Conductive heat exchange

Conduction - [HEAT-EXCHANGE TECHNOLOGY- HEAT TRANSFER] (Vol 12)... 

Analysis of the various stages in mold filling is based on two dimensionless criteria the Graetz Number, Gz, which is a measure of the ratio of convective to conductive heat exchange ... 

Figure 18.29 includes spectral properties for a paper product (i.e., the spectral, diffuse absorptivity of 62 g/cm2 paper), along with normalized Planck blackbody distributions of sources at various temperatures [156], In the absence of convection or conduction heat exchange between the source (,v) and load (L), and assuming for the moment that the source and load are in an infinite parallel plate arrangement, an expression for the heat flux delivered to an opaque load can be derived using the analyses of Chap. 7 ... 

In order to extract thermal battery parameters from a measured impedance spectrum, a thermal model of the battery is necessary. This model reproduces the effects of heat capacity, thermal conductivity, heat exchange with environment, and internal losses generation. Transient simulation is then used to calculate an impedance spectrum for the battery model which is subsequently employed for parameter identification. 

Each discretization element i consists of an element temperature T, and a lumped heat capacity, . Neighboring elements are linked by a thermal resistance representing thermal conductivity. Heat exchange with environment comprises radiation and convection. Internal heat generation is expressed by irreversible losses P,. For our simulations, a discretization with = 10 is used, which has turn out to be a good compromise between accuracy and computational effort. 

Apart from coupling the exothermic and endothermic reactions in the same volume, these reactions can be physically decoupled to run in different volumes which are separated by a solid medium allowing conductive heat exchange between the reactions. This configuration can be realized in microchannel... 

In reality, both such concerns can be overcome by dedicated monolith designs addressing the specific requirements of chemical applications as presented in Section 28.4, conductive heat exchange in monolith structures can be even more effective than convective heat transfer in packed beds, whereas washcoat catalyst loadings in excess of 25% by volume are well within the range of what is practised with monoliths nowadays. Furthermore, new structured supports, like open-celled foams, are now being considered, which show promising properties in relation to radial heat transfer. 

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. 

Values of thermal conductivity are temperature-dependent and vary widely for different materials. Table 1 summarizes the thermal conductivity values of a few materials relevant to heat-exchanger analysis (1,2). 

The Tube Wall Tubular heat exchangers are built using a number of circular (or noncircular) tubes thus, the heat-transfer rate across tubular walls, following Fourier s law of heat conduction, becomes... 

Effect of Uncertainties in Thermal Design Parameters. The parameters that are used ia the basic siting calculations of a heat exchanger iaclude heat-transfer coefficients tube dimensions, eg, tube diameter and wall thickness and physical properties, eg, thermal conductivity, density, viscosity, and specific heat. Nominal or mean values of these parameters are used ia the basic siting calculations. In reaUty, there are uncertainties ia these nominal values. For example, heat-transfer correlations from which one computes convective heat-transfer coefficients have data spreads around the mean values. Because heat-transfer tubes caimot be produced ia precise dimensions, tube wall thickness varies over a range of the mean value. In addition, the thermal conductivity of tube wall material cannot be measured exactiy, a dding to the uncertainty ia the design and performance calculations. 

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