2 - Molecular and convective transport

Abstract

The chapter discusses the formulation of molecular and convective fluxes in momentum, energy, and mass transport. Substances may behave differently when subjected to the same gradients. Constitutive equations identify the characteristics of a particular substance. For example, if the gradient is momentum, the viscosity is defined by the constitutive equation called Newton's law of viscosity. If the gradient is energy, the thermal conductivity is defined by Fourier's law of heat conduction. If the gradient is concentration, the diffusion coefficient is defined by Fick's first law of diffusion. Viscosity, thermal conductivity, and diffusion coefficient are called transport properties. The molecular mass flux expression represents the molecular mass flux with respect to the mass average velocity. Therefore, in the equation representing the total mass flux, the characteristic velocity in the convective mass flux term is taken as the mass average velocity. On the other hand, the chapter presents an equation, which is the molecular molar flux with respect to the molar average velocity. Therefore, the molar average velocity is considered the characteristic velocity in the convective molar flux term.