Mass diffusivity, abbreviated as \(D\) or \(D_m\), also called diffusivity, is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species. Diffusion is the spread of gases, liquids, or solids from areas of high concentration to areas of low concentration. It is the rate one material can disperse through another material. The higher the diffusion coefficient, the faster the diffusion will be. The diffusion coefficient for solids tends to be much lower than the diffusion coefficient for liquids and gasses.
Mass diffusivity is influenced by several factors, including temperature, pressure, and the properties of the diffusing substance and the medium. It is commonly determined experimentally or estimated using theoretical models and correlations.
Mass diffusivity plays a crucial role in various scientific and engineering applications, particularly in areas involving mass transfer and diffusion processes, such as chemical reactions, heat and mass transfer in fluids, and the movement of substances through porous media. It is an essential parameter for analyzing and predicting the rates of diffusion and transport of species in different systems and understanding how they mix or disperse over time.
Lewis Number Mass Diffusivity Formula |
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\( D \;=\; \dfrac{ \alpha }{ Le } \) (Lewis Number Mass Diffusivity) \( \alpha \;=\; D \cdot Le \) \( Le \;=\; \dfrac{ \alpha }{ D } \) |
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| Symbol | English | Metric |
| \( D \) = Mass Diffusivity | \(ft^3 \;/\; sec\) | \(m^3 \;/\; s\) |
| \( \alpha \) (Greek symbol alpha) = Thermal Diffusivity | \(ft^2 \;/\; sec\) | \(m^2 \;/\; s\) |
| \( Le \) = Lewis Number | \(dimensionless\) | \(dimensionless\) |
Schmidt Number Mass Diffusivity Formula |
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\( D \;=\; \dfrac{ \nu }{ Sc } \) (Schmidt Number Mass Diffusivity) \( \nu \;=\; D \cdot Sc \) \( Sc \;=\; \dfrac{ \nu }{ D } \) |
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| Symbol | English | Metric |
| \( D \) = Mass Diffusivity | \(ft^3 \;/\; sec\) | \(m^3 \;/\; s\) |
| \( \nu \) (Greek symbol nu) = Kinematic Viscosity | \(ft^2 \;/\; sec\) | \(m^2 \;/\; s\) |
| \( Sc \) = Schmidt Number | \(dimensionless\) | \(dimensionless\) |
Sherwood Number Mass Diffusivity Formula |
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\( D \;=\; \dfrac{ K \cdot l_c }{ Sh } \) (Sherwood Number Mass Diffusivity) \( K \;=\; \dfrac{ D \cdot Sh }{ l_c } \) \( l_c \;=\; \dfrac{ D \cdot Sh }{ K } \) \( Sh \;=\; \dfrac{ K \cdot l_c }{ D } \) |
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| Symbol | English | Metric |
| \( D \) = Mass Diffusivity | \(ft^3 \;/\; sec\) | \(m^3 \;/\; s\) |
| \( K \) = Mass Transfer Coefficient | \(dimensionless\) | \(dimensionless\) |
| \( l_c \) = Characteristic Length | \(in\) | \(mm\) |
| \( Sh \) = Sherwood Number | \(dimensionless\) | \(dimensionless\) |
