  A numerical framework implemented in the open-source tool OpenFOAM is
presented in this work combining a hybrid, pressure-based solver with a
vapor-liquid equilibrium model based on the cubic equation of state. This
framework is used in the present work to investigate underexpanded jets at
engine-relevant conditions where real-gas effects and mixture induced phase
separation are probable to occur. A thorough validation and discussion of the
applied vapor-liquid equilibrium model is conducted by means of general
thermodynamic relations and measurement data available in the literature.
Engine-relevant simulation cases for two different fuels were defined. Analyses
of the flow field show that the used fuel has a first order effect on the
occurrence of phase separation. In the case of phase separation two different
effects could be revealed causing the single-phase instability, namely the
strong expansion and the mixing of the fuel with the chamber gas. A comparison
of single-phase and two-phase jets disclosed that the phase separation leads to
a completely different penetration depth in contrast to single-phase injection
and therefore commonly used analytical approaches fail to predict the
penetration depth.
