Particle Image Velocimetry (PIV)
Particle Image Velocimetry provides planar measurements of particle, or droplet, velocities. A laser sheet is used to illuminate the particles within a plane and two components of velocity (in the laser plane) can be determined by imaging the particle (or droplet) motion. The laser sheet is typically aligned so that one velocity component is in-line with the nozzle axis. Alternatively, the laser sheet can be aligned with a cross-section of the spray; however, this is only recommended with a very strong radial velocity component. Typically many instantaneous velocity fields are determined and used to generate an ensemble average velocity field.
The simultaneous capture of 2D velocities across a spray-plane is a unique capability of this non-intrusive measurement method, and can be very useful in applications where the spray is interacting with a target or for CFD validation purposes.
Spraying Systems Co. uses a commercially available, but custom, LaVision, Inc. PIV FlowMaster system, which also serves as the LSI instrument as it utilizes a similar set of hardware.
The PIV system uses a short-duration pulsed laser, which is passed through a divergent lens, to illuminate a planar cross-section of the spray. The short laser pulse effectively freezes the spray droplets in place as they are imaged. The droplets scatter the laser light and a camera, which is triggered with the laser pulse, images the droplets within the laser plane. A band-pass light filter is attached to the camera lens, which allows only light of the wavelength of the laser to pass to the CCD sensor, essentially removing all out-of-plane droplets from view.
As the droplets traverse within the laser sheet, double-frame images are acquired with a very small and known time delay. The DaVis software package is then used to run a series of cross-correlation algorithms to determine the displacement of small groups of particles. The combination of a known displacement distance/direction and a known time delay between image-pairs allows for a direct calculation of the local velocity. This procedure is performed over the entire image domain and a planar representation of the instantaneous droplet velocity field is determined. Many instantaneous velocity fields are generally acquired and used to generate an ensemble average velocity field result.
Typically, water is used as the spray medium during PIV measurements.
The PIV instrument provides a high-spatial resolution, instantaneous spray velocity field result. Instantaneous results may be ensemble averaged into mean representations. The system may be triggered to resolve transient spray development effects over multiple cycles. The non-intrusive nature of the laser sheet illumination of the spray cross-section allows for high quality planar spray velocity measurements, which allows detailed observation of the two-dimensional spray characteristics.
The PIV system does not provide drop size information; thus, it is often used in conjunction with one of the many drop size measurement instruments to provide additional characterization.
The PIV instrument provides a unique combination of a non-intrusive measurement technique with high-resolution 2D velocity measurement through a spray cross-section. Examples of past projects using Particle Image Velocimetry at Spraying Systems Co. include detailed velocity measurements in the near exit orifice region of a prototype nozzle, and 2D velocity measurement of a spray in crossflow for CFD validation.
Prototype nozzle characterization
A prototype nozzle was developed by Spraying Systems Co. for a specific customer. In order to evaluate the design, the engineer was interested to measure the droplet trajectories in the near-exit-orifice region. The nozzle had a unique design with an external impingement surface, which prompted a heightened interest in the droplet paths. The PIV was used to illuminate a cross-section of the spray near the impingement surface, and the angle of the spray leaving the impingement surface was measured. This information allowed the design engineer to make modifications to obtain the desired spray plume shape and size for the customer.
Spray in crossflow CFD validation
A customer approached Spraying Systems Co. and was interested in performing detailed CFD modeling of a spray in their application, which included a crossflow gas. Part of this effort required a validation of the CFD methods and results, which was done, in part, by measuring the location and trajectories of the spray droplets as they exited the nozzle and interested with the crossflow gases. After validating the model, the application, which took place in a hazardous environment, was modeled with confidence to provide the customer with an optimized total spray solution.