Numerical Study Of Multi-Component Vaporization Model For Practical Fuel Droplets Under Engine-Relevant In-cylinder Conditions

The vaporization process of droplets has been highly valued because of its significance in internal combustion engines. In this research, high accuracy and efficiency multi-component vaporization models were constructed and implemented into a multi-dimensional CFD code (KTVA-3V) to calculate the vaporization processes of practical fuel droplets and development of diesel sprays under engine-relevant conditions.(1) The real-time dynamic calculation of physical property parameter of multi-component fuel was carried out. For the multi-component droplet, the composition continuously changes during the vaporization process. The empirical formula was chosen to estimate the physical parameters, including the liquid density, surface tension, viscosity, diffusion coefficient, heat capacity, thermal conductivity, and the vapor pressure of different components. The calculation results obtained from the empirical formula were compared with the reference data.(2) An improved multi-diffusion vaporization model was developed and applied to predict the vaporization characteristics of a fuel droplet under different ambient gas conditions. The present multi-diffusion vaporization model was constructed including an improved enthalpy diffusion sub-model, a corrected Stefan flow velocity, and temporal variation of thermal physical properties. The computational results were validated with the experimental data and satisfactory agreements between the predictions and measurements were achieved. Parameter studies were carried out to discuss the effects of O2, CO2 concentration, and EGR rate on the vaporization rate of fuel droplet under various ambient temperature and pressure conditions.(3) An hybrid multi-component (HMC) vaporization model was developed to improve the prediction accuracy. In this model, the practical fuel was modeled as a finite number of discrete hydrocarbon classes, and each hydrocarbon class was presented by a probability density function. The multi-component enthalpy diffusion was considered, a corrected Stefan flow velocity was used to ensure the mass conservation, and the vaporization rate of each species was obtained by the integration of the exact equation of species mass fraction. Satisfactory agreements were achieved between model predictions and experimental data. Extensive comparisons of HMC, CMC, and DMC models were carried out. It is found that the HMC model not only could improve the computational efficiency compared with the DMC model, but also illustrate significantly better accuracy than the CMC model.(4) An enhanced multi-component vaporization model was constructed to simulate the vaporization process of fuel droplets under engine-relevant conditions. In this model, a real and an ideal gas approach was used for high and low ambient pressures, respectively, and the radiation absorption was calculated by a simplified analytical solution of the radiative heating. Considering the compromise between computational accuracy and efficiency, a pressure criterion.was introduced for the choice of ideal or real gas approaches, and a diameter criterion was also defined to decide whether to consider the radiation absorption.(5) The enhanced multi-component vaporization model was implemented into KIVA-3V to calculate the vaporization processes of fuel droplets and evolution of diesel sprays. Good agreements between the model predictions and experimental measurements were achieved for the diesel spray characteristics. The implementation of the enhanced multi-component vaporization and KIVA-3V was capable well reproducing the spray characteristics under engine-relevant in-cylinder conditions.(6) A new quasi-dimensional vaporization model considering the finite thermal conductivity and mass diffusivity within droplet was constructed to improve the calculation accuracy and efficiency. In this model, an assumption of quadratic polynomial distributions of the temperature and component concentration was proposed for the liquid phase. This model was extensively validated by the experimental measurements, and good agreements were observed. By comparing with the zero-dimensional model with uniform temperature and component distributions and the one-dimensional model with finite thermal conductivity and mass diffusivity within droplet, it is found that the quasi-dimensional model has higher efficiency than the one-dimensional model, and better accuracy than the zero-dimensional model. The implementation in KIVA of the quasi-dimensional model was conducted, and the diesel spray was simulated with the acceptable computational cost and satisfactory accuracy.