Comparison of Near-Field Structure and Growth of a Diesel Spray Using Light-Based Optical Microscopy and X-Ray Radiography

A full understanding and characterization of the near-field of diesel sprays is daunting because the dense spray region inhibits most diagnostics. While x-ray diagnostics permit quantification of fuel mass along a line of sight, most laboratories necessarily use simple lighting to characterize the spray spreading angle, using it as an input for CFD modeling, for example. Questions arise as to what is meant by the “boundary” of the spray since liquid fuel concentration is not easily quantified in optical imaging. In this study we seek to establish a relationship between spray boundary obtained via optical diffused backlighting and the fuel concentration derived from tomographic reconstruction of x-ray radiography. Measurements are repeated in different facilities at the same specified operating conditions on the “Spray A” fuel injector of the Engine Combustion Network, which has a nozzle diameter of 90 μm. Long-distance microscopy at >100 kHz speeds is used to characterize the opening, steady, and closing phases of injection. X-ray radiography (5 μm beam width) is applied from multiple positions and projection angles. Tomographic reconstruction for the ensemble-average fuel mass distribution (or liquid volume fraction) shows that the near-field mixing layers and growth are related to the nozzle exit geometry, with an intact liquid core moving downstream to approximately 2.5 mm. Optical microscopy at various thresholds identifies boundaries of the spray which correspond to low mixture fraction near the detectible limit of the corresponding x-ray measurement. Defining the spray edge as that with an optical thickness of approximately one, the corresponding projected fuel mass from the x-ray measurement is on the order of only 1 μg/mm². Radiography-reconstructed planes at the spray center show local LVF of approximately 1% at the same spray width/edge. We recommend that these values be used as a quantitative projected mass or LVF for the spray “edge” derived from optical backlit microscopy.