Radiation, heat transfer mode

The heat transfer characteristics of the model are simulated via heat structures. Heat structures are one-dimensional abstract components used to represent the solid mass of the reactor plant. The one dimension is in the radial direction, perpendicular to the direction of fluid flow. Heat transfer in the axial direction (parallel to fluid flow) is represented by using several fluid volumes in the axial direction and connecting a heat structure to each. TRACE heat structures also permit two-dimensional conduction in Cartesian, cylindrical or spherical geometry. Conduction, convection, and radiation heat transfer modes can all be represented with heat structures. Heat structures, with their associated boundary conditions and neutron kinetics calculations, provide the ability to link thermal conditions among the plant structural, fuel (power components), coolant, and ambient environment (radiation enclosures). 

There are three heat-transfer modes, ie, conduction, convection, and radiation, each of which may play a role in the selection of a heat exchanger for a particular appHcation. The basic design principles of heat exchangers are also important, as are the analysis methods employed to determine the right size heat exchanger. 

Radiative heat transfer is perhaps the most difficult of the heat transfer mechanisms to understand because so many factors influence this heat transfer mode. Radiative heat transfer does not require a medium through which the heat is transferred, unlike both conduction and convection. The most apparent example of radiative heat transfer is the solar energy we receive from the Sun. The sunlight comes to Earth across 150,000,000 km (93,000,000 miles) through the vacuum of space. FIcat transfer by radiation is also not a linear function of temperature, as are both conduction and convection. Radiative energy emission is proportional to the fourth power of the absolute temperature of a body, and radiative heat transfer occurs in proportion to the difference between the fourth power of the absolute temperatures of the two surfaces. In equation form, q/A is defined as ... 

The governing heat transfer modes in gas-solid flow systems include gas-particle heat transfer, particle-particle heat transfer, and suspension-surface heat transfer by conduction, convection, and/or radiation. The basic heat and mass transfer modes of a single particle in a gas medium are introduced in Chapter 4. This chapter deals with the modeling approaches in describing the heat and mass transfer processes in gas-solid flows. In multiparticle systems, as in the fluidization systems with spherical or nearly spherical particles, the conductive heat transfer due to particle collisions is usually negligible. Hence, this chapter is mainly concerned with the heat and mass transfer from suspension to the wall, from suspension to an immersed surface, and from gas to solids for multiparticle systems. The heat and mass transfer mechanisms due to particle convection and gas convection are illustrated. In addition, heat transfer due to radiation is discussed. 

In a typical electronics system, heat removal from the ehip may require the use of several heat transfer meehanisms to transport heat to the eoolant or the surrounding environment. There are three basie heat transfer modes (ineluding phase ehange) eonduetion, eonveetion, and radiation. 

Convection, conduction, radiation, electromagnetic fields, combination of heat transfer modes Intermittent or continuous ... 

It is possible, indeed desirable in some cases, to use combined heat transfer modes, e.g., convection and conduction, convection and radiation, convection and dielectric fields, etc., to reduce the need for increased gas flow that results in lower thermal efficiencies. Use of such combinations increases the capital costs, but these may be offset by reduced energy costs and enhanced product quality. No generalization can be made a priori without tests and economic evaluation. Finally, the heat input may be steady (continuous) or time-varying also, different heat transfer modes may be deployed simultaneously or consecutively depending on the individual application. In view of the significant increase in the number of design and operational parameters resulting from such complex operations, it is desirable to select the optimal conditions via a mathematical model. 

Equation 7.12 shows that the rate of energy emitted by a blackbody increases in proportion to the absolute temperature to the fourth power, so that radiation will generally be the dominating heat transfer mode at high absolute temperatures. 

Radiation heat transfer from the flame to the product is the primary mode used in many industrial combustion systems. There are a variety of burner designs thaf rely primarily on this mechanism. Radiant wall burners are discussed in Chapter 18. Radiant tube burners are discussed in Chapter 24. Thermal radiation burners are discussed in Chapter 25. 

A more recent study of the heat fluxes inside the reactor as well as a comparison of the theoretical data predicted by a radiation heat transfer model showed that radiation is the main mode of heat transfer in the multiple hearth reactor configuration used (10). 

Heating can be done by conduction, convection, radiation, or some combination of these. The usual mode of heat transfer in thermoforming is radiation. Radiation heat transfer in polymeric systems was discussed in Chapter 4. 

The source term, S, includes combustion, that is, the heat source and the heat transfer within the system that affect temperature. In rotary kilns, the dominant heat transfer mode is radiation and there are several models to evaluate its value, some of which will be examined in detail later. 

Heat transfer in the bed of a rotary kiln is similar to heat transfer in packed beds except that in addition to the heat flow in the particle assemblage of the static structure (Figure 8.3), there is an additional contribution of energy transfer as a result of advection of the bed material itself. The effective thermal conductivity of packed beds can be modeled in terms of thermal resistances or conductance within the particle ensemble. As shown in Figure 8.3 almost all the modes of heat transfer occurs within the ensemble, that is, particle-to-particle conduction and radiation heat transfer as well as convection through the interstitial gas depending upon the size distribution of the material and process temperature. Several models are available in the literature for estimating the effective thermal conductivity of packed beds. 

Heat transfer through these various insulations can occur by several different mechanisms, but generally involves solid conduction, gas conduction and convection, and radiation. The purpose of any insulation is to minimize the summed transfer of heat by these various mechanisms. The apparent thermal conductivity of an insulation, measured experimentally to incorporate all of these heat transfer modes, offers the best means by which to compare the different types of insulation. Table 7.1 provides a listing of some accepted thermal conductivity values for the more popular insulations. 

Possible modes of heat transfer from a furnace to a specimen are by conduction, radiation and convection. In reality, however, heat transfer occurs only by convection and radiation. Heat transfer does not occur by conduction since the spedmen does not directly contact furnace walls. For convection and radiation heat transfer, the heat flow is a function of the difference in temperature of the furnace and the sample as described in Equation 1. 

The heat transfer in fluidized beds of monodisperse particle has been extensively investigated in the past. Heat transfer in a packed/fluidized bed with an interstitial fluid may involve many mechanisms as shown in Fig. 1 (Yagi and Kunii, 1957). These mechanisms can be classified into three heat transfer modes in fluidized beds fluid—particle or fluid—wall convection particle-particle or particle—wall conduction and radiation. Different heat transfer models are developed for these mechanisms, as described in the following. 

Figure 8 Relative contributions of the heat transfer modes considered to the overall heat transfer as a function of ks/kf. line 1, heat conduction Qnsfs/ line 2, heat conduction Qcsfs, line 3, heat conduction line 4, the solid-solid radiation between particle surfaces dashed-line, the percentage of total conduction. Reprinted from Cheng and Yu (2013) with permission from ACS.

In any operation in which a material undergoes a change of phase, provision must be made for the addition or removal of heat to provide For the latent heat of the change of phase plus any other sensible heating or cooling that occurs in the process. Heat may be transferred by any one or a combination of the three modes—conduction, convection, and radiation. The process involving change of phase involves mass transfer simultaneous with heat transfer. 

Heat is transferred by radiation, condurtion, and convection. Radiation is the primaiy mode and can occur even in a vacuum. The amount of heat transferred for a given area is relative to the temperature differential and emissivity from the radiating to the absorbing surface. Conduction is due to molecular motion and occurs within... 

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. 

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. 

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