TABLE OF CONTENTS When a mixture of gases or liquids is contained such that a concentration gradient of one or more of the components exists across the system, there will be mass transfer on a microscopic level. This is a result of diffusion from regions of high concentration to regions of low concentration. Mass transfer can occur on a molecular basis. In turbulent flow systems, accelerated diffusion rates can occur as a result of rapid-eddy mixing processes just as these mixing processes created increased heat transfer and viscous action in turbulent flow.
Consider the system shown in Figure 4.1.1. A thin partition separates two gases a and b. When the partition is removed, the two gases diffuse through one another until equilibrium is established and the concentration of the gases is uniform throughout the container. The diffusion rate is given by Fick's law of diffusion, which states that the mass flux of a constituent per unit area is proportional to the concentration gradient. Thus,
where the constant of proportionality, D, is called the diffusion coefficient and is measured in m 2/s. The concentration, c, is the mass of a constituent per unit volume. Notice the similarity between Equation 4.1.1, Equation 3.1.1 (Fourier's law of heat conduction), and Equation 2.1.1 (Newton's equation of viscosity). The diffusion equation describes the transport of mass, the equation of viscosity describes the transport of momentum, and the conduction equation describes the transport of energy.
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