Numerical investigation on the effect of physical properties of alternative fuels on in-nozzle cavitation in a full-scale injector for a two-stroke marine engine

A large-eddy simulation (LES) coupled with the volume-of-fluid (VOF) method and different cavitation growth models are employed to investigate the effect of physical properties of methanol and ammonia fuel on in-nozzle cavitation in a full-scale dual-hole fuel injector of a large marine two-stroke engine. The numerical approach is evaluated for hydraulic oil using particle image velocimetry (PIV) measurements and shadowgraph images from experiments with a transparent replica of the nozzle. The LES results show an accurate prediction of mass flow rates at different cavitation numbers with discrepancies less than 5% in the transition region between non-choked and choked flow conditions. The qualitative appearance of cavitation formation resembles the shadowgraph images at two different cavitation numbers. At the cavitation number of 1.3, a good agreement on transverse velocity profiles is seen between the LES results and PIV measurements, while at a higher cavitation number of 2.1, discrepancies are seen in regions where cavitation structures exist. Subsequently, the effects of non-isothermal physical properties of two alternative fuels, methanol and ammonia, are investigated and compared to n-dodecane. A thermodynamic cooling effect is seen for methanol and ammonia due to a lower critical temperature and higher latent heat of vaporization. Two different cavitation growth rates, an inertia-controlled and a thermal-diffusion controlled, are evaluated for all three fuels and the results suggest that ammonia fuel is limited by thermal effects. Finally, a comparison of wall heat transfer for all three fuels shows that the heat transfer rates of methanol and ammonia are approximately two- and sevenfold compared to that of n-dodecane, respectively, with the highest heat flux in the proximity of the cavitation region where liquid is attached to the wall.