Optical Study of Transcritical n-Heptane Injection Under Non-Reacting and Reacting Environments

A fundamental understanding of atomization, vaporization, and mixing under transcritical conditions still needs to be accomplished. In transcritical injection, the injected fluid encounters thermodynamic conditions above the critical pressure and critical temperature. Experimental techniques such as Schlieren, diffuse back-illumination (DBI), soot-DBI, and OH chemiluminescence could shed light on individual aspects of the injection subjected to these conditions. On a step toward understanding this phenomenon, this work proposes a combination of the aforementioned techniques to study reactive transcritical injection. To this end, optical measurements at high repetition rates (approximately 50,000 frames per second) are performed in a constant pressure combustion chamber. The studied operating conditions for n-heptane include both non-reacting and reacting environments. Temperatures range from 0.74 to 2 times the critical temperature, whereas chamber pressures vary from 1.04 to 4.21 times the critical pressure in order to study different mixing regimes. Different injection pressures (up to 2200¿bar) are tested for a total of 48 operating points. For the non-reacting environment, Schlieren provides information regarding penetration of n-heptane, while DBI reveals light extinction as the fluid penetrates the ambient. OH chemiluminescence permits the measurement of liftoff length (LOL) and ignition delay (ID) for the reacting environment. N-heptane penetration obtained from Schlieren follows expected trends from injection literature. The spatial extinction gradients from DBI imaging smooth out as the ambient temperature and chamber pressure increase, suggesting a thickened mixing layer where phases are not easily identified. For the reacting environment, the major finding is that LOL does not significantly change with ambient temperature nor injection pressure, and it decreases with ambient density. This fact suggests a different mixing mechanism for the studied reacting conditions.