Laser Sheet Imaging (LSI)
The Laser Sheet Imaging instrument provides planar measurements of the spray distribution. As droplets pass through a laser sheet, images are acquired; these images demonstrate the liquid distribution at an instant in time. Many images can be collected to examine the transient development of a spray, or an ensemble average can be generated from multiple images to demonstrate the average spray distribution. This method provides a uniquely non-intrusive measurement of the spray concentration distribution; often acquired at many downstream distances from the spray nozzle and used to investigate the spray’s development downstream of the nozzle.
Spraying Systems Co. uses a custom LaVision, Inc. SprayMaster measurement instrument for laser sheet imaging purposes; as well as PIV applications, which utilize a similar set of hardware.
The LSI uses a short-duration pulsed laser, which is passed through a divergent lens, to illuminate a planar cross-section of the spray pattern. 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.
Typically, water is used as the spray medium during LSI measurements. The water droplets scatter the laser light according to Mie theory, which results in illumination from each droplet equivalent to its surface area. The ensemble average result is a planar distribution representative of the total spray surface area. While this is similar to the volume distribution in many cases ( Bade_ILASS2013.pdf), it is often referenced as the spray concentration distribution and is instructive to assess local regions of high-vs-low spray concentration. For strictly volume distributions, the LIF technique must be used.
In order to obtain volume distribution measurements from the LSI instrument, it is necessary to treat the spray material with a doping agent. Laser Induced Fluorescence (LIF) measurements use a dye in the spray material, which, when subjected to the wavelength of light which is emitted by the LSI laser, causes the entire droplet to emit light; thus, the LIF method results in planar spray distributions that are representative of the spray droplet volume.
In either Mie or LIF measurements, many instantaneous images are typically acquired and ensemble-averaged in order to arrive at an average planar spray distribution.
The LSI system provides a high-spatial resolution spray distribution result that can be representative of either the concentration or the volume distribution. Instantaneous planar spray distribution results are acquired that 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 the highest quality planar spray distribution measurements, which allows detailed observation of the two-dimensional spray pattern.
The LSI 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 LSI instrument provides a unique combination of a non-intrusive measurement technique with the highest possible spray distribution resolution. Examples of past projects using laser sheet imaging at Spraying Systems Co. include detailed spray in cross flow measurements for the gas conditioning industry, and detailed transient spray distribution assessment using a pulsed injector nozzle.
Cross-flow spray distribution
In order to optimize the spray distribution in a gas conditioning application, a nozzle was placed within the wind tunnel and the liquid distribution was measured at various cross-flow angles (i.e. nozzle orientation angle relative to the wind tunnel flow). LSI planar distribution images were acquired and averaged in order to determine the optimal nozzle angle to maximize spray uniformity and coverage while minimizing any wall-wetting.
NOX injector transient spray development
A customer approached Spraying Systems Co. and was interested in characterizing the spray distribution during the injection open/close phases of their proprietary injection nozzle. The LSI system was setup to trigger with the injection pulse, with an assigned time-delay, to capture many images at multiple times relative the injection start time. These images were then ensemble averaged to provide an average spray distribution at many stages of the injection period. In general, it was found that a non-uniform spray pattern was generated at the start-and-stop portions of the injection.