massflow

Mass flow, or mass flow rate, is the amount of mass passing through a cross-section per unit time. It is commonly denoted ṁ and SI units are kilograms per second (kg/s). Mass flow is related to volumetric flow rate Q (m^3/s) by the density ρ: ṁ = ρ Q. For incompressible liquids with nearly constant density, mass flow is approximately ṁ ≈ ρ Q. For gases, density varies with pressure and temperature, so ṁ is governed by the continuity of mass.

In a steady flow, mass entering and leaving a control volume must balance, so ∑ ṁ_in = ∑ ṁ_out. In terms of a pipe cross-section, ṁ = ρ A v_n, where A is the area and v_n is the velocity normal to the cross-section. For a uniform cross-section, this reduces to ṁ = ρ Q = ρ A v.

Mass flow can be measured with various meters. Coriolis meters directly measure mass flow via vibrating tubes, while thermal mass meters infer ṁ from heat transfer. Differential pressure meters (such as orifice or venturi meters) can measure mass flow when density is accounted for. For gases, compressibility effects may cause choked flow, where the mass flow rate is limited by sonic conditions at a restriction and cannot be increased by lowering downstream pressure.

Applications of mass flow measurement include internal combustion engines, HVAC systems, chemical processing, and industrial fluid control. Mass flow is distinct from volumetric flow, yet the two quantities are linked through density and the system’s thermodynamic state.