Heat transfer mode

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. 

Batch reaclors are tanks, usually provided with agitation and some mode of heat transfer to maintain temperature within a desirable range. They are primarily employed for relatively slow reactions of several hours duration, since the downtime for filling and emptying large equipment may be an hour or so. Agitation maintains uniformity and improves heat transfer. Modes of heat transfer are illustrated in Figs. 23-1 and 23-2. 

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 ... 

Heat-transfer modes, 73 242-248 Heat-transfer printing, 9 221, 417 Heat-transfer rate... 

For the smelt-water case. Nelson suggested the water in contact with the very hot smelt was, initially, separated by a thin vapor film. Either because the smelt cooled—or because of some outside disturbance— there was a collapse of the vapor film to allow direct liquid-liquid contact. The water was then heated to the superheat-limit temperature and underwent homogeneous nucleation with an explosive formation of vapor. The localized shocks either led to other superheat-limit explosions elsewhere in the smelt-water mass or caused intense local mixing of the smelt and water to allow steam formation by normal heat transfer modes. 

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. 

It is also clear that Newton s law of cooling is a special case of Fourier s law. The foregoing provides the reason for only two commonly recognized basic heat transfer mechanisms. But owing to the complexity of fluid motion, convection is often treated as a separate heat transfer mode. 

MODE Heat transfer mode -1 for coolant countercurrent to gas... 

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. 

Now, the characteristic of the Semenov model, which is illustrated by the diagram of the left-hand side in Fig. 3, is the spatially uniform distribution of internal temperature. It seems most natural to think that this mode of temperature distribution arises from the heat transfer mode that the heat generated by the exothermic decomposition reaction occurring in a very slowly self-heating fluid filled in a container and placed in the atmosphere under isothermal conditions is transmitted uniformly throughout the fluid on account of its own fluidity. 

In most steady-state heat transfer problems, more than one heat transfer mode may be involved. The various thermal resistances due to thermal convection or conduction may be combined and described by an overall heat transfer coefficient, U. Using U, the heat transfer rate, Q, can be calculated from the terminal and/or system temperatures. The analysis of this problem is simplified when the concepts of thermal circuit and thermal resistance are employed. 

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. 

Figure 3. Typical heat transfer modes inside the LHP evaporator.

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. 

Primarily, three different test techniques are used to determine the surface heat transfer characteristics. These techniques are based on the steady-state, transient, and periodic nature of heat transfer modes through the test sections. We will cover here the most common steady-state techniques used to establish the j versus Re characteristics of a recuperator surface. Different data acquisition and reduction methods are used depending upon whether the test fluid is a gas (air) or a liquid. The method used for liquids is generally referred to as the Wilson plot technique. Refer to Ref. 15 for the transient and periodic techniques. Generally, the isothermal steady-state technique is used for the determination of/factors. These test techniques are now described. 

The flow pattern depicts a distinct topology (regarding the spatial and temporal distributions of vapor and liquid phases) of two-phase flow and greatly influences the resulting phenomena of heat transfer and friction. An important feature of a particular flow pattern is the direct relationships of the heat transfer and pressure drop characteristics to the pattern type, leading to an easy identification of important macroscopic heat transfer modes. Consequently, an approach to the selection of appropriate heat transfer and/or pressure drop correlations has to be preceded by an identification of the involved flow patterns. 

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