Boundary layer separation solid cylinder

External mass transfer, such as diffusion to particles or to the outside of pipes or cylinders, requires different correlations from those for internal mass transfer, because there is boundary-layer flow over part of the surface, and boundary-layer separation is common. The mass-transfer coefficients can be determined by studying evaporation of liquid from porous wet solids. However, it is not easy to ensure that there is no effect of internal mass-transfer resistance. Complications from diffusion in the solid are eliminated if the solid is made from a slightly soluble substance that dissolves in the liquid or sublimes into a gas. This method also permits measurement of local mass-transfer coefficients for different points on the solid particle or cylinder. 

The behavior of Co with particle Reynolds number for spheres, disks, and cylinders is shown in Figure 4-1. Note that the curves appear to pass through regions of behavior. Also, note the obvious bend in the curves for spheres and cylinders in the vicinity of Rcp = 10. These kinks are due to a phenomenon called boundary layer separation, which takes place when the fluid s velocity change is so large that the fluid no longer adheres to the solid surface. 

There are some cases where this approach fails. One such case is that in which significant regions of separated flow exist. In this case, although the boundary layer equations are adequate to describe the flow upstream of the separation point, the presence of the separated region alters the effective body shape for the outer inviscid flow and the velocity outside the boundary layer will be different from that given by the inviscid flow solution over the solid surface involved. For example, consider flow over a circular cylinder as shown in Fig. 2.16. Potential theory gives the velocity, ui, on the surface of the cylinder as ... 

In the range of Reynolds number Re = 103 to 107 (based on cyhnder diameter and free stream velocity), the flow aronnd a solid circular cylinder is periodic and transitional in character. The range of interest of the present work is located in a sub-critical flow regime (103 < Re < 105, corresponding to air velocities of - 0.1-10 m/s around a typically sized 0.1m diameter limb), in which, dne to the vortex shedding at the cylinder surface, the flow is highly unstable. The boundary layer remains fidly laminar up to the separation point and transition to turbulence... 

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