cen58933_ch06

# cen58933_ch06 - cen58933_ch06.qxd 12:05 PM Page 333 CHAPTER...

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FUNDAMENTALS OF CONVECTION S o far, we have considered conduction, which is the mechanism of heat transfer through a solid or a quiescent fluid. We now consider convec- tion, which is the mechanism of heat transfer through a fluid in the presence of bulk fluid motion. Convection is classified as natural (or free ) and forced convection, depend- ing on how the fluid motion is initiated. In forced convection, the fluid is forced to flow over a surface or in a pipe by external means such as a pump or a fan. In natural convection, any fluid motion is caused by natural means such as the buoyancy effect, which manifests itself as the rise of warmer fluid and the fall of the cooler fluid. Convection is also classified as external and inter- nal, depending on whether the fluid is forced to flow over a surface or in a channel. We start this chapter with a general physical description of the convection mechanism. We then discuss the velocity and thermal boundary layers, and laminar and turbulent flows. We continue with the discussion of the dimen- sionless Reynolds, Prandtl, and Nusselt numbers, and their physical signifi- cance. Next we derive the convection equations of on the basis of mass, momentum, and energy conservation, and obtain solutions for flow over a flat plate. We then nondimensionalize the convection equations, and obtain func- tional forms of friction and convection coefficients. Finally, we present analo- gies between momentum and heat transfer. 333 CHAPTER 6 CONTENTS 6–1 Physical Mechanism on Convection 334 6–2 Classification of Fluid Flows 337 6–3 Velocity Boundary Layer 339 6–4 Thermal Boundary Layer 341 6–5 Laminar and Turbulent Flows 342 6–6 Heat and Momentum Transfer in Turbulent Flow 343 6–7 Derivation of Differential Convection Equations 345 6–8 Solutions of Convection Equations for a Flat Plate 352 6–9 Nondimensionalized Convection Equations and Similarity 356 6–10 Functional Forms of Friction and Convection Coefficients 357 6–11 Analogies between Momentum and Heat Transfer 358 cen58933_ch06.qxd 9/4/2002 12:05 PM Page 333

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6–1 PHYSICAL MECHANISM OF CONVECTION We mentioned earlier that there are three basic mechanisms of heat transfer: conduction, convection, and radiation. Conduction and convection are similar in that both mechanisms require the presence of a material medium. But they are different in that convection requires the presence of fluid motion. Heat transfer through a solid is always by conduction, since the molecules of a solid remain at relatively fixed positions. Heat transfer through a liquid or gas, however, can be by conduction or convection, depending on the presence of any bulk fluid motion. Heat transfer through a fluid is by convection in the presence of bulk fluid motion and by conduction in the absence of it. There- fore, conduction in a fluid can be viewed as the limiting case of convection, corresponding to the case of quiescent fluid (Fig. 6–1).
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