Phase Doppler Interferometer (PDI)
The phase Doppler interferometry instruments measure drop size, velocity, and volume flux at a point within a spray plume. These instruments provide the most detailed information of the spray characteristics by characterizing each droplet that passes through the probe volume and building up accurate ensemble statistics. The probe volume is formed by two (or four) intersecting laser beams, which provide a non-intrusive, high spatial-resolution measurement point. Spray pattern characterization can be performed by moving either the instrument or the nozzle itself; collecting data at numerous specified locations. This information is then combined to represent the drop size (or velocity or volume flux) distribution throughout the spray pattern. Further analysis allows combination of the information from the many individual data points to provide volume flux and area weighted average spray pattern statistics (ref: Methods, Distributions).
While the methodology is nearly identical, there are various commercial instruments available such as the Phase Doppler Interferometer (PDI) and the Phase Doppler Particle Analyzer (PDPA); Spraying Systems Co. primarily uses the following instruments:
The phase Doppler instrument’s laser transmitter unit emits two coherent laser beams of the same wavelength (color). At the intersection of these lasers, an interference pattern is formed by the constructive and destructive interference of the two lasers, and a known interference wave frequency is generated. A droplet, acting like a prism, refracts this interference pattern and it is detected by the phase Doppler instrument’s receiver-unit and analyzed to determine the droplet characteristics. The frequency shift of the pre-determined interference pattern is proportional to the droplet’s velocity; this method is commonly known as Laser Doppler Velocimetry, or LDV. Also, there are three detectors within the phase Doppler instrument’s receiver unit that detect the interference burst pattern; the relative phase shift of this signal at each of the detectors is proportional to the droplet diameter. The phase Doppler instruments count and analyze each interference pattern as it is detected, and thus characterize each individual droplet that enters the probe volume formed by the intersecting laser beams; this is a flux-sampling method.
The PDI or PDPA instruments provide data with the highest spatial resolution, generate a unique combination of output variables, and sample each droplet to provide flux measurements. The unmatched range of output spray characteristics and the accuracy of the measurement makes the PDI or PDPA the capable of providing the most detailed measurements within a spray. The high-powered lasers allow measurement within a light to medium level of spray droplet density, and allow the characterization of points within the spray pattern without a loss in accuracy due to droplets entering the laser beams outside of the laser-intersection region.
However, due to these unique benefits, the measurement time is increased to allow multiple points to be measured within the spray pattern, and to allow extended data analysis, which is often required to process the results into actionable data.
These instruments are used in the spray community to provide the highest resolution data for both academic and industrial investigations. Examples of past projects using the phase Doppler analyzers at Spraying Systems Co. include detailed prototype nozzle analysis for the aerospace industry, and process optimization using nozzles in a cross-flow environment.
Prototype nozzle assessment
An aerospace engineering company contacted Spraying Systems Co. and was interested in spraying a liquid material at a specified flow rate. There were specific requirements for drop size and volume distribution. After modifying a standard Spraying Systems Co. nozzle design, the prototype nozzle was tested for drop size distribution at many locations within the spray pattern. These tests confirmed the spray was uniformly distributed, and validated the mean & maximum drop size characteristics.
Cross-flow spray trajectory
In order to provide detailed validation data for spray simulations (CFD) being performed by Spraying Systems Co. for a customer, detailed drop size, velocity, and volume distribution measurements were acquired using a standard Spraying Systems Co. nozzle at various angles in a wind tunnel. The PDI results allowed validation of detailed simulations at laboratory conditions; the simulation were then performed at increased temperatures and pressures that matched the customer’s process parameters.