Modes of heat transfer conduction

Tlie growfii and spread of fires occurs fiuough heat transfer or tlie migration of burning materials. There are fiuee main modes of heat transfer conduction, convection, and radiation. 

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. 

Heat exchange between the body and the environment primarily involves convection, radiation, and evaporation. Conduction is another mode of heat transfer but it is of little significance in air environments. However, in an underwater environment, conduction is the dominant mode of heat transfer. Conduction is also important when the body contacts an object of extreme temperature. 

As another example, a drop of blue liquid dye is added to a cup of water. The dye molecules will diffuse slowly by molecular diffusion to all parts of the water. To increase this rate of mixing of the dye, the liquid can be mechanically agitated by a spoon and convective mass transfer will occur. The two modes of heat transfer, conduction and convective heat transfer, are analogous to molecular diffu.sion and convective mass transfer. 

In nature there are three modes of heat transfer conduction, convention, and radiation. 

There are three modes of heat transfer. Conduction is the method of heat transfer within a solid. In closed heat exchangers, conduction is how thermal... 

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. 

We have previously discussed two of the three modes of heat transfer (conduction and convection). This chapter will consider radiation, the third mode of heat... 

Contact temperature measurement is based on a sensor or a probe, which is in direct contact with the fluid or material. A basic factor to understand is that in using the contact measurement principle, the result of measurement is the temperature of the measurement sensor itself. In unfavorable situations, the sensor temperature is not necessarily close to the fluid or material temperature, which is the point of interest. The reason for this is that the sensor usually has a heat transfer connection with other surrounding temperatures by radiation, conduction, or convection, or a combination of these. As a consequence, heat flow to or from the sensor will influence the sensor temperature. The sensor temperature will stabilize to a level different from the measured medium temperature. The expressions radiation error and conduction error relate to the mode of heat transfer involved. Careful planning of the measurements will assist in avoiding these errors. 

As the pressure increases from low values, the pressure-dependent term in the denominator of Eq. (101) becomes significant, and the heat transfer is reduced from what is predicted from the free molecular flow heat transfer equation. Physically, this reduction in heat flow is a result of gas-gas collisions interfering with direct energy transfer between the gas molecules and the surfaces. If we use the heat conductivity parameters for water vapor and assume that the energy accommodation coefficient is unity, (aA0/X)dP — 150 I d cm- Thus, at a typical pressure for freeze drying of 0.1 torr, this term is unity at d 0.7 mm. Thus, gas-gas collisions reduce free molecular flow heat transfer by at least a factor of 2 for surfaces separated by less than 1 mm. Most heat transfer processes in freeze drying involve separation distances of at least a few tenths of a millimeter, so transition flow heat transfer is the most important mode of heat transfer through the gas. 

The other mode of heat transfer is conduction. The conductive heat flux is, by Fourier s law,... 

Conduction is the primary mode of heat transfer through solid material. Conduction occurs by two mechanisms ... 

It is not necessary to assume the liquid film to be completely stagnant. Radial motion can be allowed for, but with some difficulty. It was noted in Sec. IIB2 that Forster and Zuber state that conduction is the chief mode of heat transfer (compared with convection due to radial motion). Eddies or motions of the liquid tangent to the bubble are neglected. The Zwick-Plesset theory likewise excludes eddies. The derivation is lengthy therefore the final typical equations are presented here without proof. 

Maintenance of proper temperature is a major aspect of reactor operation. The illustrations of several reactors in this chapter depict a number of provisions for heat transfer. The magnitude of required heat transfer is determined by heat and material balances as described in Section 17.3. The data needed are thermal conductivities and coefficients of heat transfer. Some of the factors influencing these quantities are associated in the usual groups for heat transfer namely, the Nusselt, Stanton, Prandtl, and Reynolds dimensionless groups. Other characteristics of particular kinds of reactors also are brought into correlations. A selection of practical results from the abundant literature will be assembled here. Some modes of heat transfer to stirred and fixed bed reactors are represented in Figures 17.33 and 17.18, and temperature profiles in... 

In this chapter, we have looked at the three modes of heat transfer, viz., conduction, convection and radiation, and the laws relating heat fluxes to temperature driving forces for each mode. From these laws we have shown how to estimate heat transfer rates in a range of geometries and situations. 

Physical situations that involve radiation with other modes of heat transfer are fairly common. If conduction enters the problem, the Fourier conduction law states that the heat flow depends upon the temperature gradient, thus introducing derivatives of the first power of the temperature. If convection matters, the heat flow depends roughly on the first power of the temperatures, the exact power depends on the type of flow. For instance, natural convection depends on a temperature difference between the 1.25 and l. 4 power. Physical properties that are temperature dependent introduce more temperature dependencies. This all means that the governing equations are highly nonlinear. 

Convection is one of the three so-called modes of heat transfer, the other two are conduction and radiation [1].[2],[3],[4]. In most real situations, the overall heat transfer is accomplished by a combination of at least two of these modes of heat transfer. However, it is possible, in many such cases, to consider the modes separately and then combine the solutions for each of the modes in order to obtain the overall heat transfer rate. For example, heat transfer from one fluid to another fluid through the walls of a pipe occurs in many practical devices. In this case, heat is transferred by convection from the hotter fluid to the one surface of the pipe. Heat is then transferred by conduction through the walls of the pipe. Finally, heat is transferred by convection from the other surface to the colder fluid. These heat transfer processes are shown in Fig. 1.3. The overall heat transfer rate can be calculated by considering the three processes separately and then combining the results. 

It is easy to envision cases in which all three modes of heat transfer are present, as in Fig. 1-9. In this case the heat conducted through the plate is removed from the plate surface by a combination of convection and radiation. An energy balance would give... 

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