Planar fluorescence imager characterizes sprays
Subramanian V. Sankar
Aerometrics Inc. (Sunnyvale, CA) has developed a planar fluorescence imaging instrument primarily for rapid evaluation of diesel and automotive fuel-injector performance; however, the system is suitable for many other sprays. The Optical Patternator provides qualitative and quantitative results on the temporal and spatial characteristics of planar and global sprays from different injectors. With these data, the system can make rapid acceptance/ rejection decisions on parts; it can also evaluate geometrical properties such as spray angle and spray symmetry. Unlike mechanical instruments, the patternator can perform injection phase-resolved measurement. Systems also can be upgraded to directly measure the Sauter mean diameter of droplets by simultaneously recording fluorescence and elastic-scattered light from the spray.
Internal-combustion-engine performance characteristics such as combustion efficiency, transient response, and pollutant emissions are affected by fuel-injection parameters such as atomization quality, injection timing, and spray targeting. The performance of diesel engines, gas turbines, and industrial furnaces is similarly dependent on the global spray characteristics. Thus, engine design and manufacturing creates a need for detailed spray analysis.
The most widely used method for spray analysis is the phase-Doppler particle analyzer (PDPA), a single-particle counter capable of high-resolution measurements of the size and velocity of individual fuel droplets in complex sprays. Combined with temporal information, these data can be analyzed to yield flow parameters such as particle number density and liquid volume flux. Fourier-transform techniques can be used to extract oscillatory characteristics of the flow from temporal data.
Although the PDPA provides a wealth of information on the behavior of steady-state and transient sprays, global spray characterization can be a time-consuming process. Time-critical applications such as quality-assurance testing necessitate alternate diagnostic techniques.
System design and operation
The patternator consists of a laser, a light-sheet projector, a camera, a frame grabber, a timing controller, a PC, and related software. The light-sheet projector illuminates a two-dimensional (2-D) plane in the spray field of interest. Fuel droplets lying in the measurement plane scatter the incident light and a 2-D array camera captures the results. The image is digitized by a frame grabber and processed by computer algorithm to yield fuel mass distribution and, optionally, the Sauter mean-diameter distribution in the spray.
The type of laser chosen depends on the application. A visible-wavelength system can be used if the fuel (or stimulant) can be doped with dyes that fluoresce when excited at the laser output wavelengths. Doping the calibration fluid is not feasible for many production-related applications, however. In such cases, the laser must be chosen to excite fluorescence from the fuel itself; usually UV wavelengths are needed.
The type of camera used in the system--high or low resolution; 8 or 16 bits; digital or RS170-compatible analog; gated, intensified, or not--is also application-dependent. Quality-assurance testing and in-line inspection, which must be performed rapidly, may require an RS170 gated, intensified camera. For research and development applications, a scientific-grade, slow-scan camera may be suitable.
A master-timing controller synchronizes the operation of the injector, laser, camera, and frame grabber. In the case of pulsed sprays, such as those produced by spark-ignition and diesel-engine injectors, the system can lock into different phases of the injection cycle to study the temporal evolution of the fuel mass distribution.
The system software controls the injector, camera, and laser during the measurement process and displays the single-shot images in real time. Ensemble averaging techniques smooth the images; built-in digital filters perform additional smoothing operations. The software also incorporates fast algorithms for perspective correction, as in cases when the camera and laser light sheet are set u¥for an oblique viewing angle.
Once the software smooths and corrects the ensemble-averaged image, various user-selectable masks can be applied to compute the spatial distribution of the liquid mass. The spray angle also can be determined. To establish acceptance or rejection of an injector, the system compares the measured parameters against user-specified values; running on a PC with Pentium Pro processor, the software can make this decision within 2 to 3 s of image acquisition. The acquired image can be compared to a master image for additional validation.
Data can be exported into a spreadsheet for further analysis, such as temporal evolution of the spray pattern, or generation of contour maps and surface plots describing the fuel mass distribution. The present software is Windows-3.1-based; a Windows NT system is slated for release in the near future.
Spray characterization options
The Optical Patternator can perform global spray characterization, useful for research and development applications. The system acquires several horizontal or vertical image slices through the spray at a specific phase of the injection cycle; this information is then analyzed to provide global performance data. The phase can be varied to record the temporal evolution of the global spray pattern. The instrument features an advanced volumetric visualization package that reconstructs the temporal and spatial characteristics of the entire spray using recorded image slices (see Fig. 2). The aerosol can be probed at any instant, and the user can slice through the plume at any angle and orientation to reveal the fuel mass distribution at that time and space.
When the spray is illuminated, a portion of the incident laser light scatters from the droplet surfaces without undergoing wavelength shift; this is referred to as elastic scattering. The Optical Patternator software combines fluorescence data with this elastic-scattering data to obtain the planar Sauter mean diameter of the droplets. The technique takes advantage of the fact that the intensity of elastically scattered light is proportional to the square of the droplet diameter, whereas the fluorescent light intensity is proportional to the cube of the diameter. The ratio of the fluorescence data and the elastic-scattering data yields a planar measurement of the Sauter mean diameter; Aerometrics has validated this technique with PDPA measurements.
The absorption and fluorescence spectra of organic molecules dissolved in nonpolar solvents such as automotive fuels are virtually identical to the spectra of the same molecules in the vapor phase; this makes it impossible to distinguish between the liquid and vapor phases of the fuel with these data alone. In many cases, however, it is possible to react the fluorescence-excited state of the monomer with an appropriate molecule to form a second emitting species called the excited state complex, or exciplex, whose emission is red-shifted with respect to that of the monomer emission.
Furthermore, the reversible reaction that gives rise to the exciplex can be manipulated such that the exciplex is the dominant liquid-phase emitter and the monomer is the dominant vapor-phase emitter. With the use of split images and different optical filters for the different images, the vapor phase and the liquid phase can be simultaneously imaged. As an option, the Aerometrics` Optical Patternator system also can incorporate exciplex-based vapor/ liquid visualization capability. o
FIGURE 1. Planar fluorescence imaging system characterizes aerosols produced by fuel injectors. A laser source illuminates a two-dimensional slice within the spray (inset), and a camera captures the resultant fluorescence.
FIGURE 2. Three-dimensional global spray images from a split injector used in four-valve engines show temporal evolution of the aerosol.