The simplest configuration for a flow with heat transfer is a uniform external flow over a flat surface, part or all of which is at a temperature different from that of the oncoming fluid (Fig. 1.1). In slightly more complicated cases the surface may be curved and the external-flow velocity u e may be a function of the longitudinal coordinate x, but in a large number of practical heat-transfer problems the variation of u e with y in the external flow is negligibly small compared with the variation of velocity in a region very close to the surface. Within this region, called the boundary layer, the x-component velocity u rises from zero at the surface to an asymptotic value equal to u e ; in practice one defines the thickness of this layer as the value of y at which u has reached, say, 0.995u e . The temperature also varies rapidly with y near the surface, changing from the surface value T w (subscript w means “wall”) to the external-flow value T e , which, like u e , can often be taken independent of y.
KeywordsHeat Transfer Shear Layer Prandtl Number Momentum Transfer External Flow
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