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Experimental study on fuel transcritical injection characteristics about macroscopic and near-nozzle microscopic structure
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on September 15, 2020 by SAE International in United States
The supercritical fluid combustion technology was regarded as an effective method to increase fuel gas mixing rate and performance. During the transcritical injection and combustion process, critical characteristics dominate the jet development to behave as different spray structure. Due to the limited researches about fuel transcritical injection characteristics, macroscopic and near-nozzle microscopic spray structure was observed respectively. In this work, a transcritical injection device was designed able to heat the fuel temperature up to 773 K and maintain the fuel injection pressure stable at 4 MPa. The experiment was conducted with the fuel injecting from supercritical condition to atmosphere condition. As a comparison, two fluids were selected to conduct transcritical injection experiment. The n-heptane was used to represent the surrogate of the supercritical diesel, while the cryogenic nitrogen was selected to represent the ideal gas. Backlight illumination and schlieren imaging technologies were applied to capture the liquid and overall jet structure images. The effect of initial fuel temperature on the spray structure was analyzed and some novel near-nozzle structure was also discussed. The results show with the initial fuel temperature increasing, the jet behaves as narrow linear structure firstly, and then transforms to gray mist along radial direction, finally almost vanish except for near-nozzle region. For the microscopic spray structure, when the supercritical fuel is injected into the atmosphere condition, there exists a closed shock structure near the nozzle exit. 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.