Heat transfer. Conduction, convection and radiation

Heat conduction is a type of transfer of heat in solids and liquids, interpreted as the imparting of kinetic energy resulting from collisions between disorderly moving molecules. The process occurs without any macroscopic motions in the body. The conductivity of diamond without traces of isotope C is the highest. The conductivity of a metals is also high. The lowest conductivity is that of a gas. 

Heat transfer by conduction is defined by the Fourier equation (1.1). The application of Eq. (1.1) in calculations encounters difficulties because the temperature gradient of the wall must be defined, as well as its increments around the whole surface S of the body. Accordingly, for practical reasons the Newton equation is usually applied  

For a description of the heat flow phenomenon on the border of the body, a differential equation is used  

Heat transfer by convection occurs in liquids and gases where there is a velocity field caused by extorted fluid motion or by natural fluid motion caused by a difference in density. The former case involves forced convection, and the latter case free convection. Combined convection occurs when both forced and free convection are present. The convection coefficient of surface heat transfer, a, defining the heat exchange in the contact boundary layer between fluid and soUd, is determined. Coefficient or is often expressed by equations containing criteria numbers, such as those of Nusselt (Nu), Prandtl (Pr), Reynolds (Re) and Grashof(Gr)  

The criteria numbers are calculated by use of material constants such as A - thermal conductivity coefficient a - thermal diffusity coefficient and V - kinematic viscosity. In the expressions in (1.50), / is a distinctive dimension of the body w is the distinctive velocity g is the acceleration due to gravity ( i - lb) is the difference in temperature, and p is the thermal expansivity coefficient. 

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There are three fundamental types of heat transfer conduction, convection, and radiation. All three types may occur at the same time, and it is advisable to consider the heat transfer by each type in any particular case. 

Heat transfer is the energy flow that occurs between bodies as a result of a temperature difference. There are three commonly accepted modes of heat transfer conduction, convection, and radiation. Although it is common to have two or even all three modes ot heat transfer present in a given process, we will initiate the discussion as though each mode of heat transfer is distinct. 

The foundations of an engineering discipline may be best understood by considering the place of that discipline in relation to other engineering disciplines. Therefore, our first concern in this chapter will be to determine the place of heat transfer among engineering disciplines. Next, we shall proceed to a review of the general principles needed for heat transfer. Finally, we shall discuss the three modes of heat transfer— conduction, convection, and radiation—and introduce a five-step methodology for an inductive formulation. 

In the preceding section we established the place of heat transfer among the engineering disciplines and distinguished the modes of heat transfer—conduction, convection, and radiation. We proceed now to the formulation of heat transfer. 

Safety The safety design objective is to provide the capability to reject core decay heat relying only on passive (natural) means of heat transfer (conduction, convection, and radiation) without the use of any active safety systems. 

Q is the amount of heat in Joules that is transferred from surroundings into the system. Although the temperature difference is the driving force, the energy transfer is Q in Joules of energy. The heat transfer is transient in nature. The study of heat transfer is a separate subject in itself and is discussed in detail elsewhere [7] and in Chapter 11. The modes of heat transfer, conduction, convection, and radiation and of late microscale mechanisms such as wave heat conduction is discussed in Chapter 9. 

There are three mechanisms for heat transfer — conduction, convection, and radiation — as depicted in Figure 3.1. These mechanisms will be discussed from Subsection 3.2.2 through Subsection 3.2.4. Several basic principles of thermod5mamics need to be addressed prior to delving into heat transfer. 

To do this we begin by recognizing that there are three fundamental mechanisms of heat transfer conduction, convection and radiation, each of which play a part in the thermal response of a laminate specimen exposed to an external heat flux. This allows us to write an equation 14.2, for the heat balance at the surface line of the laminate described in Figure 14.1. 

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