Spray and combustion characterization under an ultra-high-density condition – Multi-fuel comparison

Abstract

This study reveals for the first time the ignition and combustion characteristics of different fuel properties under ultra-high-density ambient condition. A new pre-burn type optically accessible constant volume combustion chamber was established, allowing for operation at pressures exceeding 300 bar. This chamber was utilized to generate a spray combustion environment at 150 bar and 1000 K, corresponding to a bulk-density of 50.82 kg/m3. Diesel was used as the reference fuel, and three types of primary reference fuel (PRF), i.e., PRF0 (n-heptane), PRF50, and PRF100 (isooctane), were used as the main test fuels. OH* chemiluminescence was utilized to measure the stable flame lift-off length, while the broadband chemiluminescence technique was used to measure the ignition delay time and to capture time-resolved flame development. The results show that the ignition delay of PFR100 is significantly larger than that of other fuels due to its lowest cetane number. Flame visualization results indicate that diesel spray combustion aligns well with the conceptual model proposed by Dec [SAE transactions, 1997: 1319–1348], whereas noticeable wrinkles are observed in the flames of PRF50. The flame lift-off length during the quasi-steady period shows to be independent of fuel properties, but the value has notably reduced to less than 9 mm. Comparison of the multi-fuel luminosity images reveals that diesel experiences the most pronounced combustion recession. Spatially integrated natural luminosity analysis indicates that diesel has the longest combustion duration, chiefly due to a larger rate of injection, whereas the difference in combustion duration between different PRFs is negligible. The heat release rate error range for PRF0 is the largest, suggesting stronger combustion variations. The heat release rate of diesel combustion is the most robust. The peak value of AHRR of PRF50 and PRF100 is marginally higher than that of diesel, which can be attributed to their higher low heating value.