A parametric investigation of methane jets in direct-injection compression-ignition conditions

Abstract

This work investigated the effects of ambient and injection conditions on the autoignition, flame evolution and combustion characteristics of the natural gas jets at direct-injection (DI) compression-ignition (CI) conditions. Methane (CH4, as a natural gas surrogate) was directly injected into an optically accessible constant-volume combustion chamber (CVCC). For the experiments, the ambient gas density within the CVCC was fixed at 24kgm−3, but other ambient conditions were varied, including ambient gas temperature (1060–1200K) and ambient oxygen concentration (10–21 vol.%). The effects of injection pressure (10–20MPa reservoir pressure) were also assessed at a fixed ambient temperature and ambient oxygen conditions of 1200K and 21 vol.%, respectively. High-speed schlieren imaging, heat release rate analysis and flame luminosity measurement were applied to the CH4 jet flames. The results show that the ignition delay decreases with increasing ambient temperature or ambient oxygen concentration but does not display an apparent trend with changing injection pressure. Optical images reveal that the CH4 jet combustion typically starts from a localised kernel before propagating downstream of the jet volume in most cases, other than the lower ambient oxygen concentration case. The optical results also reveal that after ignition, the CH4 jet flame recesses back towards the nozzle before stabilising at a lift-off distance. The jet becomes more lifted with a decreasing ambient temperature, a reducing ambient oxygen concentration, and an increasing reservoir pressure. The results also reveal that the heat release rate and flame luminosity profiles roughly correspond to the ignition and combustion characteristics of the CH4 jet flames.