presence of bulk fluid motion and by conduction in the absence of it Therefore conduction in a fluid can be viewed as the limiting case of convection corresponding to the case of quiescent fluid Fig 61 ID: 1024487
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2. 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 thepresence of bulk fluid motion and by conduction in the absence of it. Therefore,conduction in a fluid can be viewed as the limiting case of convection, corresponding to the case of quiescent fluid (Fig. 6–1).PHYSICAL MECHANISM OF CONVECTIONConvection heat transfer is complicated by the fact that it involves fluid motion as well as heat conduction. The fluid motion enhances heat transfer, since it brings hotter and cooler chunks of fluid into contact, initiating higher rates of conduction at a greater number of sites in a fluid. Therefore, the rate of heat transfer through a fluid is much higher by convection than it is by conduction.
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4. Experience shows that convection heat transfer strongly depends on the fluid properties dynamic viscosity , thermal conductivity k, density , and specific heat Cp, as well as the fluid velocity . It also depends on the geometry and the roughness of the solid surface, in addition to the type of fluid flow (such as being streamlined or turbulent). Thus, we expect the convection heat transfer relations to be rather complex because of the dependence of convection on so many variables. This is not surprising, since convection is the most complex mechanism of heat transfer.
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13. It is also common practice to nondimensionalize the heat transfer coefficient h with the Nusselt number, defined as
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