Thermal properties diffusivity

Keywords polyethylene, modification, thermal properties, diffusion, structure - property relation. 

Because it was not possible to explain the differences in the effectiveness of hydrogen as compared to other gases on the basis of differences in their physical properties, ie, thermal conductivity, diffusivity, or heat capacity differences, their chemical properties were explored. To differentiate between the hydrogen atoms in the C2H2 molecules and those injected as the quench, deuterium gas was used as the quench. The data showed that although 90% of the acetylene was recovered, over 99% of the acetylene molecules had exchanged atoms with the deuterium quench to form C2HD and... 

To prevent or reduce diffusion, a barrier is placed between the two materials. The ideal barrier material must not react with the materials it separates and have suitable electrical and thermal properties. Many designs call for the barrier to be deposited with constant thickness inside very narrow (0.35 im) and deep holes (aspect ratio of 2 to 1 or more). 

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). 

The high surface-to-volume ratio can also significantly improve both thermal and mass transfer conditions within micro-channels in two ways firstly, the convective heat and mass transfers, which take place at the multi-phase interface, are improved via a significant increase in heat and mass transfer area per unit volume. Secondly, heat and mass transfers within a small volume of fluid take a relatively short time to occur, enabling a thermally and diffusively homogeneous state to be reached quickly. The improvement in heat and mass transfer can certainly influence overall reaction rates and, in some cases, product selectivity. Perhaps one of the more profound effects of the efficient heat and mass transfer property of micro-reactors is the ability to carry potentially explosive or highly exothermic reactions in a safe way, due to the relatively small thermal mass and rapid dissipation of heat. 

Electrical and thermal conductivity are important diffusion layer properties that affect the fuel cell s overall performance. The maferial chosen to be the DL in a fuel cell must have a good electrical conductivity in order for the electron flow from the FF plates to the CLs (and vice versa) to have the least possible resistance. Similarly, the DL material must have good thermal properties so that heat generated in the active zones can be removed efficiently. Therefore, in order to choose an optimal material it is critical to be able to measure the electrical and thermal conductivity. In this section, a number of procedures used fo measure fhese paramefers will be discussed. 

The book by Reid et al. [9] is an excellent source of information on properties such as thermal conductivities, diffusion coefficients and viscosities of gases and liquids. Not only are there extensive tables of data, but many estimation methods and correlations are critically reviewed. 

An additional thermal property of interest is thermal diffusivity. The dental pulp sensory system is extremely sensitive to changes in temperature. These sensory inputs are interpreted only as pain. Metallic restorations of deep carious lesions of the tooth frequently need to have a low thermal conductor placed beneath them to avoid causing pulpal pain. The thermal diffusivity of composite varies from approximately that of tooth structure (0.183 mm2/s) to twice that value [204, 254], Metallic restorations of concern have diffusivities at least an... 

Recall from the beginning of the chapter that a related quantity to the thermal conductivity is the thermal diffusivity, a, which is defined as k/pCp, where k is the thermal conductivity, p is the density and Cp is the heat capacity at constant pressure per unit mass, or specific heat. Below are these thermal properties for polycarbonate. 

Next, the thermal properties of the dye must be such that absorption of the laser energy will result in dye diffusion but not in decomposition. The melting temperature Tm, the latent heat of fusion, AH, and the specific heat for these dyes were determined by differential scanning calorimetry using a DuPont 990 Thermal Analyzer. The data are given in Table II. No thermal decomposition products for these dyes were detected upon heating to 600 °C for 20 msec. 

The term thermal properties is open to more than one interpretation. Specific heat, thermal conductivity and diffusivity clearly come under this heading but the term can be taken to also include heat ageing, low temperature tests and fire resistance. However, these are more properly dealt with, as in this volume, under Effect of Temperature. Thermal analysis is a group of techniques in which a property of a sample is monitored against temperature, or time at a temperature, and, therefore, is also generally concerned with measuring the effect of temperature. Nevertheless, for convenience, a brief overview of thermal analysis is given here. 

When the ideas of symmetry and of microscopic reversibility are combined with those of probability, statistical mechanics can deal with many stationary state nonequilibrium problems as well as with equilibrium distributions. Equations for such properties as viscosity, thermal conductivity, diffusion, and others are derived in this way. 

In accordance with the usual process conditions, the initial temperature of the reactive mixture To and the upper cap temperature Tw are constant during filling, and the temperature of the insert Ti equals the ambient temperature (20°C). The model takes into account that during filling the temperature of the insert increases due to heat transfer from the reactive mix. It is assumed that the thermal properties and density of both the reactive mass and the insert are constant. It is reasonable to neglect molecular diffusion, because the coefficient of diffusion is very small 264 therefore, the diffusion term is negligible in comparison with the other terms in the mass balance equation. 

Obtain the Taylor-Prandtl modification of the Reynolds analogy between momentum transfer and mass transfer (equimolecular counterdiffusion) for the turbulent flow of a fluid over a surface. Write down the corresponding analogy for heat transfer. State clearly the assumptions which are made. For turbulent flow over a surface, the film heat transfer coefficient for the fluid is found to be 4 kW/m2 K. What would the corresponding value of the mass transfer coefficient be, given the following physical properties Diffusivity D = 5 x 10 9 m2/s. Thermal conductivity, k = 0.6 W/m K. Specific heat capacity Cp = 4 kJ/kg K. Density, p = 1000 kg/m3. Viscosity, p = 1 mNs/m2. 

The physical property monitors of ASPEN provide very complete flexibility in computing physical properties. Quite often a user may need to compute a property in one area of a process with high accuracy, which is expensive in computer time, and then compromise the accuracy in another area, in order to save computer time. In ASPEN, the user can do this by specifying the method or "property route", as it is called. The property route is the detailed specification of how to calculate one of the ten major properties for a given vapor, liquid, or solid phase of a pure component or mixture. Properties that can be calculated are enthalpy, entropy, free energy, molar volume, equilibrium ratio, fugacity coefficient, viscosity, thermal conductivity, diffusion coefficient, and thermal conductivity. 

The physical and thermal properties of the gas and liquid, interfacial area and liquid holdup, physical mass transfer coefficients, diffusion coefficients, and volumetric flow rate of the liquid are independent of temperature and conversion. 

Effect of Level of CTBN. In Table V we varied the level of CTBN at a constant amount of piperidine. At 20 parts of CTBN we find a fourfold increase in impact strength with an 11 °C loss of heat distortion temperature. This loss of thermal properties suggests that some of the CTBN flexibilizes the epoxy matrix. The morphology of these systems all shows about the same particle size. However electron micrographs of the fracture surface of the system with 20 parts CTBN show that the particles are somewhat larger and more diffuse. 

---