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8 Cards in this Set
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How is binary diffusivity defined? |
Mass diffusion is a transport process that originates from molecular motion (like conduction heat transfer). Diffusion occurs as a result of a concentration gradient of some chemical species in a mixture. Binary diffusion occurs in mixtures of two components. Dab is the binary diffusion coefficient and Dab=Dba. It is inversely proportional to pressure and increases strong with temperature (for mixtures at low pressures). For idea gases, an approximation is: Dab = 1/3*c*lambd_mfp ~ (T^3/2)/P [length^2 per second]. |
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How is self diffusion defined? |
Mass diffusion is a transport process that originates from molecular motion (like conduction heat transfer). Diffusion occurs as a result of a concentration gradient of some chemical species in a mixture. Self-diffusion is the diffusion of chemically identical species.
Daa* = 1.32*nu, nu=kinematic viscosity = mu/rho |
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Define prandtl, schidt and lewis numbers. What ranges of Pr and Sc can one expect to encounter for gases and liquids? |
Pr = nu/alpha = C_p*mu/k = viscous diffusion rate/thermal diffusion rate Sc = nu/Dab = mu/(rho*Dab) = viscous diffusion rate/mass diffusion rate Le = alpha/Dab = k/(rho*C_p*Dab) = thermal diffusivity/mass diffusivity alpha = k/rho*C_p = thermal diffusivity, a measure of the rate of heat transfer in a material from the cold side to the hot side nu = mu/rho = kinematic viscosity, the ratio of dynamic viscosity, mu, to the density of the fluid, rho Units for kinematic viscoisty and thermal diffusivity are length squared over time. Le, pr, and Sc are dimensionless. Sc ranges from .2 to 3 for gas mixtures and up to 40,000 for liquid mixtures. |
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If a cube of sugar is placed in a cup of coffee, what is the driving potential for dispersion of the sugar in the coffee? What is the physical mechanism responsible for dispersion if the coffee is stagnant? What is the physical mechanism if the coffee is stirred? |
The driving potential is a concentration gradient. If the coffee is stagnant the sugar is dispersed by diffusion. If the coffee is stirred the sugar is dispersed by diffusion and advection. |
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What is the relationship between the molar concentration and the mass density of a species in a mixture? |
rho_a = M_a*C_a [kg/m^3] M is species molecular weight. [kg/kmol] C is molar concentration. [kmole/m^3] |
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How is the mass density of a mixture defined? The molar concentration of the mixture? Mass fraction? Mole fraction? |
Mixture mass density (rho) is the sum of rho_i. Rho_i is the mass of species i per unit volume of the mixture. C is the molar concentration and is the total number of moles per unit volume of the mixture. m_i = rho_i/rho, rho = sum(rho_i) x_i = C_i/C, rho = sum(C_i) |
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In using Fick's law to determine the mass or molar flux of a species in a mixture, what specifically is being determined? |
The flux of species a relative to the mixture average velocity. |
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what is the stationary medium approximation? |
n_a'' = 0 and x_a<<1 (advection is neglected) For diffusion in a stationary medium, the species diffusion flux is equal to the absolute flux associated with transport of the species. |
