Entrainment (engineering)

Entrainment, in engineering, broadly refers to the process of capturing and transporting one substance within another. It describes the incorporation of one material into the flow of another due to momentum transfer or other physical mechanisms. The entrained substance is typically carried along by the primary flow.

Several distinct forms of entrainment exist across various engineering disciplines:

• Fluid Mechanics: In fluid mechanics, entrainment describes the process by which a turbulent flow mixes with and incorporates the surrounding fluid. For instance, a jet of fluid expelled into a quiescent environment will draw in, or entrain, the surrounding fluid due to shear forces at the jet's boundary. This leads to the jet widening and its velocity decreasing as it travels further from the source. This phenomenon is crucial in understanding mixing processes, plume dispersion, and the behavior of turbulent flows.

• Aerosol Science: Within aerosol science, entrainment refers to the lifting and carrying of particles, droplets, or other particulate matter into an air stream. This can be caused by wind, turbulence, or mechanical forces. Examples include dust entrainment from soil by wind or the entrainment of liquid droplets into a gas stream within industrial scrubbers. The efficiency of aerosol collection devices often depends on minimizing particle entrainment back into the cleaned gas stream.

• Chemical Engineering: Chemical engineering utilizes entrainment in processes such as distillation and absorption. In these contexts, entrainment describes the carryover of liquid droplets or solid particles within a rising gas stream. This phenomenon is undesirable as it reduces the separation efficiency of the process. Devices like demisters or baffles are often implemented to minimize entrainment in these separation processes.

• Environmental Engineering: In environmental engineering, entrainment is important in understanding pollutant transport. For example, contaminants can be entrained in runoff water and transported to surface water bodies, or soil particles containing adsorbed pollutants can be entrained by wind and contribute to air pollution.

The degree of entrainment is influenced by various factors including the properties of both substances (density, viscosity, surface tension), the flow rate, the geometry of the system, and the presence of any external forces. Quantifying and controlling entrainment is crucial in many engineering applications to optimize process efficiency, prevent equipment damage, and minimize environmental impact. Understanding the mechanisms of entrainment requires a solid foundation in fluid dynamics, mass transfer, and transport phenomena.