| As the problems of energy shortage and air pollution become more and more serious, it is very urgent to find new engine fuels and reduce engine emissions. Regarded as the most practical alternative engine fuels, gaseous fuels have been widely researched in engines. Besides, it has been found that hydrogen addition has a significant improvement effect on engine performance and an obvious reducing effect on engine emission. As the base of engine combustion, it is significant to study the laminar flame. Besides, it is very helpful to reduce the engine particle emission by researching the process of soot formation.The laminar premixed flame speed of hydrogen blended methane, ethane and propane along with the laminar premixed flame speed of hydrogen blended methane under different outlet gas temperature was measure on a counter-flow configuration using Particle Image Velocimetry(PIV). A numeric study was conducted to analysis the effect of hydrogen addition and outlet gas temperature on the flame speed. The soot formation in hydrogen blended acetylene premixed flame was investigated on a premixed flame configuration using Thermophoresis Sampling Particle Diagnostic(TSPD) method and Transmission Electron Microscope(TEM) analysis method.The result shows that the flame speed of hydrogen blended methane, ethane and propane increases with the hydrogen content. An approximate linear correlation was found between flame speed and the hydrogen content. The hydrogen addition shows a stronger effect on methane flame speed than ethane and propane. A sensitivity analysis shows that the reaction R1: H+O2<=>OH+O has a significant accelerating effect on hydrogen blended methane, ethane and propane flame speed. Besides, as the hydrogen content rises, the accelerating effect of R1 enhances. In addition, the mole fraction of H, O and OH free radicals in the flames increases with the hydrogen content. So, the reason why hydrogen can accelerate the flame speed of methane, ethane and propane is that the H free radical mole fraction increases when hydrogen is added to the fuels which accelerates the reaction rate of R1. Contrasted the numerical results of hydrogen blended methane, ethane and propane flame, it is found that R1 has a stronger effect on methane than the others and the mole fraction of free radicals in methane/hydrogen flame increase more obvious which all explain that the hydrogen addition has a stronger effect on methane flame speed.The flame speed of methane/hydrogen increases in all hydrogen fractions with the rose of outlet gas temperature and the approximate linear correlation was also observed between flame speed and the hydrogen content at different outlet temperature. However, the accelerating effect of R1 on methane/hydrogen flame speed decreases as the outlet temperature rises. The reason is that the reaction rate of the whole elementary reaction increases as the temperature rises, which can be demonstrated by the rise of mole fraction and the rate of production of H, O, OH and CH3 in the flame, as a result the effect of R1 weakens by contrast. Hence, the flame speed of methane/hydrogen increases with the temperature is because of the reaction rate of the whole elementary reaction increases as the temperature rises.The study of soot formation shows that the practical soot formation process is consistent with the theoretical process. The flame temperature decrease as the hydrogen fraction increases and the flame temperature tend to rise with the declination of equivalence ratio. The suppression effect on soot formation in hydrogen addition acetylene flame is due to the decrease of acetylene content and the chemical and dilution effect of hydrogen. The average diameter of soot particle decreases with hydrogen addition and decrease of equivalence ratio. A further analysis shows that hydrogen addition has a suppression effect on soot nucleation and mass growth process, while decrease of equivalence ratio can suppress the particle's mass growth process. |
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