A numerical study of the multi-component effects on the combustion and evaporation of biofuels and blends.

摘要

Biodiesel fuels and their blends with diesel are often used to reduce carbon trace and to help reducing engine emissions. However, previous studies have shown mixed effects of biodiesel on NOx emissions. Operating a compression-ignition engine in low-temperature combustion mode as well as using multiple injections can reduce NOx emissions. The effects of injection timing, spray angle and fuel composition are studied using a modified version of KIVA 3V code. The objectives of this research include: (1) to examine the effects of fuel on engine performance and emissions; (2) to study the effects of spray angle on flow patterns and pollutants formation using a discrete multi-component approach; (3) to develop a new droplet evaporation model using the continuous thermodynamics formulation, which is capable in accommodating multiple distribution functions, accounts for preferential evaporation, finite diffusion and surface regression of the droplet; (4) to demonstrate the applicability of the proposed model in engine applications. A numerical study is also conducted to study the effect of spray angle in a small bore high speed direct injection engine. Soot located in the squish region or the region above the piston bowl are readily oxidized due to abundance of oxygen. Portions of fuel are burnt in the region about the piston bowl or squish for both spray angles of 150° and 70°. Soot located within the piston bowl is oxidized at a much slower rate due to deficient of oxygen after combustion. Soot emissions are mainly due to soot remaining within in the piston bowl at the end of combustion cycle. Any strategy that pushes soot out of the piston bowl can improve the oxidation process, thus, reducing soot emission. Extra oxygen in biodiesel also helps in reducing the emission. The effects of variable cone angle spray on the performance of a diesel engine are studied. Using a variable cone angle injection extends the range of injection time without engine wall wetting, which decreases soot and unburnt hydrocarbon emissions. The numerical predictions show a 10% improvement in thermal efficiency without compromising NOx and soot emissions.;The study shows that the evaporation of the fuel affects the ignition behavior and combustion quality. Therefore, a thorough understanding of the evaporation and mixing processes is essential for further improvement in engine performance. A multi-component droplet evaporation model, as efficient as a traditional zero-dimensional model, yet preserving the correct description of the underlying physical process is developed in this study. The continuous thermodynamics formulation is used, for which the fuel (or liquid mixture) is described using a probability distribution function. The variation of composition in both liquid and vapor phases is represented by tracing the changes of the probability distribution function parameters. In the present study, the gamma distribution is used to represent the fuel fractions, and, the composition is tracked by tracing the mean and standard deviation of the distribution function. Finite diffusion, internal circulation, surface regression and high pressure effects are all accounted for. The model is shown to reproduce in a satisfactorily manner, experimental measurements adopted from the literature. The model is applied to predict the evaporation of single component (distribution) and multi-component droplets. The results show that the proposed model predicts the important distillation characteristics of practical fuel, which cannot be reproduced by a single component model.