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Combustion Diagnostics
Published in Achintya Mukhopadhyay, Swarnendu Sen, Fundamentals of Combustion Engineering, 2019
Achintya Mukhopadhyay, Swarnendu Sen
Schlieren imaging is used in combustion diagnostics to identify the flame structure. The principle depends on variation of the refractive index of the medium. Light rays are shifted due to refraction in a medium. If the refractive index varies at different places, the refractive shift will be different. The main reason for change in the refractive index is density variation. Again, density variation can be caused by temperature variation or concentration variation. If the refractive shift can be quantified, it can be calibrated with density variation and in turn with temperature variation or concentration variation (Figure 12.6).
Analysis of Markers for Combustion Mode and Heat Release in MILD Combustion Using DNS Data
Published in Combustion Science and Technology, 2019
Past DNS studies investigated the adequacy of commonly used HRR chemical markers and suggested that two-scalar markers such as or rather than a single scalar were good in identifying HRR regions in MILD combustion (Chi et al., 2018; Minamoto and Swaminathan, 2014; Nikolaou and Swaminathan, 2014; Wabel et al., 2018). However, these studies are for premixed combustion under either conventional or MILD conditions. Here, our interest is to extend those assessments of HRR markers for MILD combustion with mixture fraction variation using DNS data of Doan et al. (2018). This specific interest is because the inception of MILD combustion does not follow the classical routes due to the chemical kinetic role of radicals present in the unreacted mixture as has been shown by Doan and Swaminathan (2019). Also, the presence of both premixed and non-premixed modes in MILD combustion with mixture fraction variation was shown by Doan et al. (2018) using the Flame Index (FI) analysis (Briones et al., 2006; Yamashita et al., 1996). Recently, Hartl et al. (2018) suggested that the chemical explosive mode analysis (CEMA) of Lu et al. (2010) can be used to distinguish premixed from non-premixed regions in partially premixed combustion. Hence, a comparative analysis using the above two, FI and CEMA, techniques is of interest here. Furthermore, MILD combustion is expected to give nearly homogeneous temperature and density fields and thus, the schlieren imaging could be used to distinguish combustion under conventional and MILD conditions. This will also be explored here.
Study of Flame Kernel Development at High EGR and High Flow Speed Using Conventional Spark Igniter and Non-Thermal Plasma Under Gasoline Engine Relevant Conditions
Published in Combustion Science and Technology, 2022
Jiachen Zhai, Seong-Young Lee, Zhihao Zhao, Dan Singleton
A number of studies disclose the fact that non-thermal plasma is capable of reducing the ignition delay, enhancing the flame propagation when dealing with low-pressure conditions, and lean mixtures. However, previous studies only focus on simple gas phase fuel and air mixture, such as methane, ethylene, propane etc. In contrast, gasoline that consists of much complex components and high dilution (high EGR percentage) increases the complexity of mixture. Moreover, NRPD-based non-thermal plasma ignition has never been explored under high-speed cross flow conditions which may cause misfire. Hence, in this study, gasoline-air and gasoline-EGR mixtures are adopted to study and compare the initial flame development of two ignition systems: NRPD-based non-thermal plasma and conventional spark ignition system. In a constant volume combustion chamber (CVCC), experiments of two ignition systems are conducted under gasoline engine relevant conditions: initial ambient pressure ranges (6.5, 8.3, 11.3 bar), a range of equivalence ratios (0.7–1.0), EGR percentages (10% − 25%), and cross flow speeds (0–30 m/s). The range of equivalence ratio in the current study is determined by lean conditions in modern internal combustion (IC) engines. Modern IC engine operates at around stoichiometric air-fuel ratio with the advancement of gas after-treatment techniques. Schlieren imaging technique is utilized to capture the early flame kernel development near the ignition plug. Characteristics such as flame kernel radius, flame propagation rate, and flame front length ratio are quantified and analyzed. Concurrently, the ignition delay, combustion phase based on the chamber pressure history, and ignition probability are calculated.