heat exchanger and presentation radiation.pptx

boundary layer

1. CONVECTION BOUNDARY LAYERS The region of the flow over the surface bounded by δ in which the effects of viscous shearing forces caused by fluid viscosity are observed, is called the velocity boundary layer or hydrodynamic boundary layer or simply the boundary layer. The thickness of boundary layer δ is generally defined as a distance from the surface at which local velocity u = 0.99 of free stream velocity u ∞ . δ = Thickness of boundary layer u =local velocity u ∞ = Free stream velocity As a result of frictional effects between the fluid layers, the local fluid velocity u will vary from x = 0, y = 0 to y = δ from the surface With increasing the distance y from the surface, shear stress decreases, the local velocity u increases until approaches u ∞ . With increasing the distance from the leading edge, the effect of viscosity penetrates further into the free stream and boundary layer thickness grows (δ increases with x). Uniform velocity

2. PHYSICAL SIGNIFICANCE OF THE DIMENSIONLESS PARAMETERS

3. It is a dimensionless property, a function of temperature. It provides a measure of relative effectiveness of momentum and energy transfer in the velocity and thermal boundary layers, respectively. For gases Pr ≅ 1 ; i.e., both momentum and heat diffusion through the fluid take place at the same rate. For liquid metal Pr << 1 ; indicates heat diffuses in the fluid very quickly, and for oils, Pr >> 1; indicates heat diffusion is very slow in the fluid relative to momentum. Consequently, the thermal boundary layer is much thicker for liquid metals, much thinner for oils relative to velocity boundary layer as shown in Fig. 7.7. Further, the thicknesses of two boundary layers can be related as