In physics and engineering, mass flux is the rate of mass flow. Its SI units are kg m⁻² s⁻¹. The common symbols are j, J, q, Q, φ, or Φ (Greek lower or capital Phi), sometimes with subscript m to indicate mass is the flowing quantity. Mass flux can also refer to an alternate form of flux in Fick's law that includes the molecular mass, or in Darcy's law that includes the mass density.^[1]

Sometimes the defining equation for mass flux in this article is used interchangeably with the defining equation in mass flow rate. For example, Fluid Mechanics, Schaum's et al ^[2] uses the definition of mass flux as the equation in the mass flow rate article.

Definition

Mathematically, mass flux is defined as the limit

$${\displaystyle j_{m}=\lim _{A\to 0}{\frac {I_{m)){A)),}$$

$${\displaystyle I_{m}=\lim _{\Delta t\to 0}{\frac {\Delta m}{\Delta t))={\frac {dm}{dt))}$$

For mass flux as a vector j_m, the surface integral of it over a surface S, followed by an integral over the time duration t₁ to t₂, gives the total amount of mass flowing through the surface in that time (t₂ − t₁):

$${\displaystyle m=\int _{t_{1))^{t_{2))\iint _{S}\mathbf {j} _{m}\cdot \mathbf {\hat {n)) \,dA\,dt.}$$

The area required to calculate the flux is real or imaginary, flat or curved, either as a cross-sectional area or a surface.

For example, for substances passing through a filter or a membrane, the real surface is the (generally curved) surface area of the filter, macroscopically - ignoring the area spanned by the holes in the filter/membrane. The spaces would be cross-sectional areas. For liquids passing through a pipe, the area is the cross-section of the pipe, at the section considered.

The vector area is a combination of the magnitude of the area through which the mass passes through, A, and a unit vector normal to the area, ${\displaystyle \mathbf {\hat {n)) }$. The relation is ${\displaystyle \mathbf {A} =A\mathbf {\hat {n)) }$.

If the mass flux j_m passes through the area at an angle θ to the area normal ${\displaystyle \mathbf {\hat {n)) }$, then

$${\displaystyle \mathbf {j} _{m}\cdot \mathbf {\hat {n)) =j_{m}\cos \theta }$$

Example

Consider a pipe of flowing water. Suppose the pipe has a constant cross section and we consider a straight section of it (not at any bends/junctions), and the water is flowing steadily at a constant rate, under standard conditions. The area A is the cross-sectional area of the pipe. Suppose the pipe has radius r = 2 cm = 2 × 10⁻² m. The area is then

$${\displaystyle A=\pi r^{2}.}$$

$${\displaystyle {\begin{aligned}\Delta m&=\rho \Delta V\\m_{2}-m_{1}&=\rho (V_{2}-V_{1})\\m&=\rho V\\\end{aligned))}$$

$${\displaystyle j_{m}={\frac {\Delta m}{A\Delta t))={\frac {\rho V}{\pi r^{2}t)).}$$

Substituting the numbers gives:

$${\displaystyle j_{m}={\frac {1000\times \left(1.5\times 10^{-3}\right)}{\pi \times \left(2\times 10^{-2}\right)^{2}\times 2))={\frac {3}{16\pi ))\times 10^{4},}$$

Equations for fluids

Alternative equation

Using the vector definition, mass flux is also equal to:^[3]

$${\displaystyle \mathbf {j} _{\rm {m))=\rho \mathbf {u} }$$

where:

• ρ = mass density,
• u = velocity field of mass elements flowing (i.e. at each point in space the velocity of an element of matter is some velocity vector u).

Sometimes this equation may be used to define j_m as a vector.

Mass and molar fluxes for composite fluids

Mass fluxes

In the case fluid is not pure, i.e. is a mixture of substances (technically contains a number of component substances), the mass fluxes must be considered separately for each component of the mixture.

When describing fluid flow (i.e. flow of matter), mass flux is appropriate. When describing particle transport (movement of a large number of particles), it is useful to use an analogous quantity, called the molar flux.

Using mass, the mass flux of component i is

$${\displaystyle \mathbf {j} _((\rm {m)),\,i}=\rho _{i}\mathbf {u} _{i}.}$$

The barycentric mass flux of component i is

$${\displaystyle \mathbf {j} _((\rm {m)),\,i}=\rho \left(\mathbf {u} _{i}-\langle \mathbf {u} \rangle \right),}$$

${\displaystyle \langle \mathbf {u} \rangle }$

$${\displaystyle \langle \mathbf {u} \rangle ={\frac {1}{\rho ))\sum _{i}\rho _{i}\mathbf {u} _{i}={\frac {1}{\rho ))\sum _{i}\mathbf {j} _((\rm {m)),\,i))$$

• ρ = mass density of the entire mixture,
• ρ_i = mass density of component i,
• u_i = velocity of component i.

The average is taken over the velocities of the components.

Molar fluxes

If we replace density ρ by the "molar density", concentration c, we have the molar flux analogues.

The molar flux is the number of moles per unit time per unit area, generally:

$${\displaystyle \mathbf {j} _{\rm {n))=c\mathbf {u} .}$$

So the molar flux of component i is (number of moles per unit time per unit area):

$${\displaystyle \mathbf {j} _((\rm {n)),\,i}=c_{i}\mathbf {u} _{i))$$

$${\displaystyle \mathbf {j} _((\rm {n)),\,i}=c\left(\mathbf {u} _{i}-\langle \mathbf {u} \rangle \right),}$$

${\displaystyle \langle \mathbf {u} \rangle }$

$${\displaystyle \langle \mathbf {u} \rangle ={\frac {1}{n))\sum _{i}c_{i}\mathbf {u} _{i}={\frac {1}{c))\sum _{i}\mathbf {j} _((\rm {n)),\,i}.}$$

Usage

Mass flux appears in some equations in hydrodynamics, in particular the continuity equation:

$${\displaystyle \nabla \cdot \mathbf {j} _{\rm {m))+{\frac {\partial \rho }{\partial t))=0,}$$

Molar flux occurs in Fick's first law of diffusion:

$${\displaystyle \nabla \cdot \mathbf {j} _{\rm {n))=-\nabla \cdot D\nabla n}$$