| Marine transportation is still the largest form of transportation in international trade and transportation.Marine low-speed diesel engine has been the main propulsion device of large ships for a long time because of its economic advantages and fuel compatibility.Under the increasingly stringent emission regulations,facing the dual challenge of emission regulations and fuel economy,the optimization of diesel spray combustion is the most fundamental solution.As an important factor affecting the matching of diesel engine oil chamber,the fuel injection system of the marine low-speed engine needs to be given enough attention.Among them,the research on the special nozzle structure of the marine low-speed engine,the fuel flow characteristics of large size nozzle,and the development characteristics of spray are the key to optimize the combustion system.At the same time,as the guarantee of the economy of the marine low-speed diesel engine,the thermal properties of heavy fuel oil are obviously different from conventional diesel fuel.The study of the flow and spray of the heavy fuel oil under the condition of the marine low-speed engine is also important in optimizing the combustion system.In order to optimize the combustion process of a marine low-speed diesel engine,it is necessary to study the characteristics of nozzle internal flow and the development characteristics of heavy oil spray.In order to fully consider the effect of heavy oil thermal properties on flow and spray,a method for characterizing the thermal properties of heavy oil was first proposed.Subsequently,a nozzle internal flow and spray combustion model was established.In addition,the characteristics of internal flow and the development characteristics of near-field spray were studied from the perspective of the cavitation effect,the working fluid velocity,and the turbulence effect.Moreover,the boundary parameters of oil backpressure,gas working temperature,and pressure were used to study the characteristics of the key flow fields such as spray development and flame propagation path.The main research work of this paper is as follows:(1)In view of the imperfect characterization method of heavy oil thermal properties in simulation research,a single component model of heavy oil thermal properties was established by studying the related theories and correlations of heavy oil thermal properties.On this basis,a calculation method for characterizing the thermal properties of heavy oil by using two components instead of heavy oil was proposed to reflect the thermophysical properties of heavy oil more truly.(2)The internal flow and near-field spray simulations of the large size nozzles based on the VOF method were carried out.The nozzles’ internal and near-field spray models under large eddy simulation and VOF framework were established and verified by experimental data of visual spray research.A marine low-speed asymmetric multihole injector was taken as the research object.Based on the RANS model and the VOF method,the global flow model of large size nozzles was built and the internal and near-field spray areas of the nozzles were compared in terms of flow characteristics.The research results showed that the model could accurately reflect the development process of the initial stage of the spray and the penetration speed was close to the experimental value.Besides,the cavitation effect of both the heptane and the diesel in the internal flow of the jet hole was generated.Under the same conditions,the hydraulic reflux area and the cavitation phenomenon were not captured in the study of a single component instead of heavy oil to simulate the internal flow of the nozzle.Furthermore,the injection direction deviated from the axis of the nozzle and the spatial deflection angles of the fuel jet were 13 °,6.5 °,7.6 °,and 4.4 °,respectively.(3)In order to ensure the accuracy and applicability of the spray prediction model for the marine low-speed diesel engines,the spray model of the diesel and heavy fuel oil was developed under similar conditions of the marine low-speed diesel engine based on the research results of the nozzle internal flow.Aiming at the similarity condition of the marine low-speed diesel engine,the spray model of coupled primary spray and the spray model of diesel and heavy fuel oil was established and the experimental data of spray penetration distance were compared and verified.The results showed that the spray model could better reflect the change rule of spray penetration.In addition,the spray model was affected by the temperature,the pressure,and the eddy current.The two-component HFO spray model truly reflects the spray development process of HFO under the similar condition of the marine low speed engine.(4)Finally,the influence of boundary parameters such as fuel injection direction on the spray development and the combustion process under similar conditions of the marine low-speed diesel engine was studied.Besides,the influence of fuel injection direction and backpressure on the key parameters such as the spray penetration,the fuel evaporation rate,and the overall average diameter of the soot was emphatically analyzed.At the same time,the ignition phenomenon and the flame development path of the marine low speed engine under a similar fuel-air ratio condition were investigated and compared.The results showed that the fuel was easy to get a better spray break-up effect when the direction of the fuel injection deviated from the direction of the vortex motion.With the increase of the backpressure of fuel injection,the heavy oil spray achieved a better spray break-up effect but the high backpressure environment was not conducive to the break-up of the diesel oil droplets.Moreover,the central symmetric two heavy oil flame jet was less affected by the eddy current,and the oxygen supply in the central area was insufficient after full development.Under the influence of eddy current,the flame jet of diesel showed an arc trajectory,and there was an oxygen-rich zone in the center of the constant volume chamber at the initial stage of combustion. |
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