In physics and engineering, mass flow rate is the rate at which mass of a fluid passes through a surface over time. Its unit is kilogram per second (kg/s) in SI units, and slug per second or pound per second in US customary units. The common symbol is m ˙ {\displaystyle {\dot {m}}} (pronounced "m-dot"), although sometimes μ {\displaystyle \mu } (Greek lowercase mu) is used. Sometimes, mass flow rate as defined here is termed "mass flux" or "mass current". Confusingly, "mass flow" is also a term for mass flux, the rate of mass flow per unit of area.
Formulation Mass flow rate is defined by the limit
m ˙ = lim Δ t → 0 Δ m Δ t = d m d t , {\displaystyle {\dot {m}}=\lim _{\Delta t\to 0}{\frac {\Delta m}{\Delta t}}={\frac {dm}{dt}},}
i.e., the flow of mass Δ m {\displaystyle \Delta m} through a surface per time Δ t {\displaystyle \Delta t} . The overdot on m ˙ {\displaystyle {\dot {m}}} is Newton's notation for a time derivative. Since mass is a scalar quantity, the mass flow rate (the time derivative of mass) is also a scalar quantity. The change in mass is the amount that flows after crossing the boundary for some time duration, not the initial amount of mass at the boundary minus the final amount at the boundary, since the change in mass flowing through the area would be zero for steady flow.
Alternative equations
Mass flow rate can also be calculated by
m ˙ = ρ ⋅ V ˙ = ρ ⋅ v ⋅ A = j m ⋅ A , {\displaystyle {\dot {m}}=\rho \cdot {\dot {V}}=\rho \cdot \mathbf {v} \cdot \mathbf {A} =\mathbf {j} _{\text{m}}\cdot \mathbf {A} ,}
where The above equation is only true for a flat, plane area. In general, including cases where the area is curved, the equation becomes a surface integral:
m ˙ = ∬ A ρ v ⋅ d A = ∬ A j m ⋅ d A . {\displaystyle {\dot {m}}=\iint _{A}\rho \mathbf {v} \cdot d\mathbf {A} =\iint _{A}\mathbf {j} _{\text{m}}\cdot d\mathbf {A} .}
The area required to calculate the mass flow rate is real or imaginary, flat or curved, either as a cross-sectional area or a surface, e.g. 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 {\displaystyle A} , and a unit vector normal to the area, n ^ {\displaystyle \mathbf {\hat {n}} } . The relation is A = A n ^ {\displaystyle \mathbf {A} =A\mathbf {\hat {n}} } . The reason for the dot product is as follows. The only mass flowing through the cross-section is the amount normal to the area, i.e. parallel to the unit normal. This amount is
m ˙ = ρ v A cos θ , {\displaystyle {\dot {m}}=\rho vA\cos \theta ,}
where θ {\displaystyle \theta } is the angle between the unit normal n ^ {\displaystyle \mathbf {\hat {n}} } and the velocity of mass elements. The amount passing through the cross-section is reduced by the factor cos θ {\displaystyle \cos \theta } , as θ {\displaystyle \theta } increases less mass passes through. All mass which passes in tangential directions to the area, that is perpendicular to the unit normal, doesn't actually pass through the area, so the mass passing through the area is zero. This occurs when θ = π / 2 {\displaystyle \theta =\pi /2} :
… excerpt ends here. Continue reading the full article.
