Chapter 7 - Momentum, heat and mass transfer in boundary layers

Abstract

The liquid velocity is zero at the surface and gradually increases with distance from the surface, after which a velocity profile is established. The velocity gradient is steepest adjacent to the surface and becomes progressively less with distance from it. Although theoretically, the velocity gradient is a continuous function becoming zero only at infinite distance from the surface, the flow may conveniently be divided into two regions—a boundary layer close to the surface where the fluid is retarded and a velocity gradient exists, and in which shear stresses are significant; and the region outside the boundary layer where the liquid is all flowing at the free-stream velocity. It is thus evident that the definition of the boundary-layer thickness is somewhat arbitrary. Boundary-layer thickness is often defined as the distance (normal to flow) from the surface at which the fluid velocity reaches some proportion of the free-stream velocityDifficulties arise in comparing differently defined boundary-layer thickness, because as the free -stream velocity is approached the velocity gradient becomes very low and a small difference in the velocity criterion corresponds to a very large difference in the resulting value of the boundary-layer thickness. A thorough understanding of the flow in the boundary layer is important in a range of chemical and processing applications because the nature of flow influences not only the drag at a surface or on an immersed object, but also the rates of heat and mass transfer when temperature or concentration gradients exist.