Numerical Analysis of the Combustion and Emission Characteristics of Diesel Engines with Multiple Injection Strategies Using a Modified 2-D Flamelet Model

Gyujin Kim, Sunyoung Moon, Seungha Lee, Kyoungdoug Min
2017 Energies  
The multiple injection strategy has been widely used in diesel engines to reduce engine noise, NO x and soot formation. Fuel injection developments such as the common-rail and piezo-actuator system provide more precise control of the injection quantity and time under higher injection pressures. As various injection strategies become accessible, it is important to understand the interaction of each fuel stream and following combustion process under the multiple injection strategy. To investigate
more » ... these complex processes quantitatively, numerical analysis using CFD is a good alternative to overcome the limitation of experiments. A modified 2-D flamelet model is further developed from previous work to model multi-fuel streams with higher accuracy. The model was validated under various engine operating conditions and captures the combustion and emissions characteristics as well as several parametric variations. The model is expected to be used to suggest advanced injection strategies in engine development processes. CFD simulation is a good option for the analysis of these complex systems due to the increased computing power, and can provide quantitative information and intuitive insights into the combustion process. Several numerical approaches have been proposed to simulate multi-fuel systems by direct calculation using simple reactions [18] [19] [20] . The laminar flamelet concept [21] is an approach in which the instantaneous solution of flamelet equations is related to the corresponding turbulent flow fields. Since the single flamelet concepts do not represent the overall flame structures when multiple injection events are present during a single cycle, the 2-D flamelet model was introduced to describe both fuel streams [22] using different mixture fractions. The dimensional extension for the three-feed system could be applied using three-scale asymptotic analysis, i.e., one for each of the two fuel streams and another for the air stream. In their research, the simulation results capture the pressure and heat release rate of the experiment where split injection was applied. Although it is significant that it was the first suggestion of resolving multiple fuel streams using Representative Interactive Flamelet (RIF) concepts [23] , an increase in computational complexity requires a large amount of CPU time to solve the original 2-D flamelet model. The modified 2-D flamelet model [24] could drastically reduce CPU time without any loss of accuracy by simplifying the region where it is difficult for the reaction to occur. Although the model agrees with the experimental results for ignition delay time in a constant-volume vessel, a more specific evaluation of the robustness and availability of the model is required to apply to the engine conditions. Although Kim et al. [25] suggested a different method to solve these regions that showed good agreement with the pressure and heat release rate of the engine experimental results, the lack of validated conditions and a quantitative comparison with the previous method remains a challenge. The original 2-D flamelet model was extended to mimic a combustion process using more than three fuel streams from different times [26] , so it is applicable to the development process of commercial engines. The concept of the work in [26] is to introduce a method called the 'collapse method' where the 2-D flamelet solution can be reduced to a single dimension just before the third injection. This is because it can be considered to have already reached a steady-state if there is sufficient time between the second and third injection. Additionally, the effect of introducing the joint scalar dissipation rate (χ 12 ) in the original 2-D flamelet model has been investigated using DNS approaches, which, for simplicity, are not used in the present work [27, 28] . Based on previous research, the objective of this study is to investigate the physical mechanism of the combustion process of multiple injection using the modified 2-D flamelet model with the introduction of the collapse method. Improvements over the previous work on modified 2-D flamelet models will be shown by comparing the procedures of flame propagation as well as computational time. This paper describes a fundamental investigation of the feasibility and potential of the model being applicable in engine conditions, which presents a direction to obtain optimal injection strategies. Beginning with introducing the development process of the modified 2-D flamelet model, quantitative analysis under various engine operating conditions using the model will be provided. In addition, the applicability of the model to multi-fuel systems will be investigated by describing the computational results under changing engine parameters and suggesting advanced injection strategies that can reduce both NO x and PM emissions. Model Framework Modified 2-D Flamelet Model Much of the numerical research using the laminar flamelet concept shows successful results in diesel engine simulations [29] [30] [31] [32] . Starting with the laminar flamelet model of a single mixture fraction, the 2-D flamelet model [22] , which is described by transforming the physical coordinate of each mixture fraction Z i to the coordinate that is normal to the flame surface, was introduced. With a unity Lewis number, the original 2-D flamelet equations for the species mass fraction and temperature
doi:10.3390/en10091292 fatcat:p525kn33abfbvp2p2exswnolhi