller blades. As a result, highly unstable axial and radical fluctuating forces take place. 

Internal Flow. Depending on the atomizer type and operating conditions, the internal fluid flow can involve compHcated phenomena such as flow separation, boundary layer growth, cavitation, turbulence, vortex formation, and two-phase flow. The internal flow regime is often considered one of the most important stages of Hquid a tomiza tion because it determines the initial Hquid disturbances and conditions that affect the subsequent Hquid breakup and droplet dispersion. 

Readings given by open straight tubes (Fig. 10-ld and le are too low due to flow separation. Readings of closed tubes oriented perpendicularly to the axis of the stream and provided with side openings (Fig. 10-lg) may be low by as much as two velocity heads. 

Flow separation in boundary layer exciting blades... 

In a stationary impeller, the flow is expeeted to follow the blade shape and exit tangentially to it. A high adverse pressure gradient along the blade passage and subsequent flow separation are not eonsidered to be general possibilities. 

Matching the flow between the impeller and the diffuser is complex because the flow path changes from a rotating system into a stationary one. This complex, unsteady flow is strongly affected by the jet-wake of the flow leaving the impeller, as seen in Figure 6-29. The three-dimensional boundary layers, the secondary flows in the vaneless region, and the flow separation at the blades also affects the overall flow in the diffuser. 

When the impeller seems to be the eause of surge, the indueer seetion is where the flow separation begins. A deerease in the mass flow rate, an inerease in the rotational speed of the impeller, or both ean eause the eom-pressor to surge. 

It should be noted that the illustrations of the flow paths in Figures 6-37 through 6-39 are somewhat simplistic. Each flow path is indicated by a single streamline. The actual flow field is far more complex, with flow separation and recalculation present. Nevertheless, these figures should help with a practical understanding of the effects of changes in velocity triangles. 

Blade loading and profile loss. This loss is due to the negative veloeity gradients in the boundary layer, whieh gives rise to flow separation. 

Flow separation Formation of turbulent eddies away from boundary as... 

For a building with sharp corners, Cp is almost independent of the wind speed (i.e., Reynolds number) because the flow separation points normally occur at the sharp edges. This may not be the case for round buildings, w here the position of the separation point can be affected by the wind speed. For the most common case of the building with a rectangular shape, Cp values are normally between 0.6 and 0.8 for the upwind wall, and for the leeward wall 0,6 < C, < —0.4. Figure 7.99 and Table 7.32 show an example of the distribution of surface pressure coefficient values on the typical industrial building envelope. 

A major fraction of the aerodynamic drag in the modern streamlined car is caused by flow separation at the rear of the body. Alleviating that separation calls for a long afterbody that tapers to a point. 

Mechanism 2 of Figure 8-122B becomes apparent when the flow recirculation on the tray increases with increasing underflow clearance. The curvature of the column wall influences the movement of the liquid toward the center. High underflow clearance does not even out maldistribution due to backup where the irregular flow pattern enters into the tray below. This allows flow separation to occur on the downcomer floor, and leads to enhanced retrograde flow. 

Figure 8-122. Modification of downcomer weir at tray floor outlet (A) downcomer weir (B) mechanism of flow separation. Used by permission, Biddulph, M. W. et al. The American Institute of Chemical Engineers, Chem. Eng. Prog. V 89. No. 12 (1993), p. 56, all rights reserved.

Bypassing in a plate-type exchanger is less of a problem and more use is made of the flow separation which occurs over the plate troughs since the reattachment point on the plate gives rise to an area of very high heat transfer. 

Methods for determining the drop in pressure start with a physical model of the two-phase system, and the analysis is developed as an extension of that used for single-phase flow. In the separated flow model the phases are first considered to flow separately and their combined effect is then examined. 

The idealized picture of the flow in a heated micro-channel is shown in Fig. 8.1a. Such flow possesses a number of specific properties due to its unique structure, which forms because of liquid evaporation and the interaction of pure vapor and liquid flows separated by the interface surface. The latter has an infinitely thin surface with a jump in pressure and velocity, while the temperature is equal. One can... 

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