Why we need to care about supercritical and non-ideal injection

Injection and primary break-up in engines are typically understood as multiphase phenomena, classified using Weber and Ohneseorge numbers, and simulated using methods that preserve the liquid volume and account for surface tension forces. However, in Diesel engines, in jet engines during take-off, and in rocket engines, injection occurs at conditions that exceed the respective fuel thermodynamic critical pressures. Injection processes under supercritical conditions behave fundamentally different from injection at ambient conditions - fluids become more compressible than ideal gases, break-up and combustion are tightly coupled, fluid properties vary wildly even for small changes in temperature or pressure, and surface tension may vanish altogether. This is becoming relevant for more and more engines. While jet engines have mostly transitioned to subcritical conditions between take-off and cruise since the 1980s, the first engines with fully supercritical conditions between take-off and cruise (10km, M 0.8) are already operational. New automotive engine concepts such as spark controlled compression ignition (SCCI) and reaction controlled reaction ignition (RCCI) push gasoline engines into the realm of supercritical injection as well. Our models and analysis need to reflect this.