Effects of Back Pressure on Condensed-Phase Properties Within Supercritical Ethylene Jets

The effects of ambient pressure on the liquid contents and droplet size inside condensed ethylene jets injected at the supercritical conditions were studied both experimentally and numerically. The small angle X-ray scattering (SAXS) technique available at the Argonne National Laboratory was utilized to measure droplet size and liquid volume fraction inside the condensed jets. Two different ambient pressures inside the injection chamber were selected for testing. A companion numerical model, based on previously developed approaches, was used to provide insight into droplet properties and growth processes. It was found that the measured droplet size is on the order of 1000-2000 A (100-200 nm) for the conditions tested in the present study. The measurements also show that an elevated ambient pressure can increase droplet size and reduce the liquid volume fraction within the condensed jet for a given injection condition. This experimentally observed trend, however, contradicts the present numerical results. The contributing factors for the observed discrepancies should be addressed in the future. Both measurements and numerical calculations show that the Mach disk inside the under-expanded jet exhibits large influences on droplet size and liquid volume fraction for the condensed jet injected into an elevated ambient pressure. CFD predictions show that droplets inside the jet discharged into a low ambient pressure exhibit a higher evaporation rate to reach a smaller droplet size and a lower liquid volume fraction at the downstream locations. Both measured and predicted liquid volume fraction and mass fraction within the condensed jets are fairly small.