Ignition of hexane-air mixtures by highly under-expanded hot jets

We report an experimental study of ignition of flammable mixtures by highly unexpanded, supersonic hot jets. The high-pressure, hot-gas reservoir supplying the jet is created by impacting a projectile on a plunger to rapidly compress and ignite a rich n-hexane/air mixture, resulting in a peak reservoir pressure of more than 20 MPa. A locking mechanism was used to prevent the plunger from rebounding and the jet was created by rupturing a diaphragm covering a nozzle with an exit diameter between 0.25 and 1 mm. The jet development and ignition processes in the main chamber filled with hexane-air mixture were visualized using high-speed schlieren and OH* chemiluminescence imaging. The ignition threshold was determined as a function of composition in the jet and main chamber, the nozzle diameter, and the initial pressure in the main chamber. Unlike the case of subsonic jets in which ignition occurs at the shear layer near the nozzle exit, ignition of combustion in the main chamber was found to take place downstream of the Mach disk terminating the supersonic expansion and within the turbulent mixing region created by the startup of the supersonic jet. The results are interpreted using a constant-pressure, well-stirred reactor model simulating the mixing between the hot jet and cold ambient gas. The critical conditions for ignition are determined by the competition between energy release due to chemical reactions initiated by the hot jet gas and cooling due to mixing with the cold chamber atmosphere. The critical value (maximum for which ignition occurs) of the mixing rate was computed using a detailed chemical reaction model and found to be a useful qualitative guide to our observations.