Study On The Compressive Characterictics And Mechanisms Of Biodiesel Based On The Rod-Like Molecular Model

Compressive characterictic of injection fuel is determined by the chemical structure itself and the external condition. As renewable and clean energy, the differences in physical properties of biodiesel comparing to that of petroleum diesel result in the poor atomization. With the improvement of the working pressure in the injection system, the research of formation mechanism of compressive characteristic becomes a dominant factor in the injection flowing process and the atomization performance. Therefore, investigation of the computation of the volume viscosity under dynamic temperature and pressure of the biodiesel is critical to calculation of the injection process and obtaining the properties of the dynamic flowing, as well as control of the injection process and optimization of the atomization. Currently, the dynamic compressive characteristic is not fully considered in the numerical simulation of fluid dynamics of the injection process. This study aims at obtaining the the dynamic compressive characteristic of biodiesel in the injection process. Through engine test, the dynamic variation law of fuel in the injector chamber was achieved. Based on constructing the rod-like molecule model of saturated fatty acid ester, By establishing the volume expression and energy expression, and derivating motion equation and momentum equation, the working mechanism of volume viscosity on injection flow was made clear. The theoretical approach to adjust the temperature of the fuel supplying pipe and optimizing the atomization was proposed. The main conclusions of our research are as follows:(1) By using the self-made surface thermocouple temperature sensor and the self-developed experimental testing system, The temperature in the injector pressure chamber was examined and the variation law of temperature of the fuel in the injector was obtained. Changes in the temperature are composed with the linear decreasing part and the sine wave part. The slope of the linear part is between1.8×104K/s and3.0×104K/s, while the amplitude of the sine wave part spans from1.5K to1.5K, with a frequency between12.5kHz and8.3kHz. The experimental data suggested that the pressure of the injection process was in the form of planar damped wave, with the amplitude being2.5%～10%of the average pressure, and the frequency between5kHz and50kHz.(2) According to the chemical structure of the biodiesel molecules, which has10～25carbon atoms, the rod-like molecular of saturated fatty acid was constructed, and the expressions of molecular volume and molecular energy was proposed. Through designing the volume adjusting parameters a(P, T) and the volume energy coordinating parameters m(P, T), the computation of the volume and energy transformation between the free volume and the effective volume within molecule free space. Based on the principle of movement synthesis, the functions of molecular motion and momentum of the rod-like molecular model were constructed. In addition, the expressions of the volume compressibility under equilibrium, local equilibrium and non-equilibrium conditions were deduced.(3)The dependency of the molecular compressibility, Wada’s constant, on temperature and pressure was found, the fitting expression under the fuel working condition was obtained. According to the static compression based experimental data, the specific values of the volume adjustment parameters a(P, T) and the volume energy coordinate parameters m(P, T), as well as the fitting expressions were obtained. By comparing results from the rod-like molecular model, the molecular compressibility model and the ESD state model with the experimental data, the OARD of Methyl Caprate and Ethyl Caprate are1.1%and1.3%on the calculation values between rod-like molecular model and experimental data respectively.the results suggested that the computation of the static volume properties based on the rod-like molecular model can achieve better accuracy.(4) According to the experimental-based temperature and pressure formulas in the injector pressure chamber, the dynamic volume viscosity of the methyl and the ethyl decanoate are calculated. Comparingthese values with the volume viscosity calculated from the sound absorbing experiment gives an overall relative error of4.5%and3.6%respectively. The results indicate thatthe rod-like molecular model gives a better accuracy in accounting for the volume viscosity of a single component of biodiesel.(5) By analyzing the Navior-Stokes equation and the underlying assumptions of fluid properties, Navior-Stokes equation is deemed suitable for the fluid dynamic computation of the injection process. By introducing the dynamic volume viscosity of the biodiesel calculated from rod-like molecular model into Navior-Stokes equation, the temperature and pressure variation in the high-pressure flow of the injector pressure chamber was obtained. The computational results show that raising the temperature of fuel injection pipe can help to reduce the pressure difference between the front and the back of the orifice, and improving the fuel temperature in the orifice will contribute to the enhancement of the atomization of biodiesel.