Three-dimensional simulation of binary alkane mixture flow in a ballistic cycle of a high-pressure direct fuel injector using a mass transfer cavitation model

Abstract

A multi-species cavitation model is applied to a high-pressure injection cycle. Results of transient simulations with ballistic needle dynamics are presented. Species segregation in terms of local species demixing in the liquid phase is assessed. Therefore, the mass transfer cavitation model proposed by [101] is adopted and implemented in an incompressible large-eddy simulation flow solver. The model is based on the Rayleigh bubble dynamics equation, Raoult's and Dalton's law, vapor-liquid equilibrium, and mixing rules. It is initially tested on a hydrofoil test case and subsequently applied to a high-pressure injector. A heavier n-dodecane/n-heptane and a lighter n-octane/n-heptane mixture are considered. For both mixtures, essentially the same vortex and vapor structures are encountered. No distinct hysteresis between opening and closing phase of the cycle is present, which is related to a relatively low liquid density of both mixtures. Segregation correlates with the amount of vapor and is most pronounced for cloud cavitation at low needle lift levels and minor for string cavitation at high needle lift levels. It is considerably more pronounced for the heavier than the lighter mixture due to a wide spread of volatilities of n-dodecane and n-heptane. Even for the heavier mixture, segregation is much less pronounced than observed in the immediate environment of a single bubble. This observation is related to the homogeneous mixture approach, by which an averaged effect of segregation over the computational cell is assumed. An Euler-Lagrange approach is proposed for future studies, embedding details of mass transfer over the bubble interface into the CFD code.