Optical study on the single and multiple regions of flame propagation and combustion characteristics of methane/air mixture ignited by pilot diesel

The combustion characteristics of lean methane/air mixtures ignited by single and multiple diesel sprays were investigated using an optical RCEM test bench. The experimental approach included flame natural luminescence photography, pressure data acquisition, and heat release analysis. The results revealed that the relationships of ignition delay and orifice diameter varied under single diesel and dual fuel combustion mode. Under dual fuel mode, increasing the orifice diameter of the single-orifice nozzle resulted in a reduction in ignition delay, an acceleration in flame propagation, and an increase in the heat release rate. Increasing the number of orifices multiplied the flame regions, expanded the flame propagation pathways, and enhanced the promoting effect between the flames, resulting in faster flame propagation and increased heat release. The orifice axis angle significantly affected the ignition position and flame propagation direction. An appropriate axis angle would shorten the flame propagation distance, optimize the flame propagation direction and mitigate the impediment effect between flames.