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【正文】 Chapter Six Introduction to Convection The Convection Transfer Problem ?TV,ss TA ,qsdA Local and total convection heat transfer The local heat flux ? ???? TThq swhere h is the local convection coefficient. Because flow conditions vary from point to point on the surface, both q” and h also vary along the surface ? ?????sA ssdATTqThe total heat transfer rate by defining an average convection coefficient h? ???? TTAhq ssThe total heat transfer rate ?? sA sdAqq The average and local convection coefficients are related ?? sA ss h dAAh 1 The Convection Boundary Layers Velocity Boundary Layers ??u?u??Velocity boundary layer ??x?Free stream x y Figure Velocity boundary layer on a flat plate. Retardation Shear stresses 1). Boundary Layers thickness It is defined as the value of y for which ?? uu ). Boundary Layers velocity profile It refers to the manner in which u varies with y through the boundary layer. ?? uu ~03). Fluid flow posing The fluid flow is characterized by two distinct regions, a thin fluid layer (the boundary layer) in which velocity gradients and shear stresses are larger and a region outside the boundary layer in which velocity gradients and shear stresses are negligible. 4). Friction coefficient 22??? uCsf??5). Shear stress 0????ys yu??2. The Thermal Boundary Layers t??T?TThermal boundary layer ??xt?Free stream x y Figure Thermal boundary layer on a flat plate. ?usT1). Boundary Layers thickness It is defined as the value of y for which the ratio . With increasing distance from the leading edge, the effects of heat transfer perate further into the free stream and the thermal boundary layer grows. 2). The local heat flux The local heat flux, which conduction at y=0, may be obtained by applying Fourier’s law to the fluid. That is, 3). Heat transfer coefficient Establishing the surface energy balance at the surface, we then obtain the heat transfer coefficient by bining Equation with Newton’s law of cooling. ? ? ? ?? ???? TTTT ss0?????yf yTkq?????? TTyTkhsyf 03. Significance of the Boundary layers ? ?? ?? ???????????????mctfhyCxl a y e rb o u n d a r yi o nc o n c e n t r a thyTxl a y e rb o u n d a r yt h e r m a lCyuxl a y e rb o u n d a r yv e l o c i t yl a y e r sB o u n d a r y,???Note: For flow over any surface, there will always exit a velocity boundary, and hence surface friction. However, a thermal boundary layer exit only if the surface and freetream temperature differ. Laminar and Turbulent Flow First step in treatment of ant Convection problem Laminar boundary layer Turbulent boundary layer ? ? Turbulent region Buffer layer Laminar Sublayer x Figure Velocity boundary layer development on a flat plate. ??u?uy, v Laminar Transition Turbulent x, u u v Streamline 1. Laminar boundary layer Fluid motion is highly ordered and it is possible to identify streamlines along which particles move. Fluid motion along a streamlines is characterized by velocity ponents in both the x and y directions. cx2. Transition boundary layer The boundary layer is initially laminar, but at some distance from the leading edge, transition to turbulent flow begins to occur. Fluid fluctuations begin to development in
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