Fuel Injector Requirements to Achieve Supercritical Flow at the Exit

Advanced engine designs and alternative fuels introduce the possibility of supercritical fuel injection in aviation gas turbines and diesel engines, as is already the case for many rocket engines. Previous studies have focused mainly on fuel–air mixing in the supercritical regime after injection. However, injector requirements to achieve supercritical flow at the exit have not been investigated systematically. In this study, supercritical flow in an injector is analyzed using computational fluid dynamics with a real gas model and fluid properties derived from Helmholtz equations of state. Three operational challenges are illustrated depending upon the fuel: 1) large decreases in pressure and temperature within the injector, 2) injector choking, and 3) supersonic expansion of the supercritical jet. These challenges are addressed by developing and validating a one-dimensional, nonisentropic model of supercritical flow in the injector. This reduced-order model can guide injector designs for different fuels and applications and help decouple the injector supercritical flow from that in the downstream chamber to significantly reduce the computational effort for fuel–air mixing simulations. Results show that larger-diameter injectors are generally required to achieve supercritical injection with a fuel energy injection rate per unit area matching that of a typical diesel injector.