X-RAY RADIOGRAPHY MEASUREMENTS OF CAVITATING NOZZLE FLOW

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 have been used to make precise measurements of the projected mass distribution of sprays, 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 µm 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 µm and a temporal resolution of 3.6 µs. The vapor distributions are 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. Substantial void regions at the nozzle centerline where cavitation is not expected to occur have been investigated, and may be due to dissolved gas in the fuel coming out of solution as the static pressure drops. We propose that dissolved gas is an important variable to consider in fuel spray experiments.