Heat Transfer Modes Combined

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. 

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. 

Two-phase flows are classified by the void (bubble) distributions. Basic modes of void distribution are bubbles suspended in the liquid stream liquid droplets suspended in the vapor stream and liquid and vapor existing intermittently. The typical combinations of these modes as they develop in flow channels are called flow patterns. The various flow patterns exert different effects on the hydrodynamic conditions near the heated wall thus they produce different frictional pressure drops and different modes of heat transfer and boiling crises. Significant progress has been made in determining flow-pattern transition and modeling. 

Summary of experimental data Film boiling correlations have been quite successfully developed with ordinary liquids. Since the thermal properties of metal vapors are not markedly different from those of ordinary liquids, it can be expected that the accepted correlations are applicable to liquid metals with a possible change of proportionality constants. In addition, film boiling data for liquid metals generally show considerably higher heat transfer coefficients than is predicted by the available theoretical correlations for hc. Radiant heat contribution obviously contributes to some of the difference (Fig. 2.40). There is a third mode of heat transfer that does not exist with ordinary liquids, namely, heat transport by the combined process of chemical dimerization and mass diffusion (Eq. 2-162). 

On the basis of experimental observations (Fig. 3.27), Senda et al.[335h415] proposed six modes for water droplet deformation, and breakup during impingement on a hot surface coupled with heat transfer and evaporation (Fig. 3.28). Each mode occurs under a specific combination of surface temperatures and impact conditions, as described below. 

This section provides the solution of several examples where radiation is combined with the other modes of heat transfer. 

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. 

This book is concerned with a description of some methods of determining convective heat transfer rates in various flow situations, realizing that in many cases these methods will need to be combined with calculations for the other modes of heat transfer in order to predict the overall heat transfer rate. 

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... 

Convection is the mode of energy transfer between a solid surface and the adjacent liquid or gas that is in motion, and it involves the combined effects of conduction and fluid motion. The faster the fluid motion, the greater the convection heal transfer. In the absence of any bulk fluid motion, heat transfer between a solid surface and the adjacent fluid is by pure conduction. The presence of bulk motion of the fluid enhances the heat transfer betxveen the solid surface and the fluid, but it also complicates the determination of heat transferrates. 

The high thermal conductivity of BeO combined with other properties make it a unique material. The mode of heat transfer is by lattice waves, since the eleetrons are tightly bonded to the ions. The lack of free electrons causes the electrical conductivity and dielectric loss to be low. This combination of high thermal conductivity and low electrical conductivity is unique among commercially available materials that are economically feasible for most applications. 

The heat-transfer rate is found to be substantially higher under conditions of agitation. The heat transfer is usually said to occur by combined conductive and convective modes. A discussion and explanation are given by Holt [Chem. Eng., 69(1), 110 (1962)]. Prediction of [/ by Eq. (11-48) can be accomplished by replacing a by a, the effective thermal diffusivity of the bed. To date so httle work has been performed in evaluating the effect of mixing parameters that few predictions can be made. However, for agitated liquid-phase devices Eq. (18-19) is applicable. Holt (loc. cit.) shows that this equation can be converted for solids heat transfer to yield... 

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. 

Although not valid for large temperature differences, this linearized form of the radiative heat flux is frequently used because of its convenience, especially in problems dealing with a combination of all three modes of heat transfer. 

Figure 1.17 Examples involving combined modes of heat transfer...

Figure 1-20 (a) A model for combined modes of heat transfer, (b) two systems for the model. 

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