An experimental investigation was conducted to examine the behavior of swirling supercritical hydrocarbon fuel (SCF) jets injected into nitrogen environments whose temperatures and pressures exceeded the fuel critical values. Measurements of jet full-cone angle, mass concentration field, and penetration length were made using a schlieren system; the images were captured by a high-speed digital camera and processed using the camera’s software, plus MATLAB codes. Test parameters were the internal geometry of the pressure-swirl nozzle, fuel flow rate, and density ratio. The density ratio was varied by altering the reduced temperature of the injected fluid and nitrogen environment. SCF injections were studied at reduced temperatures ($Tjet/Tcrit$ with both reported in Kelvin) ranging between 1.01 and 1.10, a reduced pressure ($pjet/pcrit$ with both reported in bars) of 1.05, and fuel flowrates of 1.0 g/s, 2.0 g/s, and 3.0 g/s. The variable internal geometry pressure-swirl atomizer produced jets having swirl numbers $(SN)$ of 0 (straight bore), 0.25, 0.50, and 1.00 (high swirl). As expected, increasing the swirl number for a SCF jet had by far the largest effect on jet cone angle, followed by a change in the density ratio; changing the fuel flow rate had very little effect. The SCF jet penetration length increased when either the fuel flow rate or density ratio increased. The mass concentration profiles demonstrated the jets to be self-similar in nature, and correlation to a Gaussian profile showed the mass concentration field to be independent of swirl number, density ratio, and fuel flow rate. Finally, it was found that there was a linear relationship between the jet half-width and the swirl number. The current study characterized the behavior of swirling hydrocarbon fuel SCF jets for the first time. Aspects of jet behavior similar to that of gas jets include: Gaussian mass concentration profiles and jet boundaries that scale with swirl number. Finally, $CO2$ was found to be a suitable surrogate fluid for hydrocarbon fuels since the behavior of the hydrocarbon SCF jets was similar to that of $CO2$ SCF jets.

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