Combined 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. 

Combined modes of heat transfer (e.g., combined dielectric or radiation with convection or conduction)... 

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). 

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 heat transfer between a solid sur face and tile adjacent liquid or gas that is in motion,. and involves the combined effects of conduction and fluid motion. The rate of convection heal transfer is expressed by Newton s law of cooling as... 

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. 

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. 

The present section deals with a number of examples combining radiation with conduction and/or convection. Most problems involving more than one mode of heat transfer are relatively involved, as they yield nonlinear differential equations and/or boundary conditions whenever radiation is included. They are usually solved after a linearization of the Stefan-Boltzmann law. During this process, however, the quantitative nature of a problem gets lost. 

There are many applications where radiation is combined with other modes of heat transfer, and the solution of such problems can often be simplified by using a thermal resistance Rq, for radiation. The definition of Rth is similar to that of the thermal resistance for convection and conduction. If the heat transfer by radiation, for the example in Fig. 1.10, is written... 

Heat delivery. Convection and conduction from hot gas sweeping by is the leading mode of heat transfer to a drying coating to supply the latent heat of vaporization of solvent. Except when solvent evaporation is so very rapid as to produce an appreciable convective velocity away from the surface, in turbulent gas flow the mechanisms of heat transfer to and solvent transfer away from the evaporating surface are virtually identical combinations of convective action with thermal conduction on the one hand and molecular diffusion on the other. This is reflected in useful correlations, like Colburn s, of the mass transfer coefficient with the more easily measured heat transfer coefficient in turbulent flow. It is also the reason that the now fairly extensive literature on the performance and design of driers focuses on heat transfer coefficients and heat delivery rates. 

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 so-called apparent thermal conductivity of insulating materials depends upon four modes of heat transfer gas conduction and convection, radiation, and solid conduction. The principles of these four mechanisms of heat transfer are fairly well understood individually but their combined effect on heat transfer in insulating materials is complicated. Nevertheless, because of the additive nature of the heat transferred by these mechanisms, the conductivities assigned to each mechanism are additive. Thus, if each of these conductivities can be evaluated under various conditions of temperature and pressure, their sum stated as an apparent conductivity may be estimated. 

The radial heat transport is complex, involving conduction, convection, and radiation between voids and solid and between solid particles. Possible modes of heat transfer in the radial direction are shown in Figure 14.3. Different physical models result depending on whether various resistances to the heat transport are in series, parallel, or a combination of both. Here an additive model is considered, which assumes that the radial effective thermal conductivity consists of static (conduction and radiation) and dynamic contributions, the latter caused by fluid motion. These two contributions are considered to be additive ... 

As it was mentioned earlier, there are three modes of heat transfer, convection, conduction and radiation. Although two, or even all three, modes of heat transfer may be combined in any particular thermodynamic situation, the three are quite different and will be introduced separately. 

There are many commereial CFD tools available viz. FLUENT, ANSYS/FLOTRAN, CFD-ACE+ ete. The choice of the tool depends upon a few aspects viz. (i) The type of flow and the model that is to be designed (ii) ability to combine different mode of heat transfer along with the flow behavior (iii) post processing features. CFD has been extensively used to imderstand the flow field and distributions of temperature aroimd a human body in different environmental eonditions. The fabric can be modeled as a porous material consisting of fibers and air. The equation of permeability for flow through perpendieular to an array of rods by Kuwabara [23] ean be applied to find the permeability of the fabric, given by ... 

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

T. J. Chung and J. Y. Kim, Two-Dimensional Combined-Mode Heat Transfer by Conduction, Convection, and Radiation in Emitting, Absorbing, and Scattering Media—Solution by Finite Elements, ASME Journal of Heat Transfer, vol. 106, pp. 448-452,1984. 

Use of superheated steam in direct dryers Increased use of indirect (conduction) heating Use of combined (or integrated) heat transfer modes Use of volumetric heating (microwave [MW]/radio-frequency [RF] fields) in specialized situations Use of two-stage (or multistage) dryers Use of intermittent heat transfer Use of novel combnstion technologies (e.g., pulse combustion for flash drying)... 

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. 

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. 

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