Vapourair

Vapourair is a term used in fluid dynamics and heat transfer to describe a near-equilibrium two-phase mixture of vapour from a volatile liquid and surrounding air. It is not a chemical compound but a conceptual category used in modelling evaporation, aerosol transport, and related processes.

In vapourair systems, the composition is characterized by the partial vapor pressure of the liquid and the composition of the carrier air (primarily nitrogen and oxygen). The local properties—density, viscosity, and heat and mass transfer coefficients—depend on the vapour volume fraction and the overall temperature.

Formation and measurement: Vapourair forms when a liquid evaporates into an ambient gas or during atomisation and spray drying. It can be studied with optical diagnostics such as schlieren imaging, laser-induced fluorescence, and mass spectrometry, as well as with computational multiphase models.

Applications: The vapourair concept informs modelling in spray drying, agricultural spraying, inkjet printing, coating processes, and cooling towers. It helps estimate evaporation rates, droplet lifetimes, and heat and mass transfer between phases.

The concept is theoretical rather than a distinct substance. Different modelling approaches treat vapourair as a homogeneous mixture, a dispersed two-phase flow, or a quasi-steady vapor fraction, depending on the level of detail needed.

History and terminology: The term appears in parts of the fluid-mechanics literature from the late 20th century and remains non-standard. It is sometimes contrasted with pure vapour or dry air models.