Numerical Study On Near-field Spray Characteristics Of Marine Low-speed Engine

In the face of increasingly stringent emission restrictions,improving the efficiency of marine engines has now become a focus problem of governments and diesel engine manufacturers.Diesel engine efficacy and emission performance are closely related to the quality of fuel atomization,which urges researchers to continuously improve the injection system and atomization quality to reduce exhaust emissions while improving fuel economy and combustion efficiency of marine engines.The near-field spray atomization process and stability of fuel are not only influenced by surface tension and the drag force caused by the gas-liquid two-phase velocity difference,but also related to the cavitation and turbulent flow inside the nozzle orifice.In this paper,based on Open FOAM,combined with the interface compression technology under the Euler-Eulerian framework,under the conditions of high injection pressure and high ambient pressure that are closer to the actual operating conditions,a coupled numerical study of the internal flow and the primary breakup of a marine low-speed engine nozzle is carried out.The main work is listed as below:(1)Numerical study of the internal flow characteristics inside the nozzle orifice.Firstly,the cavitation phenomenon is described based on the modified Schner-Sauer cavitation model,which the bubble radius fully considerd the initial nucleation volume,and the bubble density is not set as constant relying on experience.After comparing the numerical simulation results with the experimental observations,it is found that the revised cavitation sub-model can faithfully feed back the actual evolution process of microbubbles in the orifices.In addition,developing cavitation is the main type of cavitation that occurs in the nozzle of this marine engine,from no cavitation to developing cavitation,the dimensionless number that plays a leading role is the cavitation number;from the developing cavitation to super cavitation,the main influencing factor is the Reynolds number.Secondly,using large eddy simulation to solve the mixed fluid turbulence equation,under the same pressure difference,the cavitation range at the exit of the small aperture nozzle is wider,the radial diffusion trend is obvious,and the outlet turbulent disturbance is stronger.(2)The effects of cavitation,turbulence,and aerodynamic forces on the primary breakup were quantitatively analyzed.The validation results of the jet breakup showed that the coupled numerical model could clearly capture the details such as liquid ligaments cutting off and large droplets stripping out.Meanwhile,compared with the traditional VOF model,the numerical results of the coupled numerical model are closer to the experimental values,and the calculation accuracy is higher.When the internal flow reaches super cavitation,the influence of the cavitation effect tends to be stable.At the same time,the collapse of the cloud cavitation at the orifice exit also has a positive impact on the near-field atmoziation.In addition,in the main spray area dominated by both internal flow and aerodynamic forces,the orifice exit has stronger turbulent vortex structures under high injection pressure,and the ambient gas entrainment effect on the spray is more severe,resulting in a stronger axial vortex and a smaller jet.In the aerodynamic-dominated umbrella-shaped head crushing area,increasing the injection pressure will lead to a larger velocity difference between the gas and the liquid phase,stronger drag force and better atomization quality.(3)Heavy fuel oil is a common used working fluid in ships,and its high viscosity makes it generally poor in atomization quality.In this study,a coupled numerical study of the internal flow and primary breakup of heavy fuel oil was also carried out at three different preheating temperatures.A certain amount of microbubbles occur at the nozzle entrance when heated to394 K,and the pressure in the orifice decreases approximately linearly along the flow direction and remains above the corresponding saturated vapor pressure at the other two heating temperatures.Both the liquid column breakup cone angle and the penetration increase when the the preheating temperature is enhanced.At the same time,due to the larger surface tension,the surface wave development of the heavy oil is retarded at 354 K.When heated to 394 K,the surface wave has begun to take shape,and as the flow continues,the amplitude and wavelength of the surface wave begin to increase,and the neck begins to collapse.The atomization quality improved with increasing temperature.