Convection heat transfer physical mechanism

Conductive and Convective Heat Transfer, Thermo Explosion by. There are three fundamental types of heat transfer conduction, convection radiation. All three types may occur at the same time, but it is advisable to consider the heat thransfer by each type in any particular case. Conduction is the transfer of heat from one part of a body to another part of the same body, or from one body to another in physical contact with it, without appreciable displacement of the particles of either body. Convection is the transfer of heat from one point to another within a fluid, gas or liquid, by the mixing of one portion of the fluid with another. In natural convection, the motion of the fluid is entirely the result of differences in density resulting from temp differences in forced convection, the motion is produced by mechanical means. Radiation is the transfer of heat from one body to another, not in contact with it, by means of wave motion thru space (Ref 5)... 

The convective heat transfer coefficients hi and h0 must be calculated from equations that involve the geometry of the system, the physical properties of the fluid, and the velocity with which it is flowing. These equations are obtained variously by more or less fundamental analysis of the heat transfer and fluid flow mechanisms, or by correlation of experimental data, or by combinations of these methods. A few typical values of the film coefficients are... 

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-... 

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. 

In this equation, the convective heat transfer coefficient depending on the physical properties and flow condition (hydraulics) of the coolant. Ts and Tc have been used as cladsurface and coolant temperature respectively. The variation of hydraulic parameters, like velocity or channel flow rate requires a linkage of fluid flow to the heat transfer part of HEATHYD. This feedback mechanism as depicted in Fig. 1 is mathematically modelled through outer iteration. By this procedure, the results of the heat transfer calculations are provided to the hydraulic part and vice versa. Tg, T velocity and pressure are the parameters that are mainly exchanged between the two parts. 

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). 

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. 

Transfer of heat by physical mixing of the hot and cold portions of a fluid is known as heat transfer by convection The mixing can occur as a result of density differences alone, as in natural convection, or as a result of mechanically induced agitation, as in forced convection. 

The heat transfer in a foam, as in any other physical system, occurs through thermal conductivity, heat radiation and convection [87]. It was established that in disperse systems the heat transfer through radiation is only significant at high temperature (> 100°C) and in the presence of large pores, while convection is effective only if the particles (bubbles in the foam) are large (> 1 mm). This means that thermal conductivity is the basic mechanism of heat transfer at not very high temperatures. 

This section will present one of the possible physical interpretations of these important dimensionless numbers. First, to show the meaning of Nusselt number, we consider the heat transfer flux in the x direction in the case of a pure molecular mechanism compared with the heat transfer characterizing the process when convection is important. The corresponding fluxes are then written as ... 

The result shows that the physical significance of the Nusselt number is quantity of heat transferred by the convective mechanism... 

We start this chapter with a general physical description of the convection mechanism. We then discuss (he velocity and thermal botmdary layers, and laminar and turbitlent flows. Wc continue with the discussion of the dimensionless Reynolds, Prandtl, and Nusselt nuinbers, and their physical significance. Next we derive the convection equations on the basis of mass, momentiim, and energy conservation, and obtain solutions for flow over a flat plate. We then nondimeiisionalizc Ihc convection equations, and obtain functional foiinis of friction and convection coefficients. Finally, we present analogies between momentum and heat transfer. 

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 ... 

The previous sections have dealt with a nnmber of important condnction problems, bnt all had specific geometric limitations and usually idealized assumptions. Nnmerical methods (which, for all practical purposes, require the use of a computer) can handle a much broader range of problems, inclnding any geometry that can be described in terms of surfaces and intersects, variable physical properties (including temperatnre and spatial variation), and surface heat-transfer mechanisms, inclnding convection and radiation, internal heat generation, and transient behavior. 

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. 

Let us assume that a finite time has elapsed after liquid oxygen has been placed in a vessel and that a frost layer has formed on the surface. At the outer surface of the frost (Figure 1) heat is transferred by natural or forced convection air currents, by loss of sensible and latent heat of the water vapor in the environment as it approaches the frost layer by diffusion and convection mechanisms and undergoes phase changes from the vapor to the solid state, and by radiation to the frost surface from the sun, sky, and physical surroundings. The total heat transferred to the frost surface is the sum of these three types of heat transfer. 

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