Drop Size Definition and Sampling Techniques

In order to accurately assess and understand drop size data, all of the key variables such as nozzle type, pressure, capacity, liquid properties and spray angle have to be taken into consideration. The drop size testing method should also be fully understood. The measurement techniques, type of drop size analyzer and data analysis and reporting methods all have a strong influence on the results.

Sampling Techniques

The spatial technique (i.e., spatial distribution) is implied when a collection of drops occupying a given volume is sampled instantaneously. Generally, spatial measurements are collected with the aid of holographic means such as high-speed photography or light scattering instruments. This type of measurement is sensitive to the number density in each class size and the number of particles per unit volume.

In the spatial technique, measurements are:

• averaged over a finite volume
• instantaneous sample
• sensitive to number density

Flux technique

The flux technique (i.e., flux distribution) is when individual drops pass through the cross-section of a sampling region and are examined during an interval of time. Flux measurements are generally collected by optical instruments that are capable of sensing individual drops. This type of measurement is sensitive to particle flux.

In the flux technique, measurements are:

• time-averaged
• sensitive to particle flux

Correlation between the two methods

The sampling technique is critical for understanding drop size data. Typically, nozzles measured using the spatial technique will report drops smaller on average than nozzles measured using the flux technique. When comparing data from different sources, identify the differences in sampling techniques. This should help resolve many data discrepancies. The flux distribution may be transformed to a spatial distribution by dividing the number of samples in each class size by the average velocity of the drops in that size class. If all drops in a spray are moving at the same velocity, the flux and spatial distributions are identical. However, the spray will generally exhibit differences in drop velocities that vary from class size to class size. In addition, these differences depend on the type of nozzle, capacity and spraying pressure. The table below lists the Volume Median Diameter (VMD or Dv0.5) in micrometres (microns or μm) for a single nozzle at identical conditions using both flux and spatial samples.

 Several Definitions for Drop sizes

• DV0.5: Volume Median Diameter (also known as VMD or MVD). A means of expressing drop size in terms of the volume of liquid sprayed. The VMD is a value where 50% of the total volume (or mass) of liquid sprayed is made up of drops with diameters larger than the median value and 50% smaller than the median value. This is best used for comparing the average drop size from various analyzers.
• DV0.1: A value where 10% of the total volume (or mass) of liquid sprayed is made up of drops with diameters smaller or equal to this value. This diameter is best suited to evaluate drift potential of individual drops.
• Dmin: The minimum drop size by volume (or mass) present in the spray. This diameter is also used to evaluate the drift potential of individual drops.
• DV0.9: A value where 90% of the total volume (or mass) of liquid sprayed is made up of drops with diameters smaller or equal to this value. This measurement is best suited when complete evaporation of the spray is required.
• Dmax: The maximum drop size by volume (or mass) present in the spray. This diameter is also used when complete evaporation of the spray is required.
• D32: Sauter Mean Diameter (also known as SMD) is a means of expressing the fineness of a spray in terms of the surface area produced by the spray. SMD is the diameter of a drop having the same volume to surface area ratio as the total volume of all the drops to the total surface area of all the drops. This diameter is best suited to calculate the efficiency and mass transfer rates in chemical reactions.
• D10: Arithmetic mean diameter. This diameter is best suited for
  calculating evaporation rates.
• D20: Surface mean diameter. This diameter is best suited for surface
  controlling applications such as absorption.
• D30: Volume mean diameter. This diameter is best suited for volume controlling applications such as hydrology.
• D21: Surface mean diameter. This diameter is best suited for absorption studies.
• D31: Mean evaporative diameter. This diameter is best suited for evaporation and molecular diffusion studies.
• D43: Herdan diameter. This diameter is best suited for combustion studies.

REFERENCES

• Lefebvre, A.H., 1989. Atomization and sprays, Taylor & Francis
• NFPA 750, 2010. Standard on water mist fire protection systems, National Fire Protection Association publications, Quincy, Maryland.
• Spraying Systems Co., Url-1 <http://www.spray.com/cat75/automatic/files/29.html>, Product Catalog, date of download 04.09.2014.
• Vision Research, Inc., 2004. Phantom camera control software documentation, Rev.606.
• Artium Technologies Inc., 2008. PDI-200 MD User manual, operation of the phase doppler interferometer (PDI) for spray drop size and velocity measurement, California.
• Bachalo, W.D., 1980. Method for measuring the size and velocity of spheres by dual-beam light scatter interferometry, Applied Optics, 19/3, 363-370.
• Balık, G., 2006. Non-Newtonian spray atomization, Diploma Course Project Report, VKI PR 2006-02, Von Karman Institute, Belgium.
• Balık, G., 2010. Experimental and Theoretical Investigation of Spray Flows With Non-Newtonian Fluids, PhD Thesis, Istanbul Technical University, Graduate School of Science Engineering and Technology, Mechanical Engineering Program.
• Bayvel, L. and Orzechowski, Z., 1993. Liquid atomization, Taylor & Francis.
• Elkotb, M.M., Mahdy, M.A. and Montaser M.E., 1982, Investigation of External Mixing Airblast Atomizers, Proceedings of the 2nd International Conference on Liquid Atomization and Sprays, Madison, Wisconsin.