Taylor dispersion is an effect in fluid mechanics in which a shear flow can increase the effective diffusivity of a species. Essentially, the shear acts to smear out the concentration distribution in the direction of the flow, enhancing the rate at which it spreads in that direction.[1][2][3] The effect is named after the British fluid dynamicist G. I. Taylor, who described the shear-induced dispersion for large Peclet numbers. The analysis was later generalized by Rutherford Aris for arbitrary values of the Peclet number. The dispersion process is sometimes also referred to as the Taylor-Aris dispersion.
The canonical example is that of a simple diffusing species in uniform Poiseuille flow through a uniform circular pipe with no-flux boundary conditions.
Description
We use z as an axial coordinate and r as the radial coordinate, and assume axisymmetry. The pipe has radius a, and the fluid velocity is:
\( {\displaystyle {\boldsymbol {u}}=w{\hat {\boldsymbol {z}}}=w_{0}(1-r^{2}/a^{2}){\hat {\boldsymbol {z}}}} \)
The concentration of the diffusing species is denoted c and its diffusivity is D. The concentration is assumed to be governed by the linear advection–diffusion equation:
\( {\displaystyle {\frac {\partial c}{\partial t}}+{\boldsymbol {w}}\cdot {\boldsymbol {\nabla }}c=D\nabla ^{2}c} \)
The concentration and velocity are written as the sum of a cross-sectional average (indicated by an overbar) and a deviation (indicated by a prime), thus:
\( {\displaystyle w(r)={\bar {w}}+w'(r)} \)
\( {\displaystyle c(r,z)={\bar {c}}(z)+c'(r,z)} \)
Under some assumptions (see below), it is possible to derive an equation just involving the average quantities:
\( {\displaystyle {\frac {\partial {\bar {c}}}{\partial t}}+{\bar {w}}{\frac {\partial {\bar {c}}}{\partial z}}=D\left(1+{\frac {a^{2}{\bar {w}}^{2}}{48D^{2}}}\right){\frac {\partial ^{2}{\bar {c}}}{\partial z^{2}}}} \)
Observe how the effective diffusivity multiplying the derivative on the right hand side is greater than the original value of diffusion coefficient, D. The effective diffusivity is often written as:
\( {\displaystyle D_{\mathrm {eff} }=D\left(1+{\frac {{\mathit {Pe}}^{2}}{48}}\right)\,,} \)
where \( {\displaystyle {\mathit {Pe}}=a{\bar {w}}/D} \) is the Péclet number, based on the channel radius a. The interesting result is that for large values of the Péclet number, the effective diffusivity is inversely proportional to the molecular diffusivity. The effect of Taylor dispersion is therefore more pronounced at higher Péclet numbers.
In a frame moving with the mean velocity, i.e., by introducing \( {\displaystyle \xi =z-{\bar {w}}t} \) , the dispersion process becomes a purely diffusion process,
\( {\displaystyle {\frac {\partial {\bar {c}}}{\partial t}}=D_{\mathrm {eff} }{\frac {\partial ^{2}{\bar {c}}}{\partial \xi ^{2}}}} \)
with diffusivity given by the effective diffusivity.
The assumption is that \( {\displaystyle c'\ll {\bar {c}}} \) for given z , which is the case if the length scale in the z direction is long enough to smooth the gradient in the r direction. This can be translated into the requirement that the length scale L in the z direction satisfies:
\( {\displaystyle L\gg {\frac {a^{2}}{D}}{\bar {w}}=a{\mathit {Pe}}}. \)
Dispersion is also a function of channel geometry. An interesting phenomenon for example is that the dispersion of a flow between two infinite flat plates and a rectangular channel, which is infinitely thin, differs approximately 8.75 times. Here the very small side walls of the rectangular channel have an enormous influence on the dispersion.
While the exact formula will not hold in more general circumstances, the mechanism still applies, and the effect is stronger at higher Péclet numbers. Taylor dispersion is of particular relevance for flows in porous media modelled by Darcy's law.
References
Probstein R (1994). Physicochemical Hydrodynamics.
Chang, H.C., Yeo, L. (2009). Electrokinetically Driven Microfluidics and Nanofluidics. Cambridge University Press.
Kirby, B.J. (2010). Micro- and Nanoscale Fluid Mechanics: Transport in Microfluidic Devices. Cambridge University Press. ISBN 978-0-521-11903-0.
Other sources
Aris, R. (1956) On the dispersion of a solute in a fluid flowing through a tube, Proc. Roy. Soc. A., 235, 67–77.
Frankel, I. & Brenner, H. (1989) On the foundations of generalized Taylor dispersion theory, J. Fluid Mech., 204, 97–119.
Taylor, G. I. (1953) Dispersion of soluble matter in solvent flowing slowly through a tube, Proc. Roy. Soc. A., 219, 186–203.
Taylor, G. I. (1954) The Dispersion of Matter in Turbulent Flow through a Pipe, Proc. Roy. Soc. A, 223, 446–468.
Taylor, G. I. (1954) Conditions under which dispersion of a solute in a stream of solvent can be used to measure molecular diffusion, Proc. Roy. Soc. A., 225, 473–477.
Brenner, H. (1980) Dispersion resulting from flow through spatially periodic porous media, Phil. Trans. Roy. Soc. Lon. A, 297, 81.
Mestel. J. Taylor dispersion — shear augmented diffusion, Lecture Handout for Course M4A33, Imperial College.
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