Synchrotron X-Ray Measurements of Cavitation

Cavitation plays an important role in the formation of sprays from small nozzles such as those found in fuel injection systems. However, cavitation occurs over very short time and length scales, and is difficult to measure in-situ. Precise experimental measurements of cavitation vapor distributions in three-dimensional nozzle geometries are valuable tools for the improvement and validation of numerical simulations. The primary quantity of interest is void fraction or local density, which is difficult to measure using visible light diagnostics. X-rays scatter very weakly and can be used to make precise measurements of the projected mass distribution of a spray, and these same techniques can be extended to cavitating flows. In this paper, we present the preliminary results of an x-ray radiography experiment on a model nozzle of 500 microns diameter. The advantages of a focused x-ray raster scanning method over traditional flat-field x-ray imaging are demonstrated. The raster scan radiography experiments achieve a spatial resolution of 5 micron and a temporal resolution of 3.6 microseconds. The projected vapor distributions indicate a very rapid migration of vapor from the wall into the core of the flow. The vapor distributions are also found to be very steady; time resolved measurements indicate that RMS fluctuations are not more than 1% of the mean. The spectral content of cavitation is concentrated at small Strouhal Numbers on the order of 0.001 to 0.1, suggesting a steady cavitation inception and mixing process without any large-scale fluctuations in the vapor distribution. ∗Corresponding Author: dduke@anl.gov