Spray (liquid drop)
The spray characteristics required must be understood in order to select the most appropriate technology, optimal device and size.
The most common method is through a spray nozzle which typically has a fluid passage that is acted upon by different mechanical forces that atomize the liquid.
Spray formation has taken on several forms, the most common being, pressure sprayers and centrifugal, electrostatic and ultrasonic nozzles.
Evaporation, cooling rates for gases and solids, and cleaning efficiency are examples of process characteristics that may depend largely on spray qualities.
The pattern is achieved by the use of an inlet orifice tangential to a cylindrical swirl chamber that is open at one end.
Hollow cone nozzles are best for applications requiring good atomization of liquids at low pressures or when quick heat transfer is needed.
These nozzles also feature large and unobstructed flow passages, which provide a relatively high resistance to clogging.
Full cone nozzles yield complete spray coverage in a round, oval or square shaped area.
Spray angle and liquid distribution within the cone pattern depend on the vane design and location relative to the exit orifice.
The exit orifice design and the relative geometric proportions also affect the spray angle and distribution.
The orifice has an external groove with a contoured internal cylindrical radius, or “cat's eye” shape.
The multiple plumes are primarily used to provide for the optimal mixing of fuel and air so as to reduce pollutant emission under different operating conditions.
Since the specific gravity of a liquid affects its flow rate, the values must be adjusted using the equation below, where Qw is the water capacity and Spg is the specific gravity of the fluid used resulting the volumetric flow rate of the fluid used Qf.
[6] Sprays angles are important in coating applications to prevent overspraying of the coated materials, in combustion engines to prevent wetting of the cylinder walls, and in fire sprinklers to provide adequate coverage of the protected property.
Sprays are typically characterized by statistical quantities obtained from size and velocity measurements over many individual droplets.
The most widely used quantities are size and velocity probability density distributions as well as fluxes, e.g., number, mass, momentum etc.
[8] The three most widely used methods of drop size measurements are laser diffraction, optical imaging, and phase Doppler.
The scattered light passes through a special transforming lens system and is collected on a number of concentric photodiode rings.
A series of three off axis detectors collects the optical signal that is used to determine the phase angle and frequency shift caused by the drops.
Liquid properties: viscosity and surface tension increase the amount of energy required to atomize the spray.
The extinction of light caused by the drops within a spray is also directly proportion to the surface area density.
The two most widely used methods of measuring the surface area density are Laser Sheet Imaging and Statistical Extinction Tomography.
Drop size is critical because the large surface area of a finely atomized spray enhances fuel evaporation rate.
Dispersion of the fuel into the combustion air is critical to maximize the efficiency of these systems and minimize emissions of pollutants (soot, NOx, CO).
Calcium hydroxide (lime) is atomized into a spray dryer absorber to remove acid gases (SO2 and HCl) from coal-fired power plants.
[17] Some typical applications are applying adhesive, lubricating bearings, and cooling tools in machining operations.
Spray application of herbicides, insecticides, and pesticides is essential to distribute these materials over the intended target surface.
Herbicides, insecticides and pesticides are spray applied to soil or plant foliage to distribute and disperse these materials.
The spray nozzle type and size and the operating pressure provide the correct application rate of the material and control the amount of driftable fines.
Spays, single fluid nozzles, are also used to cool animals Atomizers are used with pump-operated sprays of household cleaning products.
