Experimental and Numerical Investigation on Soot Behavior of Soybean Biodiesel under Ambient Oxygen Dilution in Conventional and Low-Temperature Flames

摘要

Biodiesel is a type of particularly attractive alternative fuel for diesel engines. Many studies have investigated the combustion and emissions as fueling biodiesel on diesel engines and constant volume chambers. However, the understanding of the processes of biodiesel soot formation/oxidation is still limited. Therefore, in this work, high time-resolved quantitative soot measurements were carried out on a constant volume chamber by fueling soybean biodiesel. Three different ambient oxygen concentrations (21%, 16%, 10.5%) were tested at a conventional ambient temperature (1000 K) of diesel engine combustion and a lower ambient temperature (800 K). Results showed that the soot appearance was delayed at lower ambient temperatures and oxygen concentrations. At 800 K, less soot mass was observed with decreasing in oxygen concentration. However, soot mass increased with decreasing oxygen concentration as the ambient temperature reaching to 1000 K. To further illuminate the opposite trend on soot behavior in different temperature flames, a semiempirical biodiesel soot model was proposed and implemented into computational fluid dynamics (KIVA-3V, Release 2) code. Validation results showed that the proposed biodiesel soot model could successfully reproduce the entire process of soot formation and oxidation under various oxygen concentrations and ambient temperatures. With decreasing temperature, the appearance of intermediate species about soot formation/oxidation was delayed and the time-integrated mass of C_2H_2 ,soot precursors, OH radicals, and soot was reduced. The soot formation mechanism dominated soot evolution and caused a lower soot mass as the ambient temperature decreased. The formation of soot precursors presented a stronger temperature dependence than biodiesel pyrolysis. Regardless of whether the initial ambient temperature was 800 K or 1000 K, soot oxidation was significantly suppressed as the ambient oxygen concentration was reduced. However, the temperature did change the evolutionary tendency of soot formation with decreasing ambient oxygen concentrations. At 800 K, the time-integrated mass of acetylene and soot precursors and the regions of high equivalence ratios were reduced as the ambient oxygen concentration decreased; therefore, the soot formation was inhibited effectively at lower oxygen concentrations. At 1000 K, the time-integrated mass of acetylene and soot precursors and the regions of high equivalence ratios increased with the decrease of ambient oxygen concentration; therefore, the soot formation was motivated at lower oxygen concentrations. It can be concluded that soot formation transition was the responsible factor for the nonconsistent soot behavior, because of ambient oxygen dilution in conventional and low-temperature flames.