# Introduction to transport phenomena

### From Thermal-FluidsPedia

## Contents |

## Continuum Flow Limitations

The transport phenomena are usually modeled in continuum states for most applications – the materials are assumed to be continuous and the fact that matter is made of atoms is ignored. When the characteristic dimension, *L*, is small compared to the molecular mean free path, λ, which is defined as average distance between collisions for a molecule, the traditional Navier-Stokes equation and the energy equation based on the continuum assumption have failed to provide accurate results. The continuum assumption also fails when the gas is at very low pressure (rarefied).

*See Main Article* Continuum flow limitations

## Momentum, Heat, and Mass Transfer

Transport phenomena include momentum transfer, heat transfer, and mass transfer, all of which are fundamental to an understanding of both single and multiphase systems. It is assumed that the reader has basic undergraduate-level knowledge of transport phenomena as applied to single-phase systems, as well as the associated thermodynamics, fluid mechanics, and heat transfer.

*See Main Article* Momentum, Heat, and Mass Transfer

## Introduction to Momentum Transfer

A fluid at rest can resist a normal force but not a shear force, while fluid in motion can also resist a shear force. The fluid continuously deforms under the action of shear force. A fluid’s resistance to shear or angular deformation is measured by viscosity, which can be thought of as the internal “stickiness” of the fluid. The force and the rate of strain (i.e., rate of deformation) produced by the force are related by a constitutive equation.

*See Main Article* Introduction to Momentum Transfer

## Introduction to Heat Transfer

Heat transfer is a process whereby thermal energy is transferred in response to a temperature difference. There are three modes of heat transfer: conduction, convection, and radiation. Conduction is heat transfer across a stationary medium, either solid or fluid. Convection occurs between a wall at one temperature and a moving fluid at another temperature. The transmission of thermal radiation does not require the presence of a propagating medium and, therefore can occur in a vacuum. Thermal radiation is a form of energy emitted by matter at a nonzero temperature and its wavelength is primarily in the range between 0.1 to 10 μm.

*See Main Article* Introduction to Heat Transfer

## Introduction to Mass Transfer

When there is a species concentration difference in a multicomponent mixture, mass transfer occurs. There are two modes of mass transfer: diffusion and convection. Diffusion results from random molecular motion at the microscopic level, and it can occur in a solid, liquid or gas. Similar to convective heat transfer, convective mass transfer is due to a combination of random molecular motion at the microscopic level and bulk motion at the macroscopic level. It can occur only in a liquid or gas.

*See Main Article* Introduction to Mass transfer

## Multiphase Systems and Phase Changes

A *multiphase system* is one characterized by the simultaneous presence of several phases, the two-phase system being the simplest case. The term *two-component* is sometimes used to describe flows in which the phases consist of different chemical substances. For example, steam-water flows are two-phase, while air-water flows are two-component. Some two-component flows (mostly liquid-liquid) technically consist of a single phase but are identified as two-phase flows in which the term “phase” is applied to each of the components. Since the same mathematics describes two-phase and two-component flows, the two expressions will be treated as synonymous. This book deals with a variety of multiphase systems, in which the phases passing through the system may be solid, liquid or gas, or a combination of these three.

*See Main Article* Multiphase Systems and Phase Changes

## Transport Phenomena in Micro- and Nanoscales

*See Main Article* Transport Phenomena in Micro- and Nanoscales

## Dimensional Analysis

*See Main Article* Dimensional Analysis

## Scaling

*See Main Article* Scaling