Measuring Spray Droplet Size from Agricultural Nozzles Using Laser Diffraction

The overall goal of this procedure is to evaluate the effect of nozzle type, spray pressure and aircraft flight speed on spray droplets resulting from the use of ground and aerial application nozzles. So this method can help answer a number of key questions in application technology field related to the use and performance of spray nozzles used on typical delivery systems. The main advantage of this technique is that it gives a relatively quick and precise measurement of a large number of nozzles and operational techniques.

Perform ground nozzle testing at the downwind opening of a large wind tunnel section. At the opening, mount a nozzle body, nozzle and pressure gauge on a transfer system. This protocol uses a 110 degree flat fan nozzle with its long axis oriented vertically.

Have an electronic pressure gauge just upstream of the outward facing nozzle outlet. Measurements take place just outside of the wind tunnel. Properly align and configure the laser diffraction system in front of the wind tunnel and nozzle.

Before proceeding, measure the distance from the nozzle outlet and the system's measurement zone. The distance should be 30.5 centimeters. Next, move to the stainless steel pressure tank used for the test mixture.

Use a prepared active blank mixture to fill the tank with enough fluid for the planned tests. After filling the tank, seal it and ensure it is properly connected to both the air pressure hose and the hose to the nozzle. The next step is to turn on the wind tunnel and set the airspeed to 6.7 meters per second.

Confirm the airspeed independently, as is done here with a reading from a hot wire anemometer inside the wind tunnel. Now use the inline pressure regulator of the air compressor to set the spray air pressure to 276 kilopascals. Verify the spray pressure using the readout of the electronic pressure gauge near the nozzle.

At this point, activate the linear traverse to position the nozzle at its highest possible position. Enter all the experimental parameters into the laser diffraction system software. Next, perform a reference measurement to account for dust and background particles.

Continue by initiating a measurement cycle. When the system is ready, open the liquid feed valve from the pressure tank. Once the spray starts, use the traverse mechanism to lower the nozzle until the entire spray plume has passed through the laser diffraction system's measurement zone.

After the measurement, close the liquid feed valve. Then return the nozzle to its highest possible position in order to repeat the reference and plume measurements. Conduct aerial nozzle testing using a high speed wind tunnel.

On a boom traverse system, in the air flow region, mount a nozzle body, nozzle and pressure gauge. This test uses a standard 20 degree flat fan nozzle oriented horizontally, parallel to the air stream. Place an electronic pressure gauge just upstream of the nozzle.

For this set up, a pitot tube allows for measurement of the wind speed. In front of the nozzle body, have a properly aligned and configured laser diffraction system. For these measurements, confirm that the distance between the nozzle outlet and the measurement zone is 45.7 centimeters.

Next, check on the tank providing the fluid. Ensure the tank is filled with an active blank mixture and is connected to the air compressor and nozzle body. Turn on the wind tunnel blower and set the airspeed at the tunnel outlet to 62.5 meters per second.

Confirm this airspeed with an airspeed indicator attached to the pitot tube. Next, adjust the air pump's inline regulator to set the spray pressure to 207 kilopascals. Use the readout from the pressure gauge on the nozzle body to verify the spray pressure.

Before initiating a measurement, place the nozzle at the top position of the traverse. Ensure that all experimental parameters are entered into the diffraction system's software, and initiate a reference measurement. Now initiate the measurement cycle, and when the system is ready, open the liquid feed valve on the pressure tank.

Once the spray has started, lower the nozzle with the traverse mechanism until the entire spray plume has passed through the measurement zone. Close the liquid feed valve when the measurement is over. Return the nozzle to its highest possible position in order to repeat the reference and plume measurements.

These data are for a 20 degree flat fan aerial spray nozzle with a number 15 orifice. It was operated at 207 kilopascals, and with an airspeed of about 54 meters per second. The blue curve gives the percentage of the total spray volume in droplets, within each of 31 measurement bins used in the laser diffraction system setup.

The red curve is the same data, plotted as a cumulative distribution. Use the data in this form to find the range of droplet diameters that contain a given percentage of the total spray volume. In this example, 50%of the spray volume is in droplets of diameter 551 micrometers or smaller.

For comparison, this is the data for a 40 degree flat fan aerial spray nozzle with a number 15 orifice. It was operated at 207 kilopascals and an airspeed of about 72 meters per second. The incremental distribution, in blue, is significantly shifted towards smaller droplet diameters in comparison to the first data set.

This is the result of secondary droplet breakup due to increased airspeed. Using the cumulative distribution, 50%of the spray volume is contained in droplets of diameter 350 micrometers or smaller. Once mastered, this technique can be done in 10 to 15 minutes for a single combination of nozzles and operational conditions.

When attempting this procedure, it's important to properly setup and align all equipment and nozzles to make sure that your results were accurate and repeatable. Following this procedure, other imaging techniques can be used to further explore the spray structure. After this development, this technique paved the way for researchers in the field of application technology, to develop a number of droplet sizing models.

These models can be used by applicators to setup their spray systems to comply with pesticide application regulations. After watching this video, you should have a good idea of how to evaluate ground and aerial nozzles under a wide range of operational conditions. Don't forget that working with lasers can be hazardous, and all proper safety precautions should be taken when working with this method.