Impinging flow pattern

Figure 10-21 A. Impingement baffles and fluid-flow patterns. (Used by permission Brown Root, Inc.)...

Demyanovich RJ, Bourne JR. 1989. Rapid micromixing by the photographic study of the impingement of thin liquid sheets. 1. A flow pattern. Ind. Engng Chem. Res. 28 830-839. 

When the fluid is contained in a vessel, the fluid streamlines about the particle, impinge on the walls and reflect back on to the particle causing increased drag. Also, since the fluid is stationary at the vessel walls, there is a distortion of the flow pattern that reacts back on the particle. Both effects increase the drag on the particle, leading to a reduced estimate of particle size. The modified form of equation (6.7) is [16] ... 

In finite containers, the drag force acting on a particle increases because of two effects. Because the fluid is stationary within a finite distance of the particle, there is a distortion of the flow pattern that reacts back on the particle. The second effect occurs when the fluid streaming around the particle impinges on the container walls and is deflected back on the particle. Considering these two effects, the drag force may be modified as (8)... 

Interestingly, better drying performance (lower final moisture contents and higher volumetric evaporation rates) was found for ISD with a rectangular impingement chamber than for the cylindrical one that is commonly used in laboratory studies and industrial units (Cao and Liu, 2000). This effect can be attributed to a different flow pattern that results in improved heat and mass transfer. 

T. Wang, D. Faria, LJ. Stevens, J.S.C. Tan, J.E Davidson and D.I. Wilson, Flow patterns and draining films created by horizontal and inclined coherent water jets impinging on vertical walls, Chem. Eng. Sci. 102,585-601 (2013). 

FIG. 2 Geometry of some flow cells with simple well-defined flow patterns (a) impinging jet with a stagnation-point flow (h) a slab (c) a cylindrical capillary. 

For proper use of the equations, the chamber shape must conform to the spray pattern. With cocurrent gas-spray flow, the angle of spread of single-fluid pressure nozzles and two-fluid pneumatic nozzles is such that wall impingement wiU occur at a distance approximately four chamber diameters below the nozzle therefore, chambers employing these atomizers should have vertical height-to-diameter ratios of at least 4 and, more usually, 5. The discharge cone below the vertical portion should have a slope of at least 60°, to minimize settling accumulations, and is used entirely to accelerate gas and solids for entty into the exit duct. 

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