Spray droplet analysis

Published in

Codefarm

3 min readAug 21, 2017

https://commons.wikimedia.org/wiki/File:US_Air_Force_050719-F-2585R-001_Aerial_Spraying.jpg?uselang=en-gb

Introduction

I hope you found the python thoughts on Pesticide and flow volume useful. Today I want to share some ideas on what spray is made of: droplets. Most of the times we use hidraulic spraying, produced by diferent kinds of nozzles. After the liquid (mixture of water and pesticide) is pressurized by a pump, there is a small place where it finds atmosferic pressure (the nozzle) and, as a result it is pulverized in millions of small droplets.

Droplet diameter and volume

Since they are almost made of water, droplets keeps its properties, like surface tension. When they are free in the air, they tend to be spherical and so, the most characteristic descriptor is the diameter. Because we are dealing with such small objects, the diameter is measured in micron, a 1/1000 of a milimeter. If you want to get an idea of how small it is, a human hair has 100 microns of diameter.

VMD and span

As you can imagine, it’s impossible to know the diameter of all droplet in the spray. The equipment that measures it, samples some droplets and generates a distribution of size, giving us some statistics.

VMD (or DV 0.5) : median or 50% percentile
DV 0.1 : 10% percentile
DV 0.9 : 90% percentile
SPAN : Relative span

What is relative span? Let’s define a function than calculates it:

Python ecosystem has some useful tools for data simulation and visualization.

Let’s simulate two different droplet distributions.

After that we can calculate the indicators, using a method from the distribution and show them with the “f” strings.

'The 10 percentile is 137.2 micron, the VMD is 150.0 micron and the 90 percentile is 162.8 micron.The span is 0.2'

When the pressure rises, as the flow increases, the VMD decreases and span too. We can say that we can have different distribution, depending on the pressure. A spray with a larger VMD is called coarse, but it’s better to know the classification according to it:

Very fine : < 159 micron
Fine : 159–231 micron
Medium : 231–326 micron
Coarse : 326–386 micron
Very coarse : 386–484 micron
Extremely coarse : 484–553 micron
Ultra coarse : > 553 micron

Take a look on the data for Albuz ATR nozzles:

Between diferent nozzles at same pressure

Between diferent pressures on the same nozzle

Conclusion

Understanding spray droplet size is fundamental to fully predict its behavior. There is always a trade-off between thinner, more efficient droplets (since they can cover more with less spray volume) and coarser, better environmental behaved droplets (that less drifted by the wind towards non target areas.

Spray droplet analysis

Introduction

Droplet diameter and volume

VMD and span

Conclusion

Resources

Written by Hugo Pires