The evaporation mechanism study of cylinder lubricating oil droplet under high temperature, high pressure and forced convective conditions in marine hydrogen engines

The auto-ignition of cylinder lubricating oil droplets, which is primarily governed by the long physical ignition delay (evaporation phase), is the crucial inducement of pre-ignition in marine low-speed two-stroke hydrogen engines. To reveal the evaporation mechanisms of lubricating oil droplets, a two-dimensional model of C 25 H 52 -C 30 H 62 binary surrogate droplets in nitrogen atmosphere is developed and validated against experiments covering stationary, forced convection, high-temperature and high-pressure conditions. Then the evaporation characteristics of oil droplets under high-temperature (700–1000 K), high-pressure (1–50 bar), and forced convection (0-4 m/s) conditions are illuminated. The results indicate that as ambient pressure increases from 1 bar to 50 bar, the normalized droplet lifetime decreases by 63.3% under forced convection conditions, which is opposite to the trend under stationary conditions. At high pressure, the increase of vapor phase density enhances convective heat transfer, while the decrease of vaporization latent heat lowers the energy required for phase transition. In the low inflow velocity range, the external shear forces and the Marangoni effect synergistically enhance the thermal and mass Peclet numbers, which accelerates droplet evaporation. However, when the inflow velocity exceeds 0.9 m/s, the competition between external shear and Marangoni convection suppresses the internal circulation, which prolongs the droplet lifetime. Under extremely high-pressure convective conditions, the oil droplet with large size exhibits higher heat transfer efficiency and shorter evaporation stage than that with small size. During the early and middle evaporation stages, the external shear forces dominate the internal circulation. Whereas, in the late stage, the Marangoni effect gradually overrides shear forces.