A special case of a two-phase (gas and liquid) flow in which the liquid phase is the dispersed phase and exists in the form of many droplets. The gas phase is the continuous phase, so abstract continuous lines (or surfaces) can be constructed through the gas at any instant without intersection of the droplets. The droplets and the gas have velocities that can be different, so both phases can move through some fixed volume or chamber and the droplets can move relative to the surrounding gas.
Spray flows have many applications. Sprays are used to introduce liquid fuel into the combustion chambers of diesel engines, turbojet engines, liquid-propellant rocket engines, and oil-burning furnaces. They are used in agricultural and household applications of insecticides and pesticides, for materials and chemicals processing, for fire extinguishing, for cooling in heat exchangers, for application of medicines, and for application of coatings (including paint and various other types of layered coatings). Common liquids (such as water, fuels, and paints) are used in sprays. It is sometimes useful to spray uncommon liquids such as molten metals. In the various applications, the approximately spherical droplets typically have submillimeter diameters that can be as small as a few micrometers.
Sprays are formed for industrial, commercial, agricultural, and power generation purposes by injection of a liquid stream into a gaseous environment. In addition, sprays can form naturally in a falling or splashing liquid. Injected streams of liquid tend to become unstable when the dynamic pressure (one-half of the gas density times the square of the liquid velocity) is much larger than the coefficient of surface tension divided by the transverse dimension. Typically, the liquid stream disintegrates into ligaments (coarse droplets) and then into many smaller spherical droplets. The breakup (or atomization) process is faster at higher stream velocity, and the final droplet sizes are smaller for higher stream velocities. Spray droplet sizes vary and typically are represented statistically by a distribution function. The number of droplets in a spray can be as high as a few million in a volume smaller than a liter. See Jet flow