Spray nozzles droplet size from

SPRAYS FROM PNEUMATIC NOZZLES DROPLET SIZE DISTRIBUTION OF SPRAYS FROM ATOMIZERS... 

The period of time needed for settling out depends on the droplet size distribution and the required completeness of removal. A few observations of droplet sizes are mentioned by York (1983). Walas (1988) discussed droplet sizes from various types of equipment (e.g., spray nozzles, spray disks, etc.). The droplet sizes varied from 10 m to 5,000/rm but sprays in process equipment usually range between 10 m to 20 m. 

Spray nozzle spray angles and droplet size formation are functions of the pressure drop across a spray header. Effective pressure drop range for a spray nozzle distributor varies from 5 to 20psi (from 34 to 138 kPa) for good design practice. Specific designs may operate outside this range. 

For liquid sprays, the droplet size varies at different radial and axial directions from the nozzle. The time-averaged measurement and data analysis procedures described above cannot provide information about the local structure of the droplet size distribution. Several techniques have been developed to transform ordinary laser diffraction measurements into spatially resolved local measurements along the radial directions of the spray. The data from the measurements at different radial directions are then processed using either a deconvolution method with optical extinction and scattering coefficients [45] or a tomographical transformation method [46,47], yielding pointwise droplet size and liquid concentration distribution as well as all mean diameters of practical interest. 

Fine water spray systems may be potentially superior to CO9 apphcations and may replace halon environments such as telephone central offices and computer rooms. In the fine spray dehveiy system, water is delivered at relatively high pressure (above 100 psi [0.689 MPa]) or by air atomization to generate droplets significantly smaUer than those generated by sprinklers. Water flow from a fine spray nozzle potentially extinguishes the fire faster than a sprinkler because the droplets are smaUer and vaporize more quickly. Preliminaiy information indicates that the smaller the droplet size, the lower the water flow requirements and the less chance of water damage. 

The term mist generally refers to liquid droplets from submicron size to about 10 /xm. If the diameter exceeds 10 /xm, the aerosol is usually referred to as a spray or simply as droplets. Mists tend to be spherical because of their surface tension and are usually formed by nucleation and the condensation of vapors (6). Larger droplets are formed by bursting of bubbles, by entrainment from surfaces, by spray nozzles, or by splash-type liquid distributors. The large droplets tend to be elongated relative to their direchon of mohon because of the action of drag forces on the drops. 

Derived from spray data for high-viscosity liquids (mixtures of glycerine and water) of 50flows through discharge slots and impacts both sides of a flat liquid sheet from a discharge slot inbetween the air slots) Droplet size measured by Malvern 2600HSD Spray Analyzer Effects of air slot thickness included... 

A two-component phase Doppler interferometer (PDI) was used to determine droplet size, velocity, and number density in spray flames. The data rates were determined according to the procedure discussed in [5]. Statistical properties of the spray at every measurement point were determined from 10,000 validated samples. In regions of the spray where the droplet number density was too small, a sampling time of several minutes was used to determine the spray statistical characteristics. Results were repeatable to within a 5% margin for mean droplet size and velocity. Measurements were carried out with the PDI from the spray centerline to the edge of the spray, in increments of 1.27 mm at an axial position (z) of 10 mm downstream from the nozzle, and increments of 2.54 mm at z = 15 mm, 20, 25, 30, 35, 40, 50, and 60 mm using steam, normal-temperature air, and preheated air as the atomization gas. 

The observed flame features indicated that changing the atomization gas (normal or preheated air) to steam has a dramatic effect on the entire spray characteristics, including the near-nozzle exit region. Results were obtained for the droplet Sauter mean diameter (D32), number density, and velocity as a function of the radial position (from the burner centerline) with steam as the atomization fluid, under burning conditions, and are shown in Figs. 16.3 and 16.4, respectively, at axial positions of z = 10 mm, 20, 30, 40, 50, and 60 mm downstream of the nozzle exit. Results are also included for preheated and normal air at z = 10 and 50 mm to determine the effect of enthalpy associated with the preheated air on fuel atomization in near and far regions of the nozzle exit. Smaller droplet sizes were obtained with steam than with both air cases, near to the nozzle exit at all radial positions see Fig. 16.3. Droplet mean size with steam at z = 10 mm on the central axis of the spray was found to be about 58 /xm as compared to 81 pm with preheated air and 96 pm with normal unheated air. Near the spray boundary the mean droplet sizes were 42, 53, and 73 pm for steam, preheated air, and normal air, respectively. The enthalpy associated with preheated air, therefore, provides smaller droplet sizes as compared to the normal (unheated) air case near the nozzle exit. Smallest droplet mean size (with steam) is attributed to decreased viscosity of the fuel and increased viscosity of the gas. 

The spray nozzle (or nozzles) in the production machine would need to be of a size such that this increased spray rate is within its performance envelope (similar droplet size, uniform in distribution), or equivalence in granule size would be impossible. Figure 17 illustrates the concept that atomizing air pressure must be adjusted to attain similar average droplet sizes in all three scales of process equipment at the desired spray rate (data from... 

Figure 14 Influence of spray nozzle type and atomizing air pressure on the mean droplet size of water sprayed from guns used in a Wurster process.

Tanasawa (111) investigated burning droplets from a swirl spray nozzle in an essentially quiescent atmosphere. Again the results are limited and preclude broad application. At this point one may wonder, therefore, how the equations for vaporization rate can be utilized. The solution is to deal with some particular droplet size which is representative of the point of interest. Usually this will be a size considerably greater than average for the spray, when it has diminished to zero, substantially all of the spray will have vaporized. 

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