Ignition and Combustion Characterization of Hydrogen/Methane Mixtures by Visualization in a Rapid Compression Machine (RCM)

Experiments has been carried out on a hydraulically actuated rapid compression machine (RCM) under engine like condition using both pure methane and methane with 5 and 30 vol% hydrogen addition with a wide range of excess air ratios (1.0–2.2) for spark ignition (SI) mode. The RCM has a variable compression ratio, charge air preheating and control of cylinder block temperature, allowing both spark ignition and compression ignition without any modifications other than adjustments of the control system. Cylinder pressure and volume recordings were performed. The ignition and early combustion was visualized by a schlieren optical system and a high speed video camera at a rate of 10 000 frames per second. Flame size, and hence the apparent flame propagation velocity is found from the schlieren images, but the images are also used to investigate flame morphology for the different gas mixtures and excess air ratios studied. It was found that hydrogen addition influenced the flame speed and pressure development over the whole range of excess air ratio tested but the effect was less at stoichiometric mixtures. The hydrogen addition was also found to have a strong influence on the flame morphology resulting in smaller cell size. The cell size was found to increase with increasing excess air ratio at the same flame size. The structures in the flames are assumed to be formed by a combination of hydrodynamic and diffusive-thermal instability effects. Further investigations are necessary in order to determine what governs the instability that causes the cell formation. This work is still in progress and further experiments and analysis will be performed to determine the cause and effect of the cellular like flame structures.