Experimental Investigation on Near-Nozzle and Macroscopic Structure of a Supercritical Fuel Injected into Subcritical Environment

The supercritical fluid combustion technology was regarded as an effective method to increase fuel gas mixing rate and performance. During the injection process, critical characteristics dominate the jet development to behave as different spray structure. Due to the limited researches about supercritical gasoline-like fuel injection characteristics, macroscopic and near-nozzle microscopic spray structures of supercritical n-heptane injected into atmosphere condition were observed and compared with the injection of cryogenic nitrogen in this work. A supercritical fuel injection device was designed able to heat the fuel temperature up to 773 K and maintain the fuel injection pressure stable at 4 MPa. Backlight illumination and schlieren imaging technologies were applied to capture the liquid and overall jet structure. The effect of initial fuel temperature on the spray structure was analyzed and some novel near-nozzle structures were also discussed. Results show that with the increase of initial fuel temperature, the jet behaves as narrow linear structure at first, and then transforms to gray mist along radial direction, and almost vanishes except for near-nozzle region at last. As for the microscopic spray structure, there is a closed shock structure near the nozzle. The axial distance of the Mach disk of the shock can be predicted by empirical correlations which are suitable for the ideal gas, but the radial distance of the Mach disk is larger than that of the ideal gas.