LES and RANS simulations of flow and combustion in NG/HCNG engines

The main objective of this thesis project is to give a contribution to the definition of the optimal CFD simulation methodology for NG engine investigation, so as to support the design process of innovative engines featuring a low environmental impact. The first part of this study presents the effect of hydrogen addition and EGR dilution on two different SI engines with different turbulence characteristics. The focus of the research has been to analyze the effect of different fuels and mixture composition by comparing experiments and corresponding numerical models. Natural gas has been a focus of research as an alternative fuel due to its chemistry which helps in reducing the CO2 emissions and its high knock tolerance. The slow laminar burning velocity of methane has been an associated drawback which can be overcome by addition of hydrogen in various fractions. This theory is supported by experiments performed on low a turbulence engine. Experimental results show that for stoichiometric condition, hydrogen addition to CNG has been beneficial in terms of bsfc and unburned hydrocarbon emissions [1] and increases the EGR tolerance of the engine by 33% by adding 15 % H2 (% vol). The results are confirmed by numerical results. Addition to this, the increase of turbulence in local flow field leads to greater flame wrinkling of the flame front and thus increases the turbulent burning rate. This combined effect of hydrogen and high turbulence leads to faster overall combustion and thus increases the EGR tolerance. The effect of hydrogen addition and EGR on combustion regime has been studied and is explained with the help of 3D results. In the second part of this thesis, LES simulations were studied, with reference to an injection event in a direct injection NG engine. The goal was to assess strengths and weaknesses of RANS and LES approaches by applying them to the same fluid-dynamic problem, at the same operation points. The effect of changing the turbulence model was then studied with the help of 3D results. [1]. Baratta, M., d’ Ambrosio, S., Misul, D., & Spessa, E. (2014). Effects of H2 Addition to Compressed Natural Gas Blends on Cycle-to-Cycle and Cylinder-to-Cylinder Combustion Variation in a Spark-Ignition Engine. Journal of Engineering for Gas Turbines and Power, 136(5), 051502. doi:10.1115/1.4026163