Hydrogen combustion analysis via infrared and visible optical diagnostics combined with CFD in a dual fuel engine at low load

Dual Fuel (DF) combustion concept has demonstrated to be a valid solution to ensure the utilization of diesel engines in heavy duty and naval applications. The substitution of part of diesel fuel with a cleaner gaseous one can reduce among others: carbon dioxides (CO2), nitrogen oxides (NOx) and surely particulate matter (PM). Nevertheless, the difficulties encountered by the reduced pilot liquid jet to reach out the furthest areas of the combustion chamber, especially at partial load, lead to unburned emissions of the premixed charge. They can be a serious issue to overcome when the premixed fuel is represented by a carbonaceous fuel like natural gas (NG). For this reason, hydrogen, that is carbon-free, represents the new progression to clean mobility. This work aims at deepening the in-cylinder phenomena through a synergetic methodology that involves experimental optical diagnostics and 3D numerical investigations. The experimental activity is carried out on a single cylinder research engine (SCRE) equipped with an optical apparatus allowing the visualization of the combustion process, later processed via high speed visible (VIS) and infrared (IR) imaging. Several filters applied on the IR camera permit to detect specific species such as CO2. Subsequently, these outcomes are used to validate numerical models, and in turn, the numerical results help to validate the species detected qualitatively via optical diagnostics and to identify which ones majorly affect the physical and chemical processes. Numerical simulations are performed with ANSYS Forte ® code on a geometry which accurately reproduces the shape of the combustion chamber. Combustion models include a turbulent-kinetic interaction model with a mechanism of 124 species and 660 reactions to deal with the autoignition phase and diesel surrogate oxidation, while to account for the flame propagation through the premixed charge the G-equation is considered. The reference test cases are characterized by two different engine speeds, 1500 and 2000 rpm, and a low load level, corresponding to 2 bar of brake mean effective pressure (BMEP) on light-duty vehicle. The hydrogen energy shares are between 83.2 and 81.8%. Results demonstrate that IR images and OH radical are key for the detection of ignition process of both diesel and hydrogen. The flame propagation in the air/H2 charge can be observed with the help of CFD elaborations. Finally, emission levels are adequately predicted by the chosen oxidation models.