Characterization of diesel spray combustion under micro-hole and ultra-high injection pressure conditions-analyses of diffused back-illumination imaging and OH* chemiluminescence imaging

This study investigated the spray combustion characteristics of diesel fuel using the Diffuse Back-illumination Imaging (DBI), Direct Photographic, and OH* chemiluminescence methods under different injection pressures (100–300 MPa) and different hole diameters (0.07–0.133 mm). The results indicated that at a certain point, the downstream of the spray, which holds a strong turbulent mixing, starts to disappear gradually without bright flames due to the cool flame combustion process. Based on this process, the ignition timing was determined using the space integral of intensity and optical thickness through the DBI method, which was validated by the OH* chemiluminescence method. The utilization of a micro-hole diameter injector and ultra-high injection pressure can effectively reduce ignition delay. Significant oxidation processes were observed both downstream and upstream for the micro-hole injector under ultra-high injection pressure. A parameter utilizing spatially integrated natural luminosity to OH* chemiluminescence ratio showed that increasing injection pressure and reducing hole diameter effectively reduced soot under unit fuel oxidation conditions. And predicted model results of droplet diameter and equivalence ratio (Siebers' and Hiroyasu's model) were used to analyze the experimental results. These findings contribute to the understanding of spray combustion characteristics and inform the development of efficient and low-emission combustion systems.