Combustion processes modeling: numerical investigations of an engine equipped with a Turbulent Jet Ignition system and of a scramjet combustor

Combustion is at the base of many energy production processes and, today like never before, research is involved in maximizing combustion efficiency and reducing the pollution deriving from it. Thanks to the high specific energy of fuels, combustion processes will still have a role in the future, especially in the transportation sector. Therefore, increasing Internal Combustion Engines (ICE) efficiency and reducing their polluting emissions has become an imperative object. Among all the non-conventional ignition systems that are being purposed by research, Turbulent Jet Ignition (TJI) seems to be one of the most promising capable to achieve leaner combustion and higher thermal efficiency in spark ignited engine. In a TJI system a jet of high-energy reactive gases is generated by means of a pilot combustion in a pre-chamber and used to initiate the main combustion event in the cylinder. By virtue of this, TJI devices are able to achieve a more stable combustion also with more problematic fuels, such as Methane. In the present work, a deep and innovative analysis approach is purposed and applied to a TJI prototype installed on a Methane fueled optically accessible spark ignition research engine. By means of 3D numerical simulations, the behavior of such engine has been monitored and analyzed over a whole engine cycle. Attention has been paid especially on the scavenging, filling and combustion phases in the pre-chamber. Moreover, the jets characteristics and species distribution and evolution are analyzed in order to study the reactive-jet-induced ignition mechanism of the main charge and the associated fuel conversion mechanism. Attention is also given to pollutant species formation and in-cylinder distribution. The purposed approach allowed the characterization of the main phenomena involved in the operation of such system as well as the evaluation of different parameters, such as combustion duration, flame evolution and pre-chamber ignition energy release. The same engine without the pre-chamber has been modeled as well and used for comparison. The bench test data recorded by Istituto Motori di Napoli CNR were employed both to tune the model and to compare the performance increase due to the TJI device. Parallelly, CFD studies on a scramjet combustor for hypersonic flight purposes fueled with Hydrogen were also carried out. After having validated the model against a set of in-flight representative data, the main physical mechanisms involved in the mixing and combustion processes were analyzed. Moreover, various information about the complex flow patterns and structure were retrieved.