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1、The Concept of Boundary LayerPrinciples of Chemical Engineering-CDUTBoundary layer and separation of boundary layer Most of the area away from the wall(small change of velocity,being regarded as ideal fluid)A thin layer of fluid near the wall(considering the effect of viscosity,existing the velocity
2、 gradient)Relative motion between actual fluid and solid wallPrinciples of Chemical Engineering-CDUTBoundary layer and separation of boundary layer Figure 1Principles of Chemical Engineering-CDUTBoundary layer and separation of boundary layer Laminar boundary layerLaminar sublayerBoundary layerTrans
3、ition zoneTurbulent boundary layerBuffer layerTurbulent boundary layerTurbulent bodyBoundary layer:fluid layer with large velocity gradient near the wallMain flow area:the area outside the boundary layer where the velocity gradient is close to zeroPrinciples of Chemical Engineering-CDUTBoundary laye
4、r and separation of boundary layer 1Reynolds coefficien:critical Reynolds coefficient:2For a smooth plate wall,the range of critical Reynolds coefficient is:562 10Re3 10cx Principles of Chemical Engineering-CDUTBoundary layer and separation of boundary layer Figure 2Principles of Chemical Engineerin
5、g-CDUTBoundary layer and separation of boundary layer In some cases,an important feature of the boundary layer isthat it may break away from the surface.Whenever this happens,the fluid in the boundary layer willflow backward and produce vortices,resulting in energy loss ofthe fluid.This phenomenon i
6、s called boundary layer separation,and it is one of the important reasons for the energy loss ofviscous fluid.There are necessary conditions for a boundary layer separationto happen:the fluid has to be viscous,and there has to be anadverse pressure gradient in the flow.Principles of Chemical Engineering-CDUTBoundary layer and separation of boundary layer Figure 3