Convection physical mechanism

The physical mechanisms of cold transfer are the same as heat transfer and use the same physical processes of conduction, convection, and radiation/absorption (for more information on these processes see Sec. 6.1.1). 

It is well known that a hot plate of metal will cool faster when placed in front of a fan than when exposed to still air. We say that the heat is convected away, and we call the process convection heat transfer. The term convection provides the reader with an intuitive notion concerning the heat-transfer process however, this intuitive notion must be expanded to enable one to arrive at anything like an adequate analytical treatment of the problem. For example, we know that the velocity at which the air blows over the hot plate obviously influences the heat-transfer rate. But does it influence the cooling in a linear way i.e., if the velocity is doubled, will the heat-transfer rate double We should suspect that the heat-transfer rate might be different if we cooled the plate with water instead of air, but, again, how much difference would there be These questions may be answered with the aid of some rather basic analyses presented in later chapters. For now, we sketch the physical mechan-... 

Radiation heat-transfer phenomena can be exceedingly complex, and the calculations are seldom as simple as implied by Eq. (1-11). For now, we wish to emphasize the difference in physical mechanism between radiation heat-transfer and conduction-convection systems. In Chap. 8 we examine radiation in detail. 

We may summarize our introductory remarks very simply. Heat transfer may take place by one or more of three modes conduction, convection, and radiation. It has been noted that the physical mechanism of convection is related to the heat conduction through the thin layer of fluid adjacent to the heat-transfer surface. In both conduction and convection Fourier s law is applicable, although fluid mechanics must be brought into play in the convection problem in order to establish the temperature gradient. 

Describe the physical mechanism of convection. How is the convection heat-transfer coefficient related to this mechanism ... 

The discussion and analyses of Chap. 5 have shown how forced-convection heat transfer may be calculated for several cases of practical interest the problems considered, however, were those which could be solved in an analytical fashion. In this way, the principles of the convection process and their relation to fluid dynamics were demonstrated, with primary emphasis being devoted to a clear understanding of physical mechanism. Regrettably, it is not always possible to obtain analytical solutions to convection problems, and the individual is forced to resort to experimental methods to obtain design information, as well as to secure the more elusive data which increase the physical understanding of the heat-transfer processes. 

Presentation of the subject follows classical lines of separate discussions for conduction, convection, and radiation, although it is emphasized that the physical mechanism of convection heat transfer is one of conduction through the stationary fluid layer near the heat transfer surface. Throughout the book emphasis has been placed on physical understanding while, at the same time, relying on meaningful experimental data in those circumstances which do not permit a simple analytical solution. 

We stait this chapter with a discussion of the physical mechanism of natural convection and the Grashof number. We then present the correlations to evaluate heat transfer by natural convection for various geometries, including fmned surfaces and enclosures, (finally, we discuss simultaneous forced and natural convection. 

We know that a hot boiled egg (or a hot baked potato) on a plate eventually cools to the surrounding air temperature (Fig. 9-1). The egg is cooled by transferring heat by convection la the air and by radiation to the surrounding surfaces. Disregarding heat transfer by radiation, the physical mechanism of cooling a hot egg (or any hot object) in a cooler environment can be explained as follows ... 

So far we presented some general discussions on boiling. Now we turn our attention to the physical mechanisms involved in pool boiling, that is, the boiling of stationary fluids. In pool boiling, the fluid is not forced to flow by a mover such as a pump, and any motion of the Iluid is due to natural convection currents and Ihe motion of the bubbles under the influence of buoyancy. 

What are the physical mechanisms responsible for the variability of the vertical thermohaline structure in the deep western basin As hypothesized in [1, 18], there are two major concurrent mechanisms, which can be referred to as the convective and advective mechanisms. The former is related to the evaporation from the... 

Equation (3.1.1) reflects the fact that the transfer of a substance in a moving medium is due to two distinct physical mechanisms. First, there is molecular diffusion due to concentration difference in a liquid or gas, which tends to equalize the concentrations. Second, the solute is carried along by the moving medium. The combination of these two processes is usually called convective diffusion [133, 270],... 

When two bodies having different temperatures are put in contact, there is a heat transfer from the higher-temperature body to the lower-temperature body. Heat transfer can be effected by three physical mechanisms conduction, convection, and radiation. 

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