Thermal conductivity mixture-averaged

Thermal Conductivity. The results of the thermal conductivity, k, tests are given in Table VII for four different mixture ratios representing S/A ratios of 1.3 to 10. The data indicate that additional sulfur had little effect on the thermal conductivity, which averaged 11.7 X 10 4 cal-cm/ cm2-sec-°C (3.40 X Btu-in./ft2-hr-°F). A comparison with the value obtained for the A/C system of 15.77 cal-cm/cm2-sec-°C (4.57 Btu-in./ft2-hr-°F) would indicate that the thermal conductivity is about 25% less for S-A—S than for A/C. This is attributed to the higher air void contents in the former which add to the insulative characteristics of the material. 

In this form Fourier s law is substituted for the heat flux. The thermal conductivity X is the average conductivity of the fluid mixture. In subsequent chapters we discuss the details of how the thermal conductivity is determined and the process to calculate the mixture-averaged values. 

Pure species thermal conductivities are usually only needed for the purpose of later evaluating mixture-averaged thermal conductivities the conductivity in the multicomponent case presented in Section 12.5.6 does not depend on the pure species formulas stated in this section. 

For the mixture-averaged thermal conductivity one may use the averaging formula [267]... 

Consider one particular gas composition, e.g. 50 mole per cent H2. This is equivalent to (100/(100 + 800)) x 100 = 11.1 mass per cent H2. From Volume 6, Section 8.8.4, the thermal conductivity of the mixture kmjx can be considered as a simple weighted average, i.e ... 

The thermal conductivities of most mixtures of organic liquids are usually less than those predicted by either a mole or weight fraction average, although the deviations are often small (15). Then, it can be calculated through the following expression (14), with errors of about 4% ... 

Condensation of mixed vapors of immiscible liquids is not well understood. The conservative approach is to assume that two condensate films are present and all the heat must be transferred through both films in series. Another approach is to use a mass fraction average thermal conductivity and calculate the heat-transfer coefficient using the viscosity of the film-forming component (the organic component for water-organic mixtures). 

Liquid Mixtures The thermal conductivity of liquid mixtures generally shows a modest negative deviation from a linear mass-fraction-weighted average of the pure-component values. Although more complex methods with some improved accuracy are available, two simple methods are recommended here that require very little additional information. The first method applies only to binary mixtures while the second can be used for multipfe components. 

When heat transfer is involved, the density and viscosity can be functions of temperature, and the thermal properties of the fluid have to be prescribed as well. These properties include heat capacity, thermal conductivity, molecular weight (for gases), and thermal expansion coefficient. For problems involving chemical species, all physical properties have to be specified for all of the species along with a method to calculate the average property for mixtures in each cell using the properties of the component species. 

As with the viscosity, simplistic mole-fraction averaging of pure-component values is not advisable for gas-phase thermal conductivities. The most common method for predicting mixture values has the form... 

For the thermal conductivity of liquid mixtures well below the critical point of each component, a linear mass-fraction average of the pure-component values is often a reasonable approximation. Such an average usually somewhat overpredicts the mixture value, and more complex mixing rules have been proposed [15] that give better quantitative results. 

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