Transportfluks

Transportfluks is the net rate at which a quantity passes through a defined surface per unit area and per unit time. It is a directional quantity, described by a flux vector J, and can refer to the transport of mass, energy, momentum, or electric charge. Depending on the quantity, common units include kilograms per square meter per second (kg m^-2 s^-1) for mass flux, moles per square meter per second (mol m^-2 s^-1) for molar flux, watts per square meter (W m^-2) for energy flux, and amperes per square meter (A m^-2) for charge flux.

Transportfluks results from different transport mechanisms that may act alone or in combination. Diffusive transport follows Fick's laws, where J_diff is proportional to the negative concentration gradient. Advective or convective transport involves material movement with the bulk flow, described by J_adv = C v for mass transfer, where C is concentration and v is velocity. For energy transfer, conduction, convection, and radiation can contribute to the total flux. In porous media, Darcy's law and related formulations describe how flux depends on pressure or potential differences and the medium's properties. The total flux is often written as J = J_diff + J_adv (and analogous forms for other quantities).

In practice, transportfluks is used across disciplines, including chemical engineering (membranes, reactors, distillation), environmental science (pollutant transport in air or water), meteorology (heat and moisture fluxes in the atmosphere), and biology (transmembrane transport). It is typically measured directly with sensors and tracers or computed from concentration and velocity fields using appropriate constitutive relations. The concept is governed by conservation laws and boundary conditions at interfaces. See also flux, diffusion, convection, and permeation.