Apparent thermal diffusivity

The quantity (ka/pfCPf) is termed the apparent thermal diffusivity and will here be given the symbol aa. It is not the thermal diffusivity of the fluid since it is the ratio of the apparent conductivity of the porous medium to the product of the density and specific heat at constant pressure of the fluid. 

Figure 3.5 The apparent vapour pressure of gold in gas transportation measurements as a function of the gas flow rate. Low flow rates, which were used earlier to assure equilibrium, are now known to be too high as a result of thermal diffusion in the gas mixture which is saturated with gold vapour...

Apparently hydrogenation was caused mainly by these penetrating ions and not, as originally surmised by Seager and Anderson, by thermal diffusion from the metal into the silicon (Seager, 1989). 

A five-fold increase in effective burning rate is possible by using 0.005-in diameter silver wires. The degree of increase that can be obtained with wires of various metals is apparently determined by the thermal diffusivity and melting temperatures of the metal. This is illustrated in Table II (12). The burning rate at 1000 p.s.i.a. and 70°F. 

Thermal conductivity increases with increasing apparent density, volatile matter, ash, and mineral matter content. Due to the high porosity of coal, thermal conductivity is also strongly dependent on the nature of gas, vapor, or fluid in the pores, even for monolithic samples (van Krevelen, 1961). Moisture has a similar effect and increases the thermal conductivity of coal since its thermal conductivity value is approximately three times higher than that of dry coal (Speight, 1994, and references cited therein). However, the thermal diffusivity of coal is practically unaffected by moisture since the /Cp value is not essentially changed by moisture. 

In most cases the Dufour effect is so small that it apparently often is negligible even when thermal diffusion is not negligible. Although it is omitted in the applications, this term is retained in the general equations for completeness. 

Uncertainties of the conventional parameters of H-atoms have been addressed since the early applications of X-ray charge density method. Support from ND measurements appears to be essential, because the neutron scattering power is a nuclear property (it is independent of the electronic structure and the scattering angle). The accuracy of nuclear parameters obtained from ND data thus depends mainly on the extent to which dynamic effects (most markedly thermal diffuse scattering) and extinction are correctable. Problems associated with different experimental conditions and different systematic errors affecting the ND and XRD measurements have to be addressed whenever a joint interpretation of these data is attempted. This has become apparent in studies which aimed either to refine XRD and ND data simultaneously [59] (commonly referred to as the X+N method), or to impose ND-derived parameters directly into the fit of XRD data (X—N method) [16]. In order to avoid these problems, usually only the ND parameters of the H-atoms are used and fixed in the XRD refinement (X-(X+N) method). 

Previous DTA results on powdered polystyrene have shown an endothermic peak above T. (see Fig. 2a), which has been identified as Tu by comparing it to TBA and thermal diffusivity" results. Apparently the endothermic peak is caused by the powder fusing and flowing to wet and cover the bottom of the sample pan, as postulated by Stadnicki et al. ... 

The time constant R /D, and hence the diffusivity, may thus be found dkecdy from the uptake curve. However, it is important to confirm by experiment that the basic assumptions of the model are fulfilled, since intmsions of thermal effects or extraparticle resistance to mass transfer may easily occur, leading to erroneously low apparent diffusivity values. 

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