Fundamental Research On Conjugate Heat Transfer Modeling For High Power Density Diesel Engine Simulation

Multiphysics coupled methods for internal combustion engine simulation has been developed more than20years and become an important design means of combustion chamber components. Although this numerical analysis method had applied on various types of IC engines, multi-flied in IC engine such as velocity field, temperature field and stress field still failed to calculation in a single solver. At the same time, increasing engine power density causes proliferation of the heat transfer throughout the chamber surface, which has adverse impact on the thermal loading and reliability of the combustion chamber components. In order to effectively control the thermal loading and optimize the engine performance, the fundamental problems of heat and mass transfer must be depth studied under the conditions of High Power Density (referred to as HPD).Considering the characteristics of HPD engine, such as high speed, high supercharge pressure, high-intensity combustion, a conjugate formulation to predict spray combustion in cylinder, heat conduction in combustion chamber components, and convection in coolant was developed for multi-dimensional HPD engine simulation. The formulation was first calculated in CFD code and then validated against experimental result.The main contents in this thesis are listed below:1, Combining thermodynamics, fluid mechanics and chemical kinetics, zero-dimensional and multidimensional numerical model for engine operation simulation was established in chapter two. The advantages and disadvantages of the two models were analyzed exactly. The result in this chapter provided a theoretical basis for further study of in-cylinder heat transfer models.2, Compared the numerical results between semi-empirical formula and wall function approach, a new in-cylinder heat transfer model was developed for HPD engine simulation and then validated by experimental data.With the increase of heat flux, the relative error of using traditional semi-empirical formula to predict components temperature is up to30%. Results show that the new in-cylinder heat transfer model developed in this paper is more suitable for HPD engine simulation.3, Materials thermal properties under different temperature, including thermal conductivity, specific heat capacity, expansion coefficient, were exactly tested by special equipment. Based on this data, nonlinear heat conduction equations of combustion chamber components were solved by numerical and analytical method.Results show that the temperature calculated by assuming material thermal properties as constant is larger10%than that of setting the properties as a function of temperature. For complex geometry, the non-linear heat conduction equation costs20%to30%computer resources more than the linear equation. Considering both calculation accuracy and time, it is better to use non-linear formula in cylinder head, piston and liner, at the same time to use linear formula in the rest parts of HPD engine.4, a conjugate formulation to predict heat conduction in components solid domain and convection in fluid domain was established for HPD engine simulation. The formulation had integrated the new in-cylinder heat transfer model and the nonlinear conduction model. Finally, the numerical solution of the conjugate formulation was validated against the test data of the HPD engine under different operating conditions.Results show that the conjugate formulation significantly improves the solution accuracy. The relative error between calculation data and test data is less than5%.5, the analysis of the conjugate formulation under different boundary conditions had subsequently revealed the interaction among various factors of HPD engine, such as combustion and cooling conditions, material properties and components structures.Results show that the cooling conditions have the greatest impact on the heat transfer in HPD engine among the four factors. Increasing the coolant velocity can effectively reduce the temperature at components inner surfaces. Secondly, permitting the components structural strength, the smaller distance between the cooling jacket and heated surfaces has the better cooling effect.